A combination of factors has led to the appearance of West Nile Virus in the Sacramento region.
Six mosquito samples taken in Sacramento County have tested positive for the virus, and 10 infected dead birds have been reported, according to the Sacramento-Yolo Vector Control District.
So far no incidences of West Nile Virus have been reported in Yolo County, but officials are warning residents to take precautions to protect themselves from mosquito bites that could transmit the disease.
“The intense mosquito season has started and West Nile virus is clearly active in our area,” said district manager David Brown in a written statement last week. “Once we see a few positive mosquito samples, the virus tends to amplify quickly and it becomes even more important for residents to take this seriously and do everything they can to protect themselves.“
No human cases have been reported locally in 2008, but 21 human cases were reported in Sacramento County and one human case in Yolo County were reported in 2007.
UC Davis entomologist William Riesen said it was somewhat unusual for the disease to appear so early in the season.
“This is not remarkably early, but this is the early part of the summer,” he said. “Typically we see the peak in late July and August.“
Research has shown that high temperatures are known to cause the virus to spread more rapidly. Riesen said warmer weather in June probably caused an amplification of the disease sooner than expected, but other factors are having a significant influence as well.
“I think it’s due to rice cultivation,” he said. “The Sacramento area is surrounded partially by rice culture, and the rice is now going in and they’re flooding the fields, which produces a big peak of rural mosquito.“
West Nile virus is transmitted by two types of mosquitoes, rural and urban. The rural type, culex tarsalis, is biologically more susceptible to the disease, and vector officials say they have been seeing more of it in urban areas.
Riesen thinks the housing crisis may also be to blame.
“The collapse of the housing market has led to people walking away from homes, and there are a lot of unmaintained swimming pools that become breeding grounds for rural mosquitoes,” he said.
Officials are advising residents to follow guidelines to prevent the spread of vector-borne illnesses, including paying attention to things like unattended swimming pools.
“Eliminating all sources of standing water and using an effective mosquito repellent continue to be key in order to protect yourself,” Brown said.
People should also avoid being outdoors at dawn and dusk, peak mosquito activity times. Wearing long sleeves and pants when outside at these times is advisable, and ensuring door and window screens are in working condition will ensure that the insects cannot get indoors.
JEREMY OGUL can be reached at city@californiaaggie.com.
