CDC: Current West Nile Virus Outbreak is Largest in U.S. History

Recent local outbreaks in Redwood City, Atherton, Menlo Park have contributed to the 693 cases in the US this year, the CDC reports. Two new cases in Half Moon Bay were just reported this week.


A Lesser goldfinch in Redwood City, a , two and a have all been reported carriers of the potentially fatal West Nile virus in the past month and a half. 

And, two new cases have just been reported in Half Moon Bay - a dead mourning dove tested positive on Aug. 9, followed by a dead red-tailed hawk the very next day.

These local instances are part of a larger outbreak happening across the country, including the death of a Michigan woman who is the state’s first human West Nile Virus death this year, according to the Centers for Disease Control (CDC).

“With the virus appearing earlier this year than it has in recent years, we want to remind residents to take the appropriate precautions to avoid getting bitten,” said Dr. Corinne Miller, state epidemiologist with the Michigan Department of Community Health.

Thus far in 2012, a total of 693 cases of West Nile Virus disease in people, including 26 deaths, have been reported to the CDC. Miller said this is the highest number of cases reported to the CDC through the second week in August since 1999, when the virus was first detected in the United States.

San Mateo County has seen a total of nine cases so far in 2012:

  • Red-shouldered hawk picked up June 11 in Woodside (WNV+)
  • Eastern gray squirrel picked up July 3 in Menlo Park (WNV-chronic)
  • Lesser goldfinch picked up July 22 in Redwood City (WNV-chronic)
  • House sparrow picked up July 31 in Atherton (WNV-chronic)
  • Canada Goose picked up July 31 in San Mateo (WNV-chronic)
  • American crow picked up August 2 in Atherton (WNV-chronic)
  • Eastern gray squirrel picked up August 2 in Menlo Park (WNV-chronic)
  • Mourning dove picked up August 9 in Half Moon Bay (WNV+)
  • Red-tailed hawk picked up August 10 in Half Moon Bay (WNV-chronic)

Mosquitoes breed in small collections of stagnant water, are common around people’s homes, and often bite people indoors. While few mosquitoes may be noticed outdoors, those that are present and biting are likely to be the type that potentially carry the virus.

The months of August and September are when most human cases of West Nile occur in California. The end of summer is when mosquitoes are older and more likely to carry the virus. The types of mosquitoes that transmit the virus tend to bite during the evening and nighttime hours.

Most people bitten by an infected mosquito show no symptoms of illness. However, some become sick three to 15 days after exposure. People 50 and older are more susceptible to severe disease symptoms.

San Mateo County residents are encouraged to take the following steps to avoid West Nile Virus:

  • Maintain window and door screening to help keep mosquitoes out of buildings.
  • Empty water from mosquito breeding sites such as buckets, unused kiddie pools, old tires or similar sites where mosquitoes lay eggs.
  • Apply insect repellents that contain the active ingredient DEET, or other EPA-approved repellent to exposed skin or clothing, especially during peak mosquito activity periods such as dusk and dawn.
  • Wear light-colored, long-sleeved shirts and long pants when outdoors.

For more information, visit the state's infectious diseases website at http://www.cdph.ca.gov/programs/idb/Pages/default.aspx or the CDC’s website at www.cdc.gov/ncidod/dvbid/westnile/index.htm.


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