What follows West Nile virus?

As the long, hot summer draws to a close in coming weeks, so will the mildest season yet for West Nile virus infection since the dreaded disease came to our region in 2004.

Thanks to hard-hitting mosquito-control efforts, heightened public awareness and increased natural immunity in the population, only 132 West Nile cases have been reported in humans statewide so far this year. Just four cases have been confirmed in Sacramento County, two in Placer County and none has occurred in neighboring foothills counties.

That's a big drop from the 880 human cases reported in California during the peak year of 2005, the year that Sacramento County led the nation with a case count of 177.

But don't celebrate just yet. Experts in mosquito-borne diseases tell us that West Nile may just be the first of many exotic infections coming to a stagnant water pool near you.

You may never have heard of these diseases, but you can bet that they are at the forefront of the minds of local mosquito-control officials. As the global economy makes our world ever smaller, they know that the need to remain vigilant against newly introduced infections spread by mosquitoes has never been greater.

The disease getting the most buzz among mosquito experts today is Chikungunya, an acute illness that causes a fever, skin rash and incapacitating pain in the joints. In Swahili, the name Chikungunya means "to walk bent over," a reference to the swollen, distorted joints that can persist for weeks among those suffering from the infection.

Chikungunya is caused by a virus that first was isolated in Tanzania in 1953. Until recently, it was a relatively uncommon disease, confined to certain tropical areas of Africa and South Asia.

But suddenly, starting in 2005, Chikungunya exploded onto several islands of the Indian Ocean, where it never had been seen before. By early 2006, it had spread to La Reunion, an island owned by France, where as many as 40,000 people each week became ill and were unable to work or attend school. In all, nearly one-third of the population was afflicted, and more than 250 people died.

From these islands, and from India, where the disease also hit in 2006, the infection was imported easily into Europe. By October 2007, more than 190 locally acquired cases were identified in certain Italian cities along the Adriatic Sea.

The infection was able to take hold in Italy because that country now hosts the Asian tiger mosquito, the same species responsible for the Chikungunya epidemics on Indian Ocean islands. This aggressive, day-biting mosquito can tolerate the colder winters of temperate climates, like those of its home in Japan and Korea. In recent years, the mosquito has spread through much of the world thanks to international shipping.

Since its arrival in the southern United States in 1983, the Asian tiger mosquito has become an enduring pest, broadening its range to 26 states. Mosquito controllers in these states, which stretch from Texas to New York, are making preparations for an invasion of Chikungunya, as well as other diseases transmittable by this hardy mosquito.

In California, the Asian tiger mosquito so far has been caught before it has established itself here as a permanent resident, but one day our luck may run out. We need to stay alert, with an unwavering commitment to mosquito control. Such control may sometimes require aerial spraying as well as other biological and chemical control methods.

The declining incidence of West Nile disease proves that communities can successfully organize against new mosquito-borne threats. In a world full of emerging disease risks, the biggest danger is complacency.

- Jason Eberhart-Phillips, M.D., is the El Dorado County health officer. He can be reached at jeberhart-phillips@edcgov.us.