January 14 (HealthDay News) -- A resistance to insecticides appears to be why bed bugs are making a comeback in some urban areas, a new study says.

Bed bugs in New York City, where infestations have grown in recent years, appear to have developed nerve cell mutations that weaken the effect of the pyrethroid toxins, such as deltamethrin, commonly used against them, according to a report in the current issue of the Journal of Medical Entomology. Such nervous system poisons normally would paralyze and kill the nocturnal blood suckers.

Toxicologists at the University of Massachusetts Amherst and Korea's Seoul National University found that the New York City bed bugs are now as much as 264 times more resistant to deltamethrin than an easier-to-kill type of bed bug found in Florida.

"This type of pyrethroid resistance is common in many pest insects, and the failure of the pyrethroids to control bed bug populations across the United States and elsewhere indicates that resistance is already widespread," senior researcher John Clark said in a University of Massachusetts news release.

The resistance appeared to be from the bed bugs' having an insensitive nervous system, rather than their ability to better breakdown the poisons in their system, the researchers said.

While bed bugs feed on the blood of their host about every five to 10 days, they are not believed to spread disease.

SOURCE: University of Massachusetts Amherst, news release, Jan. 8, 2009