Fighting Hemorrhagic Disease of White­tailed Deer

By Marcus Gray, Coordinator of Science­Based Conservation Programs and Research

Safari Club International Foundation (SCIF) and Mossy Oak have partnered with the University of Georgia’s Southeastern Cooperative Disease Study (SCWDS) to combat hemorrhagic disease, the most important viral disease of white­tailed deer in the United States. The disease is caused by viruses in the epizootic hemorrhagic (EHD) and bluetongue (BT) virus groups. Over the past 20 years, researchers have isolated nearly 800 EHD and BT viruses. Since the disease symptoms pro­duced by the EHD and BT viruses are indistin­guishable, the general term hemorrhagic disease is often used when the specific virus is unknown. At present, there are no vaccines, and no wildlife management tools or strategies available to pre­vent or control hemorrhagic disease.

The viruses are spread by midges, a tiny biting fly also called no­see­ums, sand flies, sand gnats and punkies. Consequently, the appearance of the disease is seasonal, and coincides with the time of year when these insects are active. The severity of disease outbreaks is highly variable, from a few sick individual deer, to major outbreaks that result in the death of over 50 percent of a local deer pop­ulation. The outbreaks come to a sudden end in the fall, with the arrival of freezing temperatures, and cessation of midge activity.

It is unknown whether outbreaks are caused by one virus strain or multiple strains. To investigate this question, Dr. Andrew Allison (SCWDS) ana­lyzed the DNA of virus strains from different regions of the United States. Numerous different strains were identified that appeared to be restrict­ed to a single state or to a region within a state. The distribution of these various strains suggests that there also may be geographical barriers, such as the Appalachian Mountains, that limit their distri­bution. However, certain strains were far more widespread than others, suggesting that specific genotypes may predominate during outbreaks. It is unknown whether some of these widespread strains are more potent than others. The SCWDS is conducting additional experiments to begin answering the many questions surrounding hem­orrhagic disease strains.

Some recent experimental results demonstrated a clear temperature­dependent relationship with the rate of replication for three viruses that is, as temperature increased the viral count in infected midges also increased. The increase appeared ear­lier after the midges fed on deer. During the study, midges with sufficient virus to theoretically infect a deer were consistently present as temperature increased. This result suggests that increasing temperatures may provide more infected insects in less time, potentially resulting in enhanced virus transmission.

Another significant part of this study relates to the persistence of an exotic strain of virus con­tributing to the EHD problem in North America. The exotic strain replicated as efficiently as endemic North American virus strains suggesting that the exotic virus can adapt to the EHD midge. SCIF and Mossy Oak are very interested to learn more about this strain which is spreading across the United States.

Understanding the role disease plays in wildlife populations helps wildlife biologists bet­ter manage game populations. Knowing the mechanics of disease spread and persistence in the environment can aide in adaptive management to address the causes of mortality or decrease the impact of an outbreak. From an economic stand­point, the white­tailed deer is the most important big game animal in the United States. New exot­ic strains of viruses have the potential to result in devastating economic damage in the United States, both to the deer, and potentially even to other species, including livestock. Improving our understanding of hemorrhagic disease will enable us to better manage this and other diseases in the future. SCIF and Mossy Oak are committed to fighting the most important disease affecting the most important big game animal in the United States.