This epidemiology page describes
the occurances of Johne's disease,
changes in rate of the disease over
time and how the infection is spread.

HOST RANGE | PREVALANCE
SOURCES | TRANSMISSION

The cause of Johne's disease, Mycobacterium paratuberculosis, has a broad host range. The type of animals most commonly infected are ruminants: hoofed herbivores with four-chambered stomachs that chew their cud. Examples of ruminants include: cattle, sheep, goats, deer, elk, antelope, bison, etc. (Camelids such as llamas, guanacos and alpacas, while considered "pseudo-ruminants", are also susceptible to the infection). Johne's disease has been reported in a number of wild ruminant species, both captive and free-ranging. In addition, there have been a few reports of M. paratuberculosis infecting pigs, horses, and nonhuman primates. One report found that the same strain of the bacterium was found to infect dairy cattle plus numerous rabbits in the region and a number of different carnivore species (fox, stoat, badger, raven) that may have been preying on the rabbits. Some reports in medical literature claim to have detected genetic components of M. paratuberculosis in humans. The significance of these findings has yet to be determined.

Johne's disease has been reported on every continent. Sweden and some states in Australia are the only regions of the world that can claim freedom from Johne's disease based upon a reliable disease reporting system and extensive surveys using laboratory tests. The reported prevalence of infected animals by country is at least partially a reflection of the diligence with which veterinarians and animal owners look for the disease.

Johne's disease is more common in dairy cattle than other ruminants. This is likely a function of animal husbandry methods used for dairy cattle that differ from most other species. Johne's disease is fairly common in small ruminants like sheep, goats in many countries but precise survey data have not been published. Only a small percentage of sheep flocks in Australia have been found to have Johne's disease prompting the sheep industry there to move aggressively to contain the infection.

M. paratuberculosis bacteria are obligate pathogenic parasites of animals. This means that the only place they can multiply in nature is inside an animal. Most accurately, it is inside cells that are part of the animal's immune system called macrophages. When M. paratuberculosis leaves an animal, for example in the feces, it can survive for a long time in environments such as soil and water, but it can not multiply once it is outside the animal. Consequently, the primary source of infection is infected animals.

Herds or flocks of animals acquire the infection by being exposed to the bacteria from an infected animal. When an infected animal is introduced to a herd, the opportunity for transmission of M. paratuberculosis bacteria to other animals increases the longer that animal remains in the herd. As the infection progresses in the animal, the frequency and number of M. paratuberculosis bacteria being excreted increases.

M. paratuberculosis infects the intestine, thus feces (manure) is the most common mode of exit of the bacterium from the animal. Ingestion of M. paratuberculosis-containing feces, or fecal contaminated feed or water is the most common way animals become infected. In manure, M. paratuberculosis can remain alive for over a year, depending on environmental conditions. For more detailed information on the survival characteristics of M. paratuberculosis see the part of this website called "Biology of M. paratuberculosis".

Milk from infected female animals is a second source of M. paratuberculosis infection. The likelihood of M. paratuberculosis being excreted into milk increases with time as the infection progresses. The probability of young animals becoming infected by drinking milk from infected cows, does, or ewes is a direct function of the time spent with the mother and/or how often they are fed milk from infected females. In animals where husbandry practices allow young animals to remain with the dams and nurse naturally, the chance for transmission of the infection from mother to off-spring is greatest. M. paratuberculosis may be excreted directly into the mother's milk and/or the surface of the teats might be contaminated with infected manure.

Pond water contaminated with manure carrying M. paratuberculosis is another potential source of infection. A less likely, but possible, infection source is pasture land contaminated with the bacteria.

Johne's disease has been reported in free ranging wildlife, but their role in the ecology of M. paratuberculosis is not known. Similarly, it is not known if wild birds can become infected or transfer M. paratuberculosis bacteria between farms.

Risk factors affecting the probability of transmission of M. paratuberculosis to other animals have not been measured. Consequently, only general statements can be made about circumstances that promote transmission of infection.

Animal age is perhaps the most well-recognized factor affecting M. paratuberculosis transmission. In cattle, there is an age-dependent increase in resistance to M. paratuberculosis infection. This means it takes a larger dose of the bacterium to infect an adult (over 2 years-old) than it does to infect a young animal (0 to 6 months-old). This may also be true for small ruminants, but it is not as well studied and clinical reports suggest there is a greater susceptibility of sheep, goats and deer to this infection, even as adults.

The extent and duration of exposure to contaminated manure and milk from infected adult animals directly affects the likelihood of M. paratuberculosis transmission. Clean, dry, birthing environments and housing of young animals away from the adult herd or flock limits the possibility of infection transmission. Conversely, dirty maternity pens or fecal contamination of feed and water supplies will promote spread of the infection. Provision of milk from animals free of the infection or pasterization of milk for feeding young animals is essential to preventing new cases of Johne's disease.