West Nile Virus
Weekly Public Health Update & Advisory
June 20,
2013
New WNV
Positives found in Lomita, Rolling Hills Estates, Rancho Palos Verdes, and
Torrance
This
is the first 2013 weekly West Nile virus (WNV) public health update and
advisory for all cities and areas within the Los Angeles County West Vector
& Vector-Borne Disease Control District.
Several dead American crows tested positive for WNV in the City of
Lomita (5), Rolling Hills Estates (3) and Rancho Palos Verdes (1). Additionally, mosquitoes caught in an adult
trap at the Exxon/Mobil in Torrance
also tested positive (1).
What does it mean?
There
were 479 human cases and 19 deaths in 31 counties throughout California for
2012. These were the highest numbers
recorded since 2005 when 805 human cases and 19 deaths were recorded. For 2012,
Los Angeles County recorded 174 human cases with 6 fatalities. These were the highest numbers recorded since
2004 (309 human cases and 13 deaths) when WNV first appeared in California.
Nationally for 2012, 48 states reported West Nile virus infections in people,
birds, or mosquitoes with a total of 5,387 human cases, including 243 deaths
making it the most active season since 2003.
Since,
WNV is endemic (constantly present) throughout Los Angeles County, California
and the United States, positives at any given site do not necessarily indicate
a specific focus in a particular area for the disease.
As
the spring and summer months heat up, mosquitoes can go from an egg to an adult
in only 7 days. A single female mosquito can produce up to 200 female eggs in a
week (Week 1), and their 200 off-springs produce 40,000 females in another week
(Week 2), and together those 40,000 mosquitoes can produce 8 million biting mosquitoes
by the end of week three (Week 3). During the cooler winter months, the
mosquito breeding cycle slows and can take as long as 3-4 weeks to go from an
egg to an adult so the probability of infection is reduced.
A
wide variety of wild birds, including the American crow, are the reservoir for
West Nile virus. Mosquitoes become
infected when they first bite an infected bird and then transfer the virus to
other birds and humans as they bite again.
The virus is more active (virulent) in wild birds when the weather is
hotter and conversely less active in birds during the cooler months.
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