Causative agent: Hemophilus somnus
Disease caused by H. somnus is very common in cattle (9). In Canada, 25% of cattle test serologically positive to H.somnus (9). H.somnus as been found to cause disease in bison (19). The bacterium has been isolated from the respiratory , urinary, and reproductive tracts of clinically normal cattle (9). H.somnus has been isolated from tonsils of clinically normal bison (24). Disease outbreaks in bison occur most commonly in the fall and winter and have been associated with very cold weather and stressful situations such as handling (19).
In cattle H.sommnus causes septicemia, meningitis, polysynovitis, pleuritis, suppurative bronchopneumonia, myocarditis, otitis media, mastitis, abortion and reproductive tract infections(9). In bison, only the meningitis form has been reported (19). In cattle, this form of H.somnus infection is called infectious thromboembolic menigioencephalitis or ITEME.
Clinical signs:
Sudden death is common. H.somnus has only been associated with disease in recently weaned bison calves. Calves may be found recumbent, depressed, and have a high or low body temperature. They may have blindness in one or both eyes. The eyes may be partially closed. Convulsions often occur prior to death. If clinically affected calves are found standing, they may be weak, staggery, or ataxic. They may knuckle over at the fetlocks. They may have muscle tremors. Their joints may be distended. Sick bison calves usually die within 24 to 48 hours.
Postmortem findings:
There are hemorrhagic infarcts in the brain and spinal chord.
Diagnosis:
Diagnosis of this disease is difficult in live bison. The clinical signs are suggestive of the disease but they are not definitive. The diagnosis is most often made from postmortem examination of dead animals. Tissues from the carcass, especially the brain, should be sent to a pathology laboratory for histopathology as well as bacterial culture and isolation.
Treatment:
Clinically affected bison calves may be treated with antibiotics. In cattle, oxytetracycline, or fluorfenicol are commonly used. Surveillance of at risk bison calves is critical to reducing mortalities once an outbreak has been established. Identifying and treating affected bison with minimal disturbance to the rest of the group can be very challenging. Disturbing the group by sorting and catching calves to assess their physical state by taking their temperature, pulse or respiration will significantly increase the stress placed on the calves. Bison respond less favorably to handling than cattle do. Often healthy bison that are handled take several days to return to their normal behavior. This makes monitoring programs that involve even minor handling of sick bison unsuitable. Bison tend to mask their expression of clinical signs when they are sick. Often they will not show any easily observable clinical signs of disease until just before death. Keen observation of calves at risk by producers who are knowledgeable of the behavior of bison, is important for the detection of even advanced cases of disease.
Once clinically affected bison calves are identified they must be very carefully removed from the group and treated. The entire group may be injected with fluorfenicol or long acting oxytetracycline. The behavior of bison calves varies greatly from farm to farm. On some farms the calves may be so excitable that the injuries and stress associated with handling may make mass injection impractical. In these instances, groups of bison calves from which clinical cases have occurred may be mass medicated with oxytetracycline in the feed or water.
Vaccinating in the face of an outbreak with commercially available H.somnus vaccines that are designed for use in cattle may reduce mortalities when used in bison calves.
Control:
There are many commercially available H. somnus vaccines that are designed for use in cattle. There is considerable variation in the efficacy of these vaccines in cattle. None of these vaccines are licenced for use in bison. The efficacy of these vaccines for the protection of bison against disease caused by H. somnus is unknown.
Vaccination may help reduce morbidity and mortalities in cattle, but probably doesn’t work well to protect against all forms of the disease. Initial vaccination must be followed by a booster vaccination in three to four weeks. The booster must be administered at least three weeks prior to a risk period such as the onset of cold weather. If the disease occurs on a farm expect to see recurrences in subsequent years.