BOSTON -- After waging three days of aerial bombardment with a chemical, Massachusetts officials have declared victory in their war on mosquitoes that spread a rare but deadly strain of encephalitis.

But some environmentalists and a few residents in the spraying zone have questioned the state's use of the pesticide malathion.

The spraying was triggered by an outbreak of Eastern equine encephalitis, a rare virus that attacks the brain and spinal cord when transmitted to humans. About one-third of those who contract the disease die from it, and most of the survivors suffer severe neurological damage. Although horses can be immunized, for humans there is neither vaccine nor cure.

"It strikes fear in the hearts of people, because when you live in a mosquito-infested area, you know how hard it is to prevent bites," said Thomas P. Monath, the chief of virology at the Army's Institute of Infectious Diseases in Frederick, Md.

A 7-year-old boy was hospitalized in Boston Aug. 20 with a 105-degree fever and seizures. The boy, who is expected to survive but who may have permanent damage, has the only confirmed case of the disease in Massachusetts this year, but state officials are conducting laboratory tests in seven other suspected cases.

The only other confirmed case in the United States this year occurred in South Carolina in June, said Ted Tsai, an epidemiologist with the federal Centers for Disease Control. Typically, he said, there are fewer than five cases a year nationwide.

The disease is generally confined to areas near freshwater swamps along the Atlantic and gulf coasts, including a pocket on Maryland's Eastern Shore. In Massachusetts, the area around the Taunton River basin about 20 miles south of Boston has long been known as a source of Eastern equine encephalitis.

Normally, the virus resides in the bloodstream of migratory songbirds and is passed among the birds by one species of mosquito, Culiseta melanura. That species does not bite humans or horses, so the disease does not spread unless other mosquito species bite infected songbirds and later bite horses or people.

The chances of that happening are low and usually occur only when the populations of all mosquitoes are booming because of heavy rain or other favorable conditions. Thus, most areas where Eastern equine encephalitis is endemic report many years with no human cases followed by two or three years of incidents.

Massachusetts, which reported its last human case in 1984, appears to be at the start of another cycle. State health officials, who have monitored mosquitoes and the virus for 34 years, said they found more infected mosquitoes earlier in the season this year than ever.

For that reason, the state hired a Virginia company, K&K Aircraft Inc., to spray malathion over 700,000 acres, from Buzzards Bay to Boston's southern suburbs, excluding downtowns, public drinking supplies and most streams, ponds and lakes.

Most people in the area welcomed the spraying, which occurred early last week. But some, including organic farmers, objected, and state officials acknowledged receiving 30 to 40 complaints of eye, skin and breathing problems linked to the spraying.

In addition, state wildlife agents confirmed an unusual excess of fish die-offs in the sprayed area, including one instance of more than 400,000 killifish dying.

When asked about the fish kills at a news conference, state Agriculture Commissioner Gus Schumacher became indignant. "We're comparing minnows to human lives? Let's be realistic," he said.

Public Health Commissioner David Mulligan told reporters that he believed spraying was called for, despite the "inconvenience" to beekeepers, organic farmers and others. Mulligan said the threat to human health was clear and compelling, and officials said it reduced the mosquito population by nearly 99 percent.

Laurie Antonelli of the Massachusetts Audubon Society argued that the spraying could kill so many natural predators of mosquitoes, such as dragonflies and minnows, that it will lead to worse problems next summer.

Priscilla Chapman, executive director of the New England Sierra Club, said the state based the spraying decision on such spotty data and had so few ways of assessing the long-term impact on the food chain that officials had no basis for declaring success.

"They declared victory, and they have no idea what they have done," she said.