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West Nile Virus Activity

The Butte County Mosquito and Vector Control District’s ongoing surveillance program has detected West Nile virus activity in Butte and Glenn counties to date this year.

  • For local human and equine cases, click here.
  • To see the latest in Butte and Glenn Counties, click here.
  • To see the latest West Nile Virus activity throughout California, click here.

 West Nile Virus Positives 2024

West Nile Positives 2024

What is West Nile virus?

West Nile virus (WNV) is a potentially serious mosquito-borne virus transmitted by infected female mosquitoes. In Butte County Culex tarsalis and Culex pipiens are the common vectors of WNV. WNV is a flavivirus and is one of the most common viruses transmitted in the United States. The virus has been found in Asia, Africa, Eastern Europe, and the Middle East for many years and was first detected in the United States in 1999 in New York City. Since it's arrival it has spread all across the United States. West Nile virus is closely related to St. Louis encephalitis virus also found in the United States. West Nile virus has been found every year in Butte County since it's arrival in 2004 and it appears it will be around for years to come.

How do people get WNV?

Most frequently, WNV is spread by the bite of an infected female mosquito. Adult female mosquitoes are the vectors that become infected when they feed on infected birds. After the mosquito becomes infected they then can spread WNV to humans and other animals they feed on. All other forms of WNV transmission are extremely rare, but it is possible that breast-fed infants, unborn fetuses, and blood and organ recipients could become infected without being bitten by a mosquito.

West Nile transmission cycle


What are the symptoms of WNV?

Most people who are infected with WNV have no symptoms whatsoever. However, of those who become ill, mild symptoms may include fever, head and body aches, nausea, a skin rash on the trunk of the body, and/or swollen lymph nodes. Symptoms of mild illness will generally last a few days. In severe cases, the disease may progress to encephalitis (inflammation of the brain), meningitis (inflammation around the brain), and meningeoncephalitis (inflammation of the brain and membrane). Severe symptoms may include high fever, headache, neck stiffness, disorientation, coma, tremors, and paralysis. Severe symptoms of illness may last several weeks, and neurological effects may be permanent. The time between the bite of an infected mosquito and the onset of illness ranges from 5-15 days in humans.

What is the District doing to prevent the transmission of West Nile virus?

May contain: animal, bow, weapon, insect, invertebrate, and mosquito

The District is actively controlling the mosquito population in Butte County, which will reduce the chances of someone being bitten by a WNV infected mosquito. On a daily basis, the District staff surveys and controls if necessary, acres of agricultural, rural, and residential areas for potential breeding habitats of mosquitoes. The District also provides public education, free home and yard inspections, and the delivery of mosquitofish to interested residents.

The District routinely surveys for mosquito-borne diseases such as West Nile virus, western equine encephalomyelitis, and St. Louis encephalitis. Adult mosquitoes are trapped and tested for these viruses each season. In addition to mosquito testing, seven sentinel chicken flocks are strategically placed throughout Butte County and are tested for infection every two weeks. The District participates in the California Department of Public Health's dead bird surveillance program. This tool provides the opportunity to test important reservoir host species for West Nile virus and is used as an early detection and monitoring tool of virus activity.

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