Buyer beware - Johne's and bull introduction

In a short time cow-calf herd owners will be adding bulls to their herds to breed their cows and heifers. The selection of a bull is an important decision to make, not only for performance but also for herd health.

Bulls can introduce Johne's into cow-calf herds. Introducing only one young bull from outside your own herd may seem low risk but this is not always true. Typically in Ontario bulls will join the cow herd when cows have new calves at their side. This means that during the breeding season bulls share pasture or housing with young calves that are highly susceptible to becoming newly infected with Johne's bacteria. Bulls shedding Johne's bacteria in their manure can contaminate bedding, pasture ground, forage and water sources. Manure with Johne's bacteria can contaminate cow udders and calf hair coats, leading to calves ingesting enough Johne's bacteria to cause infection.

Johne's is a long term, slowly progressive infection taking years to advance from calf-hood infection to disease signs. Calves are much easier to infect than older cattle. Most Johne's infections start when calves are nursing cows. The most common age for the signs of Johne's disease to occur, chronic diarrhea, is around 4 to 5 years of age. In some cow-calf herds, 4 to 5 years after a Johne's-infected cow or bull has contaminated the calf environment, a high proportion of cows exposed at that time can show up with signs of Johne's. Controlling Johne's in cow-calf herds once it takes hold is very difficult. Keeping Johne's out of the herd entirely to start with is the best strategy.

Just like any calf, bulls can become infected in two ways. The first is by ingestion of Johne's bacteria from the manure of infected cows. This route of infection is more likely when calving occurs indoors and newborn calves are in close contact with the manure from a number of cows. The calf's own dam does not have to be the source of the bacteria under these circumstances. The second way calves become infected with Johne's is by acquiring the infection prior to birth from their own infected mother. A calf is more likely to be born infected when born to a cow at a more advanced stage of the disease. Generally this will be an older cow and not a heifer dam. A Johne's-infected cow does not need appear sick to infect her unborn calf.

Herd owners need to be aware of the risks of introducing Johne's when bringing any cattle into their herd. Bulls may pose exceptional risk because they could come from herds where a lot of animal movement and exposure has occurred and because they will be in contact with most of the year's calf crop. Owners need to do the best they can to protect their herd from exposure to Johne's-infected bulls. Buying bulls from owners who have participated in Johne's testing and who readily share their herd's test results is one option for reducing risk. Unfortunately routine whole herd testing for Johne's is done by a minority of herds, however demand for such performance records could increase participation in Johne's monitoring and prevention programs by seedstock herd owners.

Bulls entering a herd, even young ones, should be blood tested prior to exposure to other members of the herd. Young cattle truly infected with Johne's can have negative test results because they are still in the early stages of the disease, however, some infected bulls, especially those infected before birth, will have positive tests. Given the low cost of the test (less than $15 laboratory fee) it is reasonable to test potential herd additions, with the understanding that a negative test in a young animal does not mean it is uninfected. When young bulls have a positive test, a follow-up with additional testing will need to be done to verify the outcome. It is worthwhile to know as much as possible about a bull's disease status prior to introduction to your herd. Then you can do the best you can to avoid bringing these potentially infected bulls into your herd. Talk to your herd veterinarian about a plan for bull introduction… Johne's is just one of several diseases that can come into your herd via introduction of new animals.


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