Fertile turf for skeeters

Disease risk hits peak season

Mosquito samplings in Malden and Merrimac recently tested positive for West Nile virus, which can be fatal in humans. (David L. Ryan/Globe Staff)

Use repellent and wear long sleeves. The heavy rain this summer is expected to yield a large crop of mosquitoes this week because the downpours have saturated freshwater wetlands.

"We'll get a pretty good brood of mosquitoes from it," said Northeast Mosquito Control and Wetlands Management District supervisor Walter Montgomery, who expects particularly heavy activity in developed areas with significant freshwater wetlands, such as Georgetown, Boxford, and Rowley.

The good news is that most of those mosquitoes will be of the common Aedes vexan type and not the Culex, a carrier of the potentially fatal West Nile virus. The bad news is that the chance of catching the virus increases in August and September.

So far in 2008, there have been no reports of humans infected by West Nile virus or Eastern equine encephalitis in the state, according to the Department of Public Health. Last week, the department reported two positive tests for West Nile in nonhumans locally, a mosquito sampling in Malden and a crow in Haverhill. Another mosquito sampling tested positive Monday in Merrimac.

"The virus gets amplified in nature, in birds and mosquitoes, so there are more and more opportunities for birds and mosquitoes to get infected as the summer goes along," said Dr. Alfred DeMaria, the state's director of communicable disease control, who noted the type of mosquito that carries West Nile virus also changes its feeding behavior in the latter part of the summer, moving from biting birds to biting humans and other mammals.

"The bottom line is that you can get West Nile anytime," he said. "We want to prevent every case. Regardless of the rain, regardless of mosquito numbers, people have to take precautions because you can be just unlucky enough that the mosquito that does bite you happens to have West Nile."

"We're in the high season for West Nile, I would say, especially in the more urban areas," said Christopher Webb, director of public health in Malden. "You see it a little bit more in the western part of the state earlier in the season, because there are more wetlands and a winter thaw that takes longer to go, so water stands longer. Around here, it's typically near the flood plains and marsh areas."

The infected mosquito in Malden was near the Rumney Marsh area, said Webb. There were no findings of the virus in the city in 2007, but there was a death from West Nile in 2003, he said.

Peter Mirandi, director of public health in Danvers, said he views West Nile as endemic to the area. That's one reason that when dead crows or blue jays are reported in town, they are no longer sent to the Department of Public Health to be tested, but their location is marked on a map.

"If we start to see clusters, we rethink our mosquito trappings," said Mirandi, who also is chairman of the board of directors for the Northeast Mosquito Control and Wetlands Management District, which covers Essex County plus Winthrop and Revere.

Findings of West Nile in mosquitoes appeared three weeks earlier this year than last year, an indication that there would be a greater risk to humans in August and September. David Henley, superintendent for the East Middlesex Mosquito Control Project, which includes Malden among its 25 communities, said this is the fourth year since 2000 that West Nile has been discovered in late June or early July. Two of those years - 2002 and 2003 - were peak years for West Nile virus, with a total of 40 human cases.

"However, we really don't know enough about it, because in 2002 and 2003 it was new, so we had a naive bird population," Henley said. "Some birds have developed a resistance to West Nile virus."

Eastern equine encephalitis, a virus that can be fatal, is usually contracted in the latter part of August until late fall, when the first frosts kill off mosquitoes. Birds are the source of infection for mosquitoes, which can transmit the infection to horses, other animals, and, in rare cases, people.

There were six human cases of West Nile in Massachusetts last year, including people in Arlington and Medford. There were no reported cases of EEE, but there were three cases in New Hampshire. There were 13 cases and six fatalities in the state from 2004 to 2006.

EEE generally appears in three-year clusters and then not for several years, with an average of 13 years between clusters in Massachusetts, DeMaria said. There have been positive tests in this region for four years. Montgomery noted that a recent cluster in upstate New York has been active for five years.

"We hope for the best, and worry about the worst," DeMaria said.

For up-to-date information, go to mass.gov/dph/wnv.