Cow Tales: The Skinny on Johnes
Many ranchers have had a cow in the herd that suddenly begins to deteriorate. Sometimes these individuals are the grandmas of the group with hardly a tooth. Yet other times a middle-aged cow suddenly begins to lose condition and develops diarrhea.
One of the most common causes of diarrhea and weight loss in cattle is Johnes disease. Johnes (pronounced “yo-knees”) disease is the result of a chronic bacterial infection. The bacterium responsible is Mycobacterium avium subspecies paratuberculosis or MAP for short.
This group of bacteria includes the causative organisms of tuberculosis and leprosy in humans. Johnes disease is estimated to be present in 68 percent of dairy herds and 8 percent of beef herds. The number of animals infected within each positive herd can vary dramatically depending on management behaviors.
The economic impact can exceed $250 per cow in severely infected dairy herds. It is harder to estimate the impact on the beef industry due to decreased monitoring of disease status and incomplete production records. However, Johnes disease has been estimated to cost the Georgia beef industry alone $2.5 to $4.9 million per year.
Infection most likely takes place during the first six months of life. Older animals are more resistant to infection but it can occur when contamination is high. Young animals are also more likely to encounter the organism through contaminated milk and environmental burden. Infection results when the bacteria are ingested. The bacteria can be taken up by specialized cells near the terminal end of the small intestine. Here the organisms are engulfed by white blood cells called macrophages. MAP survives and even flourishes within these white blood cells that usually are responsible for preventing bacterial invasion. In fact, MAP must be taken up by a macrophage in order to reproduce. Additional cells are recruited to the area as a result of the infection. The affected gut tissue swells inhibiting its normal absorptive function. Eventually the affected cow develops diarrhea and severe weight loss leading to death. During the late stages of the disease the bacteria can spread to the mammary gland, be secreted in milk, and cross the placenta. The disease process is chronic with a minimum of two years before outward signs are noticed. Often cows will survive to middle or old age before developing diarrhea and succumbing to the disease process. As a result, animals showing clinical disease often only represent a small portion of the total number of animals affected.
The most successful means to prevent the introduction of Johnes disease into a negative herd is to maintain a closed herd. But, market conditions may lead producers to purchase females as replacements or during herd expansion. Because young animals are more susceptible to Johnes infections, control measures focus on this stage of production. Dairy calves can be separated from their dams at birth. Colostrum can be used from known test-negative cows. Calves are routinely reared on pasteurized waste milk and commercial milk replacers until weaning. Minimizing the calf’s exposure to the cow helps prevent MAP infection. However, beef operations are more limited in their options for effective control due to the intimate relationship of the cow-calf pair. Steps can still be taken to minimize transmission of the infectious bacteria.
Producers should utilize pastures with the least amount of fecal contamination for calving. Individual calving pens should be separate from hospital pens. Pairing-out newborn calves to reduce the stocking density further decreases the calf’s exposure to MAP. General hygiene practices such as using separate equipment or clean equipment to handle feeds and the use of feed bunks to reduce fecal contamination enhance control measures.
There are two types of tests available to detect MAP infected cows. Some tests are designed to detect the bacterium or it’s DNA in the manure of cows. Blood tests are used to detect an immune response to the invading bacteria. Each test has its strengths and weaknesses. For example, as a chronic disease, many subclinically infected cows will not shed the bacteria in their manure. These cows will falsely be diagnosed as negative with fecal tests. In fact, the sensitivity of Johnes disease diagnostic tests range from about 25-50 percent. This means 50-75 percent of positive cows will not be detected by a single test. If we combine two tests we can raise the sensitivity of the testing to 65 percent. On the otherhand, the tests are very specific leading to vary few false positives. The application of individual and herd-based testing is quite complicated and best left to Johnes certified veterinarians. The best control programs utilize a combination of testing and management practices to deal with this costly disease.
Humans can suffer from a similar disease process called Crohn’s disease. Many people have tried to connect a link between the two diseases unsuccessfully. However, it is important to note diagnostic tests for Crohn’s disease are subject to many of the same challenges as those for Johnes disease. Conversely, the incidence of Crohn’s disease is no greater in people with an agriculture background as compared to those from an urban background. At this point there is no clear link between the two diseases. Should Johnes disease ever be labeled as a zoonotic disease, the beef and dairy industry will have to adapt to a new set of tests and regulations.
In short, Johnes disease is a bacterial infection leading to diarrhea and weight loss. Infection occurs primarily through ingestion of infectious bacteria as a calf. Diagnostic tests can be used to detect infected herds and individuals but their applications are complicated. Producers wishing to determine their Johnes disease status should work with a Johnes certified veterinarian to develop a herd management plan. Additionally, individual states may have cost share programs in place for the implementation of such programs. More information is available at http://www.johnes.org for producers and veterinarians alike.
Nerd Word: Peyers patch – oval, elevated patches of closely packed lymph follicles in mucous and submucous layers of the small intestine. The site of M-cells responsible for uptake of MAP from the intestinal tract.
Kenny Barrett Jr. is a veterinarian at the Belle Fourche Veterinary Clinic in Belle Fourche, SD and pens “Cow Tales” monthly. Learn more about the clinic on the Web at http://www.bfvetclinic.com, or drop them an e-mail at: firstname.lastname@example.org to suggest a topic for the next installment of “Cow Tales.”
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