WLSB reminds producers about testing bulls for Johne’s Disease
Johne’s, pronounced “yo-knees,” disease is a slowly progressive bacterial infection, which causes wasting in cattle and other ruminants. Although mistakenly thought of as a dairy cattle disease, Johne’s disease affects both commercial and purebred beef cattle.
The cause of Johne’s disease is a Mycobacterium avium subspecies, known as paratuberculosis (MAP). There is no cure for Johne’s disease, so if producers are purchasing bulls or replacement heifers, they need to do their due diligence to avoid accidentally introducing Johne’s disease into their herd.
Disease transmission
Calves can be born infected, but the disease is most commonly transmitted from adult cattle to young calves in the first six months of life.
Calves can ingest MAP bacteria in multiple ways: on contaminated teats, nursing colostrum or milk from an infected cow and licking bedding or other manure contaminated surfaces.
Susceptibility to Johne’s disease decreases with age, and adults over one year of age are least susceptible to infection.
Infected animals eventually shed the bacteria in their manure, perpetuating the disease. MAP bacteria can contaminate calving jugs, and it can survive for up to 18 months in damp environments.
Course and symptoms
Johne’s disease causes a gradual thickening of the gastrointestinal tract, progressively inhibiting affected animals’ ability to absorb nutrients and fluid.
Affected cattle begin to lose weight, despite having a normal appetite. The manure consistency changes to intermittent diarrhea and then progresses to severe diarrhea. Death occurs due to dehydration and cachexia, also known as wasting.
Infected cattle do not usually show signs of Johne’s disease until two to five years of age. Because of this long incubation period, it can be years before producers realize their herd is infected. In the meantime, cattle may be culled for other reasons, such as reproductive failure, before showing signs of Johne’s disease.
Types of testing
Unless testing is performed, Johne’s disease can exist undetected in a herd for years. The only way to confirm a herd does not have Johne’s disease is to test.
Tests for the disease can either identify the MAP bacteria causing Johne’s disease or identify the animal’s antibody response to MAP infection in their blood or milk.
Two diagnostic tests are available to identify MAP bacteria – culture and polymerase chain reaction (PCR) tests.
Culture tests isolate the bacterium from manure, tissue or environmental samples, while PCR tests look for MAP DNA in samples.
MAP bacteria grow slowly, so culture results can take months. Because of faster turn-around time, PCR testing has become the test of choice to identify MAP bacteria, usually in a manure sample.
The test used to look for antibodies in the blood of an animal is an enzyme-linked immunosorbent assay (ELISA) test. The ELISA test provides a numeric result. Typically, the higher the number, the more likely the animal is truly infected and shedding MAP.
ELISA test kits are also available for testing milk samples from individual cows.
In animals showing signs of Johne’s disease the PCR and ELISA tests are equally effective. As with all diagnostic tests, false positive and false negative results can occur.
Due to the complexity of Johne’s disease, assistance from the diagnostic lab and a veterinarian is recommended to interpret test results. Whole-herd testing is strongly recommended if any animal in the herd has been diagnosed with Johne’s disease.
Identifying animals in need of testing
There is no test to identify Johne’s disease in the early stages of infection. Only adult cattle 18 months of age and older will shed MAP bacteria reliably in manure and have detectable antibodies. Thus, testing is only recommended for adult cattle.
In cases when younger cattle are being purchased, herd test results can be used as guidance. Cattle buyers should ask the producer if they test for Johne’s disease and if they have had any positive cases in the past five to 10 years.
Individuals selling cattle can use herd test records or a statement from their veterinarian to demonstrate their herd’s Johne’s disease status.
Treatment options
There is no cure for Johne’s disease, and it does not respond to antibiotics or diet changes.
Once Johne’s disease is introduced to a herd, control measures can be taken to reduce spread, but elimination of the disease requires a test and cull program, which takes years.
Economic impacts
Johne’s disease is associated with substantial economic losses.
It is not just the animals with wasting and diarrhea affecting the bottom line. Infection with Johne’s disease impacts the health of animals before they show clinical signs.
Reproductive costs include decreased pregnancy rates and increased calving intervals. Affected animals have decreased milk production, increased susceptibility to other diseases, poor feed conversion and reduced weight at slaughter. This can culminate in premature culling – a major cost.
Johne’s is a reportable disease in Wyoming. Any Johne’s disease positive animal should be euthanized on the farm or taken directly to slaughter, and not sold, to minimize spread of disease.
Conclusion
Johne’s disease is much easier to prevent than to eliminate. Individuals should ask about a herd’s Johne’s disease testing history before purchasing animals, and should test adult cattle before introducing them into their herd.
Dr. Teckla Webb is the Wyoming assistant state veterinarian-field operations. She can be reached by e-mailing lsbforms-applications@wyo.gov.
