The brown recluse spider is not aggressive and only bites if something attacks it, presses it against its skin, or threatens its livelihood somehow. Luckily, brown recluse spiders are more peaceful than most people believe, and almost all bites on humans occur when the spider is caught up against the skin. Bites occur periodically as it likes hiding in shoes and clothes.
Bites vary in severity from almost no symptoms at all to very severe systemic reactions, such as renal (kidney) failure and death1. Unfortunately, there is currently no effective therapy for brown recluse bites as of 20112.
No conclusive tests that can identify bites from the brown recluse spider are available,3 - and because Loxoscelism can look like many serious types of pathologies, clinicians should be careful in their diagnosis of spider bites. It is a problem that diagnoses of spider bites are mostly based on symptoms rather than the specimen collected after the bite.
Most domestic spiders in the U.S. are not substantially venomous and are not dangerous, but the widow spiders and the brown spiders are exceptions.
The venom from a brown recluse spider is cytotoxic and contains several compounds, some of which some can cause tissue destruction and hemolysis or abnormal breakdown of red blood cells.
Consequences of envenomations from bites of the brown recluse spider (please notice that not all bites automatically lead to the injection of venom into a person) vary greatly1. The effect of a bite also depends on the volume of venom, and the ability of a spider to inject its venom depends on its size. According to a study (Vetter and Barger 2002) only 400 of 2,055 brown recluse spiders were large enough to penetrate human skin and inject venom.
Skin wounds of uncertain origin are often attributed as brown recluse spider bite wounds2.
The bite itself usually does not come with any pain, and the victim is often unaware of the bite until the bite site becomes itchy, red, and swollen. In most cases, there are no complications and the wound heals within two to three weeks.
In severe cases a necrotic lesion evolves, and 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 envenomations by the brown recluse spider; 4.6% of patients observed acute renal failure, and almost all patients experienced intravascular haemolysis. Other surveys show that deaths from the brown recluse spider are extremely rare.
1 Dillaha CJ et al. "North American loxoscelism: necrotic bite of
the brown recluse spider." JAMA Vol. 188 pp. 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 of the Royal Society of Tropical Medicine and Hygiene, Vol 92 page 546-548 (1998)