Dr. Robert Tremblay
A few years ago many dairy farmers in Canada participated in a voluntary program intended to reduce the risk of transmission of the bacterium that causes Johne’s disease, an often chronic infection of the small intestine that can cause weight loss, diarrhea and edema. That bacterium is known as MAP (Mycobacterium avium ssp paratuberculosis.) MAP is shed in the manure of infected cows. Calves and cows become infected by consuming anything that is contaminated with that infectious manure. The basic approach to controlling Johne’s is to identify on-farm management that might create opportunities for MAP to spread and change that management to lower the risk.
Dairy farmers in several other countries have also adopted management programs to control Johne’s disease. In countries such as the UK, there are intensive Johne’s control programs that also involve regular testing of the cows in an attempt to identify and remove cows that are likely shedding MAP in their manure.
Part of the difficulty in successfully controlling Johne’s disease is that we do not fully understand how MAP spreads amongst the cows and young stock on dairy farms. Recently the results of a long-term research project looking at the spread of MAP in one New York dairy herd was published. Researchers used genetic analysis, so they were able to track infections in cows over an eight-year period between 2004 and 2011. The farm was milking 330 cows.
This farm had already adopted many of the management strategies that would reduce the spread of MAP. They did not buy cattle. Their maternity pens were not used as sick cow pens. Maternity pens were used for one cow almost all the time. Calves were removed at birth and fed colostrum from cows that tested negative for Johne’s disease. Young stock was kept separate from adult cattle.
Over the eight-year study period, researchers collected manure samples from all cows twice a year and blood samples four times a year beginning at first calving. Tissue samples were collected from 170 cull cows at slaughter. Environmental samples were collected from 20 locations on farm twice yearly. All the samples were cultured for the presence of MAP. Any MAP found was then genetically analyzed to see how closely the bacteria were related. By looking at how closely related the MAP bacteria were to each other, the researchers hoped to determine how MAP spread on the farm.
They found that there were several different MAP bacteria on the farm, although only a few closely related MAP bacteria made up the majority. Interestingly, even though the farm had not purchased cattle for years, there were MAP bacteria that were so distantly related to each other that it was likely that the different MAP bacteria had been brought into the herd from different sources and at different times. In other words, MAP had come into the herd more than once and likely persisted in the herd for years each time it was introduced.
Other research on Johne’s suggested that not all cows are important in spreading MAP. A few infected cows pass MAP in their manure for long periods of time or much higher amounts than other infected cows or both – these cows are called ‘super shedders.’ The study found that there were super shedders on this farm. These super shedders were responsible for much of the spread of MAP in the herd but not for all the spread.
One unexpected finding was that cows were often infected with more than one different MAP bacterium and the different bacteria likely originated from more than just one source. In fact, there were often two to four different sources of MAP bacteria in each infected cow. Those infected cows then spread the MAP to create up to 17 other infected cows.
This herd used best practices that are often recommended and intended to limit the spread of Johne’s disease from cow to calf shortly after birth. These best practices appeared to work pretty well because spread from cow to calf was not common. Even so, spread from adult cows to other adult cows continued within the herd. Cow-to-cow spread was the most common type of spread and served to maintain Johne’s disease in the herd from year to year.
This was a unique study. It reinforced some of what we already knew about Johne’s disease such as the importance of super shedders. It did find that spread between adult cows is important not just cow-to-calf spread and that most cattle are infected from more than one source of MAP bacteria. One weakness of the study was that it relied on the researchers being able to detect MAP bacteria in manure. That is a weakness because many cows that have Johne’s will be negative for MAP in their manure using this test. In other words, some infected cows will be missed. If there are many of those cows, it will affect the study’s findings. Even still, this is the first research to give such a detailed picture of how MAP is spread in a dairy herd even though the herd used Johne’s control practices.
Dr. Robert Tremblay is a veterinarian for Boehringer-Ingelheim and lives near Guelph.