5 minute read
Equine Virology 101
Dr. Larry Swango taught me and a few thousand other veterinary students virology. And while I can’t speak for everyone else, I thought virology was a little difficult. That’s not because Dr. Swango didn’t do an excellent job of teaching us. It’s just that to be so small, viruses are very complicated entities. So, while I may not exactly be able to tell you the role of reverse transcriptase in replicating the genomes of retroviruses, I did leave veterinary school with a good working knowledge of virology. So, my goal today is to give you a general understanding about virology so that you will feel obliged to vaccinate your horses against equine encephalitis and West Nile virus.
A virus is an organism that requires a host cell to replicate or reproduce itself. A Medicine News Today website states that viruses are considered to be the most abundant biological entity on the planet. In a very elementary way of looking at it, a virus is a piece of RNA or DNA that lives in some kind of package. Now some of the packages have different bells and whistles on them that allow them to attach to the host cell and gain access. Once the virus gains access to the host cell, the RNA or DNA gets access to the nucleus or control center of the cell and begins to replicate itself.
It is not that much different from some of the old superhero or other movies when the bad guys get control of some nuclear missile control center (the cell nucleus) and turn the missile on the United States (the host cell or the host such as the horse for our purposes today.) The only problem is that with viruses there is generally no superhero who is going to come in and save the day after the virus has established itself. However, there are times in these movies that the CIA or special forces (the immune system) intercepts the bad guys before they can take charge of the controls and the threat is neutralized.
While there are some antimicrobial agents, such as Tamiflu, that work against some viruses, for the most part treatments for viral illnesses are limited to supportive care. So, we treat the dehydration that results from the diarrhea caused by a canine parvovirus infection while the virus “runs its course.” Depending on how virulent the virus is, sometimes the supportive therapy wins. Sometimes the virus wins. Just be aware that we are generally not treating with antiviral drugs but are trying to stay ahead of the damage caused by the virus with supportive care.
Another interesting thing about viruses is that they generally prefer specific types of tissues in the host. The influenza virus usually prefers respiratory tissue. The same applies to the common cold. The difference in the two viruses that affect the respiratory system is the degree of virulence.
The Eastern encephalitis virus, West Nile virus (another encephalitis virus that affects the brain) and rabies all prefer nervous tissues. For some reason the prognosis for neurological viruses is generally more pessimistic than most of those that attack other tissues. The recovery rate in horses for those viruses I just mentioned is more optimistic for the West Nile virus than for the other two. In fact, if your horse contracts Eastern equine encephalitis, it is probably going to die. If it contracts rabies, it is going to die.
I want to focus on the two encephalitis viruses I just mentioned. Both Eastern encephalitis virus and West Nile virus are carried by mosquitos. Maybe if you are reading this and live in Southwest Texas, you cannot relate, but here in Alabama, we have had a mild, wet winter. In fact, they are saying that it has been the wettest winter in parts of Alabama on record. You won’t get any argument from me on that subject. Anyway, I think we generally see a huge crop of mosquitoes following a winter like this. Therefore, we have more insects available to carry those viruses. And if your horse is not vaccinated and is bitten by a mosquito incubating the West Nile virus or the Eastern equine encephalitis virus, it has a 40% and 70% chance respectively of dying, even with aggressive supportive care. The 40 percent and 70 percent are the case fatality rates (rounded off to the nearest 10 percent) for these viruses. The case fatality rate is the percent of patients that die divided by the number that are confirmed with the illness.
As I write this article, people are clearing the shelves at Walmart of disinfectant wipes, hand sanitizer and for some reason toilet paper because they are concerned about coronavirus, which has a case fatality rate of about 3 percent. That number continues to change as more cases are reported. It is interesting to me that people get so excited about a virus with a small case fatality rate, yet many horse owners do not even vaccinate against viruses that are likely to kill their horse.
As we hear that one of the best ways to prevent the corona virus is to wash your hands, that doesn’t work for mosquito viruses – not that a horse has that option anyway. We are fortunate to have vaccines developed for these viruses in our horses because the other option for prevention is to avoid getting bitten by mosquitos. In fact, because there is no vaccine for these viruses in humans, avoiding mosquito bites and being lucky are our only two options. Playing the odds and being lucky has worked out for me so far. But I am aware of people who have died or have suffered severe brain damage because they were bitten by a mosquito carrying one of these viruses. And to be honest, avoiding mosquito bites has not worked all that well for me.
I just want to emphasize that we as horse owners have an obligation to do what we can to keep them healthy. I consider vaccinating against diseases when a vaccine is available is as much our responsibility as seeing that they get enough feed and water. If you have not already done this, please contact your veterinarian to discuss a vaccination program that is best for your horses.
