Flooded Plains States: Drying Out, Bracing for Mosquito-Borne Equine Diseases

The waters might be receding, but horse owners in Kansas, Oklahoma, Texas, and other areas affected by recent flooding shouldn't drop their guard against flood-related problems just yet. Veterinary authorities say owners in the soggy states should prepare for a potential surge in cases of mosquito-borne equine diseases, such as Eastern and Western equine encephalitis (EEE and WEE), West Nile virus (WNV), and equine infectious anemia (EIA).

The incidence of these diseases can increase in post-flood conditions as mosquitoes lay their eggs in standing water--and there's certainly plenty of opportunity for them to do so in those areas.

Due to above-average rainfall, residents in some areas of the Plains have been watching the sky and riverbanks since late May. Earlier this week, a new round of storms pummeled some of the hardest hit areas of Oklahoma and Texas. According to an Associated Press article, more than 900 homes in Oklahoma have been damaged by floodwaters, and another 1,100 homes were harmed in Texas. The river in Coffeyville, Kan., crested at more than 30 feet, spilling over levees and flooding the town, including a refinery, which sent 71,000 gallons of crude oil into the water.

What's a horse owner to do?

According to Scott Gordon, Montgomery County, Kan., county coordinator and Kansas State extension agriculture agent, livestock (including horses) can deal with the residue of flooding better than people, but they shouldn't be subjected to it if there's another option. He advocated removing horses from flooded pasture areas until fresh rain can rinse the grass and new growth can occur.

"If we can get an opportunity to get them away from that, and get a rain shower, and some sunshine, and some new growth, then you can put them back without any problems," Gordon said. "It shouldn't be a problem with just flood water, because animals aren't as clean as we are--they're used to ingesting some of that, but there's no use in (intentionally) subjecting them to a higher level of bacteria that might be in the water."

If horses are in an area affected by the Coffeyville oil spill, however, Gordon cautioned against allowing them access to formerly flooded pasture until the Environmental Protection Agency clears the area.

Most farms in the area are served by rural water districts, almost all of which have been cleared for human drinking use. If horses are in an area that has not been cleared, "animals are used to drinking water that's not as clean as what we're used to drinking," Gordon said. "What might give us diarrhea or upset stomach is probably not going to bother them.

For horse owners, the silver lining of the off-the-charts rainfall might be the abundant prairie grasses that will translate into a secure year for grass hay following the extreme drought experienced in the region last year. Gordon said the grasses are plentiful, and farmers are just waiting for a dry spell to bale.

"By this time last year things were getting burnt up," Gordon said. "We're in pretty good shape for forages at this point. Our alfalfa might be in short supply--we've just had way too much rain and too many wet periods to get a lot of good alfalfa put up."

Beyond dealing with the water and bacteria within, horses owners must also consider the potential for an increase in arboviruses, said Oklahoma Assistant State Veterinarian Michael Herrin, DVM. "Something we'll probably see is an increase in the mosquito population, which leads to the Eastern/Western encephalitis, West Nile virus, even EIA," he said.

Herrin noted that Oklahoma horse owners are normally good about vaccinating their horses against EEE and WEE, but he encouraged more to utilize the available WNV vaccines.

"The West Nile vaccine is proven to be pretty effective, and we definitely have the disease in the state, because we see the horses that are not vaccinated every year," Herrin said. "Our Eastern/Western (incidence) is extremely low--most everybody uses the Eastern/Western and tetanus vaccine. We see very few over a year's time."

According to the equine arbovirus reports compiled by the USDA's National Animal Health Surveillance System (NAHSS), a national reporting system for equine disease, there were 111 cases of EEE in the United States last year, along with 1,086 equine WNV cases.

A veterinarian should be summoned at first sight of early clinical signs of encephalitic disease, especially in unvaccinated horses. A horse with EEE can portray varied neurologic signs, including apparent blindness, loss of equilibrium, or a head tilt, or he can exhibit sudden behavior changes or even lameness due to loss of coordination, Herrin said.

For both EEE and WEE, as well as WNV, the worst case is recumbency--a horse that goes down and can't get up.

But with WNV, Herrin noted: "Often you'll see some much more mild signs--muscle twitching and fasciculations (small, localized involuntary muscle contractions) around the neck and face. If there's anything the practitioners kind of hang their hats on, it's those muscular fasciculations."

Herrin said horse owners should call their veterinarian if they notice any behavioral changes or neurologic signs, as these signs could be caused by an arboviral encephalitis, or something else: rabies.

"When you notice those nervous system changes, what you have to throw into that differential diagnosis is rabies," Herrin said. "It's not something you want to wait a week to find out what it is, because that then carries over to the human health." It is not uncommon for many people to examine and handle a rabid horse--exposing all in-contact individuals--before the disease is confirmed

Herrin also strongly urged horse owners to have their horses tested for EIA (the Coggins test), considering the expected increase in the number of insects capable of spreading the virus. There is no treatment for EIA, and infected horses must be quarantined for life or euthanatized, depending on state regulations.

While Oklahoma state law requires a current negative Coggins test on horses attending equine events, crossing state lines, or changing ownership, Herrin said many horse owners allow years to pass between tests.

"With regular testing you're much more able to prevent exposure by knowing about and taking care of any positive horses," Herrin said. "The worst thing that can happen is to somehow end up exposed or have a positive horse and have it go undetected for a couple years, so that you not only lose that one, but it can also transfer to your other horses. There are quite a few people who don't test regularly, and they could have minimized the number of horses affected and minimized their losses, because it is an untreatable disease."

According to the NAHSS, there were 66 cases of EIA in the country last year, 12 of which were reported in Oklahoma and 36 in Texas.

The NAHSS is a network of federal and state government agencies that coordinates animal health monitoring and surveillance. The agency's Web site shows up-to-date equine case counts of EEE/WEE, EIA, WNV, and vesicular stomatitis by state and county, as well as information on equine herpesvirus and equine viral arteritis. Site visitors can use interactive maps showing recent cases, and they can track disease occurrence over time. The NAHSS can be accessed at www.aphis.usda.gov/vs/nahss/equine.  

For more information on this system see www.TheHorse.com/ViewArticle.aspx?ID=7809.  

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care. She owns a portly gray gelding named Duncan and dabbles in several equestrian disciplines, with an emphasis on dressage.

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