With West Nile Virus having claimed four lives since summer, California, the most populous state in the United States, has been put on high health alert. (File Photo)

    LOS ANGELES, Aug. 3 (Xinhua) -- With West Nile Virus having claimed four lives since summer, California, the most populous state in the United States, has been put on high health alert.

    According to the state Department of Public Health, 56 people statewide have caught the virus this year, including four who died.

    Statewide, 423 birds and 317 mosquito pools have tested positive for the virus this year.

    In the latest development, two crows and some mosquitoes collected in the San Fernando Valley in central California tested positive for the virus, according to the Greater Los Angeles County Vector Control District.

    So far this year, seven mosquito pools and six crows in Los Angeles County have tested positive for the virus.

    California Governor Arnold Schwarzenegger has declared a state of emergency in three counties hit hardest by the Virus.

    The number of people infected this year with West Nile Virus has been three times higher than the same time last year, Schwarzenegger said in a statement e-mailed to Xinhua.

    The state of emergency, applied to Kern, Colusa and San Joaquin counties, was intended to prevent the spread of this mosquito-borne disease, said the governor.

    "My proclamation makes financial assistance available to the local vector control districts and directs state agencies to take proactive measures to protect Californians from further spread of this deadly virus," said the governor.

    Since taking office, Schwarzenegger has invested more than 15 million dollars to fight the West Nile Virus.

    California has one of the most comprehensive West Nile Virus surveillance and control systems in the United States. The state deploys surveillance and detection technology to track specific areas of West Nile Virus activity and alert local agencies so they can target their mosquito control activities.

    Despite those efforts to eradicate West Nile Virus, the virus remains a threat, and further efforts to control the spread of the virus and to reduce and minimize the risk of infection are needed, said the governor.

    "This summer presents classical environmental and ecological conditions for a resurgence of this virus," said Minoo Madon, scientific-technical services director for the District.

    "The prevailing warm weather pattern coupled with lack of rainfall, the potential for increased mosquito activity, and the recovery of bird species such as crows can contribute to increased transmission in the natural disease cycle of West Nile virus."

    People can avoid catching West Nile by wearing long-sleeve shirts and pants outdoors at dawn and dusk, and can slow the spread of the virus by ensuring their swimming pools, spas and ponds are properly maintained.

    West Nile is spread to humans from the bites of infected mosquitoes, which catch the disease by biting birds that carry the virus. The virus cannot be spread by person-to-person contact or directly from birds to people, according to health officials.

    In most cases, people who catch the virus either do not become sick or experience mild symptoms, including fever, headache, nausea and body aches.

    But there is no treatment for West Nile virus, which can be fatal in extreme cases.