Few US physicians have ever seen a person with the numbing aches, fever and chills of malaria. Once endemic throughout most of the United States, the disease was brought under almost complete control by the late 1940s, thanks to new drugs and insecticides and four decades of intense mosquito control programs. Nevertheless, the World Health Organization now estimates that worldwide, nearly half a billion persons are sickened each year, and 2.7 million die.
Plasmodium living in a red blood cell; the one at the upper left is younger. Malaria is caused by protozoans of the genus Plasmodium that live and multiply inside blood and other cells of the circulatory system. It is carried by any of several Anopheles mosquito vectors. The disease has been eradicated from the US, but the vector mosquitoes still occur in all the areas where malaria was once present. Increasing resistance of mosquito populations to insecticides and of Plasmodium to antimalarial drugs will make future control efforts much more difficult than they were in decades past.
Anopheles freeborni was the most important malaria vector in California while the disease was endemic. Populations of this mosquito occur in Marin and Sonoma counties, but they are especially common throughout the rice growing areas of the Central Valley.
Surveillance programs aim at keeping malaria from becoming reestablished. Each year, the Centers for Disease Control registers more than a thousand imported cases diagnosed in tourists and foreign visitors from endemic areas outside the US. Infections can last for months or even years, and may be passed to local Anopheles mosquitos. Although other, similar Plasmodium species occur in wild California birds and reptiles, they are not infective to man.
Sporadic, locally-acquired cases occur every year, and leave no doubt that malaria could easily regain a foothold. Sixteen times, locally acquired malaria has been reported from California. Each time, swift response by health officials and the mosquito abatement districts has prevented the disease from becoming reestablished.
One large outbreak occurred in Nevada County, where Anopheles freeborni around a Lake Vira campground bit a marine recently back from Asia and over several months transmitted the infection to more than 35 girl scouts and other campers. The girls had already returned to their homes scattered throughout California and Nevada before their infections began to be diagnosed. An undocumented laborer was the probable source of a similar outbreak in San Diego County that affected 30 migrant farm workers and local residents before it was stopped; Anopheles hermsi was the carrier.
Two hurricanes that battered western Mexico in 1997 were followed by a dramatic rise in malaria along the Pacific coast. Health officials for the state of Oaxaca recorded 10,755 cases by August 15, up from 586 the year before.
Helpful references. Changing paterns of autochthonous malaria transmission in the United States: a review of recent outbreaks. by Jane R. Zucker. Emerging Infectious Diseases. Volume 2:37-43, 1996. There is also an Internet on-line version. Malaria, the submerged disease. by Piero Olliaro et al., Journal of the American Medical Association. Volume 275:230-233, 1996. Resurgence of a deadly disease. by Ellen Ruppel Schell. The Atlantic Monthly, pages 45-60, August 1997.
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