Do they carry bovine Tuberculosis?

To protect cattle herds from bovine tuberculosis we need to better understand which animals can transmit the disease.

Michigan has been battling bovine Tuberculosis (bTB) since 1998, when the first cattle herd was diagnosed in more than 30 years. The major problem has been that the disease is present in more than one species. We know that deer and cattle can both get the disease and that they can both spread the disease.

Michigan is not the only place battling bTB. In the United Kingdom (UK), badgers carry, and can spread, the disease. In New Zealand, the main animal responsible for spreading bTB is the brushtail possum. Many cattle have been slaughtered as a result of becoming infected from badgers and possums. Therefore, control methods include killing badgers and brushtail possums to reduce the risk of disease spread.

In Michigan, most efforts have concentrated on the white-tailed deer as the host that threatens cattle health. We know that deer can get the disease, develop lesions and shed the bacteria in saliva and in feces (though the latter is rare). As that gets on feed supplies, whether pasture, forage that is green chopped or hay, cattle can ingest the bacteria and become infected.

A persistent question has been whether small mammals in Michigan are acting as transmitters, or vectors, of the disease. This has a bearing on what we do to control the spread of bTB. In order to transmit the disease, viable pathogens that cause bTB must be shed in some way. Therefore, the bacteria must multiply in organs that have an outlet from the body.

There have been multiple research studies to determine if small mammals in the area of bTB-infected farms are infected and to try to answer the question of whether it is possible that the disease transmission came through one of those small mammals.

Recently, USDA APHIS researchers, Are Berentsen and Mike Dunbar published the results of a study in which they trapped and examined raccoons from five counties in northeast Michigan where the disease is endemic in the wild deer population. They cultured tissue samples from trapped raccoons in order to diagnose animals that were infected. Of 144 samples, only two animals were bTB positive.

However, just because small mammals can become infected does not mean that they can pass the disease-causing organism along. To determine if these infected animals were shedding the organism, they used cultures of oral/nasal swabs and feces.  Those cultures were negative for bTB bacteria.  Even though the raccoons were infected themselves, they weren’t infecting any other animals.

Based on the low prevalence rate (1.4%) of raccoons that were bTB positive in this study, the continued low incidence of bTB-infected cattle herds and the inability to find the pathogen that causes bTB in the oral/nasal swab or fecal cultures in the raccoons, they concluded that it is highly unlikely that naturally infected raccoons are a vector of the disease. 

Researchers from USDA ARS lead by Mitch Palmer came to a similar conclusion from their laboratory studies with raccoons in 2002.  In addition, the results of multiple studies on Michigan opossums by Scott Fitzgerald of Michigan State University lead him to conclude that, like raccoons, opossums were unlikely to pass bTB along to other animals even when they were infected themselves.

While not absolutely conclusive, these studies reminds us that the primary vector of bTB that cattle producers need to be concerned about is white-tailed deer. Make every effort to keep these animals, which are known to shed the pathogen, away from your feed and farmstead.

That doesn’t mean that you should ignore small mammals around your farm. It would be good to trap, poison or scare these animals or to make the environment unsuitable or unavailable to them. But keep your risk management program focused on deer, and protect your cattle as much as possible.

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