Western Equine Encephalitis Virus (WEEV)

Western Equine Encephalitis Virus (WEEV) is an arthropod-borne virus belonging to the genus Alphavirus in the family Togaviridae. It is the causative agent of Western equine encephalitis (WEE), a disease that affects humans, horses, and certain bird species. The virus is transmitted primarily through mosquito vectors and has historically been responsible for significant outbreaks in North and South America. Although rare in recent decades, WEEV remains an important subject of study due to its zoonotic potential and historical impact.

Taxonomy and Classification

• Family: Togaviridae
• Genus: Alphavirus
• Species: Western equine encephalitis virus

WEEV is a member of the New World alphaviruses, along with Eastern Equine Encephalitis Virus (EEEV) and Venezuelan Equine Encephalitis Virus (VEEV), all of which are capable of causing encephalitic illness in humans and animals.

Structure and Genome

WEEV is an enveloped virus with icosahedral symmetry and a diameter of about 70 nm. Its genome consists of a single-stranded, positive-sense RNA of approximately 11.5 kilobases.

• Non-structural proteins (nsP1–nsP4): Involved in viral replication.
• Structural proteins (C, E1, E2, E3, 6K): Form the capsid and glycoprotein envelope, mediating host cell entry.
• Envelope glycoproteins (E1, E2): Play a key role in receptor binding and membrane fusion during infection.

Transmission Cycle

The virus maintains a zoonotic cycle involving birds and mosquitoes, with horses and humans serving as incidental hosts.

• Reservoir Hosts: Passerine birds are the primary amplifying hosts.
• Vectors: Mosquito species, particularly Culex tarsalis, serve as principal vectors.
• Dead-End Hosts: Humans and horses typically do not develop sufficient viraemia to sustain transmission.

Epidemiology

• Geographical Distribution: Historically reported across western North America, especially in the United States, Canada, and parts of Central and South America.
• Outbreaks: Major epidemics occurred in the 1930s and 1940s, affecting thousands of people and equines.
• Current Status: Human cases are now extremely rare, partly due to vector control and ecological changes. However, sporadic cases and low-level enzootic circulation persist.

Clinical Manifestations

The disease varies in severity between species and individuals.
In Humans:

• Most infections are asymptomatic or mild, presenting with fever, headache, and malaise.
• Severe cases involve encephalitis, with symptoms such as seizures, disorientation, and coma.
• Neurological sequelae, including cognitive and motor impairments, may persist in survivors.
• Case-fatality rate is around 3–7%, with higher risk in infants and elderly individuals.

In Horses:

• Symptoms include fever, lethargy, impaired coordination, paralysis, and seizures.
• Mortality rates can be high, sometimes exceeding 20–30% in severe outbreaks.

Diagnosis

• Laboratory Methods: Diagnosis is based on serological assays such as ELISA and plaque reduction neutralisation tests (PRNT).
• Molecular Detection: Reverse transcriptase–polymerase chain reaction (RT-PCR) identifies viral RNA in clinical or vector samples.
• Differential Diagnosis: Important to distinguish WEE from other encephalitic arboviruses such as EEEV, VEEV, and West Nile virus.

Prevention and Control

• Vector Control: Mosquito population reduction through larvicides, insecticides, and habitat management.
• Vaccination:
  • Equine Vaccine: Inactivated vaccines are widely used in horses, significantly reducing disease incidence.
  • Human Vaccine: No licensed human vaccine is available, although experimental vaccines have been developed for laboratory and military use.
• Protective Measures: Use of mosquito repellents, bed nets, and avoidance of outdoor exposure during peak vector activity.

Research and Significance

WEEV is studied both for its role as a zoonotic pathogen and as a model for alphavirus biology. Research areas include:

• Viral pathogenesis and neurotropism.
• Development of improved vaccines and antiviral therapies.
• Studies on viral evolution, as WEEV is a natural recombinant of Sindbis virus and Eastern equine encephalitis virus.
• Use in laboratory models for neuroinvasive viral infections.