Introduction:
Johnes disease, pronounced YO-knees (or paratuberculosis), is a chronic, bacterial disease of the intestinal tract in cattle and other animals, that is usually fatal. Johnes disease occurs in a large variety of animals and is widely distributed across all regions of the United States. Johnes disease has been reported in cattle, sheep, goats, deer, antelope and llamas, but is most commonly seen in dairy cattle. This disease is difficult to identify in its early stages and has no known cure.Causative Agent: Johnes disease is caused by a bacteria called Mycobacterium paratuberculosis. This organism grows very slowly, causes a gradually worsening condition, and is highly resistant to the infected animal’s immune defenses. Infected animals harbor the organism for years before they develop clinical signs or test positive for the disease.
Sources of Infection: The disease typically enters a herd when an infected, but healthy looking animal, is brought into a healthy herd. The infection then spreads to non-infected animals when they ingest manure from another infected cow. The severe diarrhea often associated with Johnes disease is the most common cause of infection from animal to animal. Once outside the animal, the bacteria are quite hardy, and may live for months in water, feed, and manure. In the later stages of infection, the bacteria is found in the milk of infected animals. Young animals can become infected by drinking milk from infected cows. Calves younger than 6 months old are most vulnerable to infection. Research also suggests that an infected mother can pass the disease to the fetus during pregnancy.
Clinical Signs: Clinical signs rarely appear until cattle are 2-3 years of age or older. Chronic diarrhea and weight loss are typical of the disease.
Generally, only one or two animals in a herd have clinical signs. Infected cattle frequently eat well and look bright, but appear unthrifty. They generally do not have a fever.
Usually symptoms first appear following calving, a severe stress, or under conditions of poor management.
The signs of Johnes disease can be confused with signs of several other diseases like coccidiosis, winter dysentery, and parasitism.
This disease develops so slowly, that by the time the owner becomes concerned, there may be a widespread problem within the herd.
Diagnosis:
An excellent way of diagnosing Johnes disease is to detect the organism that causes the infection in the manure of the animal. There are several tests available and a veterinarian can help collect samples for a Johnes disease fecal culture. These tests are very accurate, but take up to 16 weeks to get a definitive answer.Recently, tests have become available that are able to detect (identify) infected herds using a pooled fecal sample (either directly from a group of cows or from a manure storage area). These tests are PCR based and available in 3-4 days.
There are several different blood tests for Johnes disease. The ELISA test is considered most accurate and best standardized. This test is fast, simple, and able to detect animals that are infected before they show clinical signs. The ELISA test requires blood samples be collected in both red and purple topped tubes. At this time, a positive ELISA test alone is not considered sufficient for the diagnosis of Johnes disease; a fecal culture should also be performed. Recent studies indicate that Johnes disease can also be detected in a milk sample.
Treatment: There is no effective treatment for Johnes disease and attempts at treatment are not warranted. All positive animals should be culled.
Prevention: Johnes disease can be controlled and even eliminated from infected herds, but it takes a thorough understanding of the disease by animal owners, consultation with a veterinarian, and requires the use of available diagnostic tests. The basics of control are simple; new infections must be prevented and animals with the infection must be identified and removed from the herd.
Prevention is based on maintaining a closed herd. Do not buy or lease livestock unless they are known to come from clean herds.
Management changes that prevent transmission to non-infected cattle are also necessary. Specific actions include the following: