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Basics ⬇

Outline

BASICS

Definition!!navigator!!

EEEV, WEEV, and VEEV cause encephalomyelitides in North and South America that spread from wild (sylvatic) reservoirs to horses and humans and other mammals, most often by mosquito species. Historically there have been North and South American EEEV variants; the South American EEEV is now designated as MDV. This virus causes disease outbreaks in horses.

Pathophysiology!!navigator!!

  • Varying degrees of destructive encephalomyelitis associated with intraneuronal viral replication and severe infiltration of polymorphonuclear cells
  • EEEV is one of the most pathogenic neurotropic viruses known
  • In recent years WEEV has not caused any outbreaks; however a variant of WEEV, Highlands J virus, actively circulates in Florida
  • VEEV is the only arbovirus where, in epizootics, horses are not dead-end hosts

Systems Affected!!navigator!!

Central nervous system, especially the cerebral cortex.

Incidence/Prevalence!!navigator!!

  • In certain geographic locations, the annual locale incidence is consistent with spikes in activity depending on weather conditions
  • Large outbreaks can occur in new locations, and are occurring more frequently in the northern USA and Canada

Geographic Distribution!!navigator!!

North and South America only.

Signalment!!navigator!!

  • There is no associated breed or sex predisposition
  • Young horses may be at greater risk for development of EEE

Signs!!navigator!!

Historical Findings

  • For North American alphaviruses, EEEV occurs east of the Mississippi River while WEEV primarily occur in the west
  • VEE cases have been reported in southern Texas and South America and occasionally in nearby locales
  • Florida is also considered an at-risk state for VEEV
  • Significant MDV and VEEV outbreaks occur in Central and South America
  • Unvaccinated horses in endemic areas are at high risk
  • New arrivals of horses in endemic areas that are undervaccinated
  • Even in the southeast, while EEEV can occur year-round, peak incidence is midsummer

Physical Examination Findings

  • 48–96 h before onset of neurologic signs fever, inappetence, and depression can occur
  • Some horses may demonstrate lameness and even abdominal pain

Neurologic Findings

  • Clinical signs, which vary in severity with each virus, usually are referable to diffuse cerebral disease, but sometimes signs of spinal cord disease predominate
  • Fever, prodromal malaise, colic, and anorexia may initially be evident; then there is a progressive, but often abrupt, onset of somnolence and peracute to acute diffuse brain signs
  • Dementia, head pressing, ataxia, blindness, circling, and seizures often present
  • Signs of spinal cord or brainstem involvement may occur first, occasionally with focal sensory, reflex, and lower motor neuron signs, particularly in the brainstem
  • Gastrointestinal abnormality, severe obtundation, recumbence, and death may be noted before neurologic deficits are evident.
  • Other associated signs include abortion, oral ulceration, pulmonary hemorrhage, and epistaxis

Causes!!navigator!!

Etiologic Agent

  • EEEV, WEEV, and VEEV are single-stranded, enveloped positive-sense RNA viruses in the family Togaviridae, genus Alphavirus
  • They are all mosquito borne with birds the primary reservoir host for EEEV and rodents the reservoir host for endemic VEEV
  • When epizootic VEEV occurs, there is virus mutation, change in the mosquito vector, and reservoir changes to equids

Risk Factors!!navigator!!

Poor vaccination programs and the presence of dense populations of insects (most often mosquitoes) spreading viral particles from the sylvatic reservoirs, which often include birds, rodents, and reptiles.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

  • Immunoglobulin M capture ELISA titer ≥400 confirmatory in a clinical animal
  • A 4-fold rise in titer between acute and convalescent (7–10 days later) serum samples is considered positive for the diseases, although EEEV horses rarely survive to a second test
  • An increased single sample titer in an unvaccinated severely affected animal is a probable diagnosis
  • Any single sample analysis must be interpreted cautiously if there is a history of vaccination against the viruses
  • Vaccinal versus wild virus-induced titers historically have been used to determine EEEV exposure
  • EEE-to-WEE titer ratios of 4 or more are suspicious for EEE infection
  • Ratio of 8 or more is highly indicative of EEE

Differential Diagnosis!!navigator!!

WNV, rabies, leukoencephalomalacia, and hepatic encephalopathy—WNV encephalomyelitis in the USA tends to result in less severe cerebral and more prominent spinal cord signs than EEEV.

CBC/Biochemistry/Urinalysis!!navigator!!

Cerebrospinal fluid analysis is highly indicative—consisting of predominately polymorphonuclear cells that are nondegenerate with a corresponding increase in total protein.

Pathologic Findings!!navigator!!

  • Gross necropsy findings are nonspecific but often have extremely congested meninges
  • A gray discoloration with petechial hemorrhages of the brain and spinal cord is evident
  • Brain swelling can be present with even some occipital–subtentorial herniation and brainstem compression
  • Histologic changes are strongly definitive
  • Meningoencephalomyelitis with neuronal degeneration, gliosis, perivascular and neuroparenchymal infiltrates, and meningitis are highly suggestive for this disease group
  • The gliosis is extremely widespread, especially in the cortex extending into the corona radiata with cells composed primarily of nondegenerate neutrophils
  • Lesions can be observed in the heart and antigen has been found

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

  • Treatment is supportive and should be aimed at metabolic maintenance and care and prophylaxis of self-induced trauma
  • No specific treatment will reduce morbidity and mortality. Prognosis for EEE survival extremely poor
  • Prognosis for any recumbent and comatose horse extremely poor with death imminent

Nursing Care!!navigator!!

Many horses, if they survive, have residual neurologic deficits—the ethics of nursing these cases should be discussed with owners.

Client Education!!navigator!!

Appropriate vaccination and vector control.

Medications ⬆ ⬇

MEDICATIONS

Drug(s) of Choice

  • Fluid and metabolic support can be useful
  • No specific drug treatment is likely to alter morbidity or mortality
  • Early treatment with corticosteroids—considered anecdotally to lead to improvement in less severe cases

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Regular detailed neurologic examinations in horses demonstrating fever, inappetence, and depression during peak seasons.

Prevention/Avoidance!!navigator!!

  • Strict mosquito control and vaccination can prevent both human and equine cases
  • All equine cases must be reported to state health officials

Possible Complications!!navigator!!

Self-induced trauma may be severe.

Expected Course and Prognosis!!navigator!!

  • Complete recoveries from the neurologic deficits associated with EEEV are reported, but they are rare with mortality ranging from 85% to 100%
  • Animals that have recovered from EEEV often have residual neurologic deficits that commonly include clumsiness, depression, and abnormal behavior
  • Neurologic sequelae are similar but less common in horses that recover from WEEV and the mortality rate is similar to WNV at 20–40%
  • Depending on the VEEV strain mortality can be quite variable spanning from 40% to 80%
  • Reinfection is possible and survivors should be vaccinated

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Zoonotic Potential!!navigator!!

  • Horses do become viremic with EEEV and WEEV, but not high enough to transmit to feeding mosquitoes. Hence, they are dead-end hosts
  • Brain and cranial spinal tissues of horses with EEEV and VEEV are laden with virus. Extreme caution and rabies protocol for necropsy required
  • Horses with epizootic VEE have sufficient circulating viral concentrations and are the principal reservoirs
  • Blood and ocular and nasal secretions from infected horses contain high concentrations of VEEV and personal protection equipment should be incorporated while treating equines during epizootics

Abbreviations!!navigator!!

  • EEE = Eastern equine encephalomyelitis
  • EEEV = Eastern equine encephalomyelitis virus
  • ELISA = enzyme-linked immunosorbent assay
  • MDV = Maradiaga virus
  • VEE = Venezuelan equine encephalomyelitis
  • VEEV = Venezuelan equine encephalomyelitis virus
  • WEE = Western equine encephalomyelitis
  • WEEV = Western equine encephalomyelitis virus
  • WNV = West Nile virus

Internet Resources!!navigator!!

International Veterinary Information Service. http://www.ivis.org/advances/Carter/toc.asp

Suggested Reading ⬆ ⬇

Del Piero F, Wilkins PA, Dubovi EJ, et al. Immunohistochemical, and virologic findings of Eastern equine encephalomyelitis in two horses. Vet Pathol 2001;38:451–456.

Author(s) ⬆

Author: Maureen T. Long

Consulting Editor: Caroline N. Hahn

Acknowledgment: The author acknowledges the prior contribution Caroline N. Hahn.