To date, there are well over 300 human West Nile virus (WNV) cases in the US and California and Arizona lead the pack in the number of cases.

Culex mosquito
Culex quinquefasciatus

California, which has reported the most WNV cases in the country six times since the mosquito borne virus was reported in the state for the first time in 2002, leads the country as of late August, according to data reported by the California Department of Public Health (CDPH).

During the past week, 26 additional cases have been added to the annual tally bringing the state total to 83 from 19 counties. Butte County saw an additional six cases bringing that total to 36, highest in the state.

Two WNV-related fatalities have been reported in to CDPH from San Bernardino (1) and Nevada (1).

In neighboring Arizona, 53 human WNV cases have been reported to date with eight out of 10 cases reported from Maricopa County alone (42). Maricopa County is also battling a St. Louis Encephalitis (SLE) outbreak at the same time.

Related: West Nile virus season is upon us, how bad could it get?

West Nile virus (WNV) is a mosquito-borne disease that was originally found in Africa. In 1999, it was detected in the eastern United States; since then the virus has spread throughout the United States and is well established in most states.

WNV affects the central nervous system. However, symptoms vary:

Serious symptoms in a few people. Less than one percent of individuals (about 1 in 150 people) infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. WNV infection can be fatal.

Milder symptoms in some people. Up to 20 percent of the people (about 1 in 5) who become infected will display symptoms which can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms generally last for just a few days, although even previously healthy people have been sick for several weeks.

No symptoms in most people. Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms.

People over the age of 50 have a higher chance of getting sick and are more likely to develop serious symptoms when infected with WNV.

There is no specific treatment for WNV infection.

Robert Herriman is a microbiologist and the Editor-in-Chief of Outbreak News Today

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