Dealing With The ‘Big 2’ Deer Diseases: HD & CWD

Here's why mid summer is one of the whitetail's deadliest seasons, and the best response when your hunt area sees an offending viral outbreak.

by Mark Melotik

HuntStand Pro Contributor MORE FROM Mark

Here’s why mid summer is one of the whitetail’s deadliest seasons, and the best response when your hunt area sees an offending viral outbreak.


Every year, somewhere in the United States, hundreds if not thousands of whitetails are sickened or killed by the silent stalker known as hemorrhagic disease (HD). The story is almost always the same. It’s late summer or early fall, and sick or dead deer are found in or near water. Landowners and hunters are understandably concerned and want to know what caused it—and how to prevent it from occurring again the future.

While there are many possibilities, the time of year and proximity to water are highly suggestive of an outbreak of hemorrhagic disease (HD). However, with the increasing spread of Chronic Wasting Disease (CWD) across the whitetail’s range, it’s important to distinguish between the two. While the symptoms of both diseases are similar, the causes and impacts are quite different. CWD is caused by an abnormal protein, whereas HD is caused by a virus. A critical distinction is that 100 percent of deer infected with CWD will eventually die, while mortality rates of deer infected with HD vary widely from zero to more than 50 percent. While HD kills more white-tailed deer annually than all other infectious diseases combined, it is the lesser of two evils when compared to CWD, especially in the long run. In either case, reporting dead deer to your state wildlife agency is strongly encouraged. If they are aware of a HD outbreak in your area, they may choose not investigate, but it’s better to be safe than sorry when it comes to infectious diseases.

CWD was first discovered in the late 1960s in a deer research facility in Colorado. For nearly 40 years, it generated little attention and appeared restricted to a handful of western states including Colorado, Nebraska, and Wyoming. This changed abruptly when CWD was discovered in Wisconsin in 2002.  Since then, it has been discovered in nearly two dozen eastern states—and its spread continues.

The particularly troubling aspect of CWD is that there is no known way to eradicate the disease once established, because it can persist in the environment for years, perhaps decades—even in the complete absence of deer. About the only good news is that there is currently no evidence that consuming venison from infected animals poses any human health risks. However, to be safe, testing is highly recommended, as is avoiding consumption of an infected animal. For more information on CWD, or the precautions you should take in CWD areas, visit the CWD Alliance website at


Hemorrhagic disease can be caused by two related but distinct viruses—epizootic hemorrhagic disease (EHD) virus and bluetounge (BT) virus. Since both viruses produce similar disease conditions in deer, the term hemorrhagic disease, or HD, is often used to describe both, regardless of the virus actually responsible. Hemorrhagic disease is spread by biting midges, often called “gnats” or “no-see-ums,” which transmit the virus from infected deer, sheep, or cattle to uninfected deer. These gnats frequent mud flats near water sources, which increase in abundance during drought years. As gnat populations climb, deer become increasing restricted to fewer and fewer water sources, creating the perfect storm for infection.

Once a deer is infected, there are generally four possible outcomes. The deer can: 1) die within hours, 2) live for several days and then die, 3) struggle for weeks and recover completely, or 4) show no signs of disease. Deer that die quickly often show only swelling of the head, neck, tongue, and lungs. Deer that survive for several days before dying generally exhibit classic HD symptoms including lameness, reduced activity and appetite, loss of fear toward people, excessive salivation, ulcers on the tongue or in the mouth, and hemorrhages in the heart, rumen, and intestines. Deer that eventually recover often exhibit growth interruptions in the hooves and damage to the lining of the rumen (stomach). The most noticeable sign of HD infection for hunters is the “sloughing hooves” on deer harvested during the hunting season. As with CWD, there are no known human health concerns regarding consumption of venison from deer infected with HD.

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The impact of HD on deer herds varies according to the strain of virus involved, abundance of midges, and the level of immunity of the deer infected. In general, strains of EHD virus infect and kill more deer annually than do strains of BT virus.

Outbreaks of HD follow clear geographic patterns associated with the distribution of the midges. They are most common in the South but can occur in much of the central and eastern United States as far north as southern Canada. While less common, outbreaks occasionally occur in western states including Nebraska, Montana, Wyoming, and the Dakotas.

Deer that survive infection develop antibodies against the virus and temporary immunity against future disease. However, immunity to one virus strain does not guarantee complete immunity against other strains. The more often a deer is exposed to the disease, the more immunity it gains. Consequently, while deer are more frequently infected with HD in Florida, Louisiana, and Texas, few die. In contrast, in more northerly portions of the disease range such as Illinois, Michigan, and Pennsylvania, herd immunity is low or absent, and the percentage of infected deer that die is much higher. While mortality estimates vary widely, some herds in particularly hard-hit areas can experience 25-50% mortality.

Outbreaks of HD always occur during mid summer or early fall, when midges are most abundant and active. As a result, it is common for archery hunters to find the remains of deer near streams and ponds, because high fever associated with infection causes deer to seek the nearest available water. Deer of both sexes and all ages are equally susceptible to HD.

By now you are probably wondering if there is anything that can be done to prevent an outbreak of HD in your area? Unfortunately, there isn’t because outbreaks are primarily related to abundance of the midges rather than the number or health of the deer. The good news is that severe outbreaks—like those that occurred in 2007 and 2012—are fairly uncommon. By comparison, less-severe outbreaks occur every year somewhere in the United States, with most resulting in herd mortality rates below 10 percent. Therefore, in the long-run, HD is more of a nuisance to management than a serious impediment.

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Fortunately, the white-tailed deer is one of the most resilient species on the planet and, given adequate time and habitat, will quickly recover. By simply backing off your doe harvest a year or two, most herds will rebound to pre-infection levels in a season or two. Effective deer management requires the ability to make adjustments quickly based on weather, habitat conditions, and the myriad of other factors Mother Nature can throw your way.



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