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It's bad enough that the health risk to the public from potential Eastern Equine Encephalitis and West Nile viruses has been increased to high in four area communities as a result of findings of insect-transmitted disease.

Now a new and more aggressive type of mosquito that might survive in the area into November is being checked as a potential to spread the viruses.

Ochlerotatus japonicus, a mosquito that first appeared in the United States in 1998, is a prolific biter that attacks victims indoors as well as outdoors but has been associated with few disease cases so far. But Wayne Andrews, director of the Bristol County Mosquito Control Project, says health investigators are trapping "aggressively" to look for examples of the species that may be infected with either virus.

The common house mosquito, culex pipieus, is the insect most often associated with the spread of EEE or West Nile viruses according to state Department of Public Health reports. But Ochlerotatus has been blamed in at least two cases - a fatal human West Nile infection in Fall River and a horse infected with EEE in Wrentham in 2005.

Not only is Ochlerotatus a persistent pest, it's also a hardy one. The insect, which commonly breeds in rock pools, is present in the New England area all season and adults have been found into late November according to a paper on the insect by Rutgers University. The risk from mosquito-borne viruses was officially raised this week in Mansfield, Easton, Raynham and Brockton. A blue jay collected from Mansfield Aug. 9 tested positive for West Nile Virus. And mosquitoes carrying EEE have been found in Seekonk.

Andrews said Ochlerotatus is well-known in several parts of the world but was not discovered in the United States until recently. Some theorize the bugs immigrated to North America in the treads of tires imported from Asia.

Local residents can protect themselves from the more aggressive insects in much the same way recommended for the more common variety - by wearing insect repellent along with long-sleeved garments, Andrews said. Since breeding insects are attracted to pails of water, old tires and other places where water can accumulate, its advisable to empty or remove any sources of stagnant water.