List of medically significant spider bites

A number of spiders can cause spider bites that are medically important. Almost all spiders produce venom but only a few are classified as "venomous" and able to cause significant harm to humans. Two medically important spider genera have a worldwide distribution — Latrodectus and Loxosceles. Others have a limited distribution.

Medical reports have been criticized for poor evidence. In the last century, both white tailed and wolf spiders were considered medically significant, only to be recanted. Only four genera (Phoneutria, Atrax, Latrodectus, and Loxosceles) are considered medically significant. Bites of these spiders have a range of severity, with only a minority having severe symptoms. Deaths by verified spider bites are exceedingly rare (e.g. none in Australia for 40 years, none in the United States for 50 years).

The Brazilian wandering spider (a ctenid spider) is a large brown spider similar to North American wolf spiders in appearance, although somewhat larger. It has a highly toxic venom and is regarded (along with the Australian funnel-web spiders) as among the most dangerous spiders in the world. Based on one of the few pharmacological studies performed in the 1970s, Phoneutria's venom toxicity was more virulent than both Atrax and Latrodectus. Brazil reports approximately 6 deaths per year.

As their name suggests, Brazilian wandering spiders are active ground hunters. If the spider has a reason to be alarmed, it will bite in order to protect itself, but unless startled or provoked, most bites will be delivered dry (without venom). Venom bites will occur if the spider is pressed against something (such as fingers) and hurt. In this case, the high levels of serotonin contained in the venom, plus at minimum strong chelicera, will contribute to deliver a very painful bite.

Children are more sensitive to the venom of wandering spiders. The spiders often make threat gestures (such as raising up their legs, or hopping sideways on the ground), which might amuse a child to the point of reaching towards the spider. In male humans, bites of this spider may also result in prolonged painful penile erections (priapism). Scientists are attempting to create an erectile dysfunction treatment that can be combined with other medicines out of the peptide that causes this reaction.

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