Epizootic hemorrhagic disease (EHD) kills deer by destroying the lining of their blood vessels, triggering massive internal bleeding, organ failure, and a fatal collapse of the clotting system. Death can come shockingly fast, sometimes within 8 to 36 hours of the first visible symptoms. The disease is caused by a virus spread through the bite of tiny flying insects called Culicoides midges, and it primarily targets white-tailed deer during late summer and early fall.
How the Virus Enters and Spreads
The process starts with a single bite from an infected midge, a gnat-like insect barely visible to the naked eye. Once inside the deer, the virus first replicates inside immune cells in the nearest lymph nodes. Within 4 to 10 days, it enters the bloodstream by infecting red blood cells, which carry it to organs throughout the body, particularly the spleen and lungs.
From there, the virus targets endothelial cells, the thin layer of cells that line every blood vessel in the body. This is where the real damage begins.
The Vascular Damage That Causes Death
EHD is, at its core, a disease of blood vessels. Once the virus infects endothelial cells, it causes them to swell and break down. This damage triggers two simultaneous problems that feed off each other.
First, the injured vessel walls become leaky. Fluid that should stay inside blood vessels seeps out into surrounding tissues, causing widespread swelling (edema). The lungs are especially vulnerable. Fluid fills the airways, producing a frothy discharge and making it progressively harder for the deer to breathe. Necropsy findings consistently show pulmonary congestion and edema as hallmarks of the disease.
Second, the body responds to the damaged vessels by forming tiny blood clots throughout the circulatory system, a process called microvascular thrombosis. Normally, clotting is a targeted repair response. But when vessel damage is happening everywhere at once, the clotting system goes into overdrive and essentially burns through the body’s entire supply of clotting factors and platelets. This leads to a condition called disseminated intravascular coagulation, where the blood paradoxically loses its ability to clot at all. The result is uncontrolled hemorrhaging in virtually every organ: the heart, gut, kidneys, lungs, and skin.
Small arteries and veins throughout the body develop severe inflammation, with their walls breaking down and fibrin deposits forming in the tissue. Pathologists describe this as fibrinoid vasculitis. Downstream from these blocked or damaged vessels, tissues are starved of blood flow, leading to patches of tissue death (ischemic necrosis) in multiple organs simultaneously.
What Deer Experience Before Death
Clinical signs typically appear about 7 days after the midge bite, with an incubation range of 5 to 10 days. Visible symptoms include drooling, a swollen muzzle, nasal discharge (often bloody), lameness, and sometimes a bluish tongue caused by oxygen deprivation. Ulcers and hemorrhages develop in and around the mouth. Deer often develop high fevers and are frequently found near or in water, seeking relief from the heat.
In the most severe cases, called the peracute form, the deer may die so quickly that it shows few external signs at all. These animals are sometimes found dead near water with little obvious explanation until necropsy reveals the widespread internal hemorrhaging. In the acute or “classic hemorrhagic” form, the deer lives somewhat longer, and the bleeding becomes more visible: bloody discharge from the nose and mouth, and hemorrhages visible under the skin and in the gut lining.
Why Some Deer Survive
Not every infected deer dies. Death losses during outbreaks usually stay below 25% of the local population, though severe outbreaks have killed 50% or more. Deer that survive the acute phase can develop a chronic form of the disease. These animals show growth interruptions in their hooves, sometimes severe enough that the hoof walls slough off entirely. Other lasting effects include oral ulcers, scarring in the stomach lining, weight loss through winter, and occasionally malformed antlers. A deer with cracked or deformed hooves in the fall months may be a survivor of a summer EHD outbreak.
Populations in the southeastern United States, where EHD circulates more regularly, tend to have higher levels of immunity. Outbreaks are often most devastating in northern herds encountering the virus for the first time or after several years without exposure.
Why Outbreaks Happen When They Do
EHD outbreaks cluster in August and September, and drought years make them worse. Shrinking water sources create warm, shallow, stagnant pools that are ideal breeding habitat for Culicoides midges. At the same time, deer congregate around these dwindling water sources, putting healthy and infected animals in close proximity to large numbers of midges. The combination of more vectors and more concentrated deer herds accelerates transmission.
Outbreaks end with the weather. A hard frost kills the adult midge population, and with no vector to carry the virus between animals, transmission stops. In years with an early frost, outbreaks are shorter and less severe. A prolonged warm fall extends the window for the disease to spread. The virus does not pass directly from deer to deer, so once the midges are gone, the outbreak is effectively over.
EHD Versus Bluetongue
EHD is often confused with bluetongue virus (BTV), and for good reason. Both are spread by the same type of midge, produce nearly identical symptoms in white-tailed deer, and cause the same kind of vascular damage. The two diseases are so similar in clinical presentation that telling them apart requires laboratory testing. Both can cause the swollen muzzle, lameness, oral hemorrhages, and rapid death that hunters and wildlife managers associate with hemorrhagic disease. The main practical difference is that bluetongue more commonly affects cattle and sheep, while EHD hits white-tailed deer hardest.