Introduce a new bison, and perhaps Johne's disease too.

A bison producer family managed a 100 head herd for breeding stock and meat. A cow introduced to the herd within the year had lost more than 300 pounds and had diarrhea. When the veterinarian completed a health check, the primary findings were positive results for two Johne's disease blood tests (i.e. antibody to M. paratuberculosis was detected by both AGID and ELISA. She was euthanized but no gross signs of the infection (such as enlarged lymph nodes or corrugated gastrointestinal tract) were found when the carcass was examined at necropsy. Tissues were collected (1) to be assessed microscopically for lesions caused by the infection and (2) to try to isolate the M. paratuberculosis organism from the tissues through culture.

Johne's disease was confirmed by both methods. The organism was isolated from many sites in the gastrointestinal tract (the ileum, jejunum, duodenum, cecum, colon, liver, mesenteric lymph nodes). M. paratuberculosis was also isolated from the animal's manure, meaning she had been shedding the organism on the premises.

1. Know the Johne's disease history of the herd from which you plan to buy.

2. Internal tissues of a bison with Johne's disease can appear completely normal (no gross lesions) on necropsy! Tissue samples must be collected for both microscopic examination and culture to maximize the likelihood of detecting the infection.

2. While blood tests should not yet be relied upon as proof of infection in bison (too few Johne's disease cases have been tested as of yet to validate the tests), they can be used to screen herds and direct further testing to confirm the infection and control the spread of the M. paratuberculosis.

3. Having confirmed a case of Johne's disease in the herd, the owner should screen the herd annually for Johne's disease as it is very possible that the infection was transmitted to other bison. Attention should be focused on:

• offspring of the infected dam, as they have had the greatest exposure to the organism
• the other calves in the herd at the time because young animals are most susceptible to acquiring the infection
• any bison with weight loss and/or diarrhea