EHD vs. CWD: What’s the Difference?

A primer on two deer diseases, EHD and CWD, which are often confused for one another.

EHD vs. CWD: What’s the Difference?

This buck died from EHD (Epizootic Hemorrhagic Disease), which is caused by small biting insects called midges or gnats. It’s common to find deer that died from EHD near water. (Photo courtesy of Missouri Department of Conservation.)

With the wealth of online discussion forums available today, hunters often broadcast finding dead deer in their hunting areas. First, someone suggests it died from EHD (Epizootic Hemorrhagic Disease) while another claims it died from CWD (Chronic Wasting Disease). Indeed, there does seem to be confusion on these two deer diseases. My purpose here is to show the difference between EHD and CWD, but before I do that, I need to clarify another confusion.

Hunters often refer to EHD as “Blue Tongue” (BT). They are almost identical diseases, but they are not the same. The symptoms are the same and dead deer with EHD look like dead deer with BT, and relative to deer dying and impacting your hunting, they both have the same effect. Suffice is to say that EHD is much more common. In most cases, when a hunter refers to deer dying from BT, it really is EHD. In what follows, when I refer to EHD, what is said also applies to BT. Now to the differences between CWD and EHD (and BT).

How Long Have These Diseases Been Around?

EHD has been around since it was first isolated in New Jersey in 1955. CWD was first identified in captive deer in a Colorado research facility in the late 1960s and in wild deer in 1981. By the 1990s, it was reported in northern Colorado and southern Wyoming. Wisconsin started checking hunter harvested deer for CWD in 1999. However, CWD didn’t really make news until it was found in samples collected from harvested deer during the 2001 deer season.

How Do Deer (and Elk) Get the Disease?

Deer get EHD from small biting insects called midges or gnats. EHD is not spread from an infected deer to a healthy one. The gnats that bite them occur near mud flats around water. And the month when they get the disease depends on when these gnats hatch, which depends on the weather. Long summer droughts create mud flats around ponds and streams, and then rainstorms promote the hatching of the insects. This rain may be localized, triggering a localized gnat hatch. Thus, it can wipe out deer in small pockets while 10 miles away deer are unaffected. The years 2007 and 2012 — the two highest Drought Index events in recent history — were drastic years for EHD.

CWD is a central nervous system disease that causes brain deterioration. It is not caused by a virus or bacteria, but is caused by deformed proteins called prions. It is spread by direct contact with the saliva, urine or feces from an infected deer. These all contain prions and once on the soil, or wherever they drop, healthy deer may then eat or lick them and become infected. Prions can end up in growing plants and it is suspected that deer eating those plants could contract the disease.

When Will Deer Get the Disease?

Deer get EHD shortly after being bitten. This usually happens in August through September, but is highly dependent on the timing of the first frost of the season — frost kills the biting insects, effectively ending the localized EHD event. Conversely, CWD is not weather dependent. Deer get CWD whenever they ingest infected prions.   

Symptoms of EHD and CWD

Three days to a week after being bit by a gnat, deer with EHD develop a fever, bleed internally along the digestive tract and die. Death often occurs near water. There are various viral strains of EHD, and some deer develop an immunity to specific viral strains. In fact, does can pass this immunity on to their unborn fawns. Over time, many deer in a given area gain antibodies against the strain of virus that caused the EHD. The longer that strain is in an area, the higher the herd immunity.

Then why do deer living in that area die of EHD? Good question. The answer is that new strains of EHD viruses are coming into the United States (mostly from the tropics) every few years. So, though deer may be immune to one strain, they have to develop an immunity to the new strain to survive.

Relative to symptoms, it’s a whole new ball game with CWD. Once a deer ingests infected prions, it has the disease. These prions cause a degeneration of the brains of infected deer. The most obvious sign is a slow but steady weight loss that occurs over many months. Excessive drinking and urination are common as the deer becomes ill. However, it may take 2 to 3 years before the disease is noticeable externally. By that time, CWD-infected deer that are still alive become malnourished, thin and disoriented, causing abnormal behavior. An example of this behavior is repetitive walking in set patterns, but don’t confuse that with similar behavior in deer suffering from brain worm, a totally different disease. 

Now, why did I put use the phrase “CWD-infected deer that are still alive” in the previous paragraph? Because once infected, visually identifying a sick deer may take years, but the disease affects the deer’s ability to avoid predators, hunters and even vehicles. We know this based on a Wisconsin DNR study on deer survival and CWD. They collared deer in the wild in 2017 and 2018. The 2017 results were that CWD-positive deer had a 75 percent annual mortality rate, while those that did not have CWD had a 25 percent annual mortality rate. Larger samples from 2018 showed that annual mortality rate for CWD-positive deer was 69 percent, but only 35 percent for deer that did not have CWD.

Do These Diseases Kill Both Bucks and Does?

EHD kills all ages of bucks and does. Some infected deer will survive with subsequent immunity. All others will likely die.

CWD kills both sexes, but it has a higher infection rate in bucks. Even worse, it has a higher infection rate in older bucks. No one knows why this is, but one thought is that mature bucks move around more than younger bucks and this brings them into contact with more deer and potentially more prions — pure speculation.

What Are the Long-Term Effects on the Deer Population?

This is an important question. Outbreaks of EHD are localized, covering relatively small areas. Remember drought and rain control the hatching of the insects that transmit EHD and BT to deer. Usually rain storms are scattered across the country at any one date. Since the biting insects that live in mud flats and transmit EHD to deer hatch after rains, you will get EHD only in those areas.

In addition, if the strain of EHD that invades an area is the same as in years past, some deer will be immune and survive. Over several years, many will become immune as will the fawns born to immune does. It is uncommon for a deer population in one localized area (for example, a county) to get EHD 2 years in a row, so recovery is relatively quick. Even after a huge die-off in one localized area, it is common for the deer herd to be back to normal within 3 years.

However, several new strains of EHD have popped up in the past 10 years. Some disease experts are concerned that a particularly virulent strain of EHD may appear that kills a much higher number of deer in an area. That could happen. For now however, the overall concerns about EHD are far less than CWD. 

The long-term impact of CWD is another story. Studies show that when infection rates for mature deer reach 30 to 40 percent, large numbers of deer die. Those deaths can occur directly from the prions, or the deaths can be indirectly caused by hunters, predators, cars, etc. However, dead is dead, no matter how you look at it. And we have more and more areas in Wyoming, Wisconsin, Colorado and West Virginia where the prevalence rates are over 30 percent. In fact, there are now areas where half the deer population has CWD. Impacts there, over several years, will be devastating and eventually will lead to less deer for hunters to harvest.

Can Humans Get These Diseases?

No, humans cannot get EHD. Even if you are bitten by an infected gnat, you will not get EHD.

The answer to this question relative to CWD is not as clear. To date, it appears that there is a barrier that prevents humans from getting CWD even after eating prions from an infected deer or elk. However, there are some scientists who are concerned about CWD because of what happened with “Mad Cow” disease. In those cases, humans initially ate infected beef and did not get the disease, but about 10 years later, humans did get the disease and over 400 individuals died. The Centers for Disease Control and Prevention, and most state wildlife agencies, recommend that if you hunt in an area that has CWD, get the deer tested and eat it only if the test comes back negative.

How Likely Is a Cure to be Found?

Eliminating EHD is probably impossible. How would you eliminate a viral deer disease? I have no answer, and doubt if anyone does. But as long as EHD mortality stays the same as it has, there is no emergency.

Eliminating CWD is, in my opinion, impossible. Keeping the prevalence low is critical, but controversial, because that involves increasing deer harvests in areas that have the disease. Nothing stirs up hunters more than suggesting that more deer need to be harvested.

Are there genetic strains of deer that might be immune? It has been suggested that there might be, but it will take many years of research to determine that, and even longer to implement a management plan to enhance immunity of deer, if it’s possible.

Recently, a researcher stated that CWD is caused by a bacterium and that a cure is imminent. My research of the literature shows this to be highly unlikely, but we can all hope that CWD is caused by a bacteria. That would make finding a cure much easier. But don’t hold your breath. The reality is that prions are the culprit and other prion diseases (scrapie in sheep, Creutzfeld Jacobs disease in humans) have been around for many decades with multi-millions spent on research, to no avail.


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