Equine encephalomyelitis refers to a group of mosquito-borne viral diseases caused by alphaviruses in the family Togaviridae.
Overview and Clinical Importance
Equine encephalomyelitis refers to a group of mosquito-borne viral diseases caused by alphaviruses in the family Togaviridae. The three major viruses are Eastern equine encephalomyelitis virus (EEEV), Western equine encephalomyelitis virus (WEEV), and Venezuelan equine encephalomyelitis virus (VEEV). These diseases represent significant causes of severe neurological disease in horses with high mortality rates, making them critical topics for the NAVLE examination.
These viruses cycle naturally between mosquito vectors and avian or rodent reservoir hosts. Horses and humans are considered incidental dead-end hosts because viremia is typically insufficient to infect feeding mosquitoes. EEE/WEE/VEE vaccines are AAEP core vaccines, emphasizing the importance of understanding these diseases for preventive medicine.
High-YieldEEE has the highest mortality rate (70-90%) of any arboviral encephalitis in horses. When you see a horse with acute neurological signs during mosquito season (summer/fall), EEE/WEE/VEE should be high on your differential list, especially in unvaccinated animals.
| Characteristic |
EEE |
WEE |
VEE |
| Family/Genus |
Togaviridae/Alphavirus |
Togaviridae/Alphavirus |
Togaviridae/Alphavirus |
| Genome |
ss(+) RNA, ~11.7 kb |
ss(+) RNA, ~11.5 kb |
ss(+) RNA, ~11.4 kb |
| Equine Mortality |
70-90% |
20-50% |
50-90% |
| Primary Vector |
Culiseta melanura |
Culex tarsalis |
Multiple Culex/Aedes spp. |
| Reservoir Hosts |
Passerine birds |
Passerine birds |
Rodents (enzootic); Horses (epizootic) |
| Geographic Range |
Eastern US/Canada, Caribbean |
Western US/Canada, South America |
Central/South America, Mexico |
| Last US Outbreak |
Annually in endemic areas |
Not detected since late 1990s |
1971 (Texas) |
Etiology and Virology
Viral Classification
All three encephalomyelitis viruses belong to the genus Alphavirus within the family Togaviridae. They are enveloped, single-stranded, positive-sense RNA viruses approximately 70 nm in diameter with icosahedral nucleocapsid symmetry.
Comparison of Encephalomyelitis Viruses
Board Tip - Memory Aid: "EEE = EXTREME, EASTERN, ENCEPHALITIS" - Remember EEE as the most severe (highest mortality), located in the Eastern US. WEE is Western and relatively milder. VEE is Venezuelan and unique because horses can serve as amplifying hosts.
| Phase |
Clinical Signs |
Duration |
| Prodromal Phase |
Fever (102-107°F), anorexia, depression, lethargy |
1-5 days |
| Early Neurological |
Behavioral changes (hyperexcitability or somnolence), head pressing, aimless wandering, impaired vision |
12-24 hours |
| Progressive Neurological |
Ataxia, circling, cranial nerve deficits (droopy lip, muzzle deviation, dysphagia), inability to swallow |
Hours to days |
| Terminal Phase |
Recumbency, seizures, paralysis, coma, death |
2-3 days after onset |
Epidemiology and Transmission
Transmission Cycle
The transmission of EEE, WEE, and VEE involves a complex cycle between mosquito vectors and vertebrate reservoir hosts. Understanding this cycle is critical for disease prevention and control.
EEE Transmission
- Enzootic cycle: Maintained between Culiseta melanura mosquitoes and passerine birds in freshwater hardwood swamps
- Bridge vectors: Aedes, Coquillettidia, and Culex species transmit to horses and humans
- Seasonality: Peak transmission late summer to early fall (July-October); year-round in Florida
- Incubation period: 1-3 weeks in horses
WEE Transmission
- Primary vector: Culex tarsalis - breeds in sunlit marshes and irrigation water
- Amplifying hosts: House finches and house sparrows
- Additional vectors: Can be transmitted by the tick Dermacentor andersoni
- Epizootics: Associated with increased spring rainfall followed by warmer temperatures
VEE Transmission
- Unique feature: Horses serve as AMPLIFYING hosts during epizootic cycles (viremia sufficient to infect mosquitoes)
- Enzootic cycle: Maintained between Culex (Melanoconion) spp. and rodents
- Epizootic subtypes: Subtypes IAB and IC cause large outbreaks; up to 90% of susceptible equids may become infected
High-YieldA key distinguishing feature of VEE is that horses develop HIGH viremia and can serve as amplifying hosts, unlike EEE and WEE where horses are dead-end hosts. This is why VEE can cause explosive epizootics with thousands of horse cases.
| Disease |
Distinguishing Features |
| EEE |
Most severe; rapid progression (12-18 hrs to recumbency); severe mental depression; high fever; most horses die within 2-3 days; survivors have permanent neurological deficits |
| WEE |
Less severe; biphasic fever (days 2 and 6); slower progression; 50-90% may improve after 5 days; better prognosis but persistent neurological deficits common |
| VEE |
High fever (up to 107°F); severe depression; diarrhea may occur; high viremia allows mosquito transmission; mortality 50-90% in epizootics; can cause abortion in pregnant mares |
Clinical Signs and Pathophysiology
Pathophysiology
Following mosquito bite, the virus replicates locally in skin fibroblasts, then spreads via lymphatics causing primary viremia. The virus then targets the CNS, crossing the blood-brain barrier to cause diffuse polioencephalomyelitis with leptomeningitis.
Histopathological Findings
- Perivascular cuffing: Lymphocytic and neutrophilic infiltrates around blood vessels
- Neuronal degeneration: With neuronophagia (satellitosis by microglia)
- Gliosis: Microglial proliferation and glial nodule formation
- Distribution: Gray matter more affected than white; cerebral cortex, thalamus, hypothalamus, and brainstem most severely affected
Clinical Presentation
NAVLE TipThe classic presentation is an unvaccinated horse in late summer with acute onset of fever followed by rapid progression to neurological signs within 24 hours. Death typically occurs 2-3 days after neurological signs appear. EEE progresses fastest and has the worst prognosis.
Disease-Specific Clinical Features
| Test |
Sample/Method |
Clinical Utility |
| IgM Capture ELISA |
Serum or CSF |
Indicates recent infection; detectable 1-3 weeks post-infection; preferred ante-mortem test |
| Plaque Reduction Neutralization Test (PRNT) |
Paired sera (10-14 days apart) |
Gold standard for confirmation; 4-fold rise diagnostic; differentiates between EEE, WEE, VEE |
| RT-PCR |
CSF, brain tissue |
Highly sensitive and specific; can detect viral RNA in early infection; useful postmortem |
| Virus Isolation |
Brain tissue; Vero cell culture |
Definitive but rarely successful from serum; best from brain at necropsy |
| Immunohistochemistry |
Brain tissue sections |
Detects viral antigen in neurons, astrocytes, microglia; postmortem only |
| CSF Analysis |
Cerebrospinal fluid |
Pleocytosis (less than 500 WBC/µL); elevated protein (90-110 mg/dL); initially PMNs, later mononuclear |
Diagnosis
Clinical Diagnosis
Diagnosis is based on clinical signs, geographic location, seasonality, and vaccination history. However, definitive diagnosis requires laboratory confirmation because clinical signs overlap with other causes of equine encephalitis.
Laboratory Diagnosis
High-YieldFor NAVLE: IgM capture ELISA is the preferred ante-mortem diagnostic test because it indicates RECENT infection (vs. vaccination or prior exposure). A 4-fold rise in paired sera is confirmatory but horses with EEE often die before convalescent samples can be collected.
CBC and Chemistry Findings
- Peripheral leukocytosis: With left shift (neutrophilia)
- Lymphopenia: Especially in EEE and WNV
- Elevated muscle enzymes: CK may be elevated due to recumbency
Differential Diagnosis
The differential diagnosis for acute equine neurological disease is extensive. Key differentials to consider include:
| Differential |
Distinguishing Features |
| West Nile Virus (WNV) |
Spinal ataxia predominates; muzzle twitching in ~50% of cases; less severe mental changes; waxing/waning signs; 35-45% mortality |
| Rabies |
Variable presentation; aggression or paralytic form; history of wildlife exposure; always fatal; zoonotic - handle with caution |
| EPM (Equine Protozoal Myeloencephalitis) |
Asymmetric ataxia; focal muscle atrophy; chronic progressive course; caused by Sarcocystis neurona |
| EHV-1 Myeloencephalopathy |
Fever, then ataxia 8-12 days later; bladder dysfunction; history of respiratory disease or abortion in herd; vasculitis-mediated |
| Hepatic Encephalopathy |
Icterus; elevated liver enzymes and bile acids; low BUN; hepatotoxin exposure (pyrrolizidine alkaloids, iron) |
| Botulism |
Flaccid paralysis; dysphagia; weak tongue tone; dilated pupils; no fever; often associated with contaminated feed |
Treatment
There is no specific antiviral therapy for equine encephalomyelitis. Treatment is entirely supportive and aimed at controlling pain, inflammation, and preventing secondary complications.
High-YieldCorticosteroid use is CONTROVERSIAL in viral encephalitis. While they may reduce cerebral edema, some studies suggest poorer neurological outcomes. NSAIDs (flunixin meglumine) are the preferred anti-inflammatory agents.
Prognosis
Poor Prognostic Indicators: Recumbency, comatose state, flaccid paralysis, status epilepticus, inability to rise, severe mental depression
| Treatment Category |
Options |
Notes |
| Anti-inflammatory |
NSAIDs: Flunixin meglumine (1.1 mg/kg IV q12-24h); Phenylbutazone |
Reduce CNS inflammation and fever; cornerstone of therapy |
| Corticosteroids |
Dexamethasone (0.1-0.2 mg/kg IV) |
CONTROVERSIAL - some reports suggest poorer outcomes; may be used short-term for severe cerebral edema |
| DMSO |
1 g/kg IV as 10-20% solution |
Free radical scavenger; may reduce cerebral edema |
| Anticonvulsants |
Diazepam (0.1-0.4 mg/kg IV); Phenobarbital |
For seizure control; variable response |
| Fluid Therapy |
IV crystalloids (LRS, 0.9% NaCl) |
Maintain hydration if unable to drink; correct electrolyte imbalances |
| Nutritional Support |
Nasogastric tube feeding; parenteral nutrition |
For horses unable to eat due to dysphagia |
| Nursing Care |
Padded stall; sling support; prevention of self-trauma |
Critical for recumbent horses; rotate positioning to prevent decubital ulcers |
Prevention and Vaccination
Vaccination Protocols
EEE and WEE vaccines are AAEP CORE VACCINES recommended for ALL horses in North America. VEE vaccination is risk-based for horses near the Mexican border or traveling to endemic areas.
Vaccine Types
- EEE/WEE vaccines: Killed (inactivated) adjuvanted products; highly efficacious
- VEE vaccine: TC-83 attenuated strain or inactivated preparation; use only in high-risk areas
- Combination vaccines available: EEE/WEE; EEE/WEE/WNV/Tetanus; EEE/WEE/VEE
NAVLE TipRemember the vaccination timing: EEE/WEE boosters should be given in SPRING, before mosquito season begins. In areas with year-round mosquito activity (Florida, Gulf Coast), semi-annual boosters are recommended. Maternal antibody interference means foals should not receive their first vaccine until 4-6 months of age if the dam was vaccinated.
Vector Control and Environmental Management
- Eliminate standing water (breeding sites for mosquitoes)
- Clean water tanks and buckets weekly
- Use fans in stalls (mosquitoes are weak fliers)
- Apply equine-approved insect repellents
- Limit turnout during peak mosquito activity (dusk and dawn)
- Screen stalls and barns when possible
| Disease |
Mortality Rate |
Prognostic Factors |
| EEE |
70-90% |
Grave once recumbent; death usually within 2-3 days; survivors have permanent neurological deficits |
| WEE |
20-50% |
Fair prognosis; 50-90% improve after day 5; persistent neurological deficits common |
| VEE |
50-90% |
Variable depending on outbreak strain; case fatality 20-90% in epizootics |
Public Health Considerations
EEE, WEE, and VEE are zoonotic diseases that can affect humans. These are also reportable diseases - all cases of acute equine neurological disease must be reported to state veterinarian authorities.
- EEE in humans: 30-75% case fatality rate; severe neurological sequelae in survivors
- WEE in humans: 3-7% case fatality rate; 15-30% have neurological sequelae
- VEE in humans: Usually flu-like illness; less than 1% develop encephalitis
- Important: Horses DO NOT transmit disease directly to humans or other horses (dead-end hosts for EEE/WEE); VEE-infected horses CAN amplify virus
Memory Aid - "EEE = EVERYTHING EXTREME": Eastern = most EXTREME mortality in horses (70-90%); most EXTREME human case fatality (30-75%); most EXTREME rapid progression (death in 2-3 days). When in doubt about which encephalitis is most severe, remember EEE = EXTREME.
| Population |
Vaccination Schedule |
| Previously Vaccinated Adults |
Annual booster prior to mosquito season (spring); Semi-annual boosters in high-risk areas with year-round mosquito activity |
| Unvaccinated Adults |
Primary series: 2 doses at 3-6 week intervals; Annual boosters thereafter |
| Foals (vaccinated dam) |
3-dose series beginning at 4-6 months of age; 4-6 week intervals between doses; Third dose at 10-12 months |
| Foals (unvaccinated dam) |
3-dose series beginning at 1-4 months of age; 4-week intervals between doses |
| Pregnant Mares |
Booster 4-6 weeks pre-partum to ensure colostral antibody transfer |