Johne's disease or paratuberculosis is a common disease in cattle throughout the world. However, it is largely unnoticed on the dairy and its symptoms may be unfamiliar to most dairymen. A National Animal Health Monitoring System Dairy Study in 1996 showed that 45% of dairy producers were either unaware of Johne's disease or only recognized the name. Inspite of its hidden nature, Johne's disease can have significant economic repercussions in heavily infected dairies. The disease is caused by the bacteria Mycobacterium paratuberculosis. Most infections occurs in calves less than 6 months of age although all ages of cattle are susceptible. The most common route of infection is by ingestion of bacteria which have been shed in the feces of infected animals. Some calves are infected in the uterus while being carried by infected cows. The bacteria is known to be secreted in colostrum, milk and semen. Embryo transfer is not generally considered a significant route of transmission to recipient cows, however, it is possible for an infected recipient cow to transmit infection to the embryo/fetus she has received.

One of the factors that makes Johne's disease a difficult problem for livestock owners to deal with is the fact that readily apparent clinical signs may not be seen for 2 to 5 years after infection has taken place during calfhood. After infection has taken place but before clinical symptoms are apparent, the infected animal is likely to be shedding bacteria. These bacteria can contaminate the calving and housing areas and infect other animals in the herd.

In animals which do not have apparent symptoms the disease is still important because it may results in decreased milk production, increased culling rates, decreased fertility and increased mastitis. In one study, it was calculated that in a herd in which 10% of cull cows have clinical Johne's disease that the cost on a herd-wide basis is about $200/cow/year. This means that in such a herd with 1000 animals the indirect cost to the dairyman would be approximately $200,000 per year. The majority of these costs are due to the decrease in milk production.

Across the United States it is estimated that 22% of dairy herds are infected with the bacteria. In general, the larger the herd the more likely it is to have animals infected with Johne's disease. After the long incubation period, the main clinical signs seen in infected animals are profuse, long-term watery diarrhea, marked weight loss and, sometimes, intermittent fever. Johne's infected cows continue to eat even with severe diarrhea. The diarrhea usually has no blood or mucous in it. So the clinical symptoms of Johne's disease are largely non-specific and may be caused by several other agents. Often, even in severe clinical cases, Johne's disease is not recognized and the animals are simply sent to slaughter without concern for the underlying cause of the disease. Within any infected herd, only a very few infected cows will have diarrhea at any one time.

The two most common ways to confirm a diagnosis of Johne's disease are by culturing the causal bacteria from the feces of an animal or by detecting antibodies to the organism in an infected cow. Unfortunately, the bacteria which causes Johne's disease is very slow growing on laboratory diagnostic media. It takes from 4 to 16 weeks before the bacteria become evident on the diagnostic media. The blood tests have the advantage that they yield results more quickly. But both culture and blood tests suffer from the fact that overall they are only positive in about 50% of animals that are known to be infected with the bacteria. Therefore, a negative culture or test result does not rule out infection. Fortunately the specificity of the tests are high and a positive result is 99% certain to indicate an infected animal. As clinical signs get worse an individual animal is more likely to be positive with either test. Unfortunately, animals in the early or middle stages of infection will probably not show disease symptoms like diarrhea, but will be shedding the bacteria in low numbers in the feces. Therefore fecal culture are less likely to detect infection, and serum antibodies will not yet be present. Thus the animal is normal by all tests and clinical observations but is shedding the organism into the environment where it will potentially infect other animals. Throughout the course of infection and clinical disease, serologic response and fecal shedding can be intermittent and clinical disease often wanes in pregnant animals only to recur at parturition or with other stresses. Animals which have severe clinical disease may actually lose their immune response to the organism thus making fecal culture the only way to determine infection.

There is currently no treatment for Johne's infections and it is usually recommended that if an animal is determined to be infected that it be immediately culled. There is no preventive vaccine available to combat infection. The vaccine that is available simply minimizes clinical signs and shedding of the organism. It does not prevent new infections. The vaccine requires approval by each State for its use and usually is given to young stock.

Once the causal bacteria is shed in feces by an infected cow, it is able to live in the environment, feed and water for months without losing infectivity. Therefore, prevention of fecal-oral transmission and sanitation is very important in any effort to minimize or eradicate Johne's disease on a given dairy. Of particular concern for causing infection on a dairy are purchased animals since infected, shedding cattle may look entirely normal.

Currently there is some public health concern about Johne's disease. There is a small amount of somewhat contradictory evidence that links the causal organism of Johne's disease to a disease of humans called Crohn's disease. This evidence suggests that Mycobacterium paratuberculosis shed in the milk from dairy cattle may cause Crohn's disease in humans. Fortunately, pasteurization does kill the bacteria but, in the future, concerns may be voiced about the contamination of meat, raw milk and milk products and water. The possible association with Crohn's disease is just another reason for not drinking raw milk on the dairy.

Efforts at eradication of Johne's disease are currently fragmented with regulations varying on a state by state basis. There are several national committees which have been investigating the possibility and logistics of eradication and a National Paratuberculosis Certification Program is in place. This program is voluntary but is likely a forerunner of any future mandated programs. The certification program focuses on testing animals 24 months and older and culling positive animals until a participating dairy is entirely test negative. Animals which are purchased must also be tested prior to being added to the herd. A participating herd then passes through a series of levels depending on the number of times it has been herd test negative.

For now, dairymen should be aware that Johne's disease is around. The signal for concern should be the sporadic but consistent occurrence of few adult cows with diarrhea that doesn't respond to treatment. If this is the current situation on your dairy, you should consult your veterinarian to develop a diagnostic strategy to determine if Johne's is present and how you are going to deal with it.