Poisonous Spider Bites Are Rare In Washington

Despite scary movies like ``Arachnophobia,'' only a few of the world's thousands of known spider species can kill or seriously poison humans, says spider expert Rod Crawford.

Two of the species live in this state, but confirmed bites from them are rare.

The hobo spider, Tegenaria agrestis, is the only spider common to Western Washington whose bite is poisonous to people, says Crawford. Spread throughout the Northwest, the hobo is a species of European house spider.

Although hobo-spider bites are relatively rare and deaths from them rarer still, a person died from one last year in Spokane, says Darwin K. Vest, an Idaho toxin researcher who has worked with Crawford and whose specialty is poisonous stings and bites.

It's important to get medical attention if a hobo bite is suspected, says Vest. About 5 percent of untreated hobo bites are fatal, he says.

A hobo's bite may cause blistering and a skin ulceration that can take several months to heal. In about half of the cases, the victim can be expected

to develop systemic illness, with such symptoms as visual disturbances, muscle weakness, severe

headaches and disorientation.

If a hobo bite is suspected, the offending spider should be captured, if possible, for identification by a spider expert.

Lab tests show the male hobo spider's bite is far more toxic than the female's, Vest says. However, it's difficult to tell either the male or female hobo from the other two species of European house spider found here. All are brown with gray markings and have conspicuously hairy legs when seen against a light background. Experts can distinguish a male hobo from similar male spiders by sex-organ differences.

Often, the hobo has a herringbone stripe in brown, gray and tan on the abdomen, or rear, section of the body. The hobo has a leg span of 1/2 inch to 1 1/2 inches - larger than Tegenaria domestica, but smaller than Tegenaria gigantea, the other two local European house spiders.

In the past, hobo-spider bites have sometimes been blamed on the brown recluse, another poisonous spider, but Vest's research revealed the real culprit a few years ago. Crawford says the brown recluse does not live in Washington.

If you live in a large Washington city, the spider you find in your house is more likely to be Tegenaria gigantea rather than the hobo, says Crawford. Tegenaria gigantea help control the hobo population by eating them.

You're more likely to find a hobo in a small town or farming community, he says. In the forest, however, you'll find native Washington species, not European house spiders.

A hobo's bite usually occurs when the spider is pressed against the skin and can't escape, Crawford says. These spiders are most often found in crawl spaces under houses, or in piles of firewood, lumber or debris. They may also be found indoors, behind furniture and appliances or anywhere not regularly cleaned. Screening vents and sealing any gaps or cracks in the floor or walls can help keep these spiders out of the house.

Crawford and Vest advise wearing long sleeves tucked into gloves when working in crawl spaces or other spider habitats. A hooded jacket and rubber bands over pant legs and sleeves help, too.

Another poisonous spider, the black widow, Latrodectus mactans, is abundant in Eastern Washington, although it's not a native. This spider does not occur in Western Washington, except for rare colonies in the San Juan Islands, Crawford says.

The untreated bite of a female black widow is lethal in about 5 percent of cases, says Vest. The female black widow has a glossy black body with reddish markings underneath.

Severe muscle spasms, beginning in the abdomen and spreading to other large muscles, are the chief symptom of a black-widow bite. Nearly every bite victim suffers this painful reaction to some degree, says Vest, but effective medical treatment is available.

Crawford believes the male - which looks nothing like the female - is not poisonous to humans, although he's unaware of any laboratory tests done to confirm this.

Crawford has heard of no confirmed black-widow bites in this state except for one involving a man who brought a spider with him from another state. Some bites may occur but go unidentified, he says.

Another spider, harmless Steatoda grossa, is often mistaken for a black widow, says Crawford.