Research project pinpoints cause of bison deaths

The preliminary results are in from a bison research project examining the post-mortem findings and pathogenic agents causing bison deaths in Alberta herds. Four veterinarians have examined the causes of 100 bison deaths in herds across the province. The research group is composed of four veterinarians — Drs. Burrage, Clark, Lewis, and Tremblay. Although only the preliminary results are in, the results have a considerable impact on the bison industry.

As far as we know this is the first time many bison deaths have been followed up with lab diagnosis to determine the exact cause of death. By knowing the cause it can help us determine preventive measures.

What shocked the researchers was the main cause of death in this study. In the past, respiratory agents, parasitism and injuries were the main causes. Although the study is not fully complete and the population in the study is not huge, last year at least the cause of death in most cases was respiratory, and a specific type of pneumonia at that.

The researchers found increasing causes of death, within some instances high mortality upwards of 20-30 per cent from pneumonia — more specifically Mycoplasma Bovis pneumonia. This organism is not a virus and not really a bacteria — it fits between the two groups. It is not overly responsive to antibiotics. Over time it causes pyogranulomas — a sort of drier pus pocket throughout the lungs, which over time destroys them. In bison, especially mature ones, we primarily have seen weight loss, some respiratory embarrassment and coughing, eventually leading to death. The organism’s actions and the lesions it causes can resemble tuberculosis. It can also get into the joints, causing lameness because of the arthritis as well as causing abortions in pregnant cows or occasionally mastitis.

If we look in the literature, we find this organism is not new to the bison industry. Several outbreaks have been documented in large herds in the U.S. as well as in Saskatchewan, dating back to the mid- to late 1990s. The disease seemed to cause high death losses in all ages of bison and then disappear. Feeder bison, cows, bulls and in a few cases young calves still nursing are also susceptible.

Whether your bison are in a feedlot or on the open range, once introduced it runs its course and in most cases the next year pretty much disappears. Treatments were tried by most producers and this involved separation from the herd where possible and treatment, in most cases with Draxxin under the supervision of the veterinarian. There are almost no products approved for bison so the use of Draxxin has to be under the direction and with a prescription from your veterinarian.

Currently a study is in the planning stages for developing a known withdrawal for Draxxin in bison. Some herds in the study were also vaccinated for the common respiratory organisms in cattle but in most cases it only marginally improved the situation. There is no current vaccination for mycoplasma in bison, or cattle for that matter, as according to the vaccine manufacturers it is a very hard organism to produce a vaccine from.

In the study on the under-30-month bison, half the deaths were from mycoplasma and in the over-30-month bison most died or were put down because of chronic mycoplasma. It causes a severe pneumonia where almost all of the lungs is destroyed. When we see them at necropsy, it is no wonder antibiotics or any form of treatment is ineffective.

Whenever your veterinarian autopsies any dead bison, make sure he or she checks the tonsillar area and larynx as this can be the starting place for the mycoplasma infection. The organism has a predilection for the tonsillar area.

Introduced carriers?

In most of the herds infected with mycoplasma last year, the common denominator was introduction of new bison into the herd, or two herds purchased and mixed together or individual animals bought and introduced to a breeding herd. All these scenarios are making researchers ponder several questions about how and why the mycoplasma organism is spread and if there are carrier animals. The length of the incubation period, whether there is any predisposing stressor and what makes a population so susceptible are questions the researchers are trying to find answers to.

Because mycoplasma is both north and south of the border, an international team of investigators from the two veterinary schools in Canada as well as several veterinarians in the U.S. have teamed together with the original four veterinarians to try and decipher the answers to these and other questions about mycoplasma, which to my mind is the single most significant disease affecting the North American bison industry today.

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