Equine vector-borne diseases: Combating Eastern equine encephalitis and West Nile virus

Horses kept near stagnant ponds are at risk for West Nile virus, which is transmitted by mosquitoes commonly found near shallow standing water.West Nile virus and Eastern equine encephalitis, two virulent diseases transmitted by mosquitoes, can cause severe neurologic disorders and even death in unvaccinated horses. The good news: Properly timed vaccinations and aggressive mosquito control can protect horses and their owners from devastating disease.

Eastern equine encephalitis

Eastern equine encephalitis (EEE) is an alphavirus, transmitted via mosquitoes that mainly reproduce in freshwater hardwood swamps. The primary enzootic vector for EEE is the Culiseta melanura mosquito. Reservoir species include such birds as jays, warblers, finches and sparrows.

Horses, camelids and humans are also at risk from mosquito bites but are considered dead-end hosts for EEE.  Mosquito “bridge species,” which bite both mammals and birds and may be a source of EEE transmission to horses (and humans), include Aedes, Ochlerotatus, Coquillettidia and Culex species.

According to the Vermont Arbovirus Surveillance and Response Plan, EEE is well-established in North America, with cases in horses dating back to 1831.1 Most EEE viral activity occurs in the Atlantic and Gulf Coast United States.

EEE causes severe neurologic disease (encephalitis) in horses, as well as other equids and camelids, with mortality approaching 90 percent in unvaccinated animals.2 Ratites and game birds also experience high mortality from EEE infections. Signs of infection in horses include anorexia, fever, depression, ataxia, wandering, seizures, head tilt or pressing, impaired vision, hyperexcitability or irritability, limb weakness or paralysis, blindness, coma and sudden death. 

“Veterinarians need to be cognizant of the geographic and seasonal likelihood of arboviral disease transmission,” says Abigail Mathewson, DVM, state public health veterinarian for New Hampshire's Division of Public Health Services. In areas with endemic arboviral disease such as EEE, veterinarians are usually familiar with the clinical signs of illness. But veterinarians who practice near these areas or those who have clients who travel to such areas with their horses should also develop a good understanding of these diseases, she says. With vector and pathogen distributions shifting and horses traveling, it's possible that veterinarians could see EEE outside of its usual range.

If a patient shows neurologic illness during the mosquito season, lives in or near an area where there is arboviral disease or has a travel history to those areas, then arboviral illness should be a part of the differential diagnosis.

West Nile virus

West Nile virus (WNV) can affect a wide range of mammals (including humans) and birds. A “cousin” of St. Louis encephalitis, WNV was first isolated in Uganda in 1937 but was also soon prevalent in the rest of Africa, the Middle East and West Asia. Human epidemics also reached Europe in the late 1990s and early 2000s.

WNV was first seen in the United States in humans, horses and birds in 1999 via mosquito transmission of Culex species. “How it got to the U.S. is a bit of a mystery,” says Mathewson, “but it's possible that a mosquito stowed away on a commercial air transport shipment. Because we are a global community, transport around the world is a source for potential pathogen introduction and establishment.”

Horses infected with WNV can show no clinical signs or experience mild to severe illness, with signs and symptoms similar to EEE. About one-third of horses with severe illness will die or have to be euthanized because of the severity of the disease.

WNV is seen in nature in a mosquito-bird-mosquito transmission cycle primarily involving Culex species (e.g., Culex pipiens, Culex tarsalis and Culex quinquefasciatus) with birds as the reservoir hosts, especially Corvidae (jays and crows), which experience greater than 90 percent mortality.1 Though these birds were easily killed by the disease, gray catbirds and northern cardinals (and fewer numbers of other common bird species in various locations) may be primarily responsible for its spread through the eastern U.S. and from the northern Midwestern states to Florida.3

Unlike the swamp-bred mosquito species that transmit EEE, WNV mosquitoes are commonly found in areas of shallow standing water, including birdbaths, horse watering tubs, stagnant pasture ponds and manure storage pits.

Combating arboviruses

The best offense against arboviruses is a good defense. Controlling mosquito breeding areas and administering the proper vaccinations can protect horses and humans alike.

Mosquito control. “We have clinical equine cases of EEE in New Hampshire, although it's more prevalent in states to our south,” says Mathewson. Mosquito control can be complicated for EEE, as those mosquitoes are swamp breeders. For WNV, however, veterinarians should encourage horse owners to eliminate possible mosquito breeding areas by cleaning horse watering tubs, buckets and birdbaths on a regular basis, removing or draining standing yard water and dumping accumulated water caught in planters and rubber tires. Using appropriate repellents and maintaining screens on the horse owners' windows and doors also will help reduce exposure to the vectors.

 “It's important that veterinarians discuss the potential for human infection,” says Michele Schalow, DVM, equine program manager at the Michigan Department of Agriculture and Rural Development. Thorough control of mosquito breeding areas is critical to help prevent disease transmission to both horses and people. “It's also important to bring horses in during evening (mosquito feeding) hours when possible and use approved mosquito repellents on outdoor animals. In 2012, Michigan even had a confirmed case of EEE in an 8-week-old puppy,” she says. Though there are different types of preferred breeding water sources for each mosquito species, the take-home message is to encourage horse owners to clean up and control all water reservoir mosquito-breeding areas.

Vaccination. In addition to mosquito control, veterinarians must discuss the huge benefit of timely vaccinations, says Mathewson. “We have these magnificent methods of prevention, and they are most effective when administered prior to arboviral season,” she says.

Through close communication with horse owners, equine practitioners can help maintain a proper EEE and WNV vaccination schedule. All horse owners-regardless of how many horses they have-should be reminded of the importance of this simple preventive measure.

Veterinarians can attempt to monitor the local risk and how often owners should be vaccinating, which varies depending on the location, but one can't necessarily predict what the arboviral season is going to be, how the weather is going to affect the mosquitoes and what the bird population is going to be doing. So it's important to be proactive in protecting horses and people against a potential outbreak. Because it's impossible to protect pastured horses from mosquito bites, vaccinations are an easy and excellent preventive measure.

“In New Hampshire, we emphasize how valuable the use of vaccination can be,” says Mathewson. “Even if a horse survives a bout of arboviral disease, it may not be considered safe to ride anymore. Horse owners could potentially lose their favored companion or show animal, either through their passing or loss of their usual functionality. They need to weigh the detrimental outcome against a low-cost preventive measure.”

Although equine arbovirus cases are numerous, without vaccination they'd be even greater. “There are still some people who are not familiar with the diseases, usually because they are new to an endemic area,” Mathewson notes. “So if you're going out to the barn for some other reason around the appropriate vaccination time, you should be reminding clients to vaccinate. And from the clinic, send out reminder cards promoting early, consistent vaccinations.”   

It's been shown that the WNV vaccine is highly effective within a two-week period of vaccination. During the two-week postvaccination period, no local or systemic adverse reactions were observed following 96 percent of the vaccinations administered while mild, transient injection site reactions were noted in a small number of horses. These results indicate that the killed WNV vaccine is safe and efficacious.4

“If you're in an area where you don't have year-round transmission (i.e. the Northeast),” Mathewson suggests, “veterinarians should encourage clients to vaccinate their horses well in advance of the mosquito season, around April, to be sure the horses are well protected, as the vaccination should provide protective antibodies for six to eight months5 and possibly longer.”

If horses are in an area farther south, such as Florida, with essentially year-round mosquito exposure, those animals should be vaccinated two to three times a year. Young foals may experience interference with the vaccinations' effectiveness because of maternal antibodies, so more frequent vaccination may be helpful for younger horses.

In Michigan, the state veterinarian puts out a yearly press release reminding individuals of the importance of vaccinating their horses and advising them to talk with their veterinarian about vaccinating against WNV and EEE annually in the spring, at the onset of the mosquito season, so their animals are protected by late summer and early fall when the state sees the most cases of WNV. “We have to remind the horse industry each year, so that horse owners don't become complacent,” says Schalow. “If they don't see cases because of vaccine effectiveness, they still need to be reminded to vaccinate annually in spite of the disease prevalence, as these diseases are in the mosquito and wildlife population, and this puts their horses at risk.”

Even with the emphasis to vaccinate for both EEE and WNV, Michigan experienced an outbreak of EEE in 2012, and many unvaccinated horses succumbed to the disease, says Schalow.

Veterinary and public health issues

As West Nile virus began to emerge in both people and horses in 2002, the state of Michigan developed an Arbovirus Working Group, says Schalow. The group is composed of public health, animal health and environmental health organizations, including Michigan State University, which tests for WNV in horses and performs mosquito surveillance. The group monitors WNV activity and develops response plans accordingly.   

“As we see WNV and EEE in the horse population during the summer, it is a sign that the mosquitoes are prevalent and spreading the disease. It's the ‘canary in the mine,' a sign for veterinarians to let clients and people know that they can get those diseases from mosquitoes as well,” says Schalow. It's also important to make it clear to clients that it is the mosquitoes-not their horses-that are transmitting the disease to the human population. 

This is also an opportunity to encourage horse owners to take proper mosquito control actions, to reduce mosquito infestation and to be aware of their own mosquito protective control measures-wearing proper clothing and coming indoors during peak mosquito hours. “If people are aware of their own potential susceptibility to EEE and WNV infection, it will make them more aware of the need to vaccinate their horses,” says Schalow. “As there is no human vaccine, at least they can protect their horses.”

For more information on EEE and WNV vaccination schedules and guidelines, visit aaep.org.

References

1. Vermont Arbovirus Surveillance and Response Plan; 2013. Available at: http://healthvermont.gov/prevent/arbovirus/surveillance/index.aspx.

2. USDA APHIS. Eastern Equine Encephalomyelitis fact sheet. Available at: http://www.aphis.usda.gov/publications/animal_health/content/printable_version/fs_eastern_equine_enceph.pdf.

3. Dusek RJ, McLean RG, Kramer LD, et al. Prevalence of West Nile virus in migratory birds during spring and fall migration. Am J Trop Med Hyg 2009;81(6):1151-1158.

4. Ng T, Hathaway D, Jennings N, et al. Equine vaccine for West Nile virus. Dev Bio 2003;114:221-227.

5. Reed S, Bayly W, Sellon D. Equine Internal Medicine. 3rd ed. Philadelphia, Pa: Elsevier, 2009;624-628.