Epizootic Hemorrhagic Disease

Hemorrhagic Disease 

Hemorrhagic disease (HD) collectively refers to seasonal disease in deer caused by either Epizootic Hemorrhagic Disease virus (EHD), or bluetongue virus (BT). Both viruses are Orbiviruses and share similar signs of disease that make it impossible to distinguish between them in the field, hence the collective term, HD. Hemorrhagic disease typically occurs during the late months of summer and into early fall. Outbreaks of mortality can occur any year, but noticeable outbreaks occur on a seasonal, cyclical pattern that can be identified with multiple healthy-looking deer found dead near water sources during the end of summer or early fall months.

HD Hosts

Mortality and morbidity (signs of illness) associated with EHD or BT viruses are often referred to as ‘blue tongue’ or ‘EHD’. The term HD refers specifically to disease from EHD or BT viruses within white tailed-deer (Odocoileus virginianus). Domestic ruminant species that are affected by EHD or BT infections include cattle, sheep, goats, and some camelid species. Sheep are particularly susceptible to BT infections. Signs of BT infections in sheep have historically been labeled as sore muzzle as one prominent sign is reddening around the skin of the lips and muzzle. Cattle can also develop BT infections that result in fetal abortions, decreased milk production, and oral lesions.

Cattle can also become infected with EHD, rarely showing signs of illness. During the EHD outbreak of 2012, cattle displayed signs of EHD primarily through oral lesions in Iowa, Nebraska, South Dakota, and other midwestern states. Reported mortality in free-ranging ungulates like deer and elk were reported during the 2012 outbreak as well.

HD Transmission

Hemorrhagic disease is a vector borne disease and is vectored by Culicoides midges. Vector borne diseases are disease that require an arthropod to transmit the pathogen to the host species in order to become infected. Midges bite hosts to receive a bloodmeal and transmit virus to the host through their saliva during the process. These midges are commonly referred to as biting midges that inhabit semi-aquatic environments with rich organic material. Optimal habitat for Culicoides ranges from a moist pond or stream banks to shaded, moist tree cavities in forests. There are over 150 species of Culicoides midges, but only two species -- Culicoides sonorensis and Culicoides insignis -- are confirmed vectors for HD. While these two midge species are known vectors for EHD and BT, we suspect that other types of Culicoides midges are capable of infecting hosts and spreading EHD and BT.

Clinical Signs

Signs of HD are highly variable and depend on numerous components of the complex HD system. Host susceptibility, vector species, and climate conditions dictate how infection of EHD or BT will present in deer populations. We can classify clinical HD signs and lesions into three different categories based on the progression of infection: peracute, acute, and chronic HD. Peracute HD occurs shortly after infection and is sometimes referred to as the rapid form of HD. Mortality can occur without outward signs of disease. Some deer survive the infection long enough to develop acute signs of disease, or the classic form of HD that can generate visible mortality and morbidity. If deer can endure long-term infection, chronic HD lesions and signs may develop before mortality.

Deer that are able to survive infection from EHD or BT carry antibodies specific to the virus (EHD or BT). These antibodies offer partial protection from future infections with the respective virus. For example, if a deer was infected with EHD and survived, that deer would have partial protection from future EHD infections. That said, antibodies do not offer cross protection, or protection against the other virus. Deer with EHD antibodies do not have protection from BT infections.

HD Cycle

Hemorrhagic disease occurs in a cyclical, seasonal pattern where outbreaks of reported mortality typically occur at the end of summer or early fall. Outbreaks of HD (more than 200 deer mortality reports) historically have occurred every 3-5 years in regions like Iowa where deer populations do not have frequent exposure to EHD or BT viruses. These cyclical patterns are driven by numerous factors including climate conditions, land cover, deer density, and vector species. Because of the numerous components within the HD system, it is not easy to predict when and where annual outbreaks of HD mortality could occur.

Climate Impacts on HD

Of the components, temperature plays an important role in the seasonality of HD. Temperature can influence the amount of time the vector requires to reach the adult life stage where they can become infected with EHD or BT. As temperatures increase in the summer, the time needed for midges to reach adulthood decreases. This means that more adult midges are available at a given time to contract either EHD or BT viruses. In contrast, EHD or BT virus replication within the vector increases as temperatures increase. As the amount of virus increases within the vectors, more vectors are capable of infecting deer. More infectious vectors on the landscape equates to more deer becoming infected at the same time, therefore, creating outbreaks of infection.

These climate conditions are historically associated with drought, but drought is not a requirement for outbreaks of mortality associated with HD. Because moisture is a requirement for vector survival, long term drought conditions could become too warm and dry to maintain vector survival and virus transmission needed for HD outbreaks.

Geographic Distribution

In the United States, there are two broad disease zones when referring to HD distribution and manifestation: enzootic or epizootic zones. Enzootic zones are areas where we can predict the cycle or seasonality of a certain disease. Deer populations in enzootic regions of North America (south and southeast) are frequently infected with EHD or BT viruses, but do not typically develop clinical disease. Deer that can survive EHD or BT infections develop antibodies that protect against future infections from that same virus serotype of EHD or BT, otherwise known as cross protection. Resistance to specific virus serotypes of EHD or BT can be acquired in two different ways: acquired resistance or innate resistance. Does can pass antibodies to fawns through colostrum and provide protection to EHD or BT viruses during the crucial first few months of life. Deer can also acquire resistance by surviving previous infections and creating antibodies to fight future infections.

In comparison, epizootic areas are regions where disease does not typically occur and is less predictable (Figure 6). Most deer in these zones are unable to build resistance through antibodies to EHD or BT to prevent mortality or morbidity during future outbreaks. When ruminants are infected with EHD or BT, peracute or acute mortality and morbidity is often reported. States in the Midwest and east coast experience these outbreaks of HD mortality every few years, but resistance is rare due to the lack of constant virus circulation annually within deer populations. Mortality without signs of disease is common in these regions where deer die within days of infection, but some deer are able to progress through infection to reach the acute stage where clinical disease signs are present.

In the United States, enzootic and epizootic regions can be denoted broadly by latitude and longitude. Generally, HD is enzootic to the southern and southeastern regions below 31° latitude including the states from Texas eastward to Florida. Mortality is rare, but antibodies to EHD or BT viruses are prevalent throughout ruminant populations found in these regions. Southeastern states such as Georgia experience HD events, but deer typically recover with some signs of long-term (chronic) infection including scabbed knees due to painful cracks that developed in their hooves during an active virus infection.

HD in Iowa

Iowa monitors HD annually at the county level through voluntary reports of mortality or morbidity during the peak season of HD activity. Patterns of HD in Iowa follow the seasonal, cyclical trends of other states in epidemic zones. Reports of HD typically begin in late July, peak in September, and decrease through to November. Over the past decade, outbreaks of HD have occurred every 2-3 years in different regions of Iowa. Generally, the counties that border the Missouri and Mississippi River and the bottom third of Iowa experience HD more frequently than the upper central counties. Normally, the first frost of autumn marks the end of HD activity because vectors cannot survive the cold temperatures. The Iowa DNR encourages all sick deer to be reported to the county conservation officer so we can test it for HD, CWD, and other diseases.


Public Health Implications

Both EHD and BT viruses are not infectious to humans. Deer that develop the chronic form of HD or have secondary infections from bacteria are not recommended for human consumption.

Relationships to other deer diseases

HD is a prominent disease affecting white-tailed deer in Iowa but is far from the only disease affecting deer. Other diseases that can impact deer in Iowa include brain abscesses, pneumonia, skin fibromas, foot rot/club foot, or Chronic Wasting Disease. Chronic Wasting Disease (CWD) is the greatest threat facing white-tailed deer in Iowa. To learn more about that disease, visit our CWD information page here and to learn more about the differences between EHD and CWD visit our article on that topic here.

Additional Links

Iowa DNR Deer Health Page

Written by Emma Kring, updated March 2024

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