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Johne’s Disease Testing in Beef Herds: Looking at the Available Tests

Updated September 28, 2020
Russ Daly

Russ Daly

Professor, SDSU Extension Veterinarian, State Public Health Veterinarian

Veterinarian and rancher reviewing testing information on a tablet.

Johne’s Disease is a subject more and more beef producers are becoming familiar with. Once thought to be a dairy cattle problem, Johne’s is increasingly a topic of conversation for beef seedstock and commercial producers alike.

Johne’s Disease is a chronic, slowly progressing bacterial infection of the lower part of the small intestine of cattle and other ruminants. As the infection progresses, damage to the intestine causes diarrhea and wasting. The resulting loss of production and death losses make Johne’s an important issue for some commercial herds and especially for the seedstock producers marketing to those herds. Spread to other cattle through manure, the bacteria causing Johne’s Disease infects calves at a young age yet progresses so slowly that signs of the illness don’t appear until years later in adulthood.

Unfortunately, that long lag time between exposure and the signs of illness also applies to the lag time period between exposure and when diagnostic tests can first detect infection. Despite this challenge, diagnostic tests are the best tools available for identifying infected animals, ultimately driving management decisions to limit the impact of Johne’s on the beef herd.

Available Tests

Johne’s Disease tests can be split into two categories: blood tests that detect the body’s response (antibodies) to infection, and fecal tests that detect the presence of the Johne’s Disease bacteria. Neither is perfect, but understanding the differences between the two can help veterinarians and producers choose the best tool for a particular situation.

  • Blood Tests

    These tests detect the presence of antibodies specific to the Mycobacterium that causes Johne’s Disease. The most commonly used of these procedures is an ELISA test. Serum, harvested from a blood sample taken from an individual animal, is placed in a plate coated with material from Johne’s bacteria. If antibodies are present in the serum sample, they’ll strongly bind to the plate, which is then treated with enzyme-linked antibodies and a color-inducing reagent.

    It’s the intensity of this color change, measured by an optical reader, that indicates the relative amount of antibodies in the serum sample. The color intensity is compared to negative and positive controls, simultaneously run by the lab, to calculate a number called an S/P ratio. For the ELISA procedure used by the SDSU Animal Disease Research and Diagnostic Laboratory, an S/P ratio above 0.3 is considered “positive” – an indication that antibodies against Johne’s Disease are present in the serum sample; the higher the S/P number, the more antibodies are present and – likely – the more profound the infection in the animal.

    Because of the slow progression of Johne’s infections, using blood tests for detecting Johne’s Disease in an individual animal is not without challenges. Generally, blood tests aren’t positive very long in advance of animals coming down with signs of Johne’s. Cows and bulls can be expelling the bacteria in their manure, yet be negative on a blood test. And because of the sometimes-intermittent nature of bacterial shedding in infected animals, a blood test-positive animal might not have the bacteria in their feces.

  • Fecal Tests

    Genetic material specific to the Johne’s Disease bacteria is detected in manure samples by a procedure called Polymerase Chain Reaction (PCR). This is a very sensitive test that can detect very small numbers of bacteria. This sensitivity means that it is possible for the lab to combine (“pool”) several (usually 5) samples together for a test, which saves money (important because PCR tests are 5-6 times more expensive than blood tests). Fresh fecal samples should be placed in individually labeled plastic tubes or vials and sent to the lab chilled.

    The PCR procedure uses cycles of heating, cooling, and addition of reagents to amplify the specific bacterial segment to a level it can be detected. The number of these cycles is counted to get a rough estimate of how much of the genetic material was initially present in the sample: fewer cycles correlates to more bacteria in the sample, while higher numbers means there was less. These tests are reported out as positive or negative, along with the number of cycles (Ct level). Typical Ct levels for positive animals range from 23-37.

    Veterinarians are increasingly gravitating toward fecal tests because they offer a picture of the actual shedding of the Johne’s Disease bacteria, rather than the body’s response to the infection. In a given animal, they’re generally considered to detect infections earlier than blood tests, but there are challenges with PCR tests too.

    While considered an advantage, the high sensitivity of PCR tests can also create some interpretation difficulties. Unless strict attention is paid to sampling (using separate gloves for fecal collection and attention to cleanliness), it’s possible to cross-contaminate samples during collection, resulting in false positives. False positive results can also occur if cows ingest bacteria from a contaminated environment; the bacteria may be simply passing through the gut rather than from an active infection. Veterinarians use the sample’s Ct level to help sort some of these issues out, but cows that are positive at high Ct levels should be retested later.

In Summary

An understanding of the choices available for Johne’s testing is a good first step in devising a herd testing strategy. Those considerations are covered in the article, Johne’s Testing in Beef Herds: Testing Plans and Responses.