Every year, I seem to write another article about Johne’s disease. It is one of the most common topics of phone calls and questions that I receive from veterinarians and producers.
Johne’s disease is a relatively easy disease to diagnose in an individual cow either via clinical signs or at post mortem. The characteristic symptoms of progressive weight loss and chronic watery diarrhea are easily identifiable. On postmortem, the thickened intestines and corrugated surface of the intestinal wall are also fairly characteristic of this disease.
The major challenge is not usually the individual animal diagnosis in the beef cow that has clinical Johne’s disease. The dilemma most veterinarians and producers face is what to do next with the rest of the herd if they want to attempt to minimize or reduce the level of disease in the herd.
Most of our diagnostic labs use two common methods of diagnostic testing for the detection of cattle that may have become infected with the MAP bacteria. A blood test can be used to detect antibodies in the blood that have formed in response to an infection with the MAP bacteria. There is also a PCR test that can be performed on fecal samples, which identifies the genetic material of the MAP bacteria if it is being shed in the feces.
A third test that involves culturing the organism is also available at some labs in North America but is expensive and time consuming.
Neither of the more common diagnostic tests are very good at identifying cows that have subclinical infections. They both work better as the disease progresses, but fail to identify lots of positive cows early in the disease process. Both tests have about the same level of performance and the easiest way to demonstrate the problem of controlling Johne’s disease is with an example.
Imagine that I am a cow-calf producer with a herd of 1,000 cows (cows that have had at least one calf). In my herd, we have recently seen a clinical case of Johne’s disease in a five-year-old cow that had chronic diarrhea and weight loss. My veterinarian diagnosed Johne’s disease in this cow and after it died a post mortem was performed and tissues were sent to a diagnostic lab to confirm the diagnosis. I’m interested in controlling this disease and limiting its spread and so we have decided to test my entire herd.
For the sake of this example, let’s imagine that 10 percent of the cows in my herd actually have become infected with the MAP bacteria. This would be quite a high level of infection for a beef herd, but a 10 percent level of infection will make the math easier.
We obviously don’t actually have a way of determining this level ahead of time, but let’s imagine we have some omniscient powers that allow us to determine the true level of infection. This means that there are 100 cows in my herd that have become infected with the MAP bacteria and it would be ideal to identify them early so we can remove them from the herd to limit the spread of the bacteria.
We have decided to use the blood test in my imaginary herd, but the performance of the fecal PCR test is very similar and so the results would not change dramatically. There are a variety of estimates for the performance of the tests, but for this example we will use an estimate of test sensitivity of 30 percent and a specificity of 99 percent.
Sensitivity of the test refers to the ability of the test to identify infected animals. You can see that for Johne’s disease, the test sensitivity is quite poor. It will only identify 30 of the 100 infected cows as positive. So after one round of testing, if I cull all of the cows that tested positive, I will still leave about 70 infected cows behind in my herd. These 70 “false negative” cows may still be shedding the MAP bacteria into the environment and infecting other animals. These cows may show up as positive in future rounds of testing, but the test failed to identify them as positive in this first test.
The specificity of the test (or the ability to identify uninfected animals) is much better at 99 percent. However, this means that of the 900 uninfected animals in my herd, the test will correctly identify only 99 percent of those 900 negative cows, or 891 cows, as test-negative. The other nine cows will show up as positive cows on my blood test and I will be culling them even though in reality they aren’t positive.
You can easily see why Johne’s disease is a frustrating and difficult disease to control. I used a diagnostic test that only found 30 of the 100 positive cows and it also gave me results with nine false positives to cull that I didn’t need to get rid of.
Because the diagnostic testing is so imperfect, biosecurity principles need to be put in place 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. Using systems such as the Sandhills calving system or the Lacombe system, spreading out the calving environment and minimizing 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.
In commercial herds with a low prevalence of MAP infected cows, it may not be cost effective to use whole herd diagnostic testing. In these herds, early culling of clinical cases and the implementation of biosecurity principles, which 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. In those herds where it is economically advantageous to implement control programs, a long-term approach to control should be considered. None of the diagnostic tests offers a high enough sensitivity to identify all of the carriers of MAP infection and you 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.