A professor at the University of Wisconsin-Madison School of Veterinary Medicine has developed a testing method that more precisely describes an animal's level of infection, thereby enabling farmers to make more informed decisions about disease management that could improve herd productivity.
Johne's disease, named after the veterinarian who first isolated the disease-causing bacteria (Mycobacterium paratuberculosis) in 1895, affects one in 50 cows in the United States and one in 20 among the country's dairy cows. Clinical signs include diarrhea, weight loss and lowered milk production. As the disease progresses, it becomes increasingly contagious and, as a result, could devastate a dairy farm by diminishing the herd's productivity.
While two accurate methods for detecting Johne's disease infection exist, both have drawbacks. The fecal culture, which is the most accurate, can take three to four months to complete. This method, as well as the other one - a blood test that takes only a few hours to process - provides only positive or negative results, which are based on a certain number of antibodies detected in the animal's blood.
Usually, all cows that test positive - even though the results might be borderline and the animal could be productive for another two years - are culled for slaughter.
"I think culling for slaughter all cows that test positive for Johne's disease is a little too expensive, a little too drastic," says Michael Collins, a UW-Madison pathology professor who has studied Johne's disease for more than 20 years. "The economic consequences of mistakenly culling a cow due to a false-positive test result are high - roughly $1,300 a cow."
For this reason, Collins has developed a technique, used in conjunction with the standard blood test, that classifies test results into five levels: negative, suspect, weak positive, positive and strong positive. Each level has a recommendation for the cow's management, from segregating those with weak positive results to immediately culling all cows that test strong positive.
Such classification, says Collins, enables farmers to make more informed decisions on how to handle each cow.
In addition to saving farmers money, the method also allows diagnostic laboratories to handle more samples more efficiently: Collins' lab tested some 35,000 blood samples last year using the classification method at one-third to one-half the cost of fecal culture tests.
"The test is as good or better than other blood-based tests," says Collins. "Because it is so much faster and cheaper than a fecal culture, it is a very useful test for management of Johne's disease in dairy herds." He stresses, however, that this test method is less sensitive than fecal culture methods.
Details about the improved testing method for Johne's disease are published in the November issue of Clinical and Diagnostic Laboratory Immunology. Collins' method will be made available to diagnostic laboratories nationwide within six months.
- Jenny Bryers, 608-263-6914, bryersj@svm.vetmed.wisc.edu
CONTACT: Michael Collins, 608-262-8457; mcollin5@facstaff.wisc.edu
Journal
Clinical and Diagnostic Laboratory Immunology