Johne's (pronounced Yo-nees) disease is among the top animal health priorities of the Canadian dairy industry. The disease is also known as paratuberculosis because the bacterium that causes it is Mycobacterium avium paratuberculosis, or MAP. The bacterium effects the gastrointestinal tract of cattle and while most animals never show clinical disease (diarrhea), there can be significant milk production losses, increased risk of culling, and reduced reproductive performance. Research (1 and 2) indicates that the prevalence of the disease in Atlantic Canada is similar to other regions of the country. A full scientific review of the disease in Canada was presented by our research group in two papers in the Canadian Veterinary Journal (1 and 2)
Infection typically happens in the first few months of life. The disease is spread when calves ingest material contaminated with the MAP bacterium. The main source of MAP on the farm is manure of mature infected animals. These cows seed the environment with the MAP bacterium directly through their feces. MAP is very hardy and survives heat, cold and drying for one year or longer. Calves come in contact with this manure through fecal contamination of feed or water. Infected dams can also be a source of MAP for calves through their colostrum and milk, or if the calf nurses a manure contaminated teat.
For uninfected herds the main risk for introduction of the disease is the purchase of infected animals. All dairy biosecurity programs discourage the movement of animals between farms. Maintaining a closed herd is the only way to ELIMINATE RISK of Johne's disease introduction though purchased animals. If purchasing animals is necessary, specific precautions are required to REDUCE RISK of introducing Johne's disease. Because of problems with test accuracy, it is most important that the HERD OF ORIGIN is test negative for the disease. Current tests are much better at categorizing herds than individual cows. If the herd is negative, pre-purchase testing of animals can be considered as a secondary screen. Despite calfhood infection, the disease (and our ability to detect it) develops slowly with age. Consequently, it is of very little value to test heifers as a pre-purchase screen. Much greater value would be gained by testing the herd where the heifer originated. If the status of the herd of origin is unknown, the purchaser should assume that it is INFECTED.
Rigorous hygiene and biosecurity measures, identified through a farm specific risk management assessment, are the best way to control the disease
Infected calves show no symptoms of disease for many months to years. Typically, after these calves become mature lactating cows, they suffer the effects of lost production and begin to shed the bacterium to infect the next generation.
Johne's disease can have a substantial financial impact for farmers through production losses and can impact market opportunities for heifer sales. Infected cattle typically produce between 1 and 4 kilogram less milk per day than uninfected herdmates. In Atlantic Canada, we conservatively estimate the direct production loss to be approximately $5000 per year on an infected 100 cow dairy. The Dairy Farmers of Canada, in collaboration with industry partners, has developed the Canadian Johne´s Disease Initiative. The Atlantic Johne's Disease Initiative is a collaboration of the dairy boards in New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland and Labrador and Maritime Quality Milk at the Atlantic Veterinary College. AJDI is consistent with the tenets of the national program and focuses on education, strategic surveillance and risk management.
Education and Research