Limited interdecadal variation in mosquito (Diptera: Culicidae) and avian host competence for Western equine encephalomyelitis virus (Togaviridae: Alphavirus).

The research article investigates the competence of mosquitoes and birds as carriers for the Western equine encephalomyelitis virus (WEEV) over six decades. Despite evolutionary changes, the research found little change in these species’ ability to transmit the virus, which couldn’t explain the recent absence of human and animal related virus cases.

Background

• Western equine encephalomyelitis virus (WEEV) historically caused large-scale outbreaks in both humans and horses across the Americas.
• Despite increased surveillance intended to detect the presence of West Nile virus, there have been very few reports of horse or human cases of WEEV, and little recorded evidence of the virus circulating among animals and insects (also known as enzootic activity).
• In the past three years, some WEEV activity has been detected in California, but it has not been associated with any cases in horses or humans.

Research Purpose and Methods

• The researchers aimed to study whether the virus’ ability to infect hosts (host competence) and its ability to be transmitted by mosquitoes (vector competence) has changed over time.
• They compared virus samples taken during each decade over the past 60 years.
• They used white-crowned sparrows, house sparrows, and mosquitoes of the species Culex tarsalis Coquillett to test whether the virus’ infectivity and transmissibility have changed.

Findings

• The results indicated that there has been little variation over time in the virus’ virulence, or harmfulness, in birds.
• Furthermore, the competence of Cx. tarsalis mosquitoes as vectors for the virus, or their ability to transmit it, has also shown little change.
• These results suggest that even though some genetic changes over time and across different locations have been documented for the virus, these changes do not seem to have brought about significant changes in the virus’ interactions with its hosts and vectors.

Implications

• The findings could not explain why there have been fewer cases of the disease in recent years.
• Given the limited changes observed in the virus’s virulence and vector competence, other factors must be contributing to the recent decline of WEEV infection in humans and equines.