Infection starts with ingestion and leads to intestinal pathology that causes the animal to become thin.

This contagious mycobacterial disease spreads from infected adult bison to the most susceptible age group (calves) by a number of routes. Although less susceptible than young animals, adult bison may also acquire the infection. The organism can be shed by an infected animal into milk and manure; the manure then contaminates water, wallows and solid feed. Bison may be able the infection from other infected species (cattle, sheep and goats).

Caveat: most information on M. paratuberculosis infection and Johne’s disease has been obtained through studies on domestic species (for example cattle, sheep, goats). While it is believed that this information likely is applicable to bison as well, there may be some facets that differ for which we do not yet have a good understanding. Please bear this in mind as you use this web-site.

Johne’s disease is an M. paratuberculosis infection primarily of ruminants and has been documented in a majority of these hoofstock species, including bison. It is likely that all ruminants are susceptible to infection. The infection has also been reported in primates (macaque, mandrill), rabbits and the carnivores preying on these infected rabbits (fox, stoat). It is believed that the vast majority of M. paratuberculosis infections in non-domestic hoofstock species occur in the first few months of life. As bison mature, it is thought that their resistance to infection increases although complete resistance is unlikely. Adult animals can also become infected if given a sufficient dose of M. paratuberculosis at a period of immune insufficiency. In most cases, adults serve as the source of infection to young animals, shedding the organism in manure, if not milk and colostrum as well.

Most bison calves acquire the organism by suckling from manure-soiled teats, by licking contaminated flooring/fencing/feed bunks or by eating off of ground contaminated by manure from an infected animal. They also can take up the organism by drinking water contaminated by manure from infected animals.

A second method of exposure is through drinking contaminated milk. Since M. paratuberculosis is thought to be excreted in the milk of infected animals, as has been shown to be the case in cattle, calves can become infected by nursing from or being bottle-fed milk from an infected cow. The risk that the organism is being shed into the milk is higher for dams that have moved into later stages of the infection. Thus clinically affected hoofstock (as shown by weight loss and perhaps diarrhea) are more likely to infect their offspring than dams still in good condition.

The third, but believed least common, route of exposure occurs in utero. Again during the later stages of infection, the organism can spread beyond the gastrointestinal tract. At that time, if the cow is pregnant, the fetus can also become infected. This infected fetus appears completely healthy at birth, although spontaneous abortion of fetuses with this infection has been reported in cattle.

While different strains of the organism have been described ("bovine" and "sheep") strains, it is likely that a least a majority if not all strains can infect any ruminant.

While not directly pertinent to bison in some aspects, see the web pages under Dairy-Epidemiology for a broader description of this topic.