Encephalitis in Horses

Horses are susceptible to deadly encephalitis viruses spread by mosquitoes, but prevention goes a long way in protecting horses from disease.
A mosquito against a green background. Mosquitos transmit encephalitis to horses.
A new mosquito species in Florida might be partially responsible for the spike in encephalitis cases in recent years. | Getty Images

Horses are not immune from the ravaging diseases carried by mosquitoes, such as encephalitis. Unless immunized against specific mosquito-borne equine viruses, a horse is at risk of contracting fatal neurologic disease from Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE) and Venezuelan equine encephalomyelitis (VEE). As each encephalitic name suggests, there are regions where the different types are more prevalent. 

Mosquitoes obtain their infectious viral load from birds. After biting a bird, the mosquito may turn to a human or horse for a blood refill. Horses and humans are “dead-end hosts” for these viral diseases, meaning that if a mosquito bites a horse or person that is infected with EEE, WEE or VEE, it will not pick up enough viral load to spread that infection to the next human or animal that it bites. 

Sleeping Sickness

Viral encephalitis infection goes by the moniker “sleeping sickness” because horses with encephalitis appear sleepy. An infected horse develops a fever and experiences involuntary muscle twitching and an ataxic gait. Progressive encephalitic signs develop, such as head pressing, aimless wandering, seizures, hyperexcitability and coma. Eventually, the horse goes down and is unable to rise. In temperate climate regions, most cases begin to appear in late summer into the fall months but can appear year-round in southern states.

Eastern Equine Encephalitis

Eastern equine encephalitis (EEE) is an extremely virulent virus, with fatality rates of 70-90% in non-vaccinated horses even with aggressive treatment. In recent years, there have been spikes in EEE cases in horses and humans. A variety of factors contribute to increasing incidence:

  • Warm, wet weather early in the year and/or extending longer into the fall favors a robust mosquito population. This facilitates transmission of the virus to birds and from birds to humans and horses via mosquito bites. 
  • A new variant of EEE virus might be circulating. In Florida the virus persists year-round, providing opportunities for genetic changes in the EEE virus, which could impact susceptible bird populations. More birds become infected and then carry the new variant north during migration. 
  • EEE often shows cyclical trends. After a year with particularly high levels of viral activity, surviving bird populations develop long-lived immunity to that viral 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 of birds to EEE contributes to increased EEE activity.
  • In recent years, a new mosquito species has become prevalent in Florida. At this time, it isn’t known if it has a role in transmitting Arboviruses.

In Florida and South America, rodents might also play a mosquito vector role for EEE. However, birds and rodents don’t develop clinical disease from EEE. 

Western Equine Encephalitis

Western equine encephalitis (WEE), a similar neurologic disease to EEE, is transmitted to horses by the Culex tarsalis mosquito, which lives in the western part of the US. The fatality rate from WEE is about 20-30%. However, in recent years, there has been a dramatic drop in equine cases of WEE. None have been reported in North America since 1998, and none in mosquito reservoirs since 2008. WEE is no longer listed on the CDC annual data page for veterinary or human cases.

Venezuelan Equine Encephalitis

Venezuelan equine encephalitis (VEE) is a reportable foreign disease in horses. It causes death in 50-80% of horses infected by the virus. VEE is endemic in Latin America so there is a potential risk of infection in states bordering Mexico. While no cases of VEE have occurred in the US in over 50 years (since 1971 in Texas), this could alter due to climate change if an influx of VEE-infected vectors moves further north out of Mexico. Rodents are vectors for VEE, and opossums and bats may also contribute, as can ticks and black flies. 

VEE may develop a different epizootic cycle in which virus amplification in the horse or human host has the possibility of leading to mosquito infection. In those situations, a horse or human is no longer a dead-end host. Such a mutation might be caused by an E2 gene that amplifies virulence of various VEE strains. There is also a possibility of aerosol transmission of VEE.

Geographic Separation

Different mosquito species prosper in different habitats. Mosquito vectors and the specific host species they bite (birds, mammals or amphibians) define the endemic regions of all Arboviruses. Although no longer a concern, WEE in the US was mostly transmitted by Culex tarsalis mosquitoes that are limited to states west of the Mississippi River where they proliferate on the Great Plains. In the United States, EEE flourishes best in shaded, freshwater, hardwood swamps in eastern North America and the Atlantic and Gulf Coasts. It is transmitted by Culiseta melanura and other East Coast mosquito species. In particular, the wood thrush, tufted titmouse and a few other avian species serve as primary reservoir hosts for mosquitoes to amplify the spread of EEE. 

The Mississippi River forms a natural boundary between Eastern and Western equine encephalitis-carrying species of mosquitoes. The Mexican border appears to be a relative stopping point for VEE circulating into the US. Areas potentially at risk for encroachment of Venezuelan equine encephalitis (VEE) into the US include the southern borders of New Mexico, Texas, California, Louisiana, Mississippi, Alabama and along the west coast of Florida. 

Each mosquito species is adapted to its particular climate and habitat. For example, Culex tarsalis doesn’t range east past the Mississippi River into woodland habitats. Instead, it flourishes where it is well adapted to drier, treeless plains littered with sunlit grassy areas, streambeds or irrigated agricultural land. Mosquito species can become invasive and colonize new habitats. However, it is likely that the range of each virus is more restricted due to the prevalence of mosquito species rather than migrating birds.

Climate and Habitat Effects on Mosquitos

A horse in a meadow surrounded by mosquitoes, which transmit encephalitis to horses.
Mosquitoes are most active at dawn and dusk, so it might be a good idea to keep horses inside during these hours. | Getty Images

Climate and habitat affect the likelihood of outbreaks, particularly of EEE. Hot summers without drought create breeding conditions for most mosquitoes. In contrast, a deluge from a large storm can flush out mosquito larvae that were previously developing in ponds and water-filled ditches. Climate and weather patterns also greatly influence yearly clutches of birds. Susceptible hatchlings that can’t yet fly have poor defensive behavior against biting mosquitoes. With no immunity, these individuals often develop high levels of viremia.

Temperature ranges have a further effect on the success of specific mosquito species. Larvae of many mosquito species in the mountains and northern states grow in melting snow pools in the spring. Larvae tolerate being frozen for a bit to emerge as adults in early to middle summer. On the Plains, Culex tarsalis develops in spring rain puddles but doesn’t emerge in large numbers until late June, depending on ambient temperature. (While currently not a threat for WEE transmission, this species of mosquito can still transmit West Nile virus.) Drought in late hot summer might not affect mosquito numbers since they are already adults by then. However, availability of water in early to middle summer is crucial to the survival of developing larvae.

Seasonal Transmission of Encephalitis to Horses

Typically, transmission of mosquito-borne encephalitis viruses to humans or horses occurs towards the end of the transmission season in late summer and early fall. This is likely mediated by locally abundant and contiguous “bridge vector” mosquitoes that prefer to feed on mammals. A bridge vector is a species that bites both bird (reservoir) hosts and mammals. It therefore transmits infection from the reservoir host (birds) to mammals.

Sentinel chicken flocks are used by public health agencies to monitor the current circulation of certain Arboviruses. Chickens housed in outside cages are accessible to mosquitoes and their bites. Chicken blood is regularly screened for development of antibodies against Arboviruses. The chickens don’t fall ill from the viruses so sentinel screening is more efficient than catching and testing mosquitoes.

Prevention

horse shaking head flies
Mechanical barriers such as fly masks can deter mosquitoes from bitins horses. | iStock

There are two important avenues for the prevention of mosquito-borne encephalitis infections in horses. One efficacious strategy, following an initial vaccine series, is through annual immunization in the spring month or two preceding the expected re-emergence of mosquitoes. In more southern climates of the US, it might be necessary to boost twice annually since mosquitoes tend to be present year-round in these geographic areas. 

Elimination of Mosquito Breeding Grounds

Another important control method relies on elimination of breeding grounds for mosquitoes. These insects don’t just propagate in large, still bodies of water but can also be found in any container that holds water. Examples abound: horse troughs, wheelbarrows, clogged gutters, ditches, non-chlorinated swimming and wading pools, decorative ponds, bird baths, flower pots, buckets, abandoned tires, tarps, holes in trees, manure piles that collect water, culverts, and discarded jars, soda cans or water bottles, to name just a few possibilities. 

Mosquito species vary in habitat preferences for laying eggs. Some prefer to congregate in irrigation slow-moving waters, while others propagate as “container breeders.” Rafts of mosquito eggs are laid on top of standing water, especially water laced with organic debris. It takes about 7-10 days for mosquito larvae to develop and hatch in standing water. Cleaning water tanks regularly (once or twice weekly) removes hay, food particles and egg clusters and freshens the water. Applying larvicides in areas that can’t be drained weekly is a helpful measure to deter hatching. 

Biological Controls

Biological controls like mosquito-eating fish (fathead minnows or Gambusia species) consume mosquito larvae for management of ornamental ponds, irrigation and watering ponds. Standing bodies of water may also be managed by applying mosquito “dunks” (broken into cereal-size pieces) or granular “bits” containing Bacillus thuringiensis sub. israelensis (BTI), a soil bacteria. In hot and humid climates, mosquito life cycles often shorten from 7-10 days to just 3-4 days. So, it is important to apply BTI weekly until the first killing frost.

Insectisides

Insecticides—topical, spray and misting systems—also minimize the number of mosquitoes around the barn and on horses. Mowing tall grasses and shortening shrubbery removes resting places for adult mosquitoes. Permethrin applied around eaves and corners of the stabling area deters adult mosquitoes from overwintering in these locations. 

On-Horse Protection

Flysheets and fly facemasks further provide effective mechanical barriers to deter insects. Because mosquitoes are most active at dusk and dawn, it helps to stall the horses and use fans during those times—mosquitoes have trouble flying against air turbulence. Also, turning off stabling lights limits mosquito attraction.

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