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

PREVALENCE | SOURCES
TRANSMISSION | INCUBATION

Johne's disease has been reported on every continent. Virtually no country or region of the world can claim freedom from the disease. The reported prevalence of infected animals or herds is at least partially a reflection of the diligence with which veterinarians and animal owners look for the disease.

In the USA, a 1996 national survey found that 22% of dairy cattle herds had at least a 10% infection rate (called herd prevalence). Forty-one percent of herds had at least one ELISA- positive cow. Of the over 31,000 cows tested for serum antibodies by ELISA, 3.4% were estimated to be M. paratuberculosis-infected (called individual cow prevalence). For the full survey report is available from the APHIS website.

The results were also the subject of an article in Dairy Herd Management, December, 1997. This article is shorter and more interesting to read for a lay audience than the full report. It can be found inthe Articles and Brochures section of this website.

These results were comparable to those found in a survey of Wisconsin dairy herds in 1990 where one-third of herds showed evidence of having one or more infected animals (Collins et al., JAVMA 204:636, 1994). In another Wisconsin study (Nordlund et al., JAVMA 208:1872, 1996), the average percentage of cows testing ELISA-positive in 23 infected herds having 1,653 cows in total was 8.9% (called within herd prevalence). Herd prevalence estimates for dairy cattle other countries are as follows:

Until regulations are created to limit the chances for spread of Johne’s disease among herds, M. paratuberculosis will continue infecting more herds in the U.S. and likely most every other country with a significant dairy industry.

Eventually, almost all herds could become infected. The epidemic spread of Johne’s disease is illustrated in the adjacent graph.

In the U.S. survey it was found that dairies in which more than 25% of cows were born on other dairies were 2.1 times more likely to be infected than dairies in which none of the cows were born elsewhere (Wells, et al. JAVMA 216:1450, 2000). During this time of restructuring in the U.S. dairy industry, herd expansions are common resulting in purchase of large numbers of cattle. These animals are usually bought without first testing the source herd for Johne’s disease.

Consequently, infected cattle are frequently bought and large expansion herds are frequently infected. This is supported by data from the U.S. survey showing frequency of ELISA-positive herds based on herd size shown in the adjacent graph.

...in general

M. paratuberculosis is an obligate parasitic pathogen of animals. This means that the only place they can grow and multiply in nature is inside an animal (actually inside the cells of the animal). When M. paratuberculosis leaves an animal, for example in the feces, it can survive for a long time in the environment, but it can not multiply once outside the animal. Consequently, the primary source of infection is infected animals and proportionately, dairy cattle appear to have the highest infection rate. See the "Biology of M. paratuberculosis" topic for detailed information.

....for herds

Dairy herds acquire the infection by accidental addition of an infected animal. The longer that animal remains part of the herd, the greater the opportunity for transmission of M. paratuberculosis infections to other animals.

...for individual animals

M. paratuberculosis bacteria infect the intestine, thus feces (manure) is the most common vehicle for exit of the bacterium from the animal. In manure, M. paratuberculosis can remain alive for over a year, depending on environmental conditions. See the section of this website on "Biology of M. paratuberculosis" for a more compete discussion of environmental survival. Ingestion of manure containing M. paratuberculosis is a common way calves become infected.

Milk from infected cows is a second, but possibly even more important, source of M. paratuberculosis infection for calves. The likelihood of M. paratuberculosis being excreted in milk of animals increases with time as the infection progresses. The probability of young calves becoming infected by drinking milk from infected cows is a direct function of time spent with the mother and/or how often they are fed milk from infected, or possibly infected, cows. In beef cattle where husbandry practices allow young animals to remain with their mothers and nurse naturally, the chances for transmission of the infection from mother to offspring are greatest. M. paratuberculosis may be excreted directly into the mother's milk or, it might be on the outside of the cow’s teats if they are contaminated with infected manure. Pond water contaminated with manure from cattle infected with M. paratuberculosis is another potential source of infection. A less likely, but possible, infection source is contaminated pastures. 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 between farms.

As the infection progresses in cows, the frequency and number of M. paratuberculosis bacteria being excreted in milk and manure also increases. These infected cows then serve as the source of infection for calves. Heifer (female) calves become members of the adult milking herd after calving and, if infected, serve to further amplify the infection of the herd. Paratuberculosis follows basically the same epidemic pattern as other infectious diseases, one major difference is the time course. Because the rate if increase in herd infection rate (prevalence) over time is measured in years, it is often not perceived as a spreading infection (epidemic). The graph shown here, created by a computer simulation model (Preventive Veterinary Medicine, 11:131-146, 1991), illustrates the spread of Johne’s disease in a typical dairy herd.

The curve shown is for a herd of 100 milking dairy cows with average quality herd management (effective contact rate of 2.0), where one replacement heifer is bought per year from a source with a 10% prevalence of M. paratuberculosis infections (typical of the random source dairy replacement cattle market). The model illustrates that it takes a long time for the infection to get started in a herd, but once that happens M. paratuberculosis can spread through the herd quickly if no actions are taken to control it. 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. Extent and duration of exposure to feces 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 limits the possibility of infection transmission. Conversely, dirty maternity pens or fecal contamination of feed and water supplies will promote spread of the infection. See the "Control" section of this website for additional details.

After becoming infected there is a prolonged incubation period. During this phase of infection animals will appear healthy and grow normally. The incubation period varies widely among cows and the reasons for this degree of biological variation are unknown.

The adjacent graph based on data from Australian researchers show that clinical signs of Johne’s disease in dairy cows can be seen as early as 2 years of age and as late as 12 years of age. The age at which Johne’s disease most commonly is seen in dairy cattle is 5 years old which generally coincides with 2nd, 3rd, or 4th lactation.