A mosquito believed to be the most efficient carrier of the West Nile virus (WNV) to human populations is lurking about Bermuda.

The Department of Health issued a warning yesterday that the Asian Tiger Mosquito - which has been present in Bermuda since 2000 - was recently found to be carrying the dangerous virus in Maryland.

Over the two years the mosquitoes have been in Bermuda "these aggressive ankle-biters have have become widely distributed through the Island," it said.

The mosquitoes are most prominent in Southampton and Warwick according to government data but Pest Control Supervison David Kendall said they've been spotted throughout the central parishes.

While Mr. Kendall said the risk Bermuda's Asian Tiger mosquitoes are carrying the sometimes deadly virus is low, the foothold the virus has achieved across the United States should serve as a potent warning - it could happen here.

According for the US Center for Disease Control (CDC), the continued expansion of the virus shows it is now permanently established in the Western Hemisphere after first appearing in New York City just three years ago. The latest CDC counts for this year list 251confirmed and probable cases in humans and 11 deaths to date in the US.

The virus poses a deadly threat to exposed humans only rarely - less than one percent of those bitten by an infected mosquito will become seriously ill - but persons over age 50 tend to be most at risk.

It is transmitted to humans when an infected mosquito blood-feeds and multiplies inside the infected individual.

In its less serious form it can cause fever, head and body aches and, occasionally, a body rash and swollen lymph glands.

Symptoms will develop three to 15 days after infection.

A more severe infection will show itself through higher fevers, neck stiffness, stupor, disorientation, coma, convulsions and muscle weakness.

But WNV infection becomes most lethal when it crosses the blood-brain barrier and begins to swell the brain tissue (encephalitis) and interfere with the work of the central nervous system.

The situation has now reached epidemic status in the US and the CDC field teams have been deployed to the three hardest hit states - Lousiana, Mississippi and Arkansas. Louisiana alone has recorded 147 cases and eight deaths.

And scientists believe casualties of the epidemic will mount if past peak patterns hold true.

If West Nile were to gain a foothold in Bermuda, it might be some time before it was known as the Island lacks facilities to test for the virus.

Mr. Kendall said local mosquitoes would have to be captured, processes and tested overseas as the Island lacks capacity to test for it.

Finding the virus in the Asian Tiger mosquitoes has increased transmission fears as the mosquito has a special taste for the blood of humans, Mr. Kendall said.

"Previously it was found in mosquitoes which preferred birds which kept the virus circulating in the bird population," he said.

The mosquitoes share similar colouring to those which carry dengue and yellow fever (Aedes aegypti).

Both have a distinctive black and white striping, Mr. Kendall but the dengue mosquito's back stripe is hooked while the WNV-carrier's is straight.

The common mosquito (Culex) has a significantly different appearance. It is light brown in colour and whines loudly.

The best defence is a potent offence, Mr. Kendell said and residents are urged to check their homes and properties for collections of standing water - the preferred home of the pest.

"Reduction of the Asian Tiger mosquito population needs to be aimed at removal of standing water, buckets, plant flower pot saucers, barrels, used tires, wading pools, bromeliad plants and other containers that hold water for longer than a week," Government's warning said.

"Because Asian Tiger mosquitoes usually fly no more than 300 yards from the containers where they are hatched, the most effective way of controlling them is to remove or drain all containers where they might breed.

"Removal of containers is encouraged because they tend to fill up in the next rain."

Simply dumping standing water is more effective in killing the mosquitoes than spraying the areas with pesticides.

"They are aquatic and die very quickly once out of water," Mr. Kendall said. "But they spend two-thirds of their life cycle in water and if you spray a container, they will simply hatch a day or two later."

Residents with fish ponds and turtle ponds should add guppies to the system to control for mosquito-larvae, he added.

"In turtle ponds, they should add a protective area for the guppies."

Mr. Kendell said Bermuda is in its seasonal peak for mosquito numbers. "August and September are the worst hit because they have the greater humidity and heat the mosquitoes need to complete their life cycle faster," he explained.

While dawn and dusk hours are the most popular time for the pests to seek their prey.

While insect repellent containing DEET may help, Mr. Kendell said, in the worst-affected neighbourhoods the only relief will come from eliminating homes for the container-breeding mosquitoes.

"If your neighbourhood is exposed to a serious mosquito problem you will know," he said. "You can't go outside and repellent will have no effect. You'll have bumps all over."