Equine Encephalitis Virus

Family Togaviridae

Small, lipid- and protein-enveloped RNA viruses. The family includes pathogenic arboviruses (insect-borne virsuses) of the Alphavirus genus.

Genus Alphavirus

Single-stranded, linear, positive-sense RNA viruses, 60-70nm in diameter.

(Surface of an Alphavirus. This image is a computer-generated model of the surface of an alphavirus derived by cryoelectron microscopy. The spike-like structures on the virion surface are trimers composed of heterodimers of the virion surface glycoproteins E1 and E2. These spikes are used by the virus to attach to susceptible animal cells. Sourced from Wikimedia Commons, Copyright of the Centers for Disease Control and Prevention (CDC) Division of Vector-Borne Infectious Diseases, 2007)

Important Serotypes

Several alphavirus strains have been isolated during equine and human epidemics of encephalitis in the Western Hemisphere. These epidemics have most often been attributed to:

• Eastern Equine Encephalitis virus (EEV)
• Western EEV
• Venezuelan EEV

Eastern EEV has North and South American antigenic variants. Western EEV is a recombinant between an Eastern EEV-like virus and a Sindbis-like virus. Western EEV also has two antigenic subtypes - WEE and Highlands J viruses. Considerable overlap exists between the various strains in terms of their geography, and potentially also in their antigenic properties and biological behaviour. Of the 6 subtypes of Venezuelan EEV (I-VI), significant outbreaks of equine encephalitis in the Western Hemisphere over the last two decades have been caused by IAB, IC and IE. Variant ID from Central America and variant IF from Brazil are considered endemic and typically demonstrate low pathogenicity for horses. These features are also typical of subtype II (Everglades) virus in Florida and types II, IV, V ad VI viruses.

Diseases

Equine Togaviral Encephalitis

Epizootiology

Distribution

Virus distribution is largely dictated by vector distribution. EEE virus is found as far north as eastern Canada, south throughout the Caribbean and in parts of Central and South America. EEE has been identifed in the Philippines and it may have a presence in Europe. WEE virus is recognised in reservoir avian hosts in the western United States but clinical disease has been rare here in the last 20 years. As well as Mexico, Venezuela and colombia, VEE virus has been recently isolated in Trinidad, French Guiana, Peru and Brazil.

Reservoirs

Togaviridae generally persist by asymptomatically infecting wild animals (sylvatic hosts) such as birds, small mammals and reptiles. Viruses overwinter in sylvatic populations.

Vectors

The major disease vectors for each serotype of veterinary significance are:

• EEE: Aedes spp.
• WEE: Culex tarsalis
• VEE: Culex melanconium, Aedes spp., Phosphora spp.

Culiseta melanura is a vector for EEE. It is largely confined to freshwater swamps, feeds primarily on swamp birds and is rarely found in areas of increased horse density. This mosquito generally serves as a vector for the enzootic cycle involving swamp birds. Aedes spp. are more important in epizootics and epidemics. Culex tarsalis is the primary vector that maintains WEE virus in an enzootic cycle with passerine birds. Dermacentor andersoni ticks, Triatoma sanguisuga (assassin bug), and the cliff swallow bug (Oeciacus vicarius) may also be involved as vectors or overwintering reservoirs fro WEE. Several species of mosquitoes from at least 11 geerna have been determined ot be naturally infected with epidemic strains of VEE virus. Ticks may also be capable of viral trasnmission. Vectors transmit viral particles between sylvatic hosts when takin ga blood meal. If the virus can peentrate the gut of the vector, then it may pass throhg the haemolymph to the oral glands, multiply and subsequntrly be shed in the saliva and other oral secretions. If th eblood meal octnains adequate numbers of viral particles, multiplication may not be rrequired for transmission. It is likely tha the mosquito remians infected for life.