A Perfect Storm for EHD Outbreaks

Late summer is a time of discovery for deer hunters. Some of those are good. The big buck that disappeared right after rut last year that you were sure the neighbor killed might show back up. Or a new buck you’ve never seen before might become a regular on camera. But other summer discoveries aren’t so good. Like the smell of dead deer around ponds, waterholes, and slow moving creeks on your property.

EHD infected deer experience high fevers and are attracted to water. Image by M Riley

Many deer hunters immediately recognize that smell or the sight of bloated deer carcasses near water for what it is. Epizootic Hemorrhagic Disease (EHD), and the nearly identical bluetongue virus, are among the most common deer diseases. Fatality rates in infected deer populations can reach up to 90 percent and, in many cases, entire herds can be decimated in a matter of a few months. EHD is also one of the least understood deer diseases among hunters. Many confuse it with CWD because of the similar acronym and the increased amount of recent press regarding that disease.

What is EHD and How Does it Spread?

CWD is caused by a prion that damages portions of the brain, eventually killing the host. EHD has nothing to do with the often talked about prions. It is spread by a biting insect known as a midge. The midges lay eggs in the slick mud along water edges. As the deer come to the water, the midges attack, sometimes biting by the thousands.

A Southeastern Cooperative Disease Study found that as few as four bites can infect a deer, meaning most of the deer that come to the water hole will get infected.

As few as four bites from infected midges can transmit the disease. Image by M Riley

Since the midges hatch in late summer, EHD is a seasonal disease. It doesn’t happen every year, and impacts can vary from just a few deer to nearly entire herds when it does. Unlike CWD, EHD isn’t 100 percent fatal. A few infected deer will survive and develop immunity. Over time, that immunity starts to build across entire herds.

The first EHD die offs in wild deer herds were officially reported in the 1950s, but die offs consistent with EHD have been happening in the southeastern U.S. since the late 1800s. Because they have been exposed to the disease for over 100 years, many southeastern herds display a strong immunity. Unfortunately, warmer summers and milder winters have allowed the midge that causes EHD to move north, exposing herds that have little to no natural immunity. Over the past twenty years, areas as far north as the Canadian border, where EHD had never been observed, have experienced mass die-offs from the disease. In 2015, an EHD outbreak killed an estimated 90 percent of the famed Milk River deer herd in Montana.

EHD has been existed in the southeastern U.S. for over 100 years and is moving northward with warmer temperatures. Image by M Riley

While there isn’t a way to know for certain when, or how bad, an EHD outbreak in a given area will be, Lindsay Thomas Jr. with the National Deer Association reports on some new research that may help more accurately predict outbreaks.

A team of scientists that includes Brett Skelly, State Deer Biologist for West Virginia DNR; Dr. Mark Ruder and Dr. David Stallknecht of the Southeast Cooperative Wildlife Disease Study at the University of Georgia; and Dr. Sonja Christensen of the Christensen Lab for Wildlife Population Health at Michigan State University has reviewed weather patterns leading up to numerous heavy EHD outbreaks. They’ve found that it takes a pretty specific set of weather events for widespread outbreaks to occur. This pattern holds true across the whitetail deer’s home range.

What Kind of Weather Leads to an EHD Outbreak

What three conditions should hunters, land managers and biologists watch for that will likely lead to an outbreak? Dr. Christensen says it boils down to this:

• Higher than average precipitation in July (and late June down south). This leads to numerous standing water areas that provide the perfect breeding grounds for large numbers of biting midges.

• Above-average spring temperatures that allow the insects to become active sooner and build populations early around standing water from the increased rains. If the average spring temperature is higher, then your probability of disease presence is much higher, going from 10 percent probability to 30 percent with just a few degrees more on average.

• Finally, drier than average conditions in August. This leads to waterholes drying up, leaving vast mud flats—the perfect condition for the midges to breed into astronomical numbers. This also leads to more deer being concentrated around the available water sources and high midge populations, which leads to more bites.

The researchers found a high correlation between these exact weather conditions and mass outbreaks. Flip any of the factors, say a dry July and a wet August, or a cool early summer and hot later months, and the likelihood of a mass outbreak falls considerably. Unfortunately, these exact weather conditions seem to be becoming more and more common across the whitetail range.

Know The Signs of an EHD Outbreak in Your Area

When deer are infected with EHD or bluetongue virus, they experience high fever and dehydration, so they often move to water to drink and cool off. Sick and dead deer are usually found in or near water during outbreaks. If you suspect you have an outbreak, a quick trip around ponds, creeks or standing water should confirm it.

Hemorrhagic disease viruses strike deer of both sexes and all ages randomly. While dead bucks often get more attention on social media and in conversations among hunters, the disease does not strike bucks more than does or older deer more than younger deer.

Deer of all ages and both sexes are equally susceptible to EHD. Image by M Riley

Unfortunately, there is no magic bullet to prevent EHD. Anytime these conditions coincide, hunters and land managers should be on the lookout for dead or dying deer and make adjustments to upcoming harvest numbers. After a bad outbreak on our western Kentucky farm two years ago, we halted our usual heavy antlerless deer harvest, giving survivors a chance to rebuild population numbers.

The good news is, while it may take a few years, herd numbers will bounce back, even after devastating outbreaks. Take note of the deer you kill in the years following an EHD event. If that deer has damaged, cracked or even missing hooves, chances are good that it was an EHD survivor. Hopefully it passed a bit of that survival ability on to another generation or two.

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