High season for mosquito-borne diseases begins
Buy repellent and wear long sleeves: The heavy rain last month is expected to yield a large crop of mosquitoes this week because the downpours saturated freshwater wetlands that had dried out in the summer heat.
"We'll get a pretty good brood of mosquitoes from it," said Walter Montgomery, director of the Northeast Mosquito Control and Wetlands Management District, one of three mosquito control agencies in this region.
The good news is that most of those mosquitoes will be of the type Aedes vexan - not the Culex, which is a carrier for 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, another mosquito-borne virus, in the state, according to the Department of Public Health. This year, there have been 18 findings of West Nile in mosquito samples, 10 in dead birds. Eleven of the mosquito findings have come from samples collected in the last week of July, including two in Worcester and Springfield, and one each in Watertown, Merrimac, Boston, Raynham, Abington, Chicopee, and Barnstable. West Nile was also discovered in crows in Dedham and Haverhill.
"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 that the type of mosquito that carries West Nile virus also changes its feeding behavior in the latter part of the summer: from biting birds to biting humans and other mammals.
"The bottom line is that you can get West Nile any time," 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."
The areas most affected will be those near freshwater wetlands, according to David Henley, superintendent for the East Middlesex Mosquito Control Project, which serves 25 communities in this area. He said he expects the number of mosquitoes to be highest near river flood plains such as those on the Concord, Sudbury, or Ipswich rivers, or the Great Meadows National Wildlife Refuge.
"We want people to protect themselves, no matter what part of the season it is," said Thomas Carbone, Andover's director of public health. "We worry more about West Nile and EEE in August than in June, but we want people to be in the habit of protecting themselves. We want them to look around their backyards to make sure there's no standing water in kids' sand pails, gutters, bird baths, or an extra tire in the backyard. We want them to remember to wear mosquito repellent, and, if they can stand it, to wear long sleeves and long pants."
Findings of West Nile in mosquitoes appeared three weeks earlier this year than in 2007, an indication that there would be a greater risk to humans in August and September. Henley said that 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 naïve bird population," he said. "Some birds have developed a resistance to West Nile virus."
Henley said that, in an ironic way, the presence of the nuisance mosquitoes might protect humans from the disease-carrying vectors, by making them more vigilant. Reading health services administrator Larry Ramdin agreed.
"One of the unfortunate things about the public health profession is that when there's no crisis and we're doing our job well, no one knows we exist," he said. "People are not seeing the services because the protection is there. When we have a nuisance and people are getting bitten, they're more likely to respond to our messages about taking precautions."
While West Nile is contracted throughout the summer and fall, Eastern equine encephalitis, a virus that can be fatal, is usually contracted in the latter part of August through 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, humans.
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 from EEE in the state from 2004 to 2006.
"We hope for the best, and worry about the worst," DeMaria said.
People are also asked to call the Department of Public Health (1-866-MASS-WNV) to report dead birds. More information is available at www.mass.gov/dph/wnv.
Peter Mirandi, director of public health in Danvers and chairman of the board of commissioners for the Northeast Mosquito Control and Wetlands Management District, said that people have become more vigilant: "I think people are tuned in to mosquito as a vector of disease," he said. "They used to be more tuned in to mosquito as nuisance."
