West Nile virus has an interesting transmission cycle because humans are not its primary host. The virus circulates naturally between birds and mosquitoes. Certain mosquitoes become infected when they feed on an infected bird. An infected mosquito can later transmit the virus when it bites a person or another animal. This is also why reports of infected mosquitoes and dead birds matter. They can reveal increasing viral activity before large numbers of human illnesses appear.

California monitors West Nile virus through several sources, including mosquito testing, dead bird testing, sentinel chickens, horses, and reported human infections. Together, these provide public health officials with a picture of how actively the virus is circulating.

One of the more surprising things about West Nile virus is how differently people can respond to the same infection. About 80 percent of people infected with West Nile virus develop no symptoms at all. Only one in five develops what is often called West Nile fever. Symptoms may include fever, headaches, body aches, fatigue, nausea, vomiting, and sometimes a rash. Most people with this form of illness recover. The much smaller group that concerns physicians is the group that develops neuroinvasive disease, meaning the virus affects the brain, spinal cord, or surrounding tissues.

Fewer than one percent of infected individuals develop serious neurologic disease, but when this occurs, the consequences can be significant. West Nile virus can cause meningitis, an inflammation of the tissues surrounding the brain and spinal cord, or encephalitis, inflammation of the brain itself. It can also affect the nerves and spinal cord and produce profound muscle weakness or paralysis. Some patients develop an illness resembling acute flaccid paralysis, and severe neurologic disease can require prolonged hospitalization and rehabilitation. Symptoms such as severe headache, neck stiffness, confusion, unusual sleepiness, tremors, significant muscle weakness, or paralysis warrant prompt medical attention.

Anyone can become infected, but the likelihood of developing severe disease increases with age. People with weakened immune systems and certain underlying medical conditions are also at greater risk. This distinction is important. Being older does not necessarily mean someone is more likely to be bitten by an infected mosquito. Rather, if infection occurs, the person's risk of progressing to serious neurologic disease is greater. For families caring for older parents or medically vulnerable relatives, mosquito protection therefore deserves particular attention during periods of high West Nile activity.

West Nile virus activity varies considerably from year to year. The number of mosquitoes, environmental conditions, bird populations, mosquito species, and immunity to the virus among birds can all influence how much virus circulates during a particular season.

This year, California's surveillance system has detected unusually high activity. The percentage of tested dead birds and mosquito samples that are positive for West Nile virus has exceeded recent five year averages. That does not mean everyone bitten by a mosquito will be exposed to West Nile virus. It does tell public health officials that the virus is circulating more actively and that prevention is particularly important this season.

Unlike some infectious diseases, there is currently no vaccine available to prevent West Nile virus infection in people. There is also no specific antiviral medication that eliminates the virus. Treatment is primarily supportive and depends upon the severity of the illness. That makes prevention especially important.

The mosquitoes that transmit West Nile virus in California are generally most active around dawn and dusk. Using an EPA registered insect repellent, wearing clothing that reduces exposed skin when practical, and maintaining intact window and door screens can reduce bites. Standing water also matters because mosquitoes need water to reproduce. Even relatively small containers around a home can become breeding sites. Emptying water from flowerpots, buckets, containers, and other places where water collects helps reduce mosquito populations around the home.

If you find a dead crow, jay, raven, magpie, sparrow, finch, hawk, or other bird, there is another way you can help. California uses reports of dead birds as part of its West Nile surveillance program. Certain birds are particularly susceptible to the virus, and an increase in infected birds can provide an early indication that West Nile virus is circulating in an area. Residents can report dead birds through California's West Nile virus surveillance program. Those reports help public health officials determine where additional investigation and mosquito surveillance may be needed.

West Nile virus is already established in Southern California, so the goal is not alarm. It is awareness. This year's surveillance is telling us that the virus is circulating more actively than it has in recent years, at the same time that California is entering the months when West Nile transmission is typically highest.

Most infections will never cause illness. Serious neurologic disease remains uncommon. But because the consequences can be severe for the small percentage of people who develop it, particularly older adults and people with weakened immune systems, preventing mosquito bites remains worthwhile. Sometimes public health surveillance gives us an opportunity to act before illness becomes widespread. West Nile virus activity in California this summer is a good example.