John H. Kirk, DVM, MPVM
Extension Veterinarian
University of California, Davis
Tulare, CA

A recent National Animal Health Monitoring Service (NAHMS) survey suggested that only half the dairy producers in the US were familiar with Johne's Disease. In the near future, many more dairymen will know a lot more about this difficult disease. At this time, the National Johne's Working Group is attempting to organize working groups in each state. Recently, with the leadership of the California Department of Food and Agriculture, an advisory group has been formed in California. The overall goal of this advisory group is to encourage the formation of a voluntary, market driven plan to reduce the prevalence of Johne's Disease in California. Within that plan, there will be focus on education, improved diagnostics, increased research and efforts to assists producers with infected herds.

Why all the fuss about Johne's Disease in California? Isn't Johne's supposed to be a problem with dairies in East? The NAHMS survey found that the prevalence of infected herds was the same in California as elsewhere in the US. Their survey suggests that 40% of the herds larger than 300 cows have Johne's and everyone knows that California has large herds (1998 average - 620 cows/herd ). Due to the high cull rates in most California dairies, the final progressive stage of this disease, chronic diarrhea and dramatic weight loss despite a normal appetite with no response to treatment, is not often seen. However, each year a few cases of clinical Johne's is seen by the pathologist in the California Veterinary Diagnostic Laboratory System. Despite the lack of clinical cases in older cows, infected cows without obvious clinical signs of disease are surely present in our herds and are also known to produce less milk and be at a higher risk of early culling. So while we may not see the terminal disease, Johne's Disease has a hidden economic impact on California herds which at this time remain undefined.

Johne's Disease is a tough disease to deal with for both dairymen and their veterinarians. First, it is a slowly, progressive disease which only has visible symptoms, the diarrhea and weight loss, as a part of the terminal portions of the disease just prior to death. Infections most often take place in the young heifer replacement calf shortly after birth. Visible signs do not appear for 3-5 years following the initial infection. During this time, the Johne's bacteria invade the cells lining the small intestine. In response to this invasion, the cells mount an immune response to attack the bacteria. The end result is that the lining cells of intestine become thicken and no longer capable of efficient absorption of essential nutrients from the gut into the body. So the animal have a good appetite but can't absorb the nutrients they are eating. Again, most of this is progressively happening without any outward signs for several years.

A second big challenge is with diagnostics. Like most bacterial diseases, it is possible to culture the bacteria and to do blood tests for titers indicating exposure to the bacteria. The appropriate sample for culture is feces or manure. As the disease progresses, the amount of Johne's bacteria shed in the feces increases and therefore the chances of finding the Johne's bacteria in the feces increases. The downside of culture is that it takes up to 15 weeks before the results are known. Under the typical California management systems, a dairyman is bring 2 year old heifer replacements into the herd. Many of these calves have been off to a calf ranch at some point in their lives. If infected, these 2 year olds many be shedding so few bacteria in their feces that they can not be detected by culture. However, as the clinical signs of diarrhea begin to appear, it is very ease to detect by culture as the amount shed is very high.

The other diagnostic option frequently used is a blood test, which is often the ELISA. The blood test is much quicker than the fecal culture. Results of the blood test are known in 2-3 days. But, here again, there are some drawbacks or limitations. Due to the location of the bacteria within the gut lining cells, there is no immune response to the infection which can be detected until about the time that the first clinical signs of the disease appear. Thus the test is not very useful to the dairyman who would like to screen incoming heifer replacements for Johne's. However, the blood tests can give a quick answer for the older cow with diarrhea and weight loss.
With progressive nature of the disease in mind, how can a dairymen try to protect his herd from Johne's Disease? The National Johne's Working Group suggests the following preventive measures:

•	Close your herd to animals of unknown Johne's infection status by buying only from test-negative herds or pre-testing mature cows replacements.
•	Secure replacements from herds with low risk for Johne's by buying from test negative herds or herds with low prevalence.
•	Reduce exposure to calves in the maternity area by providing a clean, dry area; remove calves before they can nurse their dams; provide clean feed and water for calves; keep manure from mature animals away from calves.
•	Feed calves "low risk" colostrum from test-negative cows.
•	Identify and remove infected animals from the herd.
•	Work with your veterinarian to develop a Johne's management plan for the dairy.

Johne's Disease is a difficult disease to control, however, many of the management strategies which dairymen should use to control other bacterial and viral diseases can be used to reduce the economic impact of Johne's Disease. Infected animal can be detected, however, it is important to keep the limitations of each diagnostic test in mind so that the results of testing can be optimized. To learn more about Johne's Disease, watch for future information from the California Johne's Disease Advisory Committee.