The Mycobacterium Complex

Why Keeping Goats with Chickens Could Result in a Positive Test for Johne's Disease

The Mycobacterium Complex

There were no signs or symptoms, but Stacy did a blood test on her goats just in case.  

A friend recently had to cull her entire herd due to poor biosecurity measures, and Stacy was taking no chances. Since her herd seemed healthy in every way, she was in total shock when one of her beloved goats had a low positive result for Johne’s disease. Pronounced “Yoh-nez,” this disease can have an extremely long incubation period, but it is always deadly. Stacy immediately put her goat into isolation and sent in a sample for fecal testing. For two and a half weeks, she listened to her goat crying and calling for her friends. Once, the goat got her head caught in the fence and almost killed herself in her frantic attempts to rejoin the herd. If the results came back affirmative for Johne’s, it could mean losing Stacy’s entire herd of nine goats, three sheep, a cow, and a horse because Johne’s is easily spread between those species through fecal contamination. 

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There are four stages of Johne’s disease. In the first stage, the disease is dormant, yet slowly building. Usually, this is when an animal is under two years old because they are most susceptible in the first six months of life. This stage can last for months or years. This animal will not test positive either by ELISA blood test or by fecal culture. It is unknown if any animals recover during this stage because we do not yet have a test that is sensitive enough to detect Johne’s at stage 1.  

In stage 2, the disease still has no symptoms, but has progressed enough that the animal is shedding the bacteria in their feces. A fecal culture would detect the disease, but a blood test would not pick it up until stage 3. Again, this stage can last for years, in which your goat is likely infecting others.  

At stage 3, your goat may have signs of illness usually brought on by stress but then disappearing for a time. They may have decreased milk production and are losing weight even though their appetite stays the same.  

Pronounced “Yoh-nez,” this disease can have an extremely long incubation period, but it is always deadly.

Once an animal reaches stage 4 of Johne’s disease, they look emaciated and will soon die (Johne’s Disease, 2017).  

While goats are not prone to diarrhea like cattle with Johne’s, their stool may change consistency. There is no cure for Johne’s disease. Some have attempted to treat it with antibiotics, but soon after treatment ended, the disease came right back. Johne’s disease is caused by Mycobacterium avium subspecies paratuberculosis. Yes, this is similar to the bacterium that causes human tuberculosis and also leprosy. This disease is found worldwide, though some northern European countries have made excellent advances against it. 

The easiest, fastest, and cheapest test to scan for Johne’s disease is the ELISA blood test. ELISA stands for enzyme-linked immunosorbent assay. This test looks for antibodies to the Mycobacterium in either an animal’s blood or milk. If there are antibodies found, the amount will be compared against controls of both positive and negative tests in order to give a numeric value result. A higher number means that there is a greater chance that the animal does in fact have an infection of Johne’s disease. However, ELISA is not the most reliable test for Johne’s disease (University of Wisconsin-Madison School of Veterinary Medicine). It usually cannot detect the disease until it is in stage 3, and it can even produce a false positive result. This is what happened to Stacy. 

For two and a half weeks, Stacy searched for answers on how her seemingly healthy goat could have contracted Johne’s disease. She had received the goat from a reputable source and always took very careful precautions to keep her herd healthy. When the fecal test results came back negative for Johne’s, she only had more questions. As her goat rejoiced to be back with the herd, Stacy continued to search for answers. Her answer brought another hard decision to make. It turns out, that because Stacy also owned chickens that were kept in close proximity to the goats, a bacterium from the chickens that is very similar to what causes Johne’s was picked up by the goat and produced a false positive. 

There are a good handful of subspecies in the family of Mycobacterium avium. Several of these are zoonotic, or can jump between species including humans. These are grouped into the Mycobacterium avium complex. In particular, Stacy’s goat likely picked up Mycobacterium avium subspecies avium (yes, you read that correctly). This particular subspecies is prevalent in domestic poultry and often carried in wild birds, particularly sparrows. While goats have been found to be fairly resilient to this strand of mycobacterium, that doesn’t mean that the goat wouldn’t pick up the bacteria and develop antibodies against it because the body still sees it as a foreign invader. Because the different subspecies of Mycobacterium avium complex are so similar, it is reasonable to think that an antibody test, especially one that is not known to be the most reliable such as ELISA, would have a false positive result in reaction to one of the other subspecies of bacteria.  

Johne’s disease is caused by Mycobacterium avium subspecies paratuberculosis. Yes, this is similar to the bacterium that causes human tuberculosis and also leprosy. This disease is found worldwide, though some northern European countries have made excellent advances against it. 

From this false positive result in her goat, Stacy now knows that her chicken flock has been exposed to avian tuberculosis. Because avian tuberculosis also has a long latency period in which it is hard to detect, it can be extremely difficult to singularly test and cull infected birds from the flock. Not only can it hide in a bird before becoming detectable, it can survive in the soil for up to four years. Mycobacterium avium can survive most disinfectants, cold and hot temperatures, dryness, and pH changes. The most reliable way to eradicate this bacterium is with direct sunlight (Dhama, et al., 2011).  

Stacy now faces the decision of culling her entire flock of chickens in addition to changing her animal housing layout. From now on, her chickens will be kept far away from all other animals to eliminate the possibility of disease transfer. While she was already taking good biosecurity measures, she is planning to increase all measures with any new animals being quarantined until they can be proven free of disease. She will have all animals annually tested for disease via blood tests. Stacy recommends anyone with livestock to take these measures. It only takes one sick animal to possibly infect and wipe out our entire herd. The cost of testing and taking safety precautions is insignificant in comparison to the cost of replacing an entire herd. 

While Stacy’s story has a (mostly) happy ending, it could have been drastically different. If she hadn’t been able to send in a fecal sample for the much more expensive yet more precise test, she likely would have had to cull her goat at the very least. Stacy’s story gives an excellent example of why and how we need to observe biosecurity measures in our animal operations.  

References 

Dhama, K., Mahendran, M., Tiwari, R., Dayal Singh, S., Kumar, D., Singh, S., et al. (2011, July 4). Tuberculosis in Birds: Insights into the Mycobacterium avium Infections. Veterinary Medicine International

Johne’s Disease. (2017, August 18). Retrieved April 2, 2019, from USDA Animal and Plant Health Inspection Service: https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/nvap/NVAP-Reference-Guide/Control-and-Eradication/Johnes-Disease 

University of Wisconsin-Madison School of Veterinary Medicine. (n.d.). Goats: Diagnosis. Retrieved April 2, 2019, from Johne’s Information Center: https://johnes.org/goats/diagnosis/ 

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