Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne's Disease (JD), is prevalent worldwide. The NAHMS Dairy 1996 study, estimated 21.6% of the dairy herds in the US were infected with MAP, resulting in annual economic losses for the dairy industry of $200-250 million.
Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne's Disease (JD), is prevalent worldwide. The NAHMS Dairy 1996 study, estimated 21.6% of the dairy herds in the US were infected with MAP, resulting in annual economic losses for the dairy industry of $200-250 million. Cattle generally become infected with MAP as young calves, but do not exhibit signs of the disease until years later. Due to the chronic nature of the disease, and its long incubation period, testing and culling infected animals as a method for controlling JD has been relatively ineffective by itself. Instead, strategies for controlling JD have focused on minimizing the exposure of calves, the animals most susceptible to becoming infected, to MAP, thereby preventing new infections. The role of the environment as a source of Map transmission has long been understood, but the importance has been unknown
While calves can become infected with MAP in utero, or through ingestion of colostrum or milk from infected cows, this generally only occurs when the dam is in the latter stages of the disease. It is believed most post-natal infections occur through the ingestion of feces from a contaminated environment. Thus, factors playing a role in transmission include the amount of MAP being shed into the environment, the location contaminated, and the length of time the bacteria survives in that environment.
As an obligate intracellular pathogen, MAP does not replicate outside the, but it can survive for months to over a year in the environment. There is evidence that MAP is less likely to survive in environments of higher pH, thus supporting the use of lime in areas of potential MAP contamination.
Wildlife, birds, even invertebrates such as flies and worms commonly found around dairy farms can become infected with MAP, and occasionally shed the bacterium into the environment. While the amount of MAP shed by these nontraditional hosts is negligible compared to that shed by cattle, it does represent a way by which the bacterium can persist and multiply outside of the primary host.
Recently, the discovery of "dormancy-related genes" in the MAP genome suggests that, in the absence of essential nutrients, MAP may enter a state of dormancy and then return to a viable, infectious state when conditions again become favorable. Under field conditions in Australia, using the sheep strain of MAP, the bacterium was cultured from pasture twelve months after destocking a property.
Studies have been conducted to determine the extent of MAP contamination on infected dairy farms. The bacterium has been found in numerous locations on dairy farms including calving pens and post-weaned calf pens, high-risk areas for transmitting the disease to the next generation of herd replacements. The area's most commonly culture positive for MAP are those where manure accumulates from adult cattle, the animals most likely to be shedding the bacterium. These include manure storage areas (lagoons, manure spreaders) and high-traffic, common cow areas (feed alleys, holding pens, return alleys, etc.). As a result, targeted culturing of these areas can be used to identify MAP infected herds. In the most recent revision of the USDA's Johne's Program Standards, targeted environmental culturing was approved as an entry-level screening test for dairy herds desiring to participate in the Voluntary Johne's Disease Control Program (Table 1). Evidence also suggests the number of positive environmental cultures, and the amount of MAP in those samples, is positively correlated with the within herd prevalence.
After adult cow manure storage/accumulation areas, the next most common place to find MAP in the environment is the maternity area. This is important as new born calves are most susceptible to infection and maternity area contamination with MAP can significantly increase the risk of infection.
Environmental contamination with MAP is an important mode of transmission between infected cows and susceptible calves. Efforts need to be taken to reduce environmental contamination especially in areas where young stock are housed.
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