ohne’s Disease is a slowly progressing bacterial infection of the lower part of the small intestine in cattle and other ruminants, resulting in chronic diarrhea and wasting in infected animals. Its effects on production losses and mortality spurs seedstock and commercial beef herds to consider testing strategies to reduce the disease’s impact. The article Johne’s Testing in Beef Herds: Looking at the Available Tests outlined the different types of tests used to detect Johne’s Disease in infected animals.
The decision whether to test a herd isn’t always straightforward and should be made after careful discussion with the herd’s veterinarian. For a commercial herd that does not retain heifers or bulls, having a Johne’s-infected animal or two may not warrant a herd test; long-term effects can be minimal. For seedstock producers or commercial herds that retain heifers, though, long-term implications are more profound. In particular, seedstock producers have their herd’s reputation at stake and may have a lower threshold for making the decision to test. Even without clinical evidence of infection, some herds may be interested in screening their herd to ensure their infection level is low.
Testing Considerations
Blood tests and fecal tests both perform well at detecting their targets, but the long incubation period and chronic nature of Johne’s Disease create challenges for any test strategy. Antibodies and bacteria shed in the manure often do not appear until just before clinical signs show up in an animal, possibly years after the initial exposure and infection. Any test strategy should be employed with the understanding that some of the test-negative animals could very well be infected, just not yet detectable.
Timing
If the decision is made to test a beef operation for Johne’s Disease, one of the first considerations is when to test. Pregnancy testing time in the fall provides an opportunity to sample each animal and gives producers a chance to cull or otherwise manage infected animals prior to calving season. However, because cows are subject to more stresses around calving time (with increased antibody production and fecal shedding), chances for detecting Johne’s are greater when animals are sampled post-calving compared to other stages of the production cycle.
Age
The long incubation period for Johne’s Disease not only affects the age at which animals become ill, it also has implications for testing. Cattle in early stages of infection will rarely test positive. A practical implication of this is that testing heifers or bulls before 2 years of age is of very limited usefulness. (The same can be said for testing individual yearling bulls or heifers prior to sale).
Using Results
When cows or bulls test positive for Johne’s Disease, the natural response is to cull those animals from the herd before they become clinically infected and/or infect calves. The question often arises whether to cull any offspring of positive cows that may be present in the herd. Johne’s Disease transmission from infected dam to offspring is not universal and depends upon several factors. Cows with advanced infections can pass the Mycobacterium bacteria to calves in utero, or in colostrum and milk after birth. Perhaps more likely is the fecal-oral transmission to a calf nursing and living in close proximity to their Mycobacterium-shedding mother. Although this transmission is not inevitable, a conservative approach would be to assume progeny born right before the dam’s positive test result have been sufficiently exposed and cull them from the herd as well.
Before any samples are ever taken, producers (with their veterinarians) should formulate a clear plan for how to use the results. If producers are going to retain “favorite” cows or bulls in the herd regardless of their Johne’s test result, testing will be pointless. Producers and their veterinarians should also have clear plans for dealing with “suspect” test results, possibly re-testing animals with a different test method.
Using test results for management decisions should center around the concept that the most common way for Johne’s Disease to perpetuate itself in a herd is for calves to become infected from cows and bulls shedding the bacteria (See Johne’s Disease Management: Preventing Manure Contact is Key). Removing or segregating those positive animals from the main herd should be a priority.
In Summary
While still somewhat imperfect, testing herd animals for Johne’s Disease is an appropriate tool for producers and their veterinarians to begin to get a handle on the level of infection and to implement sound strategies for limiting the disease’s effects. Any effort at testing should be done with appropriate advice and planning with the herd veterinarian.
