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Published by the Florida Association of Equine Practitioners, an Equine-Exclusive Division of the Florida Veterinary Medical Association Issue 4 • 2019

A LOOK AT NON-TRADITIONAL CAUSES OF POOR PERFORMANCE

UPDATE ON THE EPIDEMIOLOGY OF RHODOCOCCUS EQUI FOAL PNEUMONIA

ERIN K. CONTINO MS, DVM, DACVSMR

NOAH D. COHEN VMD, MPH, Ph.D., DACVIM-LA

STANDARD AND ALTERNATIVE TREATMENT OF RHODOCOCCUS EQUI FOAL PNEUMONIA

DIAGNOSIS AND MANAGEMENT OF EQUINE SPINAL ISSUES: HOW THE FOOT MATTERS

NOAH D. COHEN VMD, MPH, Ph.D., DACVIM-LA

TRACY A. TURNER DVM, MS, DACVS, DACVSMR


EXECUTIVE COUNCIL

The President's Line

RUTH-ANNE RICHTER

BSc(Hon), DVM, MS FAEP COUNCIL PAST PRESIDENT

Dear Fellow Equine Practitioners,

rrichter@surgi-carecenter.com

With 2019 wrapped up and 2020 just beginning, the Florida Association of Equine Practitioners has had another exciting year! From successful programs to engaging content in each issue of The Practitioner, we continue to strive to provide world-class, equine-exclusive educational programs and content for our members.

ADAM CAYOT

Our 15th Annual Promoting Excellence Symposium (PES), which was held in October 2019 in Fort Myers, Florida, proved once again to be a success. Hosted this past year at the Sanibel Harbour Marriott Resort & Spa, our attendees enjoyed an exquisite venue while attending exceptional CE lectures presented by distinguished equine veterinary faculty from across the country. The Comprehensive Equine Ultrasound Wet Lab at PES 2019 was held at Track to Trail, Inc. in Naples, Florida. The sold-out wet lab was a favorite for many attendees. We would like to thank all PES 2019 speakers, wet lab instructors, resort staff, the FAEP Council and our FAEP staff for putting on another great program.

DVM

adamcayot@hotmail.com

AMANDA M. HOUSE DVM, DACVIM

housea@ufl.edu

COREY MILLER DVM, MS, DACT

cmiller@emcocala.com

ANNE L. MORETTA VMD, MS, CVSMT

maroche1@aol.com

JACQUELINE S. SHELLOW DVM, MS REPRESENTATIVE TO FVMA EXECUTIVE BOARD

jackie@shellow.com

The beginning of 2020 also means the 57th Annual Ocala Equine Conference! OEC 2020 was held, once again, at the Hilton Ocala in Ocala, Florida from January 24-26. We had another full and exciting program this year, including the ever-popular Comprehensive Equine Ultrasound Wet Lab. The wet lab was held at the Peterson & Smith Equine Reproduction Center on Friday, January 24, and it received rave reviews from attendees. Thank you to all OEC 2020 speakers, wet lab instructors, hotel staff, the FAEP Council and our FAEP staff for helping to make this year a smash! We encourage you to mark your calendars and save the date for the 16th Annual Promoting Excellence Symposium from October 8-11, 2020, at the Sawgrass Marriott Golf Resort & Spa in Ponte Vedra Beach, Florida. The FAEP Council wouldn’t be able to provide you with the quality educational programs we do without the support of our educational partners. We thank them for all they do to provide us with the resources necessary to put on our educational programs. If you’re interested in becoming a 2020 FAEP Educational Partner, contact the FAEP/FVMA at 800.992.3862 for more information on how to connect with equine veterinary professionals across Florida and the U.S. I hope everyone enjoyed a wonderful holiday season, and I send everyone warm wishes going into this year!

PHILIP J. HINKLE EXECUTIVE DIRECTOR

phinkle@fvma.org

Armon Blair, DVM FAEP Council President

Opinions and statements expressed in The Practitioner reflect the views of the contributors and do not represent the official policy of the Florida Association of Equine Practitioners or the Florida Veterinary Medical Association, unless so stated. Placement of an advertisement does not represent the FAEP’s or FVMA’s endorsement of the product or service. FAEP | 7207 MONETARY DRIVE, ORLANDO, FL 32809 | PH: 800.992.3862 | FAX: 407.240.3710 | EMAIL: INFO@FVMA.ORG | WEBSITE: WWW.FAEP.NET

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Issue 4 • 2019


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A LOOK AT NON-TRADITIONAL CAUSES OF POOR PERFORMANCE ERIN K. CONTINO | MS, DVM, DACVSMR Proceedings adapted from Contino, EC. “Going the extra mile — working up cases of poor performance” 2019 AAEP Focus on Sports Medicine and Rehabilitation Meeting Proceedings.

A 5-year-old Warmblood gelding that presented for poor performance. Despite refusing to go forward with a rider, he would willingly go forward when tacked and fitted with elastic resistance bands (seen going from the saddle pad around the hindquarters and under his abdomen) and long side reins. He became increasingly resistant to go forward as the side reins were shortened, demonstrating the importance of evaluating cases of poor performance under as many circumstances as possible. Photo courtesy of Dr. Erin K. Contino

Introduction

Pain Versus Behavior

Sport horses commonly present for poor performance rather than The first question that often arises in cases of poor performance overt lameness, and these cases can be challenging to workup. is whether it is behavioral or due to pain. While many riders and While the underlying cause is often musculoskeletal in nature, trainers are quick to assume the issue is behavioral, it is the author’s there are many other potential causes that should be considered. opinion that most of these cases have an underlying physical reason Even with a thorough musculoskeletal and lameness examination, for the behavioral changes. It has been documented that riders are and diagnostics including diagnostic analgesia and diagnostic poor judges of the presence of lameness. In a prospective study imaging, a definitive diagnosis is not always reached. In such cases, of 506 horses considered sound by their riders, only 54.3% were the practitioner needs to be open-minded and often creative in free of overt lameness, as assessed by an experienced veterinary order to get to the root of the cause. There are a variety of ways to examiner.1 work through these difficult cases, many of which will be discussed in this article.

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Issue 4 • 2019


Recently, Dyson and colleagues have developed an ethogram for A positive response to treatment can be used to help confirm the ridden horses with the goal of determining if facial expressions diagnosis. Some cases of axial skeleton pain require treatment with can be used to differentiate lame horses, or those in pain, from a systemic course of corticosteroids (1 mg/kg prednisolone PO q those that are sound.2 The 24-point ethogram includes evaluation 24 hours for 14-30 days, followed by 0.5 mg/kg PO q 24 hours for of head and ear position, eyelids, sclera, mouth expression, tail 14-30 days, followed by 0.5 mg/kg PO q 48 hours for 14 days) to activity, gait patterns and evasions — such as bucking and get clinical improvement. The author hypothesizes that these cases rearing — among several other expressions. Of 21 horses that may have a component of neuritis (e.g. inflammation of the nerve were evaluated by experienced and non-experienced observers, root ganglion), in which case the addition of gabapentin (typically there was a significant difference between the behavioral scores 10-15 mg/kg PO q 8 hours) may also be beneficial. before diagnostic analgesia (mean score nine out of 24) and after (mean score four-and-a-half out of 24). These results In the absence of overt lameness and/or in horses not amenable suggest that musculoskeletal pain can be manifested in various to diagnostic analgesia, nuclear scintigraphy can be considered. It facial expressions and behaviors. Integrating the ethogram in the is important to note that nuclear scintigraphy should not be used evaluation process may help practitioners determine whether an issue in isolation. A recent study evaluated 480 horses that underwent is behavioral or pain related. nuclear scintigraphy for lameness or poor performance. Of 1,222 diagnoses made, only 56.4% were associated with increased An older and more traditional way of trying to differentiate pain radiopharmaceutical uptake on the nuclear scintigraphic from behavior is a “bute trial,” where the horse is placed on a examination. Sensitivity was only 43.8% but specificity was high course of phenylbutazone (4.4 mg/kg q 24 hours for five to 10 at 94%, making nuclear scintigraphy a poor screening tool with days) and then ridden consistently throughout the trial period to a high rate of false negatives. Conversely, with high specificity, if determine if the behavior changes. If the horse improves during the there is increased pharmaceutical uptake, it is likely to indicate a trial, it is assumed that there is a pain or inflammatory component clinically relevant anatomic area.5 to the behavioral issues. In the author’s experience, horses with axial skeletal pain do not respond as well to bute trials compared The Ridden Examination to horses with pain in the limbs; therefore, failing a bute trial does A recent study reports that horses are often lamer under saddle not rule out the presence of pain. In other words, a bute trial can than in hand6; therefore, in the absence of overt lameness in hand, help rule in pain as a factor but should not be used to definitively a ridden examination is warranted. Additionally, this enables rule out pain as a factor. A trial of oral corticosteroids could be the practitioner to evaluate the fit of the tack and to evaluate the considered over a bute trial if axial pain is suspected. horse under different conditions — with different saddles, bits (or without a bit), riders, environments and levels of collection. The Musculoskeletal Causes of Poor Performance results of these experiments can help the practitioner troubleshoot There are several musculoskeletal conditions that can present as possible contributing factors. For example, it has been shown that poor performance rather than overt lameness. In sport horses, saddle slip is highly correlated with hind limb lameness with the sacroiliac joint-region pain and hind limb proximal suspensory saddle usually slipping toward the side of the lameness.6 desmopathy are two such examples. For practitioners that are comfortable performing diagnostic analgesia of the sacroiliac Some conditions may be dependent on head position and region, there can be a dramatic difference in the horse’s way of sometimes this can be more easily evaluated on the lounge with a going before and after analgesia of the region. Notably, horses lounging surcingle and side reins. This may also be a safer way to are often less spooky, have improved gait quality, carry a more evaluate horses that are showing severe disobediences under saddle. consistent rein contact and have increased hind limb propulsion.3 Similarly, if a ridden examination is not possible, the horse can be Similarly, hind limb proximal suspensory desmopathy may present evaluated on the lounge with a weighted surcingle. Some examples as poor performance rather than lameness. Signs can include of conditions that would be expected to worsen due to certain head resistance to move freely forward, lack of hind limb engagement positions, tack and/or with ridden exercise include mouth and jaw and impulsion, and difficulty or evasion when asked to perform issues (i.e. teeth, tongue and/or temporomandibular joint), kissing certain movements.4 Evaluating the horse under saddle before and spines, certain upper airway conditions and cervical articular after analgesia of the deep branch of the lateral plantar nerve can facet joint osteoarthrosis that may or may not be associated with aid in the diagnosis of this condition. cervical nerve root neuritis. Some additional and perhaps more surprising conditions that have manifested as gait abnormalities Additional musculoskeletal conditions that can present as poor include temporohyoid osteopathy and gastric ulcers. performance or behavioral issues include axial skeleton issues, such as osteoarthrosis of the cervical and/or thoracolumbar articular Other Causes of Poor Performance facet joints or impinging spinous processes. If abnormalities are While the number of conditions leading to poor performance found on imaging studies of the axial skeleton, diagnostic analgesia are seemingly endless, additional major categories to consider of the region can be performed, but trial treatment — often with are cardiovascular issues, respiratory issues and muscle diseases. corticosteroid infiltration of the affected region — is often elected. Cardiovascular and respiratory issues will most commonly present

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Photo A: A 15-year-old Warmblood mare being ridden on a straight line at the walk. Note the right front limb crossing in front of the left front rather than tracking straight — a gait abnormality that, in this horse, presented only when ridden. The gait could be reproduced with a weighted surcingle. Photo courtesy of Dr. Erin K. Contino

as exercise intolerance and will not be discussed further in this article. Muscle diseases are a common source of poor performance in sport horses, and an excellent summary is available in the recent edition of Veterinary Clinics: Equine Practice.7 A good example is polysaccharide storage myopathy (PSSM). Clinically, horses with PSSM can present with a reluctance to move forward under saddle, chronic back and lumbar muscle pain, lack of bascule over fences, undiagnosed lameness, reluctance to collect and poor performance.7 An initial screening test that can easily be performed in the field is an exercise test where a baseline blood sample is drawn twice: once before the test begins and again four hours after a lounging exercise (15 minutes of walk and trot on the lounge). A three-fold or greater increase in serum CK is suggestive of PSSM. Unfortunately, Type II PSSM requires a muscle biopsy for diagnosis. As this is a more invasive procedure, a muscle biopsy should be reserved for when other potential causes have been ruled out and/or there is a high level of suspicion for a primary muscle disorder.

6  The Practitioner 

Photo B: A computed tomography image of the cervical spine at the level of C6-7 of the same horse. Left is to the left; dorsal is at the top. There is narrowing of the left intervertebral foramen (solid arrow) compared to the right (dashed arrow), and it was suspected that this mare suffered from nerve root impingement at this location. The gait abnormality partially improved, but did not resolve, following corticosteroid injection of the left C6-7 articular facet joint and corticosteroid infiltration around the nerve. Photo courtesy of Dr. Erin K. Contino

Summary

While many overt lamenesses can be accurately diagnosed with a thorough musculoskeletal and lameness examination, blocking and diagnostic imaging, traditional diagnostics may be insufficient in cases of behavioral issues and/or poor performance. In such cases, it is encouraged to evaluate the horse under saddle; to attempt diagnostic analgesia even in the absence of overt lameness; and to evaluate the horse, rider and tack under as many circumstances as possible. Additional diagnostic tools that can be considered, as guided by the clinical examination, include imaging of the skull and axial skeleton, nuclear scintigraphy, dynamic endoscopy, gastroscopy and trial treatment with systemic medications. These cases can be time-consuming and financially draining, yet equally rewarding if a diagnosis is reached.

Issue 4 • 2019


References 1. Greve L and Dyson S. 2014. The interrelationship of lameness, saddle slip and back shape in the general sports horse population. Equine Vet J Suppl; 46:687-694. 2. Dyson S and Van Dijk J. 2018. Application of a ridden horse ethogram to video recordings of 21 horses before and after diagnostic analgesia: Reduction in behavior scores. Equine Vet Educ. Doi: 10.1111/eve.13029.

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3. Barstow A and Dyson S. 2015. Clinical features and diagnosis of sacroiliac joint region pain in 296 horses: 20042014. Equine Vet Educ; 27(12):637-647. 4. Dyson S. 2007. Diagnosis and management of common suspensory lesions in the forelimbs and hindlimbs of sport horses, Clin Tech in Equine Pract; 6:179-188. 5. Evaluation of the diagnostic accuracy of skeletal scintigraphy in lame and poorly performing sports horses. 2018. Vet Radiol Ultrasound; 59:477-489. 6. Greve L and Dyson S. 2014 The interrelationship of lameness, saddle slip and back shape in the general sports horse population. Equine Vet J 46(6):687-694. 7. Valberg SJ. 2018. “Muscle conditions affecting sport horses”. In: Veterinary Clinics: Equine Practice. Ed: Garcia-Lopez 34(2):253-276.

Erin Contino MS, DVM, DACVSMR Dr. Erin Contino graduated f r o m C o l o ra d o S t a t e University (CSU) in 1999 with a Bachelor of Science in equine science. She worked professionally in the equine industry as a three-day eventer and barn manager prior to returning to CSU, where she obtained a master’s degree in equine radiology in 2009 and her DVM in 2010. Following an internship at Pioneer Equine Hospital, she completed a residency in equine sports medicine and rehabilitation followed by a fellowship in equine musculoskeletal ultrasound. She is currently an assistant professor in equine sports medicine and rehabilitation at CSU. Her research interests focus on clinical diagnosis and treatment of lameness and performance issues in sport horses. She still enjoys competing in eventing.

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UPDATE ON THE EPIDEMIOLOGY OF RHODOCOCCUS EQUI FOAL PNEUMONIA NOAH D. COHEN | VMD, MPH, Ph.D., DACVIM-LA This article will cover new results pertaining to the epidemiology of R. equi pneumonia, including incidence, risk factors and emergence of macrolide resistance. Incidence: Pneumonia caused by R. equi pneumonia is a slowly

progressive disease. Because of its insidiousness, many equine breeding farms have implemented thoracic ultrasonographic screening programs for earlier detection of pneumonia. The rationale for this approach is that earlier detection resulting in earlier implementation of treatment will reduce case-fatality rates and possibly the duration of its treatment.1,2 While objective data for this concept is exiguous, there’s ample evidence that a large proportion of foals with thoracic abscess formation or consolidation identified sonographically will not progress to develop clinical pneumonia.3-8 This phenomenon results in an increased cumulative incidence of pneumonia. When farms elect to treat all foals with thoracic lesions detected songraphically, the cumulative incidence of R. equi pneumonia is increased and more foals are treated with antimicrobials. The cost-benefit of this strategy will be discussed.

Risk Factors: The predominant risk factor for R. equi remains

age with evidence that susceptibility is diminished with age and increased with dose.9-11 The mechanism of this age-related susceptibility is unknown. Although much emphasis has been placed on deficits in T cell adaptive immunity,12 evidence from our laboratory indicates that antibodies, neutrophils and complement play an essential role in protecting neonatal foals from R. equi infection.13-15 Unpublished data from our laboratory indicates that the GI microbiota differs between foals that subsequently develop pneumonia and those that don’t. The GI microbiota is known to drive immune development in other species,16,17 largely through interactions of microbes and microbial metabolites with host intestinal cells. Consistent with the hypothesis of host-microbial interactions promoting immunity, a recent study identified that foals with prior disease — predominately infectious diseases — had decreased odds of developing pneumonia.18 Airborne concentrations of virulent R. equi are higher in stalls of foals that subsequently develop pneumonia than those that don’t, indicating that the magnitude of environmental exposure impacts risk.19,20 This is consistent with evidence that dose, as well as age, impacts the risk of pneumonia following experimental infection.10 Because R. equi are ubiquitous in the environment and commonly shed by horses in their feces,9 and because the virulence plasmid can be readily shared by conjugation,21,22 elimination of the organism from the environment is improbable, if not impossible. Nevertheless, certain management practices can impact the burden of environmental R. equi. Density of mares and foals is a well-established risk factor;9 thus, reducing density would 8  The Practitioner 

Photo courtesy of Dr. Ruth-Anne Richter

likely reduce the incidence of R. equi pneumonia. Transfusion with hyperimmune plasma — discussed in previous research regarding prevention and control of R. equi pneumonia — has been documented to reduce fecal shedding by foals,23 although the impact of this on disease incidence is unknown. Macrolide treatment has been demonstrated to reduce shedding of R. equi in feces of treated foals.24 The risks associated with mass antimicrobial treatment, however, outweigh the benefits.

Antimicrobial Resistance: Our laboratory has been

collaborating with the laboratory of the late Steeve Giguère to describe and investigate the emergence of resistance of R. equi to macrolides and rifampin. In central Kentucky, we have documented that the prevalence of R. equi resistant to macrolides and rifampin has been increasing over the past three decades,25 and these resistant isolates can participate in the environment at affected farms.26 In a recent survey of horse-breeding farms in central Kentucky, macrolide-resistant R. equi were isolated from the majority of farms, and the use of macrolides associated with ultrasound screening was positively associated with isolating macrolide-resistant isolates at these farms (unpublished data).

Despite these gloomy findings, there is some cause for hope. The resistant genotype of R. equi isolated from foals, to date, carries a fitness cost.27 In experiments with soil plots, we have demonstrated that susceptible R. equi will outcompete macrolideresistant isolates in the absence of macrolide exposure, thus reducing macrolide use could lead to a reversal of the problem. This strategy, however, requires the availability and use of effective alternative treatments, which existing research also addresses. Issue 4 • 2019


Cited Literature: 1. Slovis NM, McCracken JL, Mundy GD. How to use thoracic ultrasound to screen foals for Rhodococcus equi at affected farms. Proc. Am. Assoc. Equine Pract. 2005;51:274-278. 2. McCracken JL and Slovis NM. Use of thoracic ultrasound for the prevention of Rhodococcus equi pneumonia on endemic farms. Proc. Am. Assoc. Equine Pract. 2009;55:38-44. 3. Chaffin MK, Cohen, ND, Blodgett GP, Syndergaard M. Evaluation of ultrasonographic screening methods for early detection of Rhodococcus equi pneumonia in foals. J. EquineVet. Sci. 2012;32:S20-S21. 4. Venner M, Kerth R, Klug E. Evaluation of tulathromycin in the treatment of pulmonary abscesses in foals. Vet. J. 2007;174:418-421. 5. Venner M, Rodiger A, Laemmer M, Giguère S. Failure of antimicrobial therapy to accelerate spontaneous healing of subclinical pulmonary abscesses on a farm with endemic infections caused by Rhodococcus equi. Vet. J. 2012;192:293-298. 6. Venner M, Astheimer K, Lammer M, Giguère S. Efficacy of mass antimicrobial treatment of foals with subclinical pulmonary abscesses associated with Rhodococcus equi. J. Vet. Intern. Med. 2013;27:171-176. 7. Venner M, Credner N, Lammer M, Giguère S. Comparison of tulathromycin, azithromycin and azithromycin-rifampin for the treatment of mild pneumonia associated with Rhodococcus equi. Vet. Rec. 2013;173:397. 8. Rutenberg D, Venner M, Giguère S. Efficacy of tulathromycin for the treatment of foals with mild to moderate bronchopneumonia. J. Vet. Intern. Med. 2017;31:901-906. 9. Cohen ND. Rhodococcus equi foal pneumonia. Vet. Clin. North Am. Equine Pract. 2014;30: 609-622. 10. Sanz M, Loynachan A, Sun L, et al. The effect of bacterial dose and foal age at challenge on Rhodococcus equi infection. Vet. Microbiol. 2013;167:623-631. 11. Wada R, Kamada M, Anzai T, et al. Pathogenicity and virulence of Rhodococcus equi on foals following intratracheal infection. Vet. Microbiol. 1997;56:301-312. 12. Giguère S, Cohen ND, Chaffin MK, et al. Rhodococcus equi: clinical manifestations, virulence, and immunity. J. Vet. Intern. Med. 2011;25:1209-1220. 13. Cywes-Bentley C, Rocha JN, Bordin AI, et al. Antibody to polyN-acetyl glucosamine provides protection against intracellular pathogens: mechanism of action and validation in horse foals challenged with Rhodococcus equi. PLoS Pathog. 2018;14(7):e1007160. 14. Liu M, Bordin AI, Liu T, et al. Gene expression of innate Th1-, Th2-, and Th17-type cytokines during early life of neonatal foals in response to Rhodococcus equi. Cytokine 2011;56:356-354. 15. Berghaus LJ, Giguère S, Bordin AI, Cohen ND. Effects of priming with cytokines on intracellular survival and replication of Rhodococcus equi in equine macrophages. Cytokine 2018;102:7-11. 16. Nash MJ, Frank DN, Friedman JE. Early microbes modify immune system development and metabolic homeostasis – the “restaurant” hypothesis revisited. Frontiers Encocrinol. 2017;8:349. 17. Dominguez-Bello MG, Godoy-Vitorino F, Knight R, Blaser MJ. Role of the microbiome in human development. Gut 2019;68:1108-1114. 18. Coleman MC, Blodgett GP, Bevevino KE, et al. Foal-level risk factors associated with development of Rhodococcus equi pneumonia at a Quarter Horse breeding farm. J. Equine Vet. Sci. 2019;72:89-96. 19. Kuskie K, Smith JL, Sinha S, et al. Associations between the exposure to airborne virulent Rhodococcus equi and the incidence of R. equi pneumonia among individual foals. J. Equine Vet. Sci. 2011;31:463-469. 20. Cohen ND, Chaffin MK, Kuskie KR, et al. Association of perinatal exposure to airborne Rhodococcus equi with risk of pneumonia caused by R. equi in foals. Am. J. Vet. Res. 2013;74:102-109. WWW.FAEP.NET |

21. Tripathi VN, Harding WC, Willingham-Lane JM, Hondalus MK. Conjugal transfer of a virulence plasmid in the opportunistic intracellular actinomycete Rhodococcus equi. J. Bacteriol. 2012;194:6790-6801. 22. Stoughton WB, Poole T, Kuskie KR, et al. Transfer of the virulence-associated protein A-bearing plasmid between field strains of virulent and avirulent Rhodococcus equi. J. Vet. Intern. Med. 2013;27:1555-1562. 23. Sanz MG, Bradway DS, Horohov DW, Baszler TV. Rhodococcus equi-specific hyperimmune plasma administration decreases faecal shedding of pathogenic R. equi in foals. Vet. Rec. 2019; doi: 10.1136/ vr.105327. 24. Chaffin MK, Cohen ND, Martens RJ. Chemoprophylactic effects of azithromycin against Rhodococcus equi-induced pneumonia among foals at equine breeding farms with endemic infections. J. Am. Vet. Med. Assoc. 2008;232:1035-1047. 25. Huber L, Giguère S, Slovis NM, et al. Emergence of resistance to macrolides and rifampin in clinical isolates of Rhodococcus equi from foals in central Kentucky, 1995 to 2017. Antimicrob. Agents Chemother. 2018;63:e0714-18. 26. Huber L, Giguère S, Cohen ND, et al. Identification of macrolideand rifampicin-resistant Rhodococcus equi in environmental samples from equine breeding farms in central Kentucky during 2018. Vet. Microbiol. 2019;232:74-78. 27. Willingham-Lane JM, Berghaus LJ, Berghaus RD, et al. Effect of macrolide and rifampin resistance on the fitness of Rhodococcus equi. Appl. Environ. Microbiol. 2019;85:e02665-18.

Noah D. Cohen, VMD, MPH, Ph.D., DACVIM-LA Dr. Noah Cohen received his A.B. from the University of Pennsylvania (Penn) in 1979 and his V.M.D. from Penn in 1983. He was employed in a private equine practice in Toronto for two years. In 1985, he enrolled at the Johns Hopkins University School of Hygiene and Public Health (now the Johns Hopkins Bloomberg School of Public Health) and completed an M.P.H. (1986) and Ph.D. (1988) in epidemiology with emphasis on infectious disease epidemiology. Dr. Cohen then went to Texas A&M University, where he completed a residency in large animal internal medicine. In 1991, he was appointed assistant professor of equine internal medicine in the Department of Large Animal Clinical Sciences at Texas A&M. Dr. Cohen is currently professor of equine internal medicine, the Patsy Link Professor of Equine Research, and the associate department head for research and graduate studies in the Department of Clinical Sciences at the College of Veterinary Medicine & Biomedical Sciences. He also serves as director of the Equine Infectious Disease Laboratory at Texas A&M. Dr. Cohen was named a Texas A&M University Faculty Fellow in 2002. His research interests include clinical applications of epidemiology with emphasis on observational and experimental epidemiological studies, infectious disease epidemiology with emphasis on enteric and respiratory tract bacterial pathogens, and epidemiological studies of racing injuries and colic in horses. He considers his most important professional accomplishment to be mentoring graduate students to become veterinary medical clinician-scientists and/or equine scientists. In addition to being an equine clinical-scientist, Dr. Cohen also owns and rides horses, and he produces hay at his small farm in Texas.

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STANDARD AND ALTERNATIVE TREATMENT OF RHODOCOCCUS EQUI FOAL PNEUMONIA NOAH D. COHEN | VMD, MPH, Ph.D., DACVIM-LA This article will cover practical and recent aspects of standard and alternatives to standard treatments for R. equi pneumonia in foals.

Standard Treatment: For decades, the standard treatment of foals with pneumonia attributable to R. equi has been the combination of a macrolide with rifampin (MaR).1 There are, however, no randomized, controlled clinical trials comparing the efficacy of antimicrobial treatments for foals with clinically manifested R. equi pneumonia. The rationale for the MaR combination was the observation of in vitro synergy between the two drugs and historical cases-series.2-4 More recently, evidence has emerged that co-administration of rifampin with a macrolide antibiotic results in reduced concentrations of the macrolides in plasma, pulmonary lining fluid and bronchoalveolar lavage fluid (BALF) cells.5-8 This results from both decreased intestinal Photo courtesy of Dr. Justin A. Phillips absorption and increased hepatic metabolism of macrolides induced by rifampin. Despite these decreases, the concentrations of macrolides in pulmonary lining fluid with severe pneumonia. For this reason, the combination of a and BALF cells exceed the minimum inhibitory concentration macrolide (erythromycin, azithromycin or clarithromycin) with (MIC) and the mutant prevention concentration (MPC) for rifampin remains the recommended treatment of choice for foals R. equi. These findings likely explain the apparent clinical with moderate to severe pneumonia.1,11 efficacy of the combination of macrolides with rifampin despite decreased concentrations. Administration of rifampin four hours In three separate clinical trials, a macrolide alone (either after administration of clarithromycin results in a statistically azithromycin, gamithromycin or tulathromycin) was nonsignificant improvement in the bioavailability of clarithromycin;8 inferior to the combination of azithromycin and rifampin for however, the increase in concentration is small and unlikely to be treating subclinical pneumonia.12-14 It is noteworthy that, in these of clinical relevance. Studies in immunodeficient mice indicate studies, up to 78% of the foals receiving a placebo recovered that the combination of a macrolide with rifampin is superior to without antimicrobial treatment;12-14 therefore, the apparent either drug used in monotherapy.9,10 What is needed to address efficacy of macrolide monotherapy in foals with subclinical this question is a well-designed clinical trial evaluating the relative ultrasonographic lesions should not be interpreted as evidence of efficacy of combining a macrolide with rifampin for treating similar efficacy in foals with severe pneumonia. Because in vitro foals with severe R. equi pneumonia. Such a study is unlikely to evidence indicates that resistance is less likely to occur with the be conducted because it would require a very large sample size combination of a macrolide and rifampin,2,15 monotherapy cannot with attendant logistical and financial requirements that would be recommended for subclinical pneumonia. More importantly, be large. Currently, we have more than 30 years of experience the practice of treating subclinical pneumonia should be and retrospective data, as well as animal models, indicating the questioned considering the large proportion of foals that are superiority of the combination of a macrolide with rifampin and affected and the association of increased use of macrolides and no evidence that macrolide monotherapy is effective in foals the emergence of resistance. 12  The Practitioner 

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Alternative Treatment: The emergence of resistance to MaR in Another possible alternative antimicrobial is gallium maltolate R. equi indicates the need for alternative antimicrobial treatments. Currently, there is insufficient data available to specify a preferred antimicrobial to treat foals infected with MaR-resistant R. equi. In vitro, MaR-resistant isolates are susceptible to fluoroquinolones, gentamicin, linezolid and vancomycin.16-18 Some resistant isolates are also susceptible in vitro to chloramphenicol, minocycline and trimethoprim-sulphamethoxazole.18 Vancomycin, imipenem or linezolid in foals should NOT be used unless there is a lifethreatening infection with R. equi caused by an isolate confirmed to be resistant to all other possible alternatives. A good option for foals with MaR-resistant infection is systemic and/or nebulized gentamicin. Gentamicin has been considered to be poorly effective against R. equi based on the results of a retrospective case series of foals with pneumonia caused by R. equi, in which all 17 foals treated with gentamicin died whereas all 10 foals treated with erythromycin in combination with rifampin survived.3 In contrast, another retrospective case series of 39 foals with pneumonia caused by R. equi indicated that all 19 foals that survived were treated with gentamicin; whereas, non-survivors were treated with various other antimicrobials including erythromycin, kanamycin and chloramphenicol.19 These studies were not controlled, did not account for lesion severity and reflected dosages of gentamicin lower than those currently recommended. In fact, Steeve Giguère and his colleagues have demonstrated that gentamicin is bactericidal against R. equi in culture and is highly effective against intracellular R. equi.20 Intravenous or nebulized gentamicin administered at a dose of 6.6 mg/kg once daily results in pulmonary epithelial and bronchoalveolar cell concentrations well in excess of the MIC90 for isolates of R. equi.21 Unfortunately, we lack studies documenting the clinical efficacy of gentamicin intravenous or nebulized in foals infected with R. equi.

(GaM). Gallium is a trivalent semimetal that mimics ferric iron, thereby inactivating iron-dependent enzyme systems involved with bacterial DNA synthesis and ultimately leading to the death of a variety of bacteria.22 We have demonstrated the activity of gallium against R. equi in vitro, when grown intracellularly, and in vivo in a mouse model of infection.23-25 Furthermore, minimum inhibitory concentrations of GaM are similar for macrolide-susceptible and macrolide-resistant isolates of R. equi.25 Gallium maltolate is bioavailable and safe when administered orally to foals at a dosage of 30 mg/kg bwt q. 24 h.26-29 In a randomized and controlled trial, oral GaM (30 mg/ kg bwt q. 24 h for 30 days) was non-inferior to standard therapy with a combination of clarithromycin (7.5 mg/kg bwt q. 12 h) and rifampin (5 mg/kg bwt q. 12 h) for the treatment of foals with subclinical R. equi pneumonia.30 A major limitation of this study was that it lacked a placebo control group such that it could not be determined whether the proportion of either treatment group was significantly different than what might be achieved using a placebo alone. Further investigation of the clinical efficacy of gallium maltolate is clearly warranted.

Another alternative to standard antimicrobials is the use of hostdirected therapy (HDT).31 The underlying principle of HDT is that the host’s immune response is capable of being stimulated to prevent or resolve infectious disease. Our laboratory has been exploring strategies for HDT, including nebulized immunomodulators and plasma for immunotherapy.

Cited Literature: 1. Giguère S, Cohen ND, Chaffin MK, et al. Diagnosis, treatment, control, and prevention of infections caused by Rhodococcus equi in foals. J. Vet. Intern. Med. 2011;25:1209-1220. 2. Prescott JF and Nicholson VM. The effects of combinations of selected antibiotics on the growth of Corynebacterium equi. J.Vet. Pharmacol. Ther. 1984;7:61-64. 3. Sweeney CR, Sweeney RW, Divers TJ. Rhodococcus equi pneumonia in 48 foals: response to antimicrobial therapy. Vet. Microbiol. 1987;14:329-336. 4. Hillidge CJ. Use of erythromycin-rifampin combination in treatment of Rhodococcus equi pneumonia. Vet. Microbiol. 1987;14:337–42. 5. Peters J, Block W, Oswald S, et al. Oral absorption of clarithromycin is nearly abolished by chronic comedication of rifampicin in foals. Drug Metab. Dispos. 2011;39:1643-1649. 6. Peters J, Eggers K, Oswald S, et al. Clarithromycin is absorbed by an intestinal uptake mechanism that is sensitive to major inhibition by rifampicin: results of a short-term drug interaction study in foals. Drug Metab. Dispos. 2011;40:522-528. 7. Berlin S, Spieckermann L, Oswald S, et al. Pharmacokinetics and pulmonary distribution of clarithromycin and rifampicin after concomitant and consecutive administration in foals. Mol. Pharm. 2006;13:1089-1099. 8. Venner M, Peters J, Hohensteiger N, et al. Concentration of the macrolide antibiotic tulathromycin in broncho-alveolar cells is influenced by comedication of rifampicin in foals. Naunyn Schmiedebergs Arch. Pharmacol. 2010;381:161-169.

Photo courtesy of Dr. Tiffany Hall

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9. Nordmann P, Kerestedjian JJ, Ronco E. Therapy of Rhodococcus equi disseminated infections in nude mice. Antimicrob. Agents Chemother. 1992;36:1244-1248. 10. Burton AJ, Giguère S, Berghaus LJ, Hondalus MK. Activity of clarithromycin or rifampin alone or in combination against experimental Rhodococcus equi infection in mice. Antimicrob. Agents Chemother. 2015;59:3633-3636. 11. Giguère S. Treatment of infections caused by Rhodococcus equi. Vet. Clin. North Am. Equine Pract. 2017;33:67-85. 12. Hildebrand F, Venner M, Giguèere S. Efficacy of gamithromycin for the treatment of foals with mild to moderate bronchopneumonia. J. Vet. Intern. Med. 2015;29:333-338. 13. Venner M, Credner N, Lammer M, Giguère S. Comparison of tulathromycin, azithromycin and azithromycin-rifampin for the treatment of mild pneumonia associated with Rhodococcus equi. Vet. Rec. 2013;173:397. 14. Rutenberg D, Venner M, Giguère S. Efficacy of tulathromycin for the treatment of foals with mild to moderate bronchopneumonia. J. Vet. Intern. Med. 2017;31:901-906. 15. Berghaus LJ, Giguère S, Guldbech K. Mutant prevention concentration and mutant selection window for 10 antimicrobial agents against Rhodococcus equi. Vet. Microbiol. 2013;166:670-675. 16. Berghaus LJ, Giguère S, Guldbech K. et al. Comparison of Etest, disk diffusion, and broth macrodilution for in vitro susceptibility testing of Rhodococcus equi. J. Clin. Microbiol. 2015;53:314-318. 17. Carlson K, Kuskie K, Chaffin K, et al. Antimicrobial activity of tulathromycin and 14 other antimicrobials against virulent Rhodococcus equi in vitro. Vet. Ther. 2010;11:E1-E9. 18. Giguère S, Lee E, Williams E, et al. Determination of the prevalence of antimicrobial resistance to macrolide antimicrobials or rifampin in Rhodococcus equi isolates and treatment outcome in foals infected with antimicrobial-resistant isolates of R. equi. J. Am. Vet. Med. Assoc. 2010;237:74-81. 19. Falcon , Smith BP, O’Brien TR, et al. Clinical and radiographic findings in Corynebacterium equi pneumonia of foals. J. Am. Vet. Med. Assoc. 1985;186:593-599. 20. Giguère S, Berghaus LJ, Lee EA. Activity of 10 antimicrobial agents against intracellular Rhodococcus equi. Vet. Microbiol. 2015;178:275-278. 21. Burton AJ, Giguère S, Berghaus LJ, et al. Efficacy of liposomal gentamicin against Rhodococcus equi in a mouse infection model and colocalization with R. equi in equine alveolar macrophages. Vet. Microbiol. 2015;176:292-300. 22. Bernstein LR. Mechanisms of therapeutic activity for gallium. Pharmacol. Rev. 1998;50: 665-682. 23. Harrington JR, Martens R, Cohen ND, Bernstein LR. Antimicrobial activity of gallium against virulent Rhodococcus equi in vitro and in vivo. J. Vet. Pharmacol. Ther. 2006;29:121-127. 24. Martens RJ, Miller NA, Cohen ND, et al. Chemoprophylactic antimicrobial activity of gallium maltolate against intracellular Rhodococcus equi. J. Equine. Vet. Sci. 2007;27:341-345. 25. Coleman M, Kuskie K, Liu M, et al. In vitro antimicrobial activity of gallium maltolate against virulent Rhodococcus equi. Vet. Microbiol. 2010;146:175-178.

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26. Chaffin MK, Fajt V, Martens RJ, et al. Pharmacokinetics of an orally administered methylcellulose formulation of gallium maltolate in neonatal foals. J. Vet. Pharmacol. Ther. 2010; 33:376-382. 27. Chaffin MK, Cohen ND, Martens RJ, et al. Evaluation of the efficacy of gallium maltolate for chemoprophylaxis against pneumonia caused by Rhodococcus equi infection in foals. Am. J. Vet. Res. 2011;72:945-957. 28. Martens RJ, Mealey K, Cohen ND, et al. Pharmacokinetics of gallium maltolate after intragastric administration in neonatal foals. Am. J. Vet. Res. 2007;68:1041-1044. 29. Martens RJ, Cohen ND, Fajt VR, et al. Gallium maltolate: safety in neonatal foals following multiple enteral administrations. J. Vet. Pharmacol. Ther. 2010;33:208-212. 30. Cohen ND, Slovis NM, Giguère S, et al. Gallium maltolate as an alternative to macrolides for treatment of presumed Rhodococcus equi pneumonia in foals. J. Vet. Intern. Med. 2015;29:932-939. 31. Rao M, Dodoo E, Zumla A, Maeurer M. Immunometabolism and pulmonary infections: implications for protective immune responses and host-directed therapies. Front. Microbiol. 2019;10:962.

Noah D. Cohen, VMD, MPH, Ph.D., DACVIM-LA Dr. Noah Cohen received his A.B. from the University of Pennsylvania (Penn) in 1979 and his V.M.D. from Penn in 1983. He was employed in a private equine practice in Toronto for two years. In 1985, he enrolled at the Johns Hopkins University School of Hygiene and Public Health (now the Johns Hopkins Bloomberg School of Public Health) and completed an M.P.H. (1986) and Ph.D. (1988) in epidemiology with emphasis on infectious disease epidemiology. Dr. Cohen then went to Texas A&M University, where he completed a residency in large animal internal medicine. In 1991, he was appointed assistant professor of equine internal medicine in the Department of Large Animal Clinical Sciences at Texas A&M. Dr. Cohen is currently professor of equine internal medicine, the Patsy Link Professor of Equine Research, and the associate department head for research and graduate studies in the Department of Clinical Sciences at the College of Veterinary Medicine & Biomedical Sciences. He also serves as director of the Equine Infectious Disease Laboratory at Texas A&M. Dr. Cohen was named a Texas A&M University Faculty Fellow in 2002. His research interests include clinical applications of epidemiology with emphasis on observational and experimental epidemiological studies, infectious disease epidemiology with emphasis on enteric and respiratory tract bacterial pathogens, and epidemiological studies of racing injuries and colic in horses. He considers his most important professional accomplishment to be mentoring graduate students to become veterinary medical clinician-scientists and/or equine scientists. In addition to being an equine clinical-scientist, Dr. Cohen also owns and rides horses, and he produces hay at his small farm in Texas.

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There’s nothing else like it. Over the past 30 years, Adequan® i.m. (polysulfated glycosaminoglycan) has been recommended millions of times1 to treat degenerative disease, and with good reason. From day one, it’s been 2, 3 the only FDA-Approved equine PSGAG joint precription available, and the only one proven to. Restore synovial joint lubrication Repair joint cartilage Reverse the disease cycle Reduce inflammation When you start with it early and stay with it as needed, horses may enjoy greater mobility 2, 4, 5 over a lifetime. Discover if Adequan is the right choice. Talk to your American Regent Animal Health sales representative or call (800) 458-0163 to order. BRIEF SUMMARY: Prior to use please consult the product insert, a summary of which follows: CAUTION: Federal law restricts this drug to use by or on the order of a licensed veterinarian. INDICATIONS: Adequan® i.m. is recommended for the intramuscular treatment of non-infectious degenerative and/or traumatic joint dysfunction and associated lameness of the carpal and hock joints in horses. CONTRAINDICATIONS: There are no known contraindications to the use of intramuscular Polysulfated Glycosaminoglycan. WARNINGS: Do not use in horses intended for human consumption. Not for use in humans. Keep this and all medications out of the reach of children. PRECAUTIONS: The safe use of Adequan® i.m. in horses used for breeding purposes, during pregnancy, or in lactating mares has not been evaluated. For customer care, or to obtain product information, visit www.adequan.com. To report an adverse event please contact American Regent, Inc. at (800) 734-9236 or email pv@americanregent.com. Please see Full Prescribing Information at www.adequan.com.

1 Data on file. 2 Adequan® i.m. Package Insert, Rev 1/19. 3 Burba DJ, Collier MA, DeBault LE, Hanson-Painton O, Thompson HC, Holder CL: In vivo kinetic study on uptake and distribution of intramuscular tritium-labeled polysulfated glycosaminoglycan in equine body fluid compartments and articular cartilage in an osteochondral defect model. J Equine Vet Sci 1993; 13: 696-703. 4 Kim DY, Taylor HW, Moore RM, Paulsen DB, Cho DY. Articular chondrocyte apoptosis in equine osteoarthritis. The Veterinary Journal 2003; 166: 52-57. 5 McIlwraith CW, Frisbie DD, Kawcak CE, van Weeren PR. Joint Disease in the Horse.St. Louis, MO: Elsevier, 2016; 33-48. Adequan and the Horse Head design are registered trademarks of American Regent, Inc. © 2019, American Regent, Inc. PP-AI-US-0222 2/2019


DIAGNOSIS AND MANAGEMENT OF EQUINE SPINAL ISSUES: HOW THE FOOT MATTERS TRACY A. TURNER | DVM, MS, DACVS, DACVSMR

Introduction

Lameness affects the horse’s back. Forelimb lameness is relieved by transferring weight to the rear limbs; conversely, rear-limb lameness is relieved by transferring weight to the forelimbs. One could say all lameness goes through the back.

The effect of hoof imbalance on forelimb lameness has been welldocumented. The forelimb hoof has little effect above the fetlock, but the rear hoof has much greater effect on the upper leg and back. The purpose of this article is to discuss the common hoof imbalances that have been described in the rear hoof, compare those to the forefoot, discuss possible pathophysiology and discuss the effect these rear hoof imbalances have on lameness of the rear leg. Different biomechanics exist between the forelimb and rear limb. Also, rear hoof biomechanics affect the movement (biomechanics) of the upper limb, and these alterations in biomechanics can result in lameness. Only three types of hoof imbalances have been described for the rear hoof compared to the front feet, which minimally have at least six.1 The rear feet develop dorsoplantar imbalance, under-run heels or plantar surface imbalance, and medial to lateral imbalance. Unlike the front feet, contracted heels are a very rare problem, as are horses with mismatched rear feet. It’s not that this can’t occur, but the incidence appears to be so low that it usually isn’t of significance. Finally, in the front feet, small hoof to body size is a significant problem with important prognostic implications, but it has yet to be recognized in the rear feet. Generally, the rear feet are smaller than the front feet, and there are no formulas to evaluate this parameter in the rear feet.

Broken Hoof-pastern Axis

This is the most common rear limb hoof imbalance.1 The hind hoof is normally slightly more upright than the forefoot, usually by about five degrees; however, the dorsal hoof wall and the pastern, like the front foot, should be in the same alignment. Many factors can affect this. Some examples include: 1. Older horses often show fetlock hyperextension due to suspensory weakness (age related) and this, in turn, causes a broken forward hoof pastern axis. 2. Pain in the front feet can cause the horse to place its feet further under the body.

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Lateral to medial radiograph showing negative sole angle. Photo courtesy of Dr. Tracy A. Turner

3. Long toes and under-run heels in the front feet cause a similar postural change. The latter two postural changes cause the heels in the rear feet to be overloaded and, subsequently, slows the growth of the heels. The heels are the first part of the hoof to distort. Unlike the forelimb where hoof imbalances most frequently lead to foot pain, rear hoof imbalance causes problems higher up in the limb.2 A broken back hoof axis with a negative solar angle is the most common hoof imbalance of the rear foot.1 This imbalance causes the hoof to stay on the ground longer than normal. Further, the strain on the deep flexor will be markedly increased over normal before the heel is lifted. This can lead to many lameness issues. The most serious problem is tenosynovitis of the distal tendon sheath. Although not the most common lameness caused by this imbalance, because of its effect on the

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Correcting this imbalance would appear simple. One would think they could simply raise the heels to correct the negative solar angle; however, the author has noted this causes other problems. The increased heel height further exaggerates heelfirst landing, increasing the heel pressure. Increased heel height also increases heel pressure during midstance.2 In addition, this alters the hoof flight and riders have complained that it changes the gait. A different approach would seem appropriate. The author has used radically rockered shoes to improve rollover and reduce tendon strain. This has been beneficial for tendon sheath swelling cases.

Plantar Surface Imbalance

Dorsal plantar radiographic projection showing medial to lateral imbalance. High on the medial side with a lateral flare (arrow). Photo courtesy of Dr. Tracy A. Turner

deep flexor tendon, it is the most serious. This can lead to marked swelling in the sheath, pain and even disruption of the blood supply to the tendon.1 The most common lameness associated with this imbalance is tarsitis, or inflammation of the hock. In an unpublished retrospective study conducted by the author, the risk factors for horses needing hock injections were evaluated. The negative solar angle was a common risk factor among all horses needing hock injections. This is probably due to this imbalance causing an increased stance time for the limb. In a different study, this imbalance was associated with a high incidence of gluteal pain in horses.4 The hind hooves have a more exaggerated, heel-first landing than the front hooves.2 During landing, the rear hooves slide horizontally after the hoof touches the ground. At midstance, the coffin joint is maximally flexed. Maximal extension occurs at the termination of stance; therefore, anything that increases the terminal stance phase will increase joint extension and, therefore, dorsal rim pressure on the joints. It may possibly take the extra work of the flexor tendon muscle and increased strain on the tendon to overcome the negative solar angle of the coffin bone that causes the stress and damage within the tendon sheath. In addition, it is likely the increased stance phase and increased dorsal rim pressure on the tarsus that creates inflammation in the distal tarsal joints.

Abnormal heel conformation of the hind feet is easy to recognize.3 When looking at the limb from the side, the digit will show a broken back hoof-pastern axis. The slope of the coronary band from the toe to the heel will have an acute angle. The bulbs of the heels will have a bending appearance and can be seen lying against the shoe palmar to the end of the heel. The dorsal hoof wall begins to take on a "bullnosed" appearance. Looking at the foot from behind, the frog is situated well below the hoof wall, and the frog can be seen to prolapse down between the two branches of the shoe. The frog is generally large from the constant stimulation with the ground. Upon removing the shoe, the end of the heel of the hoof wall is located well forward from the base of the frog.3 The horn tubules will be parallel with the ground. The hoof wall at the heel will be thin. There will be no angle to the sole, and the bars will be absent. The whole frog will be pushed down below the hoof wall. When the foot is placed on the ground, total weight bearing will be placed on the frog and many horses are reluctant to stand on it when the opposing limb is lifted off the ground.

Bullnose hoof. Conformation is associated with negative sole angle. Photo courtesy of Dr. Tracy A. Turner

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to the frog wedge, and two 4.5 race nails are placed through this strip into the hoof wall at the toe quarters to hold the frog wedge directly over the frog.

Dorsal view of a hoof showing the upright inside wall and flared outside wall. Typical medial to lateral imbalance of the rear foot. Photo courtesy of Dr. Tracy A. Turner

When viewing the ground surface of the foot, there will be a "trough" noted between the apex of the frog and the inner branch of the shoe at the toe.3 Hoof testers placed on either side of the heel at the angle of the sole will elicit a painful response. Damage to the heels of the hind feet is often easier to improve than damage to the forefeet, possibly due to the difference of the load encountered on the hind limbs versus the forelimbs. Three methods can be employed to treat this condition.

An Animalintex self-contained poultice that is saturated with water and applied so it envelops the whole foot. It is secured to the foot with a bandage. The horse is placed in a stall with a firm surface for 24-48 hours. During this time, the feet are submerged in a bucket of water a few times to keep the poultice saturated. At the onset of applying the frog wedge, the horse is given 2 grams of phenylbutazone because some horses will show discomfort and others may develop a digital pulse. When the poultice is removed, the frog will be compressed between the heels forming a flat even surface that includes the frog and both heels. The horse can be shod immediately or can be placed in a stall bedded with sawdust for an additional day to let the feet dry out. The frog will be soft and can be shaped further. Any additional horn at the heels can be removed so the heels of the hoof wall are solid and approach the base of the frog. It’s important to be careful to keep the frog and both heels in the same plane. A shoe can be fitted and applied. The same principles apply when shoeing hind feet. A line is drawn across the widest part of the foot, and the shoe is fitted so the line is placed in the middle of the shoe. In the hind feet, the branches of the shoe may extend marginally beyond the end of the heels. If additional heel elevation is necessary, a wedge pad or a bar wedge can be placed under the heels if the shoe is fitted in the manner described. This will concentrate the load under the frog and heels rather than behind the heels, which is the case with a long shoe. A third approach is to trim the heels as suggested above but trim the front half of the hoof in a different plane.3 This maximizes rollover of the hoof. The effect shows a reduced strain on tendon sheaths and the deep flexor tendon, but it has not been successful in restoring the hoof capsule.

Medio-lateral Imbalance

Most people find medio-lateral balance more difficult to assess in the hind feet than in the front feet.1 The balance of the hoof dictates its placement and affects hoof flight.2 Because the pelvis, stifle and hock all have some lateral movement, it’s easy for a hoof that has been worn unevenly, or has been trimmed out of First, allowing a horse to go without hind shoes — if possible — balance, to cause the entire limb to move out of line. Because for four to eight weeks can be very effective.3 This approach can of the asymmetrical movement of the hock, the hind limb has also be used with horses that are resting due to lameness issues. a slight rotating action as it moves forward over the foot. This is The shoes are removed, and the hoof wall at the heels is moved necessary to ensure that the feet are wider than the forelegs when in a plantar direction until solid structures of the hoof wall are galloping. In many animals, this leads to an unnatural wear on encountered. The hoof wall at the toe is lowered appropriately the lateral branch of the hoof, causing the hoof capsule to be high and the edges are rounded. Over the next few weeks, the pressure on the inside.1 on the frog will compress and displace the frog until it assumes the same plane as the heels on either side. The author’s assessment of rear limb hoof imbalances has If the horse needs to continue in work and wear shoes, a different corroborated the high medial observation. The rear feet are approach has been described.3 The shoes are removed, and much more commonly high medially. In contrast, the front the heels are trimmed in a plantar direction until solid horn is hooves are more likely to be high laterally. This causes changes established. Excess dorsal hoof wall is removed from toe quarter in the horse’s stance. The tendency is for the leg to rotate inward to toe quarter. The goal is to compress the prolapsed frog to have and the limb to be placed more medially. In addition, there are a flat, even plane that includes both the heels and the frog. The effects on the limb in general. With a high medial wall, the limb back section of a degree pad is cut out to fit over the frog as a develops a varus deformity, and the fetlock, hock and stifle can mirror image. A thin strip extending across the toe is left attached each be involved. Many different issues of lameness have been 18  The Practitioner 

Issue 4 • 2019


attributed to this conformation.2 Injury to the lateral branch of the suspensory ligament has been noted.1 In addition, increased pressure on the inside of the joints can cause lameness of the fetlock, hock and stifle. In evaluating rear hoof imbalance cases, the author has noted an increased association with both distal hock lameness and medial femoro-tibial lameness. Accurate identification of key points on the foot — allowing for evaluation of dorsal hoof wall and heel angles, sole depth, and medial and lateral wall height — isn’t always possible depending on the technique used and the conformation of the foot.4 Digital radiography allows improved visualization of soft tissues; however, accurate identification of the coronary band and heels can still be difficult. Edge burn-through (saturation artifact) at the periphery of soft tissues is a common artifact with digital radiography. This can result in an inaccurate assessment of hoof wall thickness. The author has used lower-power techniques with digital radiography so the edge burn-out does not occur and the contrast can then be adjusted digitally so that markers are less necessary; however, markers are the most reliable identifiers. Rigid metallic markers or barium paste can be used. The goal is to accurately identify the true border of the dorsal wall and contour. The marker should equal the length of the toe and should be contoured to the true shape of the wall. Running a 2-mm bead of barium paste directly over the dorsal median hoof wall, extending from the coronary band to the tip of the toe, allows for accurate identification of the toe length, wall contour and border, and it provides for an appreciation of hoof wall distortion. A halo artifact (Uberschwinger) may be seen surrounding the barium, but this will not preclude accurate border identification. Spot marking at the widest point of the proximal (coronary) and distal wall in the quarters, at the proximal and distal wall in the heels, and at the apex of the frog will aid in the evaluation of quarter angles, quarter wall height, heel angle and height, sole depth, and toe-to-heel ratio. It is crucial that the positioning of the patient, foot and X-ray beam be accurate when evaluating the foot for the purposes of podiatry.4 True assessment and measurement of the dorsal hoof wall and heel angle, sole depth, joint congruity, medial-to-lateral balance, and toe-to-heel ratio is dependent on proper positioning. Slight abduction or adduction of the limb or shifting of weight can cause joint incongruity on the horizontal beam dorsalpalmar view. The horse should be placed on firm, level footing with the limbs squarely beneath the horse. Limb conformation should also be evaluated prior to taking radiographs. When placing the foot on the positioning blocks, it’s important to allow the foot to position itself as dictated by the limb’s conformation (toed-in, toed-out). To reduce magnification, the foot should be placed on the positioning block in such a way that the foot is as close as possible to the cassette or sensor plate. The lateromedial and horizontal dorsopalmar projections are the most useful views to perform when evaluating the foot for conformation and balance.4 Consideration of the interest area, as well as having solid anatomical knowledge of the horse's foot, is important when performing these radiographic views. The lateromedial (L-M) projection is performed with the horse WWW.FAEP.NET |

standing squarely on a flat, level surface with each foot on a positioning block of equal height. It's important that the cannon bone be perpendicular to the floor in both the medial-to-lateral and dorsal-to-palmar planes. Keeping the horse's head and neck straight is also important to reduce the influence of uneven loading of any one limb.4 Focal-film distance usually ranges between 24-28 inches, and it is important to be consistent. Once the technique is established, the focal-film distance remains constant. Once the horse is positioned squarely, proper beam alignment and positioning is the next step in obtaining a workable image. If the area of interest is the distal phalanx and the purpose of the study is evaluating foot balance and symmetry, the center of the beam should be aimed 1.5-2 cm proximal to the weight-bearing surface and midway between the toe and the heel. The beam angle should be parallel with the heel bulbs and the ground surface. This beam alignment will produce a film that shows the medial and lateral solar margins and palmar processes of the distal phalanx superimposed on one another (in the "normal" foot). Any obliquity in the image can be corrected by raising or lowering the central beam to adjust for variation in sole depth or by adjusting the beam angle in relation to the heel bulbs. Correct positioning reduces the likelihood of artifactual changes to the joint space that might otherwise be interpreted as joint asymmetry and foot imbalance.4 This projection allows evaluation of medial-to-lateral balance and conformation of the foot with observation and measurement of the medial and lateral wall length and angle, along with the orientation of the distal phalanx within the hoof capsule. Orientation of the distal phalanx can be assessed by measuring the distance from the articular surface of the distal phalanx to the ground surface. The solar canal can also be used as a reference point, but it’s less consistent. Using the solar margin as a point of reference can be variable due to changes that can occur in the bone. In horses with "ideal" conformation, the articular surface of the distal phalanx is parallel to the ground, as is a line between the medial and lateral coronary band, and the medial and lateral walls are of equal thickness and the distance from the medial and lateral solar margins to the ground are similar. In horses with significant rotation or angulation in the distal limb, the relation of the distal phalanx with the ground may not be as symmetrical. Furthermore, the distal interphalangeal joint space should be approximately even across its width regardless of angulation of the phalanges. It is normal for the medial quarter wall to be at a slightly steeper angle and subsequently measure shorter in length; however, caution in overinterpretation of joint incongruency is recommended because any malpositioning of the limb or foot can create the appearance of medial to lateral imbalance. Using radiographs to assess the relationship between the hoof and the underlying osseous structures, as an aid in assessing foot balance, is about developing an understanding of the relationships between the position of the hoof capsule, the angle of the distal phalanx within the hoof capsule, the symmetry of the interphalangeal articulations and the alignment of the phalanges.

FLORIDAAEP |

@FLORIDA_VMA | The Practitioner  19


Conclusion

Hoof imbalance of the rear hooves should be considered as important as hoof imbalance of the front hooves. Although the imbalances may not cause specific hoof pain, they will increase the likelihood of lameness farther up the limb. Assessment by radiography is essentially the same as the foreleg, but visual identification is different since the front leg and rear leg move differently.

Acknowledgements

The author declares they have adhered to the Principles of Veterinary Medical Ethics of the AVMA.

References: 1.

Colles C, Ware R: The Principles of Farriery, London, J.A.Allen, 2010, pp 157-162.

2. Back W, Clayton H: Equine Locomotion, Edinburgh, Elsevier, 2013, pp 127-147. 3. O’Grady SE, Merriam JG: Low Heels in the Hind Feet-An Often Overlooked Problem, Am Farriers J, March/April 2007, 45-48. 4. Eggleston RB: Value of Quality Foot Radiographs and Their Impact on Practical Farriery, in Proceedings. Am Assoc Eq Practnr, 2012, 58, 164-175. 5. Mannsman RA, James S, Bilkslager AT, et al: Long Toes in the Hind Feet and Pain in the Gluteal Region: An Observational Study of 77 Horses. J Eq Vet Sci, Dec 2010, pp 720-726.

I

Tracy A. Turner DVM, MS, DACVS, DACVSMR Dr. Tracy A. Turner received his DVM degree from Colorado State University in 1978 and interned at the University of Georgia. He completed a surgical residency and master’s degree at Purdue University. He has served on the faculties of the University of Illinois, University of Florida and the University of Minnesota. He joined Anoka Equine Clinic in Elk River, Minnesota in 2004, where he practices sports medicine, lameness and surgery. Dr. Turner's primary areas of research interest have focused on equine lameness with interest in equine podiatry and thermography. He has spoken nationally and internationally on lameness topics. He has written more than 100 peer-reviewed manuscripts, more than 250 non-peer-reviewed papers, and more than 30 book chapters on equine lameness, podiatry and thermography. Dr. Turner is a Diplomate of the American College of Veterinary Surgeons, a Diplomate of the American College of Sports Medicine and Rehabilitation, and is a Fellow of the American Academy of Thermology. He is an active member of the AVMA, the AAEP, the American Horse Council and the Minnesota Horse Council.

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Practitioner Issue 4, 2019  

Published by the Florida Association of Equine Practitioners, an Equine-Exclusive Division of the Florida Veterinary Medical Association.

Practitioner Issue 4, 2019  

Published by the Florida Association of Equine Practitioners, an Equine-Exclusive Division of the Florida Veterinary Medical Association.

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