Johne’s disease is a relatively easy disease to diagnose in an individual cow, either through clinical signs or at post mortem. The characteristic symptoms of progressive weight loss and chronic watery diarrhea are easily identifiable.
This slowly developing bacterial infection is most often contracted by very young calves and may not show clinical signs until the animal is four to five years old.
The infection causes chronic thickening of the intestinal mucosa, which affects the ability of the intestine to absorb nutrients. No effective treatments are available.
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The dilemma most veterinarians and producers face is what to do next with the rest of the herd. The choices of a variety of diagnostic tests and approaches can be confusing.
None of the diagnostic tests for Johne’s disease are very good at identifying cows that have subclinical infections. They all work better as the disease progresses but can fail to identify infected cows early in the disease process, which results in false negative tests.
Three potential tests are available that can be used to try to identify positive cows in the herd: the fecal culture test, the fecal PCR (polymerase chain reaction) test and the blood ELISA antibody test.
The fecal culture test is best able to identify infected cows. The sensitivity of this test can reach 60 percent, which means it will identify only 60 percent of the infected cows as positive.
However, because Mycobacterium avium paratuberculosis (MAP) is a very slow growing organism, the culture method can take months before results are available.
The other disadvantage is that this test is the most expensive of the three, costing $65 to $70 per sample. However, the fecal samples can be grouped into five cow pools, which can reduce the cost of testing significantly, although positive pools will have to be retested individually.
The availability of this test in Canada is becoming limited because fewer diagnostic labs are offering it. As a result, it may become more difficult to access.
The other two tests perform at approximately the same sensitivity of 30 to 35 percent. Note that this means that if there are 10 cows infected in your herd, the testing method will find only three to four cows positive.
The fecal PCR test uses molecular techniques to identify the MAP bacteria in the feces, and the blood ELISA test looks for antibodies to the MAP bacteria in the blood.
The fecal PCR costs $35 to $40 per sample and can also be pooled in groups of five cows to reduce the cost somewhat. Positive pools have to be retested to identify which cows are positive.
The blood ELISA is the least expensive of the three tests, costing approximately $12 per sample and cannot be pooled. However, there is an additional cost of having to blood sample the cows versus just collecting fecal samples.
In the end, these two tests are very similar in performance and overall cost, and the decision probably comes down to whether you prefer to collect fecal samples or blood samples from your herd.
It should be noted that testing and culling for Johne’s disease is a slow process, and it may take years to lower the prevalence of the infection in a heavily infected herd. Each time you test a herd, the poor sensitivity of the tests means that you are leaving positive cows in the herd.
Regardless of the testing protocol, biosecurity principles need to be part of the control strategy to limit the spread of the infection. This is based on two main principles:
- minimize the exposure of susceptible calves to the feces of infected cattle
- reduce the environmental contamination by eliminating animals that shed MAP
The basic principles of minimizing fecal exposure for young calves that are implemented for control of neonatal diarrhea are essentially the same management principles for the control of MAP infections.
Systems such as the Sandhills calving system or the Lacombe system to spread out the calving environment and minimize fecal contamination are important aspects of MAP control. Testing and culling alone will prove to be a wasted effort if attempts are not made to minimize spread through biosecurity and management.
It may not be cost effective to use whole herd diagnostic testing in commercial herds with a very low prevalence of MAP-infected cows. In these herds, early culling of clinical cases and the implementation of biosecurity principles that reduce the exposure of susceptible calves to adult feces may be enough to minimize transmission.
The control of Johne’s disease in beef cow-calf herds has not been studied as extensively as in dairy herds. A long-term approach to control should be considered in herds where it is economically advantageous to implement control programs.
None of the diagnostic tests offer a high enough sensitivity to identify all of the carriers of MAP infection, and your veterinarian will need to consider economics as well as your motivations to control the disease when making decisions about implementing diagnostic testing strategies.
