A look at current trends in mosquito-borne diseases affecting horses

Recent news reports describe accelerated wildlife species loss because of significant changes in habitat and food sources. But one creature that seems to be thriving is one of the world’s most dangerous: the mosquito. Mosquito-borne disease affects all mammals, with some of the most virulent infecting horse and human similarly: Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE), and West Nile virus (WNV).

The good news is horses can’t spread EEE, WEE, or WNV to other horses or humans, and vice versa. Female mosquitoes are the intermediate or “bridge” vectors that cause infection. Birds (and sometimes rodents) carry the viruses but don’t always show clinical signs. Mosquitoes bite the birds and pick up the virus; then those carrier mosquitoes pass the virus on to horses, humans, or other birds when they take their next blood meal. People and horses are considered dead-end hosts because they don’t have enough infective virus in their blood to be transferred through mosquitoes or body fluids to other humans or animals.

Let’s examine recent trends with these three encephalitides—diseases causing brain inflammation—so you can understand how to best protect your horse.

Eastern Equine Encephalomyelitis

Culiseta melanura mosquitoes transmit EEE, causing serious neurologic disease in horses, with a 90% fatality rate. These mosquitoes reside east of the Mississippi, localizing EEE to states in and around that region. Besides having a fever, infected horses develop an uncoordinated gait (ataxia) and often experience involuntary muscle twitching. Progressive encephalitic signs develop, such as headpressing, aimless wandering, seizures, hyperexcitability, and coma. Once a horse goes down, he’s unable to rise.

Through 2019, most equine cases of EEE were seen in Michigan (29), Florida (28), Louisiana (18), and near the Michigan border in Indiana (11). The earliest cases appeared in March and continued throughout the year.

Wendy Vaala, VMD, Dipl. ACVIM, director of life-cycle management equine and companion animal at Merck Animal Health and a member of the American Association of Equine Practitioners’ vaccination guideline review group, says while officials reported far more human EEE cases in 2019 than in any previous year, they did not see a similar spike in equine cases. As of mid-December, an unprecedented number of 38 human cases with 15 fatalities had been reported in nine states, and 182 equine cases of EEE had been reported in 24 states. (For comparison, EEE reportedly infected 712 horses in the high-level year of 2003.)

“Increased spikes in EEE cases have been associated with a variety of factors,” says Vaala, including:

• Warm, wet weather early in the year and/or extending longer into the fall favors a robust mosquito population capable of transmitting the virus among birds and from birds to humans and horses. The northern Midwest experienced a very wet 2019 beginning in early spring and extending into the fall.
• A new variant of the EEE virus might be present. In Florida the virus circulates year-round, providing opportunities for it to undergo genetic changes that could impact susceptible bird populations. More birds become infected and might carry the new variant north during normal migration. For example, in Massachusetts two of the last outbreak cycles of EEE involved new variants of the virus.
• Health officials often document EEE cyclical trends. After a year with particularly high levels of viral activity, surviving bird populations develop longlived immunity to that strain, resulting in a transient decrease in EEE cases for a number of years. Once that bird population dies off, another period of increased susceptibility to EEE infection and carrier status in younger birds contributes to further EEE activity.

Usually, the number of WNV cases in horses is double the number of EEE cases. But in 2019 the number of EEE cases far outnumbered the 88 WNV cases reported by early December. Most were horses that were unvaccinated, undervaccinated, or had an unknown vaccination history, says Vaala.

Western Equine Encephalomyelitis

The Culex tarsalis mosquito, which lives primarily in the western part of the United States, transmits WEE, a neurologic disease similar to EEE. Fatality rate among infected horses is 40-50%. Recent years have seen a dramatic drop in equine cases, with none reported in the western U.S. since 2004. However, birds and mosquitoes in this area still harbor the virus, so vets recommend vaccinating horses annually as a safeguard.

West Nile Virus

As the primary reservoirs for WNV, many avian species (corvids, such as crows, ravens, and jays; house finches; and owls and hawks) often succumb to the infection. Some birds retain viremia (virus in the blood) for three months, possibly resulting in overwintering of the virus; in other words, even if mosquitoes have died off for the winter, new generations can pick up virus from those birds in which it’s been lying in wait. Mosquito-infecting WNV titers have been found in the blood of infected cottontail rabbits, some squirrels, and chipmunks; however, scientists don’t yet know their importance as reservoirs for the virus. 

A horse exposed to WNV through the bite of an infected mosquito does not always develop clinical signs, but when he does they can be serious. Along with fever, many horses experience problems with their cranial nerves, which are responsible for functions of the head, such as chewing, swallowing, blinking, and facial muscle tone. Infection with WNV causes other neurologic problems, including hind-limb weakness, ataxia, an inability to stand, and paralysis. Encephalitic signs such as seen with EEE are also possible. About 33% of horses with WNV die. Of those that recover, about 40% experience persistent neurologic deficits.

Horses older than 15 are more at risk of developing severe neurologic signs from WNV and dying as a result. Horses that develop signs from WNV later in mosquito season (which varies by region but occurs when temperatures are consistently above 50 degrees) have a less favorable prognosis for survival than horses that fall ill earlier in the season.

In contrast to EEE, WNV isn’t as localized to specific regions of the country because various mosquito species that live throughout the United States and Canada carry and can transmit the virus. In temperate climates WNV tends to appear in mid- to late summer, lasting until a killing frost eliminates mosquito vectors. Cases in 2019 occurred primarily in California (15), Colorado (10), and Florida (10).

Preventing Disease

Owners can protect their horses from EEE, WEE, and WNV with annual vaccination. All horses are at risk of contracting mosquito-borne viruses if not immunized correctly. Owners are fortunate to have access to effective EEE and WEE vaccines and several effective WNV vaccines developed and labeled specifically for horses. To date, no vaccines have been developed for humans against these diseases. The equine vaccines are very safe, inexpensive, and highly efficacious. Properly vaccinated horses appear to be adequately protected, says Vaala.

Horses that have not received a full series (two to three injections spaced three to six weeks apart) of the initial immunizations or a yearly booster are at risk of contracting disease. The Indiana state veterinarian’s office released a statement about its 11 EEE cases in 2019, noting “all affected horses were unvaccinated, and clinical signs progressed quickly. The one horse that survived had a history of vaccination but was not up to date.”

A similar trend occurred in Michigan: “Eighty-six percent of the equine cases were either unvaccinated or had not received an annual vaccination for at least a year,” says state veterinarian Nora Wineland, DVM, MS, Dipl. ACVPM.

Indeed, horses that have received the initial two- to three-part series of EEE/WEE or WNV vaccines are not protected forever. Your veterinarian must administer the vaccine booster annually, preferably at least two to four weeks prior to your region’s anticipated mosquito hatch.

These vaccines provide effective viral immunity for four to six months. Veterinarians recommend twice-annual boosters in areas of the country where mosquito populations persist due to mild weather year-round.

“In an unusually warm and wet year, more frequent EEE vaccination is warranted,” says Vaala. “Also, owners shipping horses south for winter shows should make certain their horses are properly immunized against mosquito-borne diseases.”

“Vaccination frequency is best discussed with a horse owner’s veterinarian,” says Wineland. “Vaccines are licensed based on studies completed by the manufacturing company as part of the approval process. Studies that review duration of immunity inform your veterinarian about recommendations for vaccination frequency relative to your geographic environment.”

It is also important to involve your veterinarian in the immunization process. “To optimize an immune response, the vaccine must have been handled appropriately (distribution to administration), not be expired, and be properly administered to a healthy horse,” says Vaala, something the veterinarian is best-equipped to do. Moreover, “major vaccine producers offer a ‘vaccine guarantee’ that provides financial support for diagnostic work-ups and treatment of horses that have contracted one of the core diseases despite having been properly immunized with a veterinarian-­administered ­vaccine.”

Another point to consider for prevention revolves around turnout time—­horses turned out at dusk or dawn are exposed to more mosquitoes than if they were confined in stalls with fans and/or insect sprays during those hours. Vaala says mosquitoes have difficulty landing and feeding in a constant breeze. She also recommends applying mosquito repellent labeled for equine use.

“Comprehensive mosquito control remains an important part of any prevention strategy,” she says.

Remove all standing or stagnant water that can serve as mosquito breeding habitats to reduce populations on your farm. Even the least suspected “container” is a risk, including persistent puddles around water tanks or in tire tracks. Water troughs, wheelbarrows, clogged gutters, ditches, nonchlorinated pools, decorative ponds, bird baths, flowerpots, buckets, abandoned tires, tarps, holes in trees, manure piles that collect water, culverts, and even discarded jars, soda cans, or water bottles are potential breeding sites for mosquitoes. Discard or drain such water-holding receptacles regularly.

Potential Effects of Climate Change

While scientists have not yet come to a consensus about climate change and how it does or will affect various ecosystems, recent trends could shed light on how mosquito-borne disease might fare in the future. Over a 20-year period Canadian researchers reported seeing a 10% increase in human cases of mosquito-borne disease. The United Nations’ Intergovernmental Panel on Climate Change anticipates that mosquitoes will not only expand their range but also become more abundant. It attributes this to warming temperatures, higher amounts of precipitation, and extreme weather events.

“Michigan experiences equine cases of EEE each year, typically fewer than 10 per year,” says Wineland. “High amounts of rainfall contribute to an increased number of cases, and (in spring 2019) Michigan experienced abnormally heavy rainfall.”

Larvae of many mosquito species in the mountains and northern states grow in melting snow pools in the spring, can tolerate a bit of freezing, and emerge as adults early to mid-summer. In Plains states, C. tarsalis mosquitoes flourish in spring rain puddles, emerging in large numbers in late June.

More rainfall amplifies the amount of standing water available to mosquitoes for laying eggs. Hot summers without drought breed the most mosquitoes, but rainfall can confound populations with its variability and intensity. Deluge-type storms, for instance, can flush out mosquito larvae that were developing in ponds and ditches, leading to reduced numbers.

Higher temperatures increase the rate of larval development, which means more mosquitoes to breed and exponential population increases. High seasonal temperatures also shorten incubation times, with more rapid mosquito hatches increasing numbers of potentially infective mosquitoes. Plus, infected females seek hosts for blood meals over a longer life span.

Climate and weather patterns also influence yearly bird clutches, especially susceptible hatchlings that have poor defensive behaviors against biting mosquitoes, in part because they can’t yet fly. These young birds often develop high viremia because they have no immunity. 

To add to the issue, some mosquito species are likely to extend their range into new areas. And, reservoir bird populations are likely to move in conjunction with climate-related changes to food supplies and habitat. The Canadian report cites that over the next 60 years, duration of mosquito-borne disease transmission in Canada is likely to extend beyond the typical three-month period to five months.

Concern over the ongoing loss of insect and bird biodiversity (think bees) could also potentially lead to reduced pesticide use. This could affect mosquito curtailment, with greater numbers and higher risk of disease exposure. At this time, however, “there is no reason to think that mosquito control measures have decreased in the U.S. or that the ‘bridging’ mosquito populations are becoming resistant to the pesticides used,” says Vaala.

Take-Home Message

Annual or twice-annual vaccination strategies and comprehensive mosquito control efforts on horse properties can go a long way toward protecting both human and horse from mosquito-borne disease.