Whitetail Deer | Michigan DNR
The state of Michigan has confirmed its third positive detection of bovine tuberculosis in the last two weeks, marking the first positive cases of the new year and again highlighting Michigan as an anomaly among other states in terms of its “reservoir” of the disease.
On Feb. 3, the Michigan Department of Agriculture and Rural Development (MDARD) reported two cases of bovine TB at privately owned captive cervid facilities in Sanilac and Alcona counties — the first cervid facilities affected by the disease in Michigan since 2009.
Cervids include all species of deer, elk and moose as well as many other members of the Cervidae family of hoofed ruminant mammals. MDARD does not specify which species of cervid are at the facilities for privacy reasons.
On Tuesday, MDARD then confirmed the presence of a third afflicted herd, this time in a dairy herd in Oscoda County. Detection of bovine TB is much more common in cattle than captive cervid facilities.
According to Dr. Nancy Barr, assistant state veterinarian and TB program coordinator at MDARD, said that Oscoda County’s most recent finding is the 82nd cattle herd since 1998 to have a confirmed case of the infectious disease.
State efforts to curb bovine TB are monitored by the federal government, as the disease is known to jump to humans. It has been nearly eliminated in every state besides Michigan.
“We’re the only place in the United States,” said Dr. Michael VanderKlok, MDARD’s bovine tuberculosis eradication manager.
“We’re the only place that has it endemic in a wild population, in our deer up in that small area of northeastern Michigan.”
A Michigan-specific problem
“We do things differently here in Michigan than most people do in the remainder of the country because of our reservoir of disease,” Barr said Friday in a phone interview.
That “reservoir” of bovine TB exists in free-range white-tailed deer in northern Michigan. Since it would be virtually impossible to test every one of those wild deer for disease, the state encourages hunters to check carcasses for signs of legions and report those appropriately, but Michigan is forced to focus mainly on surveilling the disease in captive and farmed herds of cervids.
We do things differently here in Michigan than most people do in the remainder of the country because of our reservoir of disease.
– Dr. Nancy Barr, assistant state veterinarian and TB program coordinator at MDARD
“Generally, that’s how our cattle herds become infected, is a spillover from those wild deer,” Barr said.
Other states do not have this same issue.
The U.S. Department of Agriculture (USDA) has long kept a database of where each state is in terms of eradicating disease like bovine TB in cattle.
In 2000, Michigan lost its entire “accredited free” status and has been the only state in the country not classified as such by the federal government ever since.
Michigan was instead designated as “modified accredited” (MA) for its cattle herds. In the years following, the state has been able to reduce the area of that modified accredited zone (MAZ) down to a few counties rather than the entire state, making the state’s current designation “MA/Free.”
Those four counties in the MAZ include Alcona, Oscoda, Montmorency and Alpena counties. All are in the northeast region of the Lower Peninsula.
“We have a special designation in that four-county area because we are aware that we have the potential risk for exposure to our cattle all of the time,” Barr said.
That designation “allows us to continue to focus our effort on that area, so maintaining that split-state status is very important,” VanderKlok added.
Barr said that the disease reservoir of bovine TB also extends to white-tailed deer in the neighboring Presque Isle County, but the USDA does not include that in its formal MAZ designation because the federal program only looks at the disease in cattle.
The state routinely tests facilities for the disease and generally finds about two cases in bovine per year.
The fact that bovine TB was discovered this month in Sanilac County, however, is unusual. The county is located in Michigan’s Thumb region — about 200 miles south of the state’s four-county MAZ.
“We do not have a known wildlife reservoir [of bovine TB] in Sanilac County,” Barr said. “When we get that whole-genome sequence back on that bacteria, that will help us to understand how that herd became infected.”
Whole-genome sequencing results can take up to three months to be completed. The Sanilac County herd will not be included in the federal bovine TB eradication program, as that facility farms cervids rather than cattle, but the state will still use the data to limit further spread of the disease.
“It’s rare for us to find it in the privately-owned cervid herds. So this is an unusual year,” Barr said.
According to VanderKlok, the story behind why Michigan is the only state with a “disease reservoir” of bovine TB goes back to the 1950s and 1960s.
Back then, he said, Michigan didn’t have very large deer populations.
“So in order to increase the deer populations up in those areas, they started heavily feeding deer — by the semi-load,” VanderKlok said.
Normally, the interactions between wild populations of deer is just low enough to keep diseases like bovine TB under control.
But: “When you take them and start to, in essence, put them in feedlots … you take away natural competition. The deer don’t have to compete for the feed. You’re bringing them in close concentration with each other and making it so easy for me to transmit the disease,” VanderKlok said.
A link to humans
Bovine tuberculosis is caused by the Mycobacterium bovis bacteria and primarily affects cattle. It can be transmitted between animals in the wild and those raised on a farm, and has the potential to infect humans who also come into contact.
The infectious disease presents in the form of nodules or lesions that grow in the lymph nodes and other tissues. Bovine TB has a slow progression, meaning that it can take many years for an infected animal to shed the virus in a way that could spread it to others.
And because Michigan tests cattle regularly for it, this generally means that the state can curtail it early in those animals. Wild deer, on the other hand, live longer with the disease and will generally show more signs of being infected.
“We definitely find the disease very early in our cattle herds, so it’s rare for them to progress to the point of the disease where they’re actually shedding it in a way that could make people sick,” Barr said.
“But we always make sure that we follow up there, because people can get it and it can lead to serious disease in people.”
Humans can be infected via contact with infected tissue from an animal or by ingesting milk from an infected cow. Bovine TB does act like other tuberculosis bugs in humans, Barr said, which could prove fatal if left untreated.
It’s rare for us to find it in the privately-owned cervid herds. So this is an unusual year.
– Dr. Nancy Barr
“Raw milk can be a source of bovine tuberculosis, but we have mitigation strategies in place to prevent that from getting into the milk,” she said. “…Anytime we find an animal and a dairy herd, that milk from the entire herd is immediately sent away.
“That happens until we’re sure that that herd is uninfected.”
The state will then work with farmers who have been exposed to the infected animals to make sure they are tested and receive follow-ups with their local health department.
Hunters are also encouraged to inspect the bodies of deer for legions before consuming the meat. If there are lesions or abnormalities present, they should submit the carcass to the state for testing.
“Worldwide, about 30% of all the human cases are bovine tuberculosis,” VanderKlok said. “… TB is a very, very difficult disease to work with. It’s actually the most significant infectious disease of humans in the world.”
But the bottom line, Barr said, is that Michiganders are safe despite the state’s uniquely infected deer population thanks to many different safeguards against the disease.
“Milk and meat from cattle in Michigan is safe,” she said. “We have a lot of protective measures in place to ensure that the food chain is safe. I think people need to know that there’s not a risk that they’re going to be exposed. It’s very tightly controlled.”
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