Causative agent: Mycobacterium avium, subspecies paratuberculosis
Johne’s disease occurs in white-tailed deer, European red deer, reindeer, sika deer, axis deer, fallow deer, moose, aoudads, mouflons, llamas, African buffalos, water buffalos, camel, stonebucks, antelopes, yaks, gnus, zebu cattle, tule elk, bighorn sheep, mountain goats, American bison, and wapiti (25), as well as domesticated cattle, sheep and goats (9).
The disease is very common in cattle and sheep worldwide (9). When tested, 5.5% of cull cows in Ontario had Johne’s (9). Sheep can carry and spread the bacteria, showing minimal disease themselves (9). The prevalence of this disease is increasing worldwide (9). The source of infection is ingestion of feed or water contaminated by the bacterium. In cattle and sheep the bacteria is shed in feces, milk, and semen. Infected cattle can shed the bacteria for years before they develop disease. In cattle, the bacteria can cross the placenta and infect the fetus. Thirty seven percent of fetuses from infected cows become infected (9). Infection of cattle usually occurs early in life, but disease does not occur until the animals are 3 to 5 years of age(9). The bacterium can persist for one year or more on pasture (9). Johne’s disease has been reported in bison (25).
Clinical signs:
The clinical signs of Johne’s disease have not been reported for bison (25). In cattle, the most common clinical signs are chronic diarrhea and loss of body weight leading to emaciation (9).Cattle may take up to a year to die (9).
Postmortem findings:
Pathological changes associated with Johne’s disease in bison have not been documented (25). In cattle, the most common findings are emaciation and thickening of the walls of the small intestine and cecum (9).
Diagnosis:
The clinical signs of chronic diarrhea and emaciation are not specific to Johne’s disease. There have been no specific diagnostic tests established for diagnosing Johne’s disease in bison. Likewise, the only reliable method of establishing a diagnosis in cattle is postmortem examination. There are many tests available for use in live cattle, none of them are very sensitive. In general, there is a high probability of identifying an infected herd but it is not possible to consistently diagnose the presence of the bacterium in healthy, live individual animals.
Serological tests are of limited value for use as screening tests, but are reasonably accurate for diagnosing the disease in clinically affected cattle.
Fecal cultures are the most reliable diagnostic tests. As with serological testing, fecal cultures can detect herd problems, but are not very reliable for detecting disease in individual animals. However in cattle, fecal cultures are thought to be able to detect the presence of the bacterium in the herd 1 to 3 years before clinical disease is seen (9). Fecal samples must be taken from a number of individuals in the herd and must be repeated on several occasions in order to determine whether the bacterium is present or absent in the herd.
Treatment:
There are no treatment protocols established for the treatment of Johne’s disease in bison. Treatment of cattle has been shown to produce temporary improvement, but has not been shown to cure animals (9). This disease is generally considered untreatable.
Control:
In all susceptible species, control has proven to be very difficult because there is no reliable test to determine if an individual animal is infected. This makes it impossible to identify and remove infected animals before they develop clinical signs. Because of the widespread nature of the bacterium, it is difficult to prevent its introduction into a herd when new stock are purchased. Fecal cultures of all animals every 6 months and culling of infected animals and their offspring has reduced, but not completely eradicated the disease from cattle herds (9). The only way to eradicate the disease is to depopulate and then repopulate from clean stock.