Mosquitoes with viruses on rise, analysis shows

This summer could be one of the worst in years for mosquito-borne diseases such as West Nile virus and Eastern equine encephalitis.

Monitoring for disease-carrying mosquitoes in Hillsborough County has yielded two to three times more positive tests than in the past three or four years, said Carlos Fernandes, manager of Hillsborough County Mosquito Control.

And the summer onslaught of mosquitoes is just starting.

"We have seen a very active beginning of the season," Fernandes said.

The county does blood tests on chickens caged at different locations, a tool used for early detection of mosquito-borne illnesses.

This year, 15 of the sentinel chickens tested positive in Hillsborough for West Nile virus or Eastern equine encephalitis, Fernandes said.

Normally at this time of year, five to seven chickens test positive.

Fernandes said more infected mosquitoes mean a greater potential for viruses to spread to humans.

However, it is too soon to tell whether the high number of mosquitoes carrying the diseases will continue through the summer.

"Nobody can say if it will subside or get worse," Fernandes said.

The last human case of West Nile virus in Hillsborough was in 2004.

So far, no human cases have surfaced in Florida, but July and August are peak months for spread of the illness.

"We are concerned about human cases," said Jonathan Day, medical entomologist with the University of Florida.

It is uncommon for the disease to spread to humans. Five cases in a year would be a lot.

Symptoms can be severe. Some people have no symptoms or minor symptoms resembling the flu.

If the virus hits the central nervous system, it can cause the brain to swell. In those cases, the disease can be fatal, and survivors suffer serious, permanent brain damage, Day said.

Doctors can only treat the symptoms, and there is no vaccine for humans, though there is for horses.

Horses are more likely to contract Eastern equine encephalitis than humans because they are more frequently exposed to mosquitoes, Day said.

The potential for mosquito-borne diseases in West Central Florida goes beyond Hillsborough, where two horses have tested positive for Eastern equine encephalitis.

Health officials have confirmed one horse in Pasco and three in Polk County have contracted Eastern equine encephalitis. Another horse in Polk is being tested.

The three cases in Polk are more than the average number for a whole summer. Some years, Pasco goes without seeing a single case.

Any appearance of Eastern equine encephalitis spurs immediate action.

"That is something that catches our attention very quickly," Fernandes said.

The Polk cases prompted the county health department to issue an advisory warning to the public that the disease is present.

"Certainly, if horses can be bit, people can be bit, especially if they're out in those rural areas," said Daniel Haight, Polk County Health Department director. "If we get human cases, we've already alerted the emergency rooms to be on the lookout."

Mosquito control spray trucks and aircraft in the three counties blanketed areas where the diseases were found.

Horses and people contract West Nile and equine encephalitis when bitten by mosquitoes that already have fed on a bird infected with one of the viruses. Birds are carriers.

Neither disease spreads from person to person, from horses to people or from either back to mosquitoes, said Dennis Moore, director of the Pasco County Mosquito Control District.

Newly hatched mosquitoes don't spread the disease because they haven't fed on an infected bird.

"The concern is (a mosquito) that's two weeks or so old and already bitten a bird and is looking for a second blood meal," Moore said.

Eastern equine encephalitis also is showing up this summer in counties around the state that rarely see any cases during an entire year.

The disease is appearing farther south than its normal hotbed in north Florida and the Panhandle.

Of the 33 confirmed cases of Eastern equine encephalitis in Florida, most are farther down the peninsula than usually seen, including five in Collier and Miami-Dade counties.

"Normally, we'd expect to see none in some of those places," Day said.

The cold winter, especially in North Florida, may be responsible for the southern shift, Day said.

Cold weather - 80 days when the temperature didn't reach 60 degrees - kept mosquitoes from flying, and winter migrating birds that carry the disease from Caribbean islands and Central and South America spent more time in southern Florida, Day said.

After a wet spring, mosquito populations dropped off in May and early June, but numbers are building.

"A lot of areas that haven't been wet are wet now. There will definitely be more flying around," said Pasco County's Moore.

West Nile virus is not as serious a health threat to humans as Eastern equine encephalitis. Most people who catch it survive.

West Nile's arrival in Florida in 2001 concerned health officials because no one knew its severity and it was new to the state.

But 2003 turned out to be the worst year, with about 100 human cases, Day said.

He believes the hurricanes of 2004 and 2005 disrupted breeding areas for birds that serve as carriers.

Another disease mosquitoes can carry has emerged in Florida after being eradicated in the state 40 years ago.

So far, a dozen people in Key West have contracted dengue fever, a disease common worldwide in the tropics but one that vanished from Florida four decades back, Fernandes said.

"Suddenly, there are cases. That's scary," he said.

Unlike West Nile or equine encephalitis, dengue, also known as break-bone fever, is transmitted person to person. Mosquitoes biting someone with the disease can transmit it to someone else.

"If somebody with dengue went from Key West to Miami, it could be a major problem," Day said.