Deer protected from deadly disease by genetic differences

URBANA, Ill. — It was the height of summer 2022 when the calls started coming in. Scores of dead deer suddenly littered rural properties and park preserves, alarming the public and inconveniencing landowners.

According to officials at the Urbana Park District, it was epizootic hemorrhagic disease, a midge-borne viral illness that pops up in white-tailed deer populations around the state every few years. And when susceptible deer are infected, they die within days.

Now, University of Illinois scientists have found gene variants in deer associated with the animals’ susceptibility to EHD. The team sequenced the gene for Toll-Like Receptor 3, a protein that spans membranes of intracellular organelles in immune cells and helps recognize double-stranded RNA viruses. When a dsRNA virus, such as the one that causes EHD, enters the cell, TLR3 activates the host’s first immune defenses, triggering inflammation and priming the rest of the immune system.

When the team sequenced TLR3 from EHD-infected and uninfected deer, they found dozens of variable sites in the DNA known as single-nucleotide polymorphisms. Two of the SNPs were significantly more common in uninfected deer. That conclusion is rooted in the probability that many white-tailed deer in Illinois are exposed to EHD in their lifetimes, but only some will die from the disease.

In many areas, outbreaks occur every 3-5 years, when environmental conditions favor the life cycle of midges that carry the virus. The midges spend their larval stages in mud under ponds and puddles where deer drink during drought conditions.

As those water sources dry up, usually during late summer, the midges’ muddy habitat is exposed and the adult flies emerge to bite and infect deer. The cycle can be interrupted locally by a soaking rain or a cold snap, which is why outbreaks don’t happen every year.

The researchers emphasize that EHD is not transmissible to humans or pets through midge bites or consumption of infected deer meat.

Although there’s not much wildlife managers can do to disrupt the cycle and prevent outbreaks in natural habitats, the team says it’s still helpful to understand the genetic underpinnings of the disease.

Theoretically, deer in captive herds could be sampled to characterize the level of vulnerability to EHD, and wild herds could be sampled during the hunting and EHD-outbreak seasons, informing managers and the public of future risk.

Another recent study shows the disease has been slowly but steadily moving northward in Illinois. The researchers don’t know whether that’s due to climate change or greater reporting, but it’s clear EHD isn’t restricted to rural parts of Illinois.