Viruses are similar to bacteria in that they are microscopic, disease causing organisms. Unlike bacteria, however, there is a constant debate about whether or not viruses are actually alive. By certain standards, viruses are not living organisms because they depend on a cell to replicate and survive, be it an animal, plant, or bacteria. Nonetheless, viruses are every bit as widespread as bacteria, and some can cause deadly diseases.
For example, Epizootic Hemorrhagic Disease Virus—or EHDV—is a virus that causes severe illness and death in wild ruminant populations, especially white-tailed deer. Within a week of infection, deer will exhibit weakness, drooling and bloody diarrhea. They have a high fever which they try to cool by laying in water, before they suddenly fall unconscious and die.[1] Up to 40% of the deer population can die of this disease.[2]
A type of gnat called Culicoides variipennis carries the virus from one host to another.[3] Luckily, this disease isn’t contagious, meaning that a sick deer isn’t going to give it directly to other deer just from being around them. The virus requires a gnat to take it up from the blood for it to be transmitted to a new host.[4] Thus, most control measures to prevent the disease focus on the intermediate, the culicoides gnat. Removing standing water and spraying insecticides where the gnats tend to mate and live are currently the most effective methods of reducing the fly population and preventing infection with the EHD virus.
Theoretically, a vaccine that could be either injected or given in the feed would be very useful for protecting susceptible animals from the virus. But there are a few problems with developing one.
Firstly, the EHD virus is an RNA virus. This means that rather than its genome being made up of DNA like in animals, it is made up of RNA. There are a few structural and component differences between DNA and RNA, but the important part is the different effect that each one has. RNA is more susceptible to mutations during replication. This, combined with the incredible numbers of virus that are produced, means that the virus changes and adapts incredibly quickly. Any vaccine produced that targets specific pieces of the virus genome would rapidly be worked around.
Secondly, our vaccines to control and prevent viral infections are already limited. Due to the fact that viruses use the host’s cells to replicate, it is difficult to develop safe and effective drugs. Studying the genetic sequence of viruses helps determine what compounds would prevent their replication, but as I said, viruses (especially RNA viruses) mutate rapidly. This is the main problem with flu vaccines; new ones must be manufactured each year, as the previous year’s vaccines are ineffective due to viral mutation.[5]
There may be some steps in the EHD virus cycle that can be targeted. Host cells are very good at recognizing and eliminating viral genome, so the virus wraps its RNA in a protective coat called a capsid, so that the immune system can’t see it. Perhaps a vaccine to the capsid could be developed. This way, the body could catch the virus before it even had a chance to enter the cells, increasing safety and efficacy. Unlike other vaccines that target cell receptors, which are very specific and highly mutable, perhaps the capsid would be less changeable.
Controlling and treating viruses has been a difficult
subject for decades. At the moment, the easiest way to prevent Epizootic
Hemorrhagic Disease is to prevent the gnat that spreads it.
[1] https://www.michigan.gov/emergingdiseases/0,4579,7-186–26647–,00.html
[2] Given, M. D. (2018) VMED 9250 Section 9-10 Orbi Birna & Paramyxoviridae 181106 [PowerPoint slides].
[3] https://animaldiversity.org/accounts/Culicoides_variipennis/
[4] https://www.michigan.gov/emergingdiseases/0,4579,7-186–26647–,00.html
[5] https://www.medicinenet.com/flu_vaccination/article.htm#what_is_influenza_flu
Vet Med and One Health 
