The Johne's ELISA: What it Can (and Can’t) Tell You

A very commonly used test for Johne’s Disease in cattle is the Enzyme-Linked Immuno-Sorbent Assay (ELISA). This test can rapidly be done on serum, plasma or milk and is relatively inexpensive when compared with other tests such as fecal culture or the Polymerase Chain Reaction (PCR) test. A number of different ELISA tests are available from different manufacturers.

It is not uncommon for herds do an ELISA test on a sub-group of the mature cows (e.g. 30 randomly selected cows), or on the whole mature cow herd, and to use the ELISA test results to identify ‘high-risk’ animals. These animals might be identified with a special ear tag and then calved in a separate pen, or the colostrum from these animals might not be fed to calves. In some situations these animals are monitored more closely at calving time so that the calves can be removed as soon as possible after birth. Some herds are culling all ELISA positive cows and/or their calves from the herd. ELISA negative animals in these herds are usually assumed to be at low (or no) risk of causing new infections and therefore the management of these animals and their offspring is often a bit more lax.

Diagnostic tests such as the Johne’s ELISA are used to tell us something about the “true” status of the animals being tested. As such, it is important to know how well they are able to do this - how well, for example, they can tell us which animals are truly infected with the bacteria that causes Johne’s disease (also know as Mycobacterium avium ssp. paratuberculosis or “MAP”). Or we may wish to use the test to determine which animals are shedding the organism in their manure, since this is thought to be one of the major ways by which uninfected animals become exposed and infected. However, very few diagnostic tests are “perfect” - that is, very few tests are able to perfectly separate the infected from noninfected animals, or the shedding from the non-shedding animals in a herd. And the Johne’s ELISA is certainly far from perfect! This doesn’t mean it is a useless test, but it does mean that it needs to be used properly and that the results must be interpreted with care.

This discussion will focus on using the ELISA test to determine which animals are shedding the MAP bacteria in their manure, since this is usually of the most immediate use for managing high risk animals in herds with Johne’s Disease. While some animals may be infected with the bacteria but not (yet) shedding, they do not currently pose a risk to the other animals in the herd.

There have been a number of PA dairy herds participating in a multi-year study of Johne’s Disease - in these herds blood and fecal samples were collected on the same day from all the mature cows in the herd. This gives us an opportunity to evaluate the ability of the blood ELISA test to tell us which cows were actively shedding the organism in their manure. A number of research papers have also been published in the past number of years which suggest that the ELISA test correctly identifies somewhere between 20 and 35 percent of the animals that are shedding the organism (i.e. are fecal culture positive) and the results from our study herds in PA generally fall within this range. For example, if there are 20 animals in “herd A” shedding MAP in their manure on a given day, the blood test (if done on all of the mature cows) would identify between 4 and 7 of these animals (“true-positives” - see table below).

In other words, while it might be assumed that the ELISA negative animals do not pose a risk to other animals in the herd, 13 to 16 of these negative animals would actually be shedding or “culture positive” (“false-negatives”). It is easy to see therefore that if the management of the ELISA negative cows is not just as rigorous as the ELISA positive cows there is a good chance of a significant number of new infections occurring because of these supposedly ‘low-risk’ animals! The ELISA tests are generally much better at classifying a non-shedding (culture negative) animal as a negative - between 95 and 98 percent of non-shedding animals will be ELISA negative. Therefore if “herd A” has 100 cows that are culture negative, 95 to 98 of these will be ELISA negative (“true-negatives”). However there will be 3 to 5 cows which the ELISA test says are positive which are not shedding the bacteria in their manure (“false-positives”). Therefore if this herd decides to cull all of its “ELISA positive” animals it is easy to see that there will be some cows culled that do not pose a significant risk to their herdmates (5 out of 9). (It is of course possible that some of these animals may start shedding the bacteria in the future.)

The following table summarizes the information from “herd A”.

Given these results, is the Johne’s ELISA test of any use in dairy herds?

Yes, there are some situations in which it makes good sense to use the ELISA test. First, in animals that are showing signs that are compatible with Johne’s (weight loss, diarrhea, good appetite) the ELISA test performs much ‘better’ - if the test comes back as positive there is a very good chance that the cow really does have Johne’s Disease. Therefore, it may be used by veterinarians as a rapid diagnostic test to confirm their diagnosis.

Secondly, while it may not be very accurate at separating infected from non-infected, or shedding from non-shedding animals, the ELISA test is useful for screening herds for the presence of Johne’s Disease. In other words, even though some cows may be incorrectly classified by the test, it is generally able to pick out herds that have infected animals in them. This is especially true when a large proportion of the herd is tested and when the testing is repeated over a number of years. Therefore the ELISA can be used to screen herds that are quite likely to be free of Johne’s Disease and want to enter the ‘status level’ of the PA Johne’s Herd Certification Process. Increasing confidence in the test-negative status of the herd is achieved through repeated testing. Given the lack of ‘accuracy’ of the test and the nature of Johne’s Disease, some herds with low levels of infection may not find any positive animals until they have tested for a few years. (When screening herds with the ELISA test, any animals that have a positive ELISA result should be retested by fecal culture.

In fact, for herds that are certified at the status level these animals must be tested by fecal culture or PCR.) Finally, the Johne’s ELISA is able to provide a ‘rough estimate’ of the level of infection within a herd. That is, a herd that has 2 percent of its animals ELISA positive on a whole-herd test is very likely to have a lower percent of its animals infected/shedding with Johne’s Disease than a herd that has 9 percent positive, since the test performs fairly similarly in different herds. (However, as discussed above, this does not provide very accurate information about the specific animals that are infected/shedding!)

Controlling Johne’s Disease involves implementing management and biosecurity practices which will minimize the number of animals exposed to the bacteria that causes this disease. Testing can be an integral part of this process, but the right test must be used for the right purpose, and the results interpreted correctly.

Ernest Hovingh
Dairy Extension Veterinarian