It’s been a rollercoaster year for weather in the United States, with record-breaking heatwaves, dirt-cracking drought, and flooding rainfalls. That pattern hasn’t only been a trial for the average citizen and livestock owners—it’s also setting the stage for a potentially larger and longer season for West Nile virus, experts say. There are some things you can do to reduce the risk to your horses, including eliminating any standing, stagnant water sources where the mosquitoes that carry West Nile virus breed. But the best way to protect your horses from West Nile virus is even simpler: vaccinate them against it.

We asked Kevin Hankins, DVM, MBA, Managing Equine Veterinarian for Zoetis Animal Health, for insights about this year’s West Nile virus outlook, how the vaccine works, and the effects of the virus on unvaccinated horses.

Why is this year being to be a potentially bad year for West Nile virus for horses?

When we look back historically over West Nile virus outbreaks, whether it's here or in Europe, a common pattern we see is an extended drought period that's followed by rain. Eighty-six degrees ambient temperatures is ideal for replication of the mosquito and replication of the virus in the mosquitoes. What happens during drought conditions—which is counter-intuitive to what people think about mosquitoes and lots of rain—is that those bodies of water become stagnant and smaller, which makes them easier to heat up. That makes them ideal breeding conditions for mosquitoes. And it makes them have ideal conditions for that virus to replicate within those mosquitoes.

So you get these small bodies of water, lots of virus replication and lots of mosquito replication. When a big rain comes, it takes all these small, infected mosquito pools and spreads them out all over the place. So now that you've got infected mosquitoes that are more widespread to start their own breeding pools and other places, and the temperature is still high. So it just makes it the ideal conditions for spread of the virus as well as for infections.

This year, everything is setting up just perfectly for not only an early season, but maybe even an extended season of West Nile, and definitely a higher number of mosquitoes carrying the virus.

When is the typical timeframe for West Nile virus season?

Traditionally, in most of the country the highest number of cases start occurring in August, especially in the Midwest. The season will go through October here. In other parts of the country, like Florida and the rest of the Southeast, the West Nile virus season can extend all year.

What we’ve seen this year is an earlier occurrence; we saw some cases in some parts of the United States starting in June. Here in the Midwest, we started seeing cases in July instead of August this year. We do worry about the length of the season, because the longer it stays warm, the longer the West Nile virus season can extend. But I worry more about the number of infected mosquitoes, because that increases the odds of a horse or human being bitten by an infected mosquito. There was a study, I believe at Texas A&M, that a horse around an infected mosquito pool can be bitten up to 5,000 times an hour. Their chances of getting West Nile virus if they’re not vaccinated are astronomical; it would be crazy to think they weren’t going to get it.

So it will depend on the weather, but we know it started earlier and that we are seeing more cases. All of that lines up with the weather patterns we’ve seen in the past that have been associated with larger outbreaks.

What does West Nile virus do to an unvaccinated horse?

West Nile is one of those core diseases that we consider a life-threatening disease. The virus attacks the horse’s nervous system. The clinical symptoms will depend on where in the nervous system the virus replicates at the highest amount. That’s what’s tough about West Nile virus: it can look like almost anything. It can vary from some fasciculations (rapid muscle tremors) and some ear twitching, and that’s it, all the way to completely recumbent and unable to get up. They can have hypersensitivity to touch or sound. They can have neurological symptoms. The clinical signs of West Nile can look like EPM [equine protozoal myeloencephalitis]; it can look like rabies; it can look like Eastern equine encephalitis; it can look like trauma. Luckily, we have very good tests for it, and if a horse is vaccinated for it, West Nile moves way down the list of likely causes.

If an unvaccinated, infected horse is standing when it comes to the clinic, it’s about a 30% death rate. If they’re down when they arrive, that goes up above 70% death rate.

Given how deadly West Nile virus can be, is vaccination uptake in the U.S. as high as it should be?

Vaccination for West Nile has probably dropped off from when the virus first came into the United States. When West Nile first was introduced into the United States, everybody was scrambling to get their horses vaccinated. As it went from epidemic to being endemic, meaning that it's found throughout North America and United States, the fear factor went down with it. Compliance for West Nile virus is still very good, but it's not what it was when it first came through the U.S. The West Nile cases that anybody sees are all in non-vaccinated horses. It’s absolutely rare to see West Nile virus in a vaccinated horse, and when you do it’s usually something to do with their immune system. The good news is that horses that are vaccinated that do come down with West Nile have very mild symptoms, and they recover.

As veterinarians, we’d all like to see higher compliance for vaccination against the core diseases: rabies, West Nile virus, Eastern and Western equine encephalitis, and tetanus. That compliance has been made much easier with the Core EQ Innovator^® vaccine, which vaccinates against all five of those core diseases in one injection.

How good is the West Nile vaccine at protecting against the disease?

The efficacy of the West Nile vaccines are very, very good. I mean, we're talking efficacies in the 90% range, or greater for the West Nile vaccines that are out there.

They're efficacious, they're safe, they're easy to use, and they protect against a disease that has a high death rate to it. It's a very cheap insurance policy against a disease that can kill a horse that you've got a lot of money, time, and personal feelings invested in.

How does the West Nile virus vaccine work?

It’s really very simple. It's a killed or inactivated vaccine, so the vaccines are not able to reproduce the disease, meaning they can't cause West Nile by vaccinating the horse. The vaccine does a very good job of stimulating the immune system to produce antibodies against West Nile. It introduces that killed virus to the horse's immune system, which then produces antibodies that will protect that horse if they see the virus in the future.

In horses that haven’t been vaccinated for West Nile before, it’s a two-dose series, given four to six weeks apart. In adult horses, after that it’s typically an injection that’s given annually.

Does that schedule differ for horses in places like the southeastern U.S. that typically have a longer mosquito season?

Absolutely. The ideal time to vaccinate is in the early spring; that’s true for all the core diseases, because the vectors that carry those diseases start showing up in late spring, and you want to make sure your horse is protected by then. If you’re in an area where you have a high incidence of West Nile or you’re in an area with a prolonged mosquito season, I actually recommend that you give a booster shot in adult horses in late summer or early fall.

And the West Nile vaccine is safe to give with other vaccines?

When we combine the West Nile vaccine with rabies, Eastern and Western encephalitis, and tetanus vaccines, as in the Core EQ Innovator, it doesn't take away from the efficacy of the West Nile or take away the efficacy of the other components. So yes, you can combine it and, and do well with it.

Giving them together in a single injection is convenient, but it’s also good for compliance.

What is your advice for horse owners in setting up their horses’ vaccination schedules?

It’s always important for the horse owner to sit down with their veterinarian and make a preventive health program for their horse. Horse owners can get a lot of information from a lot of sources, but the best source of information they can get for their horse and their area is to talk with their veterinarian, because he or she is familiar with their horse, their area, and their specific premises—and that helps them develop a preventive health program that is going to be more beneficial for that horse and for whatever activity that horse does, whether it never goes off the farm or travels regularly to compete. Together, the horse owner and veterinarian can maximize the health opportunities, which will help the horse perform better and live longer. And, in the long run, it’s going to cost the horse owner less money to have preventive health care.