Although an estimated 20 to 30 per cent of Canadian dairy herds have cows infected with Johne's Disease (JD), outwardly sick cows are a rare event in most herds. However, it's in the dairy industry's best interest to reduce JD infection for several reasons beyond cow illness.
A bacterial infection, JD starts in very young calves. Infection lasts for the animal's lifetime and we can't tell that it's present in most animals. About five per cent of infected animals become outwardly sick, developing continuous diarrhea. Despite normal appetites these animals lose weight rapidly and must be removed from the herd.
Recent evidence shows the disease is more widely spread than 15 years ago due to increased animal movement from herd to herd and larger herd sizes. If infection is spreading into more herds where it can spread to more cows, JD's impact may be on the rise. JD-infected cows, even though not obviously sick, have reduced milk production and a shorter herd life over time.
Another cause for concern is that the organism causing JD in cows is increasingly being studied for a possible link to Crohn's disease in people. While this association and the possibility that milk could expose people to infection are unproven, we need to nip this in the bud by curtailing the spread of infection among dairy cattle.
Although JD is a bacterial infection, you can't treat cattle for it. Unlike most other diseases, JD develops slowly-the interval between initial infection and bacterial shedding or sickness takes years rather than days or weeks. An infected animal can pass the disease on to a new generation of heifers before she tests positive or shows any symptoms.
Once JD is established in a herd, reducing the reservoir of infected animals over time is the only way to prevent new cases from occurring. As older infected cattle are gradually removed through normal culling or targeted removal, they can be replaced by uninfected heifers.
In the last five to 10 years, other jurisdictions have shifted from the test-and-slaughter approach for JD control to emphasizing prevention of new infections in heifer calves. These programs involve changing heifer rearing in ways that are believed to prevent exposure of the young stock to JD infection. Canada's dairy industry agrees with this approach, and has decided that the best way to control JD is a standardized national prevention approach.
Currently in Ontario and Western Canada, the JD Prevention Project funded by CanAdvance, CanWest DHI, DFO, WestGen and the Ontario Ministry of Agriculture, Food and Rural Affairs is underway. This project is the first broad-based attempt at getting veterinarians and their dairy clients to find infected herds through whole-herd testing [using the DHI JD milk ELISA test], to assess calf management and make changes to prevent the heifer calves from contacting the JD bacteria.
The two-part project has worked with 80 veterinary-producer pairs to develop and train vets in a management approach to JD control. Farm visits associated with this part of the project are complete and data entry and analysis is underway. Results should be available in early 2007.
The project's second part, continuing until 2007, helps trained vets enrol additional client herds in the management program. A final evaluation will help us understand which calf-raising strategies best ensure that young heifer calves don't become infected with JD, even if some JD-infected cows are still on the farm.
We can also learn from programs elsewhere. Once you know about recommended management strategies that apparently work in other areas and can learn about how infection starts, you can begin to look at your own calf-raising system and consider improvements. Your vet, who understands the disease, your herd dynamics and your calf-raising program, can help you make this assessment and recommend changes. Materials are available through vets.
We'll continue to monitor JD prevention developments and learn from the experiences of others around the world. Results from the Ontario and Western Canada projects, when completed, will help us fine-tune the recommendations to fit Canadian heifer rearing and dairy farming.
Wells et al. Herd level risk factors for infection with Mycobacterium paratuberculosis in US dairies and association between familiarity of the herd manager with the disease or prior diagnosis of the disease in that herd and use of preventive measures. JAVMA 2000 Vol 216 (9) 1450.
Raizman et al. Characterization of Minnesota dairy herds participating in a Johne's disease control program and evaluation of the program risk assessment tool. Prev Vet med 2006 (in press).
Berghaus et al. Factor analysis of a Johne's disease risk assessment questionnaire with evaluation of factor scores and a subset of original questions as predictors of observed clinical paratuberculosis. Prev Vet med 2005 Vol 72 291.
This article first appeared in the Ruminations column of The Milk Producer Magazine, May, 2006.
What We Can Learn from Programs Elsewhere
Evaluations of JD prevention programs elsewhere in North America and around the world are just starting to be published in research literature. It takes a fairly long time to assess farms, to adopt management changes and for the heifers to grow up to prove whether the program prevented infections.
One of the largest JD farm management studies enrolled 1,000 herds in a voluntary program 10 years ago through the U.S. Department of Agriculture. These herds were classified as either JD-positive or JD-negative, based on test results and their herd disease histories. Then the two groups were compared for the preventive management practices they used.
Herds that had more than 25 per cent of their cows raised on another farm were twice as likely to have JD in the herd. Larger herds-for example, those with over 300 cows compared to those with less than 50-had a five-fold greater risk of having JD in the herd. Using group pens for cows around calving and for housing milk-fed calves also increased the likelihood that a herd would be JD-positive.
Preventing exposure of newborn calves to JD infection right after birth is a high priority in all programs. This is when calves are in the closest contact with cows and live briefly in the cow's environment. This close contact makes the risk of exposure to JD bacteria in cow manure, cow bedding, on cow skin and in colostrum very high.
In the U.S. study, herds with owners knowledgeable about JD were more likely to wash teats and udders before collecting colostrum for calves or allowing calves to nurse. Herds where JD had been diagnosed previously were over three times more likely to remove a newborn calf from the calving pen less than one hour after birth. Both these practices are recognized as a part of an effective JD prevention program.
Experiencing JD in the past was a key factor in determining how much a producer knew about the disease and its prevention. Over 90 per cent of the producers with a herd that had a case of JD diagnosed during the past 10 years felt they understood the JD prevention basics. Just under half the producers who had never had a case felt as confident in their knowledge.
This is an important consideration in Canada as we consider a national JD prevention strategy-education about the disease must be a priority.
Disease knowledge will help producers understand the need for prevention. It will also help them to decide how important JD testing and prevention are in their herds. In some herds, JD warnings could be missed if herd owners don't know what they're looking for. Testing in most herds, at least as a preliminary step, may be warranted even if the disease isn't apparent. The opportunities to fine-tune a prevention program could be missed by owners if they are unaware of all the ways JD can be transmitted to their heifers. Without education, they may not recognize and implement effective management changes in timely fashion.
Another voluntary JD prevention program started in 2001 in Minnesota. The program herds are being followed over time to identify changes made to calf raising and to measure the impact these changes have on the percentage of cows testing positive for JD.
Like the current Ontario and Western Canada project, herds are blood tested at enrolment to identify cows with the greatest risk of transmitting infection to young stock and the infection pressure on calf raising. A questionnaire about calf-raising practices assesses JD transmission risks.
After a herd has enrolled in the project the risk assessment is repeated annually. Blood testing is done less frequently. To date, only 224 herds have been on the program long enough to have had at least two rounds of blood testing done where at least two years have elapsed between the first and the most recent one.
The risk assessment scores of the 224 herds have been compared to the blood test results. So far, the reductions in scores have not added up to a reduction in positive-testing cattle in these herds. It's likely that two years on the program wasn't enough time to let heifers raised under the new management strategy grow up and repopulate the herds.
This may be especially true in U.S. dairy herds where herd turnover is not driven by a production limit. Some herds may keep positive-testing but productive cattle for a longer time since few will actually get sick. We await the further follow-up planned for herds on this project after five years.
We have used some of this emerging information to help develop our Canadian approach but will have to make some changes so programs fit our farming systems and individual farms. Disease control programs need to be developed locally and evaluated frequently for effectiveness. This process needs to continue to keep up with the dairy farming's evolution.
By Dr. Ann Godkin, Veterinary Scientist/OMAFRA
Government of Ontario Ministry of Agriculture, Food and Rural Affairs