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What is the Risk of the Introduction of Bovine TB to an Area?

December 26, 2013
 
 

By: Phil Durst, Michigan State University Extension

The question of the risk of a beef or dairy herd in an area becoming infected with bovine tuberculosis (TB) is important to beef and dairy producers as well as government regulators. While those groups may view it differently, quantifying the risk by determining a risk level can be an important policy tool.

Therefore, the United States Department of Agriculture (USDA) developed a risk assessment model that assigns a risk value to counties for which data has been entered and compares that to a baseline county (located in another state). The risk assessment is then used, among other factors, to determine whether or not the TB status of a county may be changed, such as to upgrade a county from Modified Accredited Advanced (MAA) to TB-free status.

In Michigan, an upgrade of status is certainly on the minds of beef and dairy producers in the seven counties currently recognized with the MAA status: Antrim, Charlevoix, Cheboygan, Crawford, Emmet, Otsego and Presque Isle Counties. They want to be recognized as "TB-free". However, the question of risk is also on the minds of these beef and dairy producers in the four Modified Accredited Zone counties: Alcona, Alpena, Montmorency and Oscoda. Indeed, their livelihood depends on the risk to their herds.

The likelihood of introduction of the disease into cattle herds in a county is calculated as the mean number of possible contacts between cattle herds and infected deer or purchased herd additions. The potential contacts, either direct or indirect, with infected deer is the basis for the Michigan Wildlife Risk Mitigation program.

The Wildlife Risk Mitigation program breaks down the potential contacts by likely means of contact including feed storage, feeding location and management and water source. The purpose of this tool is to identify risks that can be reduced by a producer by decreasing the likelihood that deer, whether infected or not, will have access to feed and water used by cattle.

Once a producer knows the areas of his or her operation that are high risk, then he or she should develop a mitigation plan to reduce the risk by changing the accessibility of the feed or water to deer. That may involve fencing or enclosure, it may involve development of artificial water sources or management changes to reduce risk. The Michigan Department of Agriculture and Rural Development (MDARD) inspects changes made to mitigate, or reduce, risk and verifies a farm as risk mitigated.

As the USDA calculated the risk of farms in counties, one of the factors it used in the calculation is the percentage of cattle herds that are verified as wildlife risk mitigated. The USDA considers all the herds in a county to be at higher risk when the percentage of verified herds is lower. Therefore, all the producers in a county are impacted by the regulations that will apply to that county even though they may personally be verified as wildlife risk mitigated.

Unfortunately, in the four-county MA Zone (Alcona, Alpena, Montmorency and Oscoda Counties) the percentages of herds that are verified are the lowest among the 11 MAZ and MAAZ counties, with only 61 – 76 percent of herds being verified.

The percentage of herds does not likely reflect the percentage of cattle involved as some of these are very small herds, however, the fact that herds have not become wildlife risk mitigated increases the "risk of introduction of the disease" in the USDA model.

The percentage of herds verified is one factor in the model that also includes the deer population and prevalence of TB in the deer herd. Yet, when the model was run, the relative likelihood of cattle infections being introduced was "Extremely High" for Alpena County, "High" for Alcona County and "Moderate" for Montmorency and Oscoda Counties. In this case, Alpena was calculated as 22 percent more likely to experience the introduction of the bacteria that causes TB into a herd than the baseline county, an event that "is almost certain to occur".

The actions of any one producer do impact all other producers in a county just as the actions of one landowner or hunter impacts others. Michigan State University Extension encourages cattle producers across all of the northern Lower Peninsula to become wildlife risk mitigated and verified, whether they are large or small producers of even only a few head, or whether they are beef or dairy producers. Contact MDARD at 989-785-5616 for more information.

Lowering the risk of TB infection to herds requires that everyone take personal responsibility for what is within their sphere of influence. When every producer does that, the likelihood of improving the TB status of a county will be greatly improved. 

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