“The big issue is that we still continue to find bovine TB in whitetail deer. So, there’s that reservoir that continues to exist that isn’t getting worse or getting better and is a constant source of bacteria for our cattle.”
– Daniel Grooms
Although it was declared TB free in 1979, Michigan state animal health authorities have been battling bovine tuberculosis in cattle and whitetail deer since 1994.
“We discovered it first in our deer population, and we assume that it spilled back into our cattle,” said Dr. Daniel Grooms, a veterinarian and associate professor at Michigan State University’s large animal clinical sciences department.
Between 2000 and 2003, every dairy, beef, bison, and goat herd in the state was tested. This testing resulted in no evidence of bovine TB except in a pocket between Lake Michigan and Lake Huron.
Since 1996, progressive expansion of TB testing has occurred in northeast Michigan and annual risk-based surveillance testing continues in an attempt to “squeeze” the problem into a smaller area. Over 1.4 million TB tests have been completed, and 50 herds have been culled.
Annually, 150,000 TB tests are conducted on over 3,000 herds numbering at a cost of US$5.2 million. TB testing and movement certificate requirements are in place for animals, according to the state’s Department of Agriculture.
“The big issue is that we still continue to find bovine TB in whitetail deer. So, there’s that reservoir that continues to exist that isn’t getting worse or getting better and is a constant source of bacteria for our cattle,” said Grooms.
Shared feed, hay, or standing water where cattle and deer mix is believed to be the major source of infection, but spread from other wildlife has not been completely ruled out.
In a process virtually identical to that used by the CFIA in Manitoba’s TB eradication area near Riding Mountain National Park, testing starts with a caudal fold screening test done by an accredited veterinarian to “cast a wide net” and find evidence of infection in a herd, even if some false positives result.
If “responders” with swelling, redness, hardness or a “bump” at the test site are found within 66 to 78 hours later, the herd is quarantined.
Gamma interferon testing, which is similar to the Bovigam test used in Manitoba, involves taking a blood sample, or comparative cervical testing (CCT) via injection of both avian and bovine tuberculin in two places in the neck is used on those animals.
In some cases, the owner of the animal is allowed to choose which of the two tests they prefer.
Since 2004, CCT testing has been phased out in Manitoba.
Program specialist Rick Smith said farmers, especially those outside the “hot” zone, appreciate having a choice because they may have lost animals to false positives in the past, and have come to trust one of the two tests more than the other.
“I wouldn’t say there’s anything scientific going on here, it’s just they may think they have better luck with the CCT than with the gamma interferon.”
If an animal is deemed “suspect” after CCT, the owner must choose whether it will be removed for euthanasia and necropsy at MSU, or retested after 60 days.
An animal is considered a “reactor” if it has a higher level of response to CCT based on skin thickness, or shows “suspect” results to two CCT tests.
In gamma interferon testing, or “CCT in a test tube,” blood can be taken either at time of reading the caudal fold test, or within 30 days of the initial tuberculin injection. Suspect animals are either removed, killed and tested at the lab, or retested within 30 days, depending on the owner’s preference.
Compensation is paid to the owner based on the value of the animal in cases where animals must be slaughtered.
If a single animal is confirmed to have TB via tissue culture, the entire herd is depopulated according to USDA rules.
Classification as a “reactor” does not prove it has TB. That can only be verified by a lab necropsy and culture testing, said Grooms.
Some false negatives may occur, or in about one to five per cent of all animals tested.
“In any disease, there’s no test that’s 100 per cent specific. In other words, there’s always the risk of false negatives,” he said. “Certainly that’s true for bovine TB with the caudal fold test or whatever.”
“Misses” are rare, but they may occur because the animal was in a very early stage of infection where its immune system was not yet mobilized, or it was so far gone that its immune system had shut down.
An animal in that state is called “anergic,” and the caudal fold test may fail in such cases. Outward, visual signs of advanced disease in some cases may not be present, he added.
“Sometimes, they might not look any different,” he said. “They may not look physically terrible. I’ve done some necropsies on animals that when you find out what’s inside of them, you can’t believe that they look like that physically on the outside.”
Other, unrelated diseases that result in immunosuppression may cause false negatives under caudal fold testing, he added.
European studies have found that acute Bovine Viral Diarrhea (BVD) infection may temporarily interfere with gamma interferon testing accuracy.
Johne’s disease, caused by mycobacterium paratuberculosis, which is closely related to bovine TB, is another that may cause anergia.
“It would not surprise me that there may be other natural things that might mask that bovine TB response, but we don’t know what they all are yet,” said Grooms. “There’s certainly interest in knowing what they might be.”
It is very important to understand, he added, that the caudal fold testing procedure is only designed to screen entire herds for exposure or infection to TB – not catch every individual.
Grooms added that he was not aware of any biological or health problems that have occurred as a result of repeated, long-term testing involving tuberculin injections.
“Certainly here in Michigan, we’ve had animals that have been tested multiple times over their life in a herd, and nobody has suggested that there is a health effect. But to be honest, I’m not sure anybody has looked.” [email protected]