Spider Bites, Confused
Special to the Montana Medical Association and CA.
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 ho use 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 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.
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
Help in spider identification and control is available at your local Extension office where
you also could pick up the free MontGuide 9210 "Spider Identification and
Management." Handling costs $2.95 if the MontGuide is ordered from MSU Extension
Publications, 115 Culbertson Hall, Bozeman, Mont. 59717. Medical professionals can
review spider related information on the Virtual Medical Center, a computer bulletin
board at (406) 994-2564. (Carol Flaherty Montana State University)
Spider Bites: Does Your Doctor Know?
A survey of recent literature concerning spider bites shows that they are of minimal
medical importance. This is especially true in Montana. Of the 60 spiders that can cause
medical complications in humans only two exist in Montana, the Tegenaria agrestis or
Aggressive House spider (Hobo spider) and the Latrodectism or Black Widow spider.
Contrary to popular belief and the knee jerk reaction of many professionals the Brown
Recluse spider does not exist in Montana or a state bordering Montana. Your Doctor
The fear of Brown recluse spiders does exist in Montana because spider bite diagnosis is
very difficult and the Brown Recluse is a popular culprit. This fear exists because rarely
is the spider blamed for the bite seen or recovered. It is essential for medical and
medicolegal reasons that professionals be absolutely sure of their diagnosis.
The basic facts about the physical ability of spiders to bite humans places them near the
bottom of the list. The ability of most spiders to bite a human could be compared to our
ability to bite a plaster board wall in a house. The angles of a spiders mouth are all
wrong. 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 none of the above has occurred a doctor should state “possible
arthropod envenomation, vector unknown” in the medical record.
Of the spiders that do occur in the Pacific Northwest the Aggressive House spider is a
common biter. Aptly named since this spider is easily provoked and unlike the Black
Widow (which will die trying to escape a human) the aggressive house spider will act in
an aggressive manner when crowded. This characteristic is sometimes attributed to the
non-sticky nature of the aggressive house spiders web. Prey captured in the web are more
able to extract themselves, thus the aggressive house spider must attack the entangled
prey in a timely manner or lose it.
The Black widow spider’s venom is the most serious of the two. The venom of the Black
Widow is a neurotoxin, so it affects muscle coordinate and nerve endings. The good side
of the Black Widow is that they are not aggressive. For example, Black Widow bites and
others occur as a last act of revenge, when the spider is trapped in a piece of clothing and
being crushed by its use.
Control of spiders is best accomplished by reducing suitable environments. Spiders look
for nooks and crannies to hide in, irregular surfaces allow them to attach webs and offer
daytime hiding places to those spiders which prefer night time activities. Poorly
maintained window and door seals allow the spiders wandering around a building’s
foundation to gain access. Chemical control of spiders is possible but the cost and risks
of introducing pesticides into the living environment make chemical control a less
effective control measure.
Assistance in identification and control of spiders is available from your local Extension
office. Medical professionals can review spider related information through the Virtual
Medical Center, a computer bulletin board at (406) 994-2564.
Categories: Spider Bites, Brown Recluse, Hobo Spider, Black Widow