Histo ph il us so mni in Ca tt le

By: Jose Valles, MS and Lisa Taylor, MS

Histophilus somni, formerly known as Haemophilus somnus, was recognized as a disease-causing pathogen in 1956. While H. somni is known to affect cattle in both dairy and feedlot settings worldwide, it is most commonly found in countries with large cattle populations such as Canada and the United States.

H. somni is a gram-negative bacterium with microscopic coccobacillus characteristics that belongs to the Pasteurellaceae family. This particular facultative anaerobe is non-motile and non-spore forming. It is a commensal organism found in mucosal surfaces of cattle; non-pathogenic and pathogenic forms of the organism are present in the reproductive and respiratory tracts of both genders. The organism can be transmitted by sexual intercourse in the reproductive tract or by inhalation in the respiratory tract.

This bacterium is considered an opportunistic agent because it requires mucosal disruption in order to become infective. Mucosal breakdown is commonly triggered by stress-causing factors such as change in diet, other health issues, transportation, weaning and weather. In the respiratory tract, H. somni can enter the bloodstream during mucosal interruption and result in septicemia.

Infection with H. somni is more common in feedlot settings but has also been reported in dairy and pasture cattle. Cattle 6 to 12 months of age are at higher risk to become infected, and feedlot cattle are more susceptible to infection at the start of their feeding period. Although death due to aggressive septicemia in infected animals generally occurs in the first 21 days post-arrival to the feedlot, mortality is a risk during the entire feeding period. On average mortality of infected animals occurs between 30 and 60 days on feed.

H. somni is likely to cause other disease syndromes such as mastitis, myocarditis, otitis media, polysynovitis, reproductive tract diseases, respiratory disease and thrombotic meningoencephalitis (TME). Myocarditis and respiratory disease resulting from H. somni have become more common in recent years. Death can occur very rapidly after infection, depending on the disease manifestation.

Clinical signs of infected animals vary depending on the manifested disease. Some of these clinical signs may include abducted elbows, closed eyes, coughing, depression, fever, lack of voluntary muscle coordination, lateral recumbency, loss of appetite, nasal discharge, rapid breathing, stiffness, sudden death and weakness. Cattle affected by TME are likely to die within 24 hours of onset of the disease, and in some cases of TME, sudden death is the only clinical sign. Similarly, myocarditis cases are often rapid course with sudden death the only clinical sign. Myocarditis cases may also exhibit symptoms of left heart failure such as coughing, exercise intolerance and open mouth breathing.

Suggested medical examination for cattle suspected of H. somni infection includes rectal temperature, lung score, visual clinical signs, movement, feed intake and water consumption. Affected animals should be closely monitored.

At necropsy, affected animals can present a number of lesions dependent on the active disease prior to death. Gross lesions are likely to be found in those animals affected by myocarditis or respiratory disease (Figures 1 and 2).

Common gross lesion findings include broncho-pneumonia, edema, fibrinous laryngitis, fibrinous pericarditis, fibrinous pleuritis, focal myocardial lesion (typically in the left ventricular papillary muscle), pulmonary congestion and polyarthritis. Other less common gross lesions that can be found include fibrinopurulent meningitis, fibrinous gonitis and polyserositis.

In animals affected by TME, gross lesions can vary greatly and be difficult to identify. These TME gross lesions may include visible hemorrhagic and necrotic regions on the brain surface or cut section and swollen brain secondary to edema. Reproductive tract lesions of animals affected by H. somni include cervicitis, endometritis and vaginitis. Microscopic lesions found in animals affected by H. somni can include neutrophilic inflammation, tissue necrosis and vasculitis. In some cases, thrombi may contain colonies of gram-negative bacteria. Continued on Page 10