Three straight years of warm winters and wet summers have primed South Florida for what health officials say could be the worst onslaught of encephalitis-carrying mosquitoes the state has ever experienced.
While the state called off long-standing epidemic alerts for St. Louis encephalitis in 12 counties on Monday, officials warned that the potentially fatal disease may return in even greater force this summer when the next generation of mosquitoes hatches.
"The amount of (encephalitis) virus circulating in nature has increased every year for the past three years," said Flora Wellings, director of the state epidemiology research center in Tampa. "We have to be concerned."
St. Louis encephalitis causes a brain infection that can be fatal in some cases. The disease is spread to people by mosquitoes that have bitten infected birds and other small animals.
The only confirmed case of St. Louis encephalitis in the state last year occurred in October in Lee County. Health officials credited intensive efforts to eradicate mosquitoes for preventing the spread of encephalitis to people.
The last statewide encephalitis epidemic in Florida was in 1977 when 110 people got the disease and eight died. Palm Beach County experienced a limited outbreak of encephalitis in 1979 with five reported cases.
Encephalitis is most prevalent in the late summer and early fall when night- biting mosquitoes are common. The spread of the disease usually is halted in winter when cold weather eliminates the insects.
But several years of unusually warm winters have allowed the mosquitoes to thrive and to spread the encephalitis virus to animals who continue to carry the disease into the next mosquito season, Wellings said.
"We've had long summers, a lot of rain, a lot of mosquitoes hatching out," she said.
In the past, encephalitis had been most common in the southwestern part of the state, said Linda McCaig, an epidemiologist with the Florida Department of Health and Rehabilitative Services. "We're seeing a trend of the virus moving northward and to the east coast," she said.
Both Dade and Palm Beach were among the counties that were under epidemic alerts for encephalitis this winter.
Wellings said the encephalitis virus was as prevalent in the state this past season as it was during the 1977 epidemic, but mosquito control efforts prevented its spread to humans.
Because the mosquitoes frequently bite birds, health departments around the state keep small flocks of "sentinel chickens" out of doors and test them regularly for encephalitis to provide an early warning of the presence of the disease.
"Had we not had the sentinel chickens, we would have had epidemics this year," Wellings said.
Wellings said that because of the threat posed by encephalitis, surveillance by health departments will have to be increased this summer. Instead of putting flocks of sentinel chickens outdoors in mid-June, as is usually done, the flocks will have to be in place by the beginning of the month, she said.