Home Contra Costa County Old Mosquito-Borne Illness Returns to California

Old Mosquito-Borne Illness Returns to California

by ECT
The Contra Costa Mosquito & Vector Contra District announced a disease that disappeared from California for 12 years is back in California. Mosquitoes in Fresno tested positive for St. Louis encephalitis in June of this year. In Kern County, mosquitoes tested positive for the disease in late 2016 and one person died.
St. Louis encephalitis and its viral relative, Western equine encephalitis were common in California in the 1940s-1960s. In fact, the District’s original mosquito surveillance plan was designed because of these diseases.
“Our surveillance program existed to detect St. Louis encephalitis and Western equine encephalitis and so we were already looking for mosquito-borne diseases when West Nile virus came along,” said the District’s Scientific Programs Manager Steve Schutz, Ph.D. “We strengthened our program due to West Nile virus, but we still look for St. Louis and Western equine encephalitis as well as other potential new mosquito-borne diseases that could arrive in our county at any time.”
The last time District employees found Western equine encephalitis within Contra Costa County was in 1997.
Schutz says the data suggests St. Louis encephalitis and Western equine encephalitis were truly gone statewide from 2003 until 2015, when St. Louis encephalitis started to reappear in Southern California. Genetic testing of the current strain shows it was likely reintroduced by birds migrating from South America. Although these viruses seemingly went away for a while, the California native mosquito species that transmit it did not.
Did you know?
  • California has 53 different species of mosquitoes.
  • There are 23 different species in Contra Costa County alone.
  • Each species differs by:
    • Flight ranges.
    • Locations.
    • Preferences of water type to lay their eggs.
    • Preferences of animals or humans to bite.
    • Diseases they can transmit.
Some species have the ability to transmit more than one disease. For example, the two types of mosquitoes known to transmit West Nile virus in Contra Costa County can also transmit St. Louis and Western equine encephalitis.
So what does all of this mean to you?
It means you have to keep doing what you’re doing.
  • Dump out standing water.
  • Avoid being outside at dawn and dusk to avoid mosquitoes.
  • Report dead birds.
  • Defend yourself by wearing repellent when mosquitoes are present.
Keep up the good work. Mosquito control is in your hands and our hands.
Information provided by the Contra Costa Mosquito & Vector Control District

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