VET VIEWS Understanding Diseases Associated with Histophilus Somni by Ronald K. Tessman, DVM, Ph.D., DACVIM, DACVPM, technical beef consultant, Elanco Animal Health Histophilus Somni (H. somni), formerly known as Haemophilus somnus, is a nonencapsulated, gram-negative bacterium in cattle, most commonly infecting feedlot cattle. It is a major pathogen of the bovine respiratory disease (BRD) complex, along with Mannheimia haemolytica and Pasteurella multocida, making it an economically significant pathogen for the cattle industry. Although we most frequently associate H. somni with BRD, it also infects several other organ systems and is a causative agent for multiple other diseases that can be life threatening to cattle. Historically, H. somni has been found predominately in Canada, but in the early 2000s, it started popping up more often in diagnostic lab isolations in the Midwest and down into the Plains States where there is a high concentration of feedyards. More recently, diagnostic labs in the Midwest and Southeast are seeing an uptick in the prevalence of H. somni in isolates from BRD submissions in the area. Veterinarians and researchers have hypothesized why H. somni is becoming more widespread and increasingly prevalent, but there is not a clear culprit. Nonetheless, it is important for producers to work with their veterinarian to understand the diseases associated with H. somni, what signs and symptoms to look for, when and how to intervene and general management and prevention strategies. H. somni is an opportunistic pathogen. It breaks down the mucosal membrane’s i.e., the protective lining of organs, ability to protect and provide immunity to an organ or organ system. Stress from weaning, transportation or inclement weather along with other factors can further compromise the animal’s ability to fight off infection, leading to a variety of neurological, cardiac and respiratory conditions including: •Pneumonia or pleuritis •Septicemia •Thrombotic meningoencephalitis •Myocarditis •Tenosynovitis •Polysynovitis Septicemia occurs when H. somni enters the bloodstream, 22 California Cattleman April 2023
triggering inflammation and the disruption of blood flow. TME is a neurological disease caused by H. somni that infects the brain. Myocarditis is a condition that causes inflammation of the heart muscle, and tenosynovitis and polysynovitis infect the joints and surrounding tendons causing severe inflammation. All can be life threatening and affect the overall health and well-being of cattle. Other than BRD, these diseases caused by H. somni are hard to diagnose because infected cattle often show few clinical signs. Cattle with TME, myocarditis, septicemia and joint diseases may appear normal with sudden death being the only clinical sign. Once cattle are infected with H. somni, the disease progression can be very rapid. Other clinical signs that may be observed for TME are closed eyes, inability to get up from a lying position, fever or depression. Animals that can stand will be weak and may appear blind. Cattle experiencing myocarditis may show signs of heart failure, including cough, open mouth breathing and exercise intolerance. Since neurological and cardiac conditions caused by H. somni progress rapidly, usually leading to death within 24 hours without clinical signs, monitoring cattle for signs of respiratory disease is recommended. Respiratory disease often precedes other infections. Cattle with respiratory complications may exhibit high fevers, show labored breathing, ...CONTINUED ON PAGE 24