EHD and CWD: What’s the Difference?

February 18, 2015 By: Lindsay Thomas Jr.

The two most significant diseases affecting deer today are hemorrhagic disease (EHD and bluetongue viruses) and chronic wasting disease (CWD). Many people confuse the two. Here’s a quick guide to the major differences between EHD and CWD.

Which Deer Disease is More Serious?

The National Deer Association believes that CWD is the most serious long-term threat to the future of wild deer and deer hunting that we face today. But because CWD’s impacts are difficult to see on the landscape now or won’t become serious for many years in most areas, it is easy for some people to dismiss or disregard the danger.

Both EHD and CWD are serious matters. EHD strikes quickly, can kill a lot of deer in a short time in extreme outbreaks, then fades and may not strike again in a serious outbreak for years. CWD creeps more slowly, taking one to two years to kill an individual deer and more to build prevalence in the population, but it does not fade or rest. Because they operate differently is no reason to treat either disease lightly. If anyone urges you to disregard one of these diseases because they say it’s not as serious as the other, be skeptical.

Below is a table of the major differences between EHD and CWD. To learn more, visit our CWD Resource Center. To see a map of CWD’s most current locations, visit the National Wildlife Health Center’s CWD Distribution page.

Differences Between EHD and CWD in Deer

Hemorrhagic Disease
(EHD, Bluetongue)
Chronic Wasting Disease
BasicsEHD: Viruses spread by biting gnats in late summer. Symptoms include fever and internal hemorrhaging.CWD: A syndrome of the central nervous system in which the brain deteriorates. Cause when normal proteins call “prions” become deformed.
PathwayEHD: Cannot be spread from deer to deer, only through bites from infected insects.CWD: Spread deer-to-deer through direct contact, or contact with the saliva, urine, feces, blood and body parts of infected deer or infectious materials in soil.
VictimsEHD: Bucks and does of all ages are equally susceptible to being bitten by infected insects.CWD: Higher infection rates among bucks, particularly mature bucks, most likely because they cover more ground and contact other deer more often.
LocationEHD: The viruses are present everywhere in North America, but outbreaks are associated with drought and extreme heat, usually in late summer.CWD: Present in deer or elk herds in 31 states (as of 2023). Preventing CWD’s spread to new areas is critical. Transportation of live, infected deer/elk or their parts is the primary long-distance pathway.
Mortality RateEHD: Some deer survive infection. Herd immunity/survival is higher in areas with longer historical exposure.CWD: Always fatal. The syndrome predisposes infected deer to increased death by predators, cars, other diseases, but those that live long enough will eventually die of CWD’s direct effects.
Speed of DeathEHD: For those deer that die, death usually occurs within a few days of viral infection.CWD: Incubates in infected deer for one to two years before visible symptoms appear. During incubation, deer can spread CWD to other deer.
DurabilityEHD: Viruses cannot survive outside the bodies of the insect vector or the deer/elk host.CWD: Infectious materials remain viable indefinitely (years) in the environment and are shed in feces, urine, saliva, blood and carcasses of infected animals.
Human HealthEHD: Cannot infect people, either through insect bites or through handling or consuming infected deer.CWD: No evidence that CWD is a health issue in humans, but the Centers for Disease Control and Prevention urges hunters who harvest deer in CWD zones to submit them for testing and wait for satisfactory results before eating the venison.
Long-TermEHD: Outbreaks vary locally from mile to serious, but deer populations rebound. Whitetails have lived with and adapted to these viruses for decades.CWD: Except where prevalence rates are kept low through active management, CWD threatens to slowly erode deer productivity and reduce a population’s ability to sustain hunter harvest without declining. 

About Lindsay Thomas Jr.:

Lindsay Thomas Jr. is NDA's Chief Communications Officer. He has been a member of the staff since 2003. Prior to that, Lindsay was an editor at a Georgia hunting and fishing news magazine for nine years. Throughout his career as an editor, he has written and published numerous articles on deer management and hunting. He earned his journalism degree at the University of Georgia.