Bites

Most domestic spiders in the US are not substantially venomous to man and does not constitute a threat. Exceptions are the widow spiders and the brown spiders.

The brown recluse spider can be found in many southern states of North America, in some parts of Europe and in Israel. The spider has extended its habitat to include homes and garages and encounters between man and brown recluse spider has become more likely.

The venom from a brown recluse spider is cytotoxic and contains several compounds of which some can cause tissue destruction and hemolysis or abnormal breakdown of red blood cells.

Consequences of envenomations from the bite of the brown recluse spider (please notice that not all bites automatically lead to an envenomation) varies in range from a light itching to death¹. The effect of a bite also depends on the amount of venom injected. In so called dry bites no venom is injected. The ability of the spider to cause an envenomation also depends on its size. According to a study (Vetter and Barger 2002) only around 400 of 2055 brown recluse spiders were estimated as large enough to cause envenomations.

It is a problem that diagnosis of spider bites often are based on evolved symptoms rather than from a spider collected from the incident. This has led to doctors in places like California, where there's no population of the brown recluse spider, to be confronted with medical cases (patients) with alleged brown recluse spider bites.

Dermonecrotic wounds of uncertain origin are often attributed to the brown recluse spider, Loxosceles reclusa².  Seen from both the patients and the spiders perspective this is unsatisfying.

The bite itself is usually not accompanied by any pain and the victim is often unaware of the bite until the bite site becomes red and swollen. In most cases there are no complications and the wound heals within two to three weeks.

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In severe cases a necrotic lesion evolves. The wound will appear bluish with irregular edges. As the venom propagates in the body of the victim it destroys tissue and the wound expands.

In a brazilian study only 1.5% of the investigated 267 cases died from envominations by the brown recluse spider. 4.6 % of patients observed acute renal failure and almost all patients experienced intravascular haemolysis³. Other surveys shows that deaths from the brown recluse spider are extremely rare.

References

¹ Dillaha CJ et al. "North American loxoscelism: necrotic bite of the brown recluse spider." JAMA Vol. 188 page. 153-156. (1964)
² Vetter RS "The Diagnosis of Brown Recluse Spider Bite Is Overused for Dermonecrotic Wounds of Uncertain Etiology" ANNALS OF EMERGENCY MEDICINE, Volume 39 (5) pp. 544-546 (2002)
³ Sezeerino et al. TRANSACTIONS OFTHE ROYAL SOCIETY OFTROPICAL MEDICINE AND HYGIENE, Vol 92 page 546-548 (1998)