Once you know that your herd has Johne’s Disease (JD), is there any more reason to test? Let’s be clear, every herd owner should know whether they have the disease within the herd. If you don’t know, then taking manure samples from the holding pen or scrape alley for culture is a good way to diagnose the herd status. This sample should represent the manure from multiple cows. You should have at least three negative culture reports, taken on different days, before trusting that your herd is not infected.
Let’s say that you know that your herd is infected (which is true of the overwhelming majority of dairy herds). Not only do you know your herd status, but you have put in place practices to reduce the potential for new infections and disease spread (see Make your calves the focus for Johne’s Control). Then does testing still have a place?
The diagnostic tests for JD have limited applicability as individual animal tests. In the case of culture or polymerase chain reaction (PCR) analysis of manure from a single animal, we know that if live bacteria are present then the lab will likely be able to grow, culture and identify that bacteria. Even if only dead bacteria are present, PCR analysis may be able to magnify and identify the DNA of the JD pathogen.
However, infected animals only shed bacteria intermittently. Depending on the progression of the disease, they may go days, weeks or even months without shedding the bacteria. If the fecal sample was taken on a day that an infected animal was not shedding bacteria, then those tests will not detect the disease and a producer may wrongly conclude that the animal is not infected.
Likewise, the ELISA test on blood serum or milk is designed to detect antibodies produced by the animal in response to the infection. Yet an infected animal may not have mounted an immune response or that response has declined. Again, a negative test result does not necessarily mean an uninfected animal. In fact, the sensitively, or ability to detect positive animals by ELISA is only 30% to 50%.
The good news on any of these tests is that the specificity is high. There is a very low risk (1% to 5%) that the test results will cause a false-positive diagnosis. So is an animal tests positive for JD, she usually is.
That being said, it is not unusual to see different results from different tests or different results at different times. An animal may be positive by fecal culture and negative by ELISA or an animal may be positive by ELISA one time and negative the next time. Since this disease has no cure, it is most likely that an animal diagnosed as infected is still infected.
Though that may mean that test results can sometimes be confusing, we simply need to understand the limitations of them and use them with that understanding in mind.
Therefore, our recommendations are that routine herd testing can provide information to manage cows and to gauge the progress that a herd owner is making in combating the spread of JD within a herd. Positive results for a cow can be used to make management decisions such as to not use the colostrum for heifer calves or even not to re-breed. Animals diagnosed as infected may be identified visually with the use of a zip-tie in the ear tag so that more aggressive practices can be used in the maternity pen to reduce exposure of newborn calves.
Animals younger than 18 months are not good candidates for JD testing. You may want to target your testing to specific times to better utilize the information. For example, testing cattle at the end of lactation provides important information just prior to calving which can then be used to make decisions about where that cow calves, or whether to use her colostrum, or whether to breed her or not.
MSU encourages you to work with your herd veterinarian on a herd testing plan, interpretation of test results and a control plan for JD within your herd. In spite of the limitations of testing, it can provide some information by which to manage and feedback on herd progress. But testing can never substitute for good risk-reduction practices, consistently implemented with every animal. There is no use testing cattle if you are not willing to make changes in management to reduce the risk of disease transmission on your farm.
Source: Phil Durst, Michigan State University Extension