Effect of temperature on the transmission of western equine encephalomyelitis and St. Louis encephalitis viruses by Culex tarsalis (Diptera: Culicidae).

The research article investigates how varying temperatures impact the transmission rates of two viruses, western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE), by Culex tarsalis, a type of mosquito.

Extrinsic Incubation Rate and Transmission Thresholds

• The extrinsic incubation rate indicates the speed with which a virus can be spread by a carrier, in this case, the Culex tarsalis mosquito. The researchers found that this rate increased linearly as the incubation temperature rose between 10 to 30 degrees Celsius.
• The zero transmission thresholds, the temperatures below which the viruses can’t be transmitted, were determined to be 10.9 degrees Celsius for WEE and 14.9 degrees Celsius for SLE.
• The number of degree days (a measurement indicating how much and for how long the temperature was above a certain level) necessary for median transmission was found to be 67.6 for WEE and 115.2 for SLE.

Viral Titer and Transmission Rates

• WEE viral titer, which is the concentration of the virus in the mosquito’s body, did not change significantly throughout all incubation temperatures. However, the transmission rates dropped significantly after 7-10 days of infection when mosquitoes were kept at 20-30 degrees Celsius.
• For the SLE virus, barriers in the midgut and the salivary gland of the mosquitoes limited transmission rates at all temperatures. However, unlike WEE, the transmission rates did not decrease noticeably with time, which suggests that there was no virus modulation – a process where a virus changes its behavior or characteristics in response to the environment or host defenses.

Extrinsic Incubation Period and Potential Transmission Season

• Using degree-day models, the researchers estimated the changes in the extrinsic incubation period for both WEE and SLE across months using estimated mosquito temperatures, derived from a combination of nocturnal air and diurnal resting site temperatures.
• It was found that temperatures in the San Joaquin Valley averaged 5 degrees Celsius cooler than in the Coachella Valley, impacing the length of the potential transmission season for the two viruses: the season for WEE was shortened from 10 to 8 months and for SLE from 8 to 5 months.