West Nile Virus is the number one mosquito transmitted disease in the United States. The disease is caused by a mosquito transmitted virus.

West Nile Virus is a mosquito-borne disease that was introduced to North America in 1999 and detected in California in 2003. The affects of this mosquito-borne disease have been significant and costly to humans and wildlife throughout the U.S. including California. Researchers have discovered that many native species of mosquitoes, including some that were not known to vector diseases before, can effectively transmit this disease to humans. In Napa County there are four very common and widespread species of mosquito (the Little House Mosquito, The Foul Water Mosquito, The Tule Mosquito and the Western Encephalitis Mosquito) that are known to be excellent carriers of this disease. Two other species (the Fish Pond Mosquito and Western Treehole Mosquito) are presumed carriers although research is still pending. All of the aforementioned species of mosquitoes are specifically targeted by this District.

• Incubation: period 5 to 15 days.
• Symptoms: Many people show little or no symptoms.Those that become ill can have fever, headache, nausea, body aches and swollen lymph nodes.1 in 150 infect persons may require hospitalization.
• Susceptibility: The elderly are the most susceptible to the virus.
• Diagnosis: By physician using acute phase serum or cerebral spinal fluid.
• Treatment: There is no specific treatment for infection. Supportive care is important (contacting your physician, fluids, electrolytes, managing fever, etc.)

Mosquito vectors: efficient vectors are:

• Little House Mosquito (Culex pipiens)
• Foul Water Mosquito (Culex stigmatosoma)
• Tule Mosquito (Culex erythrothorax)
• Encephalitis Mosquito (Culex tarsalis)

Known Occurrences in Napa County

West Nile Virus was first detected in Napa County birds in the summer of 2004. This virus has been found every year since and is expected to be present in the County for many years to come. Our first human case was diagnosed in 2006 with the second and last case in 2007.

Coordinated surveillance work will continue with neighboring Mosquito Abatement District’s and the California Department of Heath Services, testing birds and mosquito populations for the presence of this virus.

Napa Country West Nile Virus Confirmed Case Data

To download detailed maps of California’s West Nile Virus cases, click the links below:

For more information about this disease go to The California Department of Public Health West Nile Virus Website.