Signalment:  
Gross Description:  
Morphologic Diagnosis:  
Lab Results:  
Condition:  
Contributor Comment:  
Eastern equine encephalitis is an alphavirus in the togavirus family that causes encephalitis in both humans and horses. The reservoir host is wild birds, where virus replicates to sufficiently high titers to facilitate vector transmission of the disease. Mosquitoes serve as the biological vector for Eastern equine encephalitis. In contrast to birds, horses and humans are dead-end hosts since a sufficient viremia to allow transmission does not occur. Infected horses often present with fever, anorexia, and lethargy that ultimately progresses to a range of neurological signs that include paresis, seizures, paralysis, and death. Mortality due to Eastern equine encephalitis is quite high, often approaching 90%.
Eastern equine encephalitis is sporadically seen in Tennessee, primarily in western portion of the state during the months of August and September. The horse in this case was euthanized in late March due to the infection, and defies a simple explanation since the biological vector would not yet be available. Iatrogenic transmission has been suspected in another recent case of EEE and administration of a killed vaccine several days prior to onset of clinical signs warrants consideration in this case. Additionally, the rapid clinical progression and the severity of inflammation seen in the brain could reflect introduction of a much larger inoculum than would be seen in association with normal vector-borne disease.
JPC Diagnosis:  
Conference Comment:  
EEE, WEE, and VEE are caused by related but distinct alphaviruses. EEE and VEE are lethal in approximately 90% of cases, whereas WEE is less virulent with approximately 40% mortality in the horse. In endemically infected areas, EEE and WEE are maintained by a wild bird-mosquito (reservoir-vector) cycle, particularly in swampy or tropical areas. Avian reservoirs maintain sufficient viremia to permit infection of mosquitoes. The infection of domestic animals and humans occurs with the movement of virus from swampy areas carried by reservoirs, vectors, or both. Culiseta and Culex sp. of mosquitoes are most important in maintaining endemic infections.
References:
2. Franklin RP, Kinde H, Jay MT, Kramer LD, Green EN, Chiles RE, Ostlund E, Husted S, Smith J, Parker MD: Eastern equine encephalomyelitis virus infection in a horse from California. Emerg Inf Dis 8:283-288, 2002
3. George, LW: Diseases producing cortical signs. In: Large Animal Internal Medicine, ed. Smith, BP, 2nd ed., pp. 1018-1021. Mosby, St. Louis, MO, 1996
4. Hahn CN, Mayhew IG, Mackay RJ: Disease of multiple or unknown sites. In: Equine Medicine and Surgery, eds. Colahan PT, Mayhew IG, Merritt AM, Moore JN, 5th ed., vol. 1, pp. 884-888. Mosby, St. Louis, MO, 1999
5. Maxie MG, Youssef S: Nervous system. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 4th ed., vol. 2, pp. 423-424. Elsevier Limited, St. Louis, MO, 2007
6. Rakich, PM, Latimer KS: Cytology. In: Duncan & Prasses Veterinary Laboratory Medicine Clinical Pathology, eds. Latimer KS, Mahaffey EA, Prasse KW, 4th ed., p. 322, Blackwell Publishing, Ames, IA, 2003
7. Summers BA, Cummings JF, de Lahunta A: Inflammatory diseases of the central nervous system, In: Veterinary Neuropathology, eds. Summers BA, Cummings JF, de Lahunta A, pp.144-146. Mosby, St. Louis, MO, 1995