Animal Health Matters: What to do with Johne’s test results

Russ Daly
Special to the Farm Forum

Johne’s Disease is a slowly progressing bacterial infection of the small intestine in cattle, causing chronic diarrhea and wasting. Its effects on production losses and mortality spurs seedstock and commercial beef herds to consider testing strategies to reduce the disease’s impact. In my last Animal Health Matters column, I outlined the different types of tests used to detect Johne’s Disease in infected animals.

The decision to test a herd usually comes after finding a Johne’s infected cow — but that should be talked over with your veterinarian. If you have a commercial herd and don’t keep heifers or bulls, encountering a Johne’s-infected animal or two may not warrant a herd test since long-term effects can be minimal if all the calves go to the feedyard. For seedstock producers or commercial herds that retain heifers, though, long-term implications are more profound. Seedstock producers have their reputation at stake and have a lower threshold for making the decision to test. Some herds without clinical Johne’s cases may be interested in herd screening to ensure their infection level is low.

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.

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 increase somewhat when animals are sampled post-calving compared to other times.

The typical years-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 rarely test positive. A practical implication of this is that testing heifers or bulls before 2 years of age is not very useful (the same can be said for testing individual yearling bulls or heifers prior to sale).

When cows or bulls test positive for Johne’s Disease, they should be culled from the herd before they become clinically infected and to limit the possibility of them infecting calves. The question often arises whether the offspring of positive cows still present in the herd should be culled, too. Johne’s Disease transmission from infected dam to offspring is not inevitable and depends upon several factors. Cows with advanced infections can pass the bacteria to calves in utero, or in colostrum and milk after birth. More likely is the fecal-oral transmission to a calf nursing and living in close proximity to their infected mother. The conservative approach is 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, you and your veterinarian should plan how to use the results. If you’re going to retain “favorite” cows or bulls in the herd regardless of their Johne’s test result, testing will be fairly pointless. You and your vet 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. Removing or segregating those positive animals from the main herd should be a priority.

While imperfect, testing herd animals for Johne’s Disease is the best tool available for you and your veterinarian 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 your veterinarian.