Widow spiders belong to the genus Latrodectus and include the black widow spider (Latrodectus mactans mactans) in the United States. The term widow spider is used because not all species in the genus Latrodectus are black. Other widow spiders in North America include the brown widow (Latrodectus geometricus), the red-legged widow (Latrodectus bishopi), Latrodectus variolus, and Latrodectus hesperus. The redback spider (Latrodectus hasselti) is endemic to Australia. Latrodectus mactans tredecimguttatus and Latrodectus pallidus are found in Europe and South America, and the button spider (Latrodectus indistinctus) is found in South Africa.
The adult female black widow spider is approximately 2 cm in length and shiny black with a red-orange hourglass or spot on the ventral abdomen. The male is much smaller, brown, and incapable of envenomating humans. Juvenile females are also brown but have the general body morphology of the adult. Males and juveniles have a pale hourglass shape, similar to adult females. The female sometimes eats the male during or after copulation. Webs are irregular, low-lying, and commonly seen in garages, barns, outhouses, and foliage. Other widow spiders are generally black but may have red spots, such as Latrodectus mactans tredecimguttatus, or a dorsal red stripe, such as the redback spider. Latrodectus geometricus is brown with red and yellow markings.
Latrodectus mactans is shown in the images below.
Envenomation is an uncommon occurrence with an extremely variable presentation. Treatment of envenomation often is based on speculation and anecdote, and much of the literature is contradictory. This article attempts to keep recommendations in agreement with the most current standards of care. This article serves as a guideline, and the clinician should use judgment for individual patient encounters.
See Arthropod Envenomation: From Benign Bites to Serious Stings and Venomous Spider Bites: Keys to Diagnosis and Treatment, Critical Images slideshows, for help identifying and treating various envenomations.
Alpha-latrotoxin causes the toxic effects observed in humans by opening cation channels (including calcium channels) presynaptically, causing increased release of multiple neurotransmitters. This results in excess stimulation of motor endplates with resultant clinical manifestations. Clinically, the predominant effects are neurological and autonomic, in contrast to the dermonecrotic local effects associated with spiders causing necrotic arachnidism (eg, brown spiders [Loxosceles species]).
According to the 2014 Annual Report of the American Association of Poison Control Centers’ National Poison Data System (NPDS), approximately 1700 widow spider bites were reported for 2014,  although this figure is probably conservative because of underreporting. No deaths caused by widow spider envenomation have been reported to the AAPCC since its first annual report in 1983.  Deaths after black widow spider bites were reported in 2001,  2003,  and 2006 in Spain, Greece, and Albania (2 deaths), respectively. 
The vast majority of patients with widow spider envenomations recover fully.
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