A number of spiders can cause spider bites that are medically important. Almost all spiders produce venom but only a few are 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 ten genera (Atrax, Phoneutria, Loxosceles, Latrodectus, Hadronyche, Macrothele, Missulena, Illawarra, Sicarius, and Hexophthalma) 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. not one in Australia since 1979).
Brazilian wandering spiders
Phoneutria nigriventer, 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. 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 without venom. Venom bites will normally occur if the spider is pressed against something and hurt. Children are more sensitive to the venom of wandering spiders. The spiders often make threatening 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 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.
Australian funnel-web spiders
The Australian funnel-web spiders (family Atracidae), such as the Sydney funnel-web spider Atrax robustus (a mygalomorph spider, not to be confused with the araneomorph funnel-weaver or grass spiders) are regarded as among the most venomous in the world. They react vigorously to threats and, reputedly, will more often attempt to bite than run away. A. robustus, a large black spider, is found within a radius of about 100 km from Sydney. Its venom contains a compound known as δ-atracotoxin which is highly toxic to primates. Approximately 10% of bites lead to serious symptoms for a total of 3–4 severe envenomations annually.
A. robustus is one of five designated species of the genus Atrax (the others being A. christenseni, A. montanus, A. yorkmainorum, and A. sutherlandii). The related genus Hadronyche is represented by 32 other dangerous species in eastern Australia, including Queensland and Tasmania. The males in this case have somewhat more potent venom than females and they also wander, making them more likely to be encountered in summer. Bites by males of two large species, the Sydney funnel-web and northern tree funnel-web, have resulted in death. One specific case denoted an individual being bitten through the heel of his leather footwear after provoking the spider, indicating the great strength of the spider's chelicerae. One other genus in the family Macrothelidae has been reported to cause severe symptoms in humans. Severe bites have been attributed to members of the genus Macrothele in Taiwan, but no fatalities. In other mammals, such as rodents, for example, the effects of funnel web spider venom are much less severe.
Tangle-web spiders One genus of the tangle web spiders has venom which is known to be medically significant. This genus, the widow spiders of genus Latrodectus, has caused human fatalities. The other genus, Steatoda, the false widow spiders, have bites that can cause pain and erythema but only around 30% of bites lead to systemic symptoms.
Widow spiders
The widow spiders (genus Latrodectus), such as the black widow, redback spider, and katipō are spiders that carry a neurotoxic venom which can cause a set of symptoms known as latrodectism. Widow spiders are large, shiny house spiders with relatively spindly legs and deep, globular abdomens. Mature females have dark and shiny abdomens with one or several red spots, either above and/or below. The spots may take the form of an hourglass, or two triangles, point-to-point. They seldom leave their webs, but if they have to, they can run fast over short distances. Male widows and immature females may have a variety of streaks and spots on a browner, less globular abdomen. The males are generally too small to be dangerous. Widows tend to be non-aggressive. Some species, especially those in North America, have no spot on their backs, with the hourglass becoming visible only when the spider hangs upside-down, and can thus be mistaken for their less dangerous cousins, which is especially true in males. The Australian red-back and North American widow species live near humans and bites are frequent, numbering thousands yearly. The venom produces very painful effects including muscle spasms, 'tetanus-like' contractions, nausea and vomiting, and severe generalized pain. A serious bite will often require a short hospital stay to control pain. Children may be less sensitive to severe effects of redback venom. Still, children as well as elderly and ill individuals are advised to seek medical attention. Fatalities were reported as high as 10% of cases in the early-20th century but deaths have not been reported in the United States for decades and only 0.5% of those bitten have had major medical complications. The last record of a red-back fatality in Australia was in 1955.
False black widows
… excerpt ends here. Continue reading the full article.
