Johne's Disease Surveillance: A Baseline Study in Beef Cattle and the Prevalence of Salmonella sp.

Johne's Disease Surveillance: A Baseline Study in Beef Cattle

Introduction
Johne's disease is recognized in both domestic and free ranging ruminants worldwide and is caused by the bacterium Mycobacterium avium subsp. paratuberculosis. It is a slowly progressive disease with symptoms of emaciation and chronic diarrhea occurring within 2-6 years of infection. There is extensive bacterial proliferation in the lining of the intestinal tract accompanied by bacterial shedding in feces. This causes insidious spread within a herd since the bacteria can survive long periods in the environment. Young animals are especially susceptible to infection and intrauterine transmission does occur. Johne's disease has been a well documented cause of production loss and culling in dairies, and its impact is now being investigated in beef cattle.

Johne's disease has clinical and pathological similarities to Crohn's disease in humans. Speculation that they may have a common cause has recently been supported by the isolation of M. avium subsp. paratuberculosis from several people with Crohn's disease and suggestions that Crohn's disease responds to anti-mycobacterial drugs. Research remains inconclusive and this uncertainty fosters public concern that water, meat or milk contaminated by the bacteria may be a human health risk.

Prevalence of Johne's disease has been conservatively estimated at 21.6% of dairy herds (NAHMS '96) and 7.9% of beef herds (NAHMS '97) based on serological surveys in the U.S.A. A herd was considered positive if 10% of the animals were serologically positive, to allow for the fact that false positive reactions occur with serology. Actual herd prevalence would be higher if this criterion was too stringent.

The United States is advocating a voluntary control program and Australia already has a program in place to protect non-infected herds in an effort to eliminate Johne's disease from low prevalence regions. It is important to increase awareness of this disease and establish baseline levels to determine the best approaches to control for the Alberta cattle industry.

Initial survey results
In 1998, The Food Safety Division of Alberta Agriculture, Food and Rural Development, with funding from the Alberta Cattle Commission, surveyed a sample of 226 cow/calf herds in Alberta. Fecal samples were obtained from beef cows during pregnancy checking, by practitioners in 62 of the 72 municipalities in Alberta. Samples were cultured using a method incorporating PCR confirmation that was developed by Agri-Food Laboratories Branch, and has been accredited by USDA for Johne's disease certification. There were 678 samples suitable for culture from the 226 herds and ten herds were designated positive based on a single positive culture. Eleven of the 678 individual animals were positive. This provides an estimate that 4.4% of the Alberta beef herds may be positive for Johne's disease, with 1.6% of the beef cows testing positive.

The initial estimate of prevalence based on this study 4.4%, is lower than the 7.9% U.S.A. beef herd prevalence, but the methods of diagnosis are not comparable (culture vs serology) and sampling was not random in either study, so bias undoubtedly exists. The initial Alberta survey does indicate that Johne's disease is a potential concern for beef producers but prevalence is likely sufficiently low that there is potential to establish Johne's free herds or regions. A herd certification program would require reliable and cost effective methods for diagnosis in early stages of infection. While not ideal, repeated herd testing is the best current method to establish negative herd status. Herds which have not had a positive animal would purchase animals only from other equivalent herds to maintain their status.

As an industry, we cannot justify complacency with this disease. At the very least, livestock producers need to be made more aware that Johne's disease may have human health implications. The economic impact of the disease in beef cattle, in terms of reduced feed efficiency, poor milk production and decreased productive life span has not been estimated, and is certainly a factor for herds which have a high prevalence. Johne's disease is a potential trade issue as well, since negative testing is being proposed as a requirement for export of bovine genetics to counties including Japan. Further surveillance, as well as studies relating culture, serology and histological lesions are underway in the Food Safety Division.

References

• Clarke CJ (1997), The pathology and pathogenesis of paratuberculosis in ruminants and other species. Journal of Comparative Pathology 116: 217-261
• Collins MT (1997), Mycobacterium paratuberculosis: a potential food-borne pathogen?. Journal of Dairy Science 80: 3445-3448
• Stabel JR (1998), Johne's disease: a hidden threat. Journal of Dairy Science 81: 283-288