Johne’s disease is relatively easy to diagnose in an individual cow, either using 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. The dilemma most veterinarians and producers face is what to do next with the rest of the herd.
The variety of diagnostic tests and approaches that are available can be overwhelming, and little guidance is available for dealing with the disease in the cow-calf herd because most of the research has been focused on the dairy herd.
Chronic watery diarrhea and significant weight loss in cattle older than two years are features of Johne’s disease, caused by persistent infection with the bacteria Mycobacterium avium subspecies paratuberculosis (MAP).
Young calves or previously unexposed mature cattle may become infected by oral ingestion of the bacterium because of environmental contamination by an infected herd mate shedding MAP in manure.
Clinical signs of infection typically become apparent only at two years, or longer after failing to clear the primary infection. Chronically infected individuals may intermittently shed bacteria in manure and milk in the absence of clinical signs for several years, increasing the risk of exposure and infection for calves and other herd mates.
Our most recent estimate of the extent of MAP infections in Western Canada comes from the recent Western Canadian Cow-Calf Surveillance Network Project. In this study, local veterinarians collected blood samples from 1,811 cows from 93 herds in Alberta, Saskatchewan and Manitoba. Serum from blood samples was tested with an ELISA test for the detection of MAP antibodies.
Evidence of infection as determined by antibody presence was observed in only 28 of the cows, or 1.6 percent.
Positive herds were defined as having at least two cows test positive, and using that definition, 5.4 percent of herds tested in the surveillance network were positive.
Making decisions on how to approach controlling this disease at the herd level must be taken in context with the economic impact. There are two basic scenarios to consider:
- diagnosis of Johne’s disease within a commercial cow-calf herd
- diagnosis of Johne’s disease in a purebred or seed stock herd
The second scenario obviously has a much greater potential economic impact, and seed stock producers have much more motivation to control or attempt to eliminate this disease from their herds. At the very least, they should be seen to be making every effort to control this infectious disease and not spread it by selling infected individuals to other herds.
In the commercial herd, it may in some cases not be economically viable to attempt to do extensive diagnostics because the clinical impact of the disease in most commercial cow-calf herds is relatively minor unless they are heavily infected.
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 miss lots of positive cows early in the disease process.
The ELISA antibody test is the least expensive option, but it does the poorest job of finding positive cows early in the disease process. It also occasionally misdiagnoses negative cows as positive.
The fecal culture is the best test, but even it is imperfect and will not identify all the early infections. It is much more expensive and takes months to grow the bacteria.
There is also a fecal PCR test that uses molecular methods to identify bacteria. It is cheaper and faster than the culture method but has the same issues as the ELISA antibody test.
Regardless of which diagnostic test is chosen, the imperfect sensitivities of the tests will not find all of the positive animals. As a result, the veterinarian and producer must be prepared for any form of control program to be a five to seven year project.
If you believe you have Johne’s disease in your herd, you will need to have a conversation with your veterinarian about which method of testing will be best for your particular situation.
Regardless of the testing protocol, biosecurity principles need to be 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 controlling neonatal diarrhea are essentially the same management principles for controlling MAP infections.
It may not be cost effective to use whole herd diagnostic testing in commercial herds with a low prevalence of MAP infected cows. In these herds, early culling of clinical cases and implementing biosecurity principles that reduce the exposure of susceptible calves to adult feces may be enough to minimize transmission.