A new pest is on the rise in Lancaster County.

It's aggressive, fond of human blood and known to carry the deadly West Nile virus.

It's the Asian tiger mosquito, an invasive species that showed up in the county in larger numbers this year than ever before.

"There now seems to be a gradual trend that these things are gaining more of a foothold in the state," said Matt Mercer, Lancaster County's West Nile virus program coordinator.

As part of the program to monitor and control local mosquito populations, a team of workers every year sets up traps all over the county to capture mosquitoes to test for the presence of West Nile virus.

From 2003 through 2007, Mercer said, those traps captured a total of eight Asian tiger mosquitoes, which get their name from the white bands on their black legs and abdomens.

This year, the traps captured 98.

None of the Asian tiger mosquitoes captured locally this year was found to carry the virus.

People in urban areas should be particularly concerned about Asian tiger mosquitoes, according to Mercer, because such places are prime breeding grounds for the biting pests.

"Besides being aggressive, daytime biters, the other notorious thing regarding the species is they don't need much of a breeding area to populate," he said. "Even as little as some water in a discarded soda can they can use.

"Any kind of standing water is a potential breeding ground for these critters, which is why they are often found in urban areas."

According to a 2001 article posted on National Geographic's Web site, Asian tiger mosquitoes accidentally were introduced to the United States in the 1980s.

One of the first places the mosquitoes are believed to have been found is in Houston in 1985.

The article states that some mosquitoes hatched from eggs believed to have been transported to Houston from Asia in old tires bound for recycling.

In about a year, the article states, Asian tiger mosquitoes were found in Jacksonville, Fla., and within the next eight years they had spread to every county in Florida.

Since the mosquitoes are now known to exist throughout the southeast United States, Mercer believes they have been, and likely will continue to be, swept north to states such as Pennsylvania by hurricanes and other tropical storms.

Mercer talked about the Asian tiger mosquitoes Wednesday morning when he asked the Lancaster County commissioners to approve the county's annual application to the state Department of Environmental Protection for the West Nile virus program funding.

In 2009, the program is scheduled to receive $108,235 — down from the $123,029 awarded this year — for mosquito-population monitoring and for efforts to control their numbers by treating likely breeding areas with substances that kill mosquito larvae.

West Nile virus can be transmitted to humans by mosquitoes, which pick it up from infected animals — usually birds.

Symptoms are flulike, including fever, headache, body aches, swollen lymph glands and, occasionally, a rash.

Severe cases may cause tremors, high fever, convulsions, muscle weakness, paralysis, coma, disorientation and even death.

Across Pennsylvania this year, 14 cases of West Nile virus in humans were confirmed by the state Department of Health. None was in Lancaster County.

A 76-year-old Philadelphia man died in September after contracting the virus.

There were only two cases in the state in which the virus was found in horses, and both of those occurred in Lancaster County — one in Fulton Township and one in the Leola area.

According to Mercer, the horse in Fulton Township was euthanized because of the virus.

In addition to the horses, the virus was found here in 10 mosquitoes collected in 2008 — three of which were trapped at the Lancaster Stockyards property in the city.

The finding of the virus was one of the reasons Mayor Rick Gray this past summer declared the site a public safety hazard and ordered the owners of the property to clean it up.

E-mail: preilly@lnpnews.com