(July 17, 2014) — The good news for most living creatures on the planet is that the West Nile Virus doesn’t affect them. The bad news is reserved for birds, humans and horses, all of which are affected by and can die from the disease.
West Nile Virus (WNV) was first discovered in Uganda in 1937 and arrived in the U.S. in 1999, where it was identified by a veterinarian at the Brooklyn Zoo. It has since spread across the country via a transmission cycle between mosquitoes and wild birds. Birds serve as hosts and the mosquitoes are vectors, moving from host to host. Horses and humans are considered “dead-end” hosts, meaning that they can’t transmit the disease further.
WNV was first identified in Texas in 2002. Eight deaths were reported in 2004, which can be put into perspective by noting that six Texans won the lottery that year and eight were struck by lightning. In 2012, there were more than 18,000 human cases, resulting in 89 deaths.
No horses have died from WNV this year, but now is the time for horse owners to begin protecting their horses from the disease. Beau Whitaker, a veterinarian with Brazos Valley Equine Hospital in Salado, noted at a recent Williamson County Texas AgriLife Extension program devoted to WNV and horses that most cases show up in September or October.
“All the cases I’ve seen have come in September or October,” Whitaker said. “The levels of concern are much higher that time of year.”
The Texas A&M Veterinary Medical Diagnostic Laboratory has documented a significant increase in the percentage of horses that test positive for exposure to the West Nile virus. During the 2010-11 fiscal year, TVMDL diagnosed West Nile virus in 2 percent of the horses tested (18 out of 916). That increased to 13.4 percent of horses tested (132 out of 985) during the 2011-12 fiscal year.
The number in Texas dipped slightly to 120 in 2012, but that was by far the most of any state in the nation; Louisiana, with 62, had the second-highest number of equine WNV cases.
Whitaker said the 120 reported cases in 2012 probably represented just a fraction of actual cases. The biggest reason for the increasing number of equine deaths from WNV is a failure to vaccinate, he said.
“The vaccine is effective,” he said. “Most of the cases we’ve seen have been unvaccinated horses. Only one had been vaccinated.”
Symptoms in horses include muscle twitches, hypersensitivity to touch and sound, fever and depression. More severe cases might include ataxia — a neurological disorder that causes a lack of coordination — impaired vision, weakness, convulsions and paralysis. If a horse goes down because of the disease, Whitaker said it’s hard to get them back up.
“If we can keep them up we can probably save them,” he said. “Once they go down, the chances of saving them also go down ... If they (horses) are recumbent, there is a 65 percent death rate. Two out of three of those horses will die.”
Treatment includes administering IV fluids, antiviral medications, anti-inflammatory drugs, NSAIDS and steroids. The most significant recovery occurs in the first five to seven days, though relapses are possible. Full recovery can take up to six months. According to the TVMDL, of horses that recover from the disease, up to 40 percent may exhibit neurological signs for six months or more after the initial diagnosis. And Whitaker said one of every three horses that gets WNV will die.
“Vaccinate,” Whitaker said. “There are several (vaccines) out there and all have proven to be effective. Just use one is all I can say.”
He added that a booster shot is sometimes recommended four to six months after the initial vaccination and that annual or bi-annual (twice a year) vaccinations are recommended in years when the disease is widespread.
The prevalence of WNV in any given year can be connected to the pervasiveness of the culex mosquito, usually called the Southern house mosquito. These are the ones that are most active at dawn and dusk. They breed and hatch in stagnant water and can generally be found where there is sufficient blood to feed on. Stables with horses inside provide an endless bounty for these mosquitoes.
Sonja L. Swiger, a Texas AgriLife Extension livestock veterinary entomologist, said control of mosquitoes around the stable includes dumping, cleaning or draining any standing water. Mosquito dunks, which disrupt the life cycle of mosquitoes but pose no threat to people or animals, are recommended for water troughs and buckets.
“You can spray the stables for adults, but it’s expensive and problematic,” Swiger said. “You need to take the horses out of the stable before you spray, but if you take the horses out the mosquitoes will move with them because that’s their food source.”
Swiger said using mosquito traps around the stable is not a bad idea.
“If you have a problem, the traps certainly won’t hurt anything, and they might do some good,” she said. She noted that prices range from $34 to the $1,000 range.
Catherine Zettel Nalen, a medical entomologist with the Williamson County and Cities Health District, said it’s important for landowners to take a look at their own property before calling about a mosquito infestation.
“In Dallas, they found that 25 percent of the complaints about mosquitoes could be traced back to the caller’s own property,” she said. “They were basically reporting themselves. We see that here, too.”
Horses, of course, aren’t the only mammals in Texas that need to worry about WNV. For humans, health professionals recommend the "Four Ds" for protection from West Nile:
n/ Dawn and Dusk. This when the mosquitoes that carry WNV are most active, and bites can be avoided by not being outside during this time of day.
n/ Dress. Long-sleeve shirts and long pants are the recommendations here.
n/ Drain. The speakers at the Georgetown event agreed that draining or refilling any standing water at least twice a week is the key to breaking the mosquitoes’ life cycle.
n/ Defend. Any EPA-approved repellant is recommended. “The most effective one is the one you actually use,” Nalen said.