Back from The Vet: Johne’s Disease
What treatment options do you have for Johne's disease in goats?
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It can lurk in your herd for years without causing any signs of illness. It can spread rapidly from animal to animal, infecting an entire herd. When animals do show signs of infection, there is no treatment or method to slow the progression of the disease. It is often fatal. There is no vaccine available to prevent it. What is this insidious disease?
Johne’s disease, or Mycobacterium avium subspecies paratuberculosis, is a bacterial infection of goats and other ruminants. It is contagious and fatal. The bacteria can survive in the environment for several months and is resistant to heat, cold, drying, and moisture. Though the prevalence of Johne’s is not well established in goat herds, some studies indicate that up to 50+% of herds may have Johne’s-infected animals. Dairy cattle herds have the highest prevalence, with 68% of herds having an animal test positive for the disease. The infection is highly contagious and may be spread from animal to animal; with cross-species, such as cow to goat or goat to sheep, transmission as well.
Goats contract the infection when a new animal, usually asymptomatic, is brought into the herd and is shedding the bacteria in their manure. Infected goats can also spread the bacteria to their kids, either across the placenta, through milk, or infected manure. Young animals appear to be the most susceptible to infection. Once swallowed, the bacteria invade the intestine, primarily the part called the ilium, and lives within white blood cells there. The infection can live in the intestine for years without causing any signs of illness. During this time, however, the animal is able to shed the bacterium in their milk and manure. As the disease progresses, so does the shedding of the bacteria. At some point, anywhere from two to 10 years after infection, the bacteria begin to spread throughout the goat. There is an increased immune response to the bacteria, causing the formation of inflammatory tissue. As the primary infection is within the intestine, this inflammatory tissue forms there. This inflammation, known as granulation, causes thickening of the intestine, impeding its ability to absorb nutrients. Eventually, the inflammation spreads to the lymph nodes of the intestine, and then throughout the body.
Animals infected with Mycobacterium avium subsp. paratuberculosis go through several stages of the disease. Initially, infected animals appear to be normal and healthy. Unfortunately, after the early stages of the infection, these apparently healthy animals can be shedding the bacteria. As the infection progresses, often after years, animals may begin to show signs of these infections. Due to the intestinal inflammation caused by the bacteria, a primary sign of infection is weight loss. Unlike cattle infected with Johne’s, goats do not often show signs of diarrhea. Occasionally, they can have a clumpy or pasty stool. As there are multiple diseases of goats that can manifest as weight loss, such as caseous lymphadenitis and intestinal parasites, it is important to work closely with a veterinarian to confirm a diagnosis.
Diagnosis of infection with Mycobacterium avium subsp. paratuberculosis can be made in several ways. In animals suspected to have died due to the infection, a veterinary necropsy is advised. Inflammation of the intestinal tissue is readily visible in these animals, as well as enlargement of the intestinal lymph nodes. When found, this abnormal tissue can be tested to assess for the presence of the bacteria. In live animals, there are multiple tests available to determine if an infection is present within your herd or a specific animal. When choosing a test, consulting with your herd veterinarian, and establishing your testing goals are imperative.
There are three different methods of laboratory tests for Johne’s. The first is PCR. PCR is a rapid and inexpensive test that assesses for the presence of the mycobacterium’s genetic material within the sample. The sample types used for this test are manure or tissue. Culture is the second method of testing. Culture also uses manure or tissue samples. These test samples are monitored for eight weeks to assess for growth of the bacteria. Heavily contaminated samples can generally be ruled positive more rapidly, but labs will continue the incubation period to be confident in a negative result. Due to its more rapid turnaround time, PCR tests have largely replaced culture tests. When used at a whole herd level, both of these tests can be performed more economically by pooling samples. Samples are pooled, generally five samples per pool, and tested as one. Should there be a positive in a pooled sample, the five samples mixed for that test are then tested individually. It is recommended to send your samples with the animals listed on the form in order of decreasing age. This allows the oldest, and most likely to be shedding, animals to be pooled together. The third method for testing for Johne’s is called an ELISA. This test assesses for antibodies to the bacteria within the animal’s milk or blood. It provides a numeric result, with the higher numbers indicating a greater likelihood of infection. The ELISA test was made to test large numbers of animals quickly, making it very inexpensive. Though it is inexpensive, it is the least sensitive of the three tests. ELISA tests will be positive in only 30-50% of animals that will show infection on PCR or culture. This is because antibody production against the bacteria occurs after shedding of the bacteria in manure.
Once identified within a herd, there is no specific treatment for Johne’s disease. There are, however, methods to control its spread. Testing is necessary to identify infected animals and careful records must be kept to ensure knowledge of positive and negative animals. Manure must be handled appropriately, ensuring that the more susceptible young animals are not exposed to contaminated manure. Kidding must occur in clean pens or on clean pastures. As the bacteria can also be found in milk, pasteurization of milk fed to kids is imperative. Milk must be pasteurized at 145 F for 30 minutes for batch pasteurization or 162 F for 15 seconds for flash pasteurization. If pasteurization is not possible, feeding of milk replacer is advised. As pasteurization of colostrum is difficult, it should only be fed from test-negative does. If possible, test-positive animals should be culled from the herd. If not possible, these animals must be carefully managed to decrease spread as they are shedding the disease.
Prevention of Johne’s within your herd is far better than having to control it. There are simple ways to prevent the transmission of the disease into your herd. First and foremost, prevent the introduction of positive animals to your herd. If possible, operate a closed herd, with the introduction of no outside animals. If you do add outside animals, ensure they come from a test-negative herd and that you are able to see the negative test results. Herds that offer multiple negative tests are more to be preferred. If you are buying kids, ensure they come from test-negative does. When considering tested animals, place more value on fecal PCR than on ELISA results. Johne’s is a costly disease, and it is much easier to prevent its introduction to your herd than it is to eradicate it.
If you have not assessed your herd for the presence of Mycobacterium avium subsp. paratuberculosis, it is advised you do so. There are states offering certification programs to establish test-negative herds. It is only by identifying shedding animals that the spread of disease is able to be stopped. Work closely with your veterinarian as you establish a Johne’s program for your herd.
The Johne’s Information Center — University of Wisconsin-Madison School of Veterinary Medicine (https://johnes.org/)
Dr. Katie Estill DVM is a veterinarian consultant for Goat Journal, Countryside & Small Stock Journal, and Countryside online. She works with goats and other large livestock at Desert Trails Veterinary Services in Winnemucca, Nevada.
Originally published in the January/February 2020 issue of Goat Journal and regularly vetted for accuracy.