Montana State University |
Montana State University |
Updated 04/01/02 BOZEMAN -- Again this year, the bite of the aggressive house spider in Montana is being confused with that of the brown recluse spider, according to Montana State University-Bozeman Extension agents asked to identify spiders.
Distinguishing between the two is important for treatment and to prevent future bites.
"Brown recluse spiders are not native to Montana or any state adjacent to Montana, though they could be present in lumber shipped in from milder climates," says Will Lanier, MSU-Bozeman Extension insect diagnostician. "However, the aggressive house spider is native to the area and causes an ulcerating bite similar to the recluse."
The aggressive house spider tends to climb downward. It is usually found on ground or basement levels, often entering through windows. Vacuuming lower levels can eliminate nests, but if this is done the bag should be disposed of so spiders cannot escape. Worn or damaged window and door seals, where spiders can enter, should be repaired. In addition, firewood should be inspected for spiders and egg-sacks before being brought into the home, and wood piles should be kept away from the house. Chemical control of spiders is possible, but both cost and risks of pesticides in the home make chemical control less suitable than other measures.
Though the literature suggests that spider bites are of minimal medical importance, the bite of the aggressive house spider may require prolonged treatment and leave a scar, says Robert Flaherty, a Bozeman physician.
Symptoms:The aggressive house spider's bite initially produces a slight prickling sensation. A small insensitive hard area appears within 30 minutes, surrounded by an expanding reddened area of two to six inches in diameter. Between 15 and 35 hours after the bite, the area blisters. About 24 hours later, the blisters break and the wound oozes. It scabs over, but tissues beneath the scab continue to die. Surgical repair is sometimes needed. The most common symptom in addition to the bite is a migraine headache.
The fully developed lesion can vary from one-half to one-inch or more in diameter and may take several months to heal.
Treatment: Aggressive house spider bites generally require only local wound cleaning, topical antibiotics and tetanus prophylaxis. Skin grafting is rarely necessary.
In contrast, treatment of brown recluse bites involves cleaning the wound, applying topical antibiotics and tetanus prophylaxis. Topical corticosteroids are not helpful, but systemic corticosteroids and dapsone may be helpful. Skin grafting is sometimes necessary.
Diagnosis: Diagnosis is difficult because the spider is rarely seen or recovered. If a spider is not captured in the act or vicinity, all evidence is circumstantial. The resulting effects of the bite should be carefully compared to those associated with systemic arachnidism. If unsure, a doctor should state "possible arthropod envenomation, vector unknown" in the medical record.
Of 60 spiders that can cause medical complications in humans, only two exist in Montana: the aggressive house spider (Tegenaria agrestis) and the black widow spider (Latrodectism), says Lanier.
The aggressive house spider is a common biter. It is easily provoked and will be aggressive when crowded. This may be due to the fact that its web is not sticky, so it must attack the entangled prey or lose it. Drawing of aggressive house spider.
The black widow spider's venom is the more serious. It is a neurotoxin, so it affects muscle coordination and nerve endings. However, the black widow spider is not aggressive. Black widow bites often occur when a spider is trapped in clothing and being crushed.
(See also "Symptoms, Diagnosis and Treatment of Aggressive House Spider Bite compared to Brown Recluse" and "Spider Identification and Management.") Help in spider identification and control is available at your local Montana State University Extension office where you also could pick up the free MontGuide 9210 "Spider Identification and Management." There is a charge for handling if the MontGuide is ordered through MSU Extension Publications, PO Box 172040, Bozeman, MT 59717; credit card orders phone (406) 994-3273..
Send questions or comments to Carol Flaherty, MSU Communications Services, Bozeman, MT 59717 or email her at carolf@montana.edu You are the 704127th person to access this page.
