Extreme cold, less threat of disease?

Lansing — Michigan DNR officials believe this winter’s bitter cold likely will help prevent the spread of epizootic hemorrhagic disease in the state’s deer this summer.

DNR wildlife veterinarian Steven Schmitt told Michigan Outdoor News that prolonged freezing this winter will bode well for keeping the larvae of biting midges in check, and likely will limit the number that successfully hatch in the spring.

That’s important because the midges are the primary vectors for EHD, which devastated Michigan’s deer herd in 2012. The acute, infectious, and often-fatal viral disease killed an estimated 15,000 deer in 30 counties across the southern half of the state in 2012, but the prevalence decreased dramatically to an estimated 100 deer in six counties in 2013.

The especially cold 2013-14 winter undoubtedly resulted in a deep freeze in mud flats and other areas where midges lay their eggs, which Schmitt said he expects to impact their numbers significantly in the coming year.

“That’s just one part of the puzzle, but that could certainly knock the larval stage of the midge population down,” Schmitt said of this winter’s deep freeze. “A lot of those midges would not survive, with fewer hatching out in the spring.

“With the winter we’re having, it’s going to be hard for the larvae of the midge to survive in large numbers,” he said. “Without a dry summer, the midge population will never build up to a population needed to transmit the disease to deer” in significant numbers.

The spread of EHD relies on numerous factors, but the initial hatching population of midges each year plays an important role. The biting flies must reproduce in significant numbers to spread EHD effectively, which typically requires a strong hatch followed by very warm weather and drought-like conditions that expose mud habitats the midges call home.

A weak hatch greatly reduces the midge’s ability to reproduce in the numbers necessarily to cause a large EHD outbreak like that of 2012. Schmitt said he believes the 2012 outbreak is also helping to prevent the spread of the disease, since animals that are exposed to each specific strain of EHD – but don’t die of the virus – typically build immunity to that strain.

“With EHD, the vast majority that become infected … either die or become protected,” Schmitt said, adding that other weather factors also could play into the midge’s life cycle this summer.

The higher the temperatures, the faster the midge reproduce, Schmitt said, particularly if summer temperatures hit triple digits.

“Going into the future, I think we will probably see … more EHD every year, but I think it will be hundreds (of deer impacted), not tens of thousands like 2012,” he said.

Schmitt said results from other ongoing deer and elk disease testing appear to have been fairly typical in 2013, according to preliminary data. The prevalence of bovine tuberculosis in northeast Michigan’s TB zone is expected to remain just under 2 percent in the most recent round of testing, conducted last year. No deer have tested positive for TB outside the special zone, he said.

All targeted testing of deer for chronic wasting disease last year has been negative, as well, Schmitt said.

DNR officials are expected to outline 2013 deer disease testing results for Natural Resources Commission soon.

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