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For decades deer have suffered from a persistent disease that attacks their blood cells and causes hemorrhaging. The disease is caused by viruses in the epizootic hemorrhagic (EHD) and bluetongue (BT) virus groups, and it is fatal. 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 no-see-ums, sand flies, or sand gnats. 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.
During the last 10 years, exotic EHD and BT viruses have been isolated consistently in the United States, and in 2007, the largest recorded outbreak in wild deer was documented. Although such events suggest that the way these viruses behave in North America may be changing, the factors responsible for successful virus introductions or large-scale regional outbreaks are complex and poorly defined. The changes may involve the host, virus, parasites or any combination of them.
One important question relates to the source of viruses responsible for outbreaks. It is unknown whether outbreaks are caused by one virus strain or multiple strains. To investigate this question, Dr. Andrew Allison (SCWDS) did a genetic analysis 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. Additionally, based on the distribution of these various strains, there also may be geographical barriers, such as the Appalachian Mountains, that limit their distribution. Experiments to begin answering the many questions surrounding the various strains of the hemorrhagic diseases are underway.
Nationwide distribution of hemorrhagic disease outbreaks in wild deer between 1980 and 2003. Map courtesy of SCWDS.
Some recent experimental results demonstrated a clear temperature-dependent relationship with the rate of replication for three viruses. In other words, as temperature increased, the viral count in infected midges increased. This is significant because these results suggest that increased temperature may produce more infected insects in less time, potentially resulting in enhanced virus transmission.
Understanding the role disease plays in wildlife populations helps wildlife biologists better manage game populations. Knowing the mechanics of how a disease spreads and its persistence in the environment will help us learn how to decrease the impact of an outbreak. New exotic strains of viruses have the potential to result in devastating damage in the United States, not just to deer, but all wildlife and livestock. Improving our understanding of hemorrhagic disease will enable us to better manage this and other diseases in the future.
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