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Bite caused by a spider
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A spider bite, also known as arachnidism, is an injury resulting from the bite of a spider. The effects of most bites are not serious. Most bites result in mild symptoms around the area of the bite. Most spiders do not cause bites that are of importance. Symptoms may include pain which may be at the bite or involve the chest and abdomen, sweating, muscle cramps and vomiting among others. Headaches, vomiting and a mild fever may also occur. and South American wandering spider. Spider bites may be overdiagnosed or misdiagnosed. Historically a number of conditions were attributed to spider bites. In the Middle Ages a condition claimed to arise from spider bites was tarantism, where people danced wildly. While necrosis has been attributed to the bites of a number of spiders, good evidence only supports this for recluse spiders. Bleeding also may occur with a bite. Signs of a bacterial infection due to a spider bite occur infrequently (0.9%). Activation of the sympathetic nervous system can lead to sweating, high blood pressure and gooseflesh. Most recluse spider bites are minor with little or no necrosis. However, a small number of bites produce necrotic skin lesions. First pain and tenderness at the site begin. The redness changes over two to three days to a bluish sinking patch of dead skin—the hallmark of necrosis. The wound heals slowly over months but usually completely. Rarely, bites may cause widespread symptoms, with occasional fatalities. There are a few spiders that can bite human skin and cause a skin reaction, but spiders are blamed for many more reactions. In particular the misdiagnosis of infections and other skin ailments are commonly attributed to brown recluses. + Spiders associated with skin reactions Scientific name Common name Venom ReactionLampona sppWhite tailedWelt Loxosceles spp Hobo Redness Lycosidae spp Wolf Redness, itching

Spiders do not feed on humans and typically bites occur as a defense mechanism. This can occur from unintentional contact or trapping of the spider. Medically significant spider venoms include various combinations and concentrations of necrotic agents, neurotoxins, and pharmacologically active compounds such as serotonin. Worldwide only two spider venoms have impact on humans—those of the widow and recluse spiders. Unlike snake and scorpion envenomation, widow and recluse species bites rarely have fatal consequences. In addition to the widespread widow and recluse spiders, some spider families found exclusively in remote tropical regions have a lethal neurotoxic venom: the wandering spider in Brazil and the funnel web in Australia. However, due to limited contact between these spiders and humans, deaths have always been rare, and since the introduction of anti venom in Australia, there have been no funnel web related deaths.

A primary concern of the bite of a spider is the effect of its venom. A spider envenomation occurs whenever a spider injects venom into the skin. Not all spider bites involve injection of venom, and the amount of venom injected can vary based on the type of spider and the circumstances of the encounter. The mechanical injury from a spider bite is not a serious concern for humans. However, it is generally the toxicity of spider venom that poses the most risk to human beings; several spiders are known to have venom that can cause injury to humans in the amounts that a spider could inject when biting. While venoms are by definition toxic substances, most spiders do not have venom that is directly toxic (in the quantities delivered) to require medical attention and, of those that do, severity is typically mild. Spider venoms work on one of two fundamental principles; they are either neurotoxic (attacking the nervous system) or necrotic (attacking tissues surrounding the bite). In some cases, the venom affects vital organs and systems. The venoms of the widow spiders, Brazilian wandering spider and Australian funnel web are neurotoxic. Heart muscle damage is an unusual complication of widow venom that may lead to death. Pulmonary edema, which is fluid accumulation in the lungs, is a feared and potentially serious but uncommon complication of funnel web venom.

Reliable diagnoses of spider bites require three conditions: first, there should be clinical effects of the bite at the time or soon afterwards, although there are no symptoms universally diagnostic of a spider bite, and bites by some spiders, e. g. Loxosceles species, may initially be painless; second, the spider should be collected, either at the time of the bite or immediately afterwards; and third, the spider should be identified by an expert arachnologist. Many of these conditions are far more common and more likely to be the source of necrotic wounds. False reports of spider bites in some cases have led to misdiagnosis and mistreatment, with potentially life threatening consequences.

Efforts to prevent bites include clearing clutter and the use of pesticides.

Most spider bites are harmless and require no specific treatment. Treatment of bites may depend on the type of spider; thus, capture of the spider—either alive, or in a well preserved condition, is useful. Treatment of spider bites includes washing the wound with soap and water and ice to reduce inflammation. Analgesics and antihistamines may be used; however, antibiotics are not recommended unless there is also a bacterial infection present. Antivenom is available for severe widow and funnel web envenomation. Dapsone, an antibiotic, is commonly used in the United States and Brazil for the treatment of necrosis. There have been conflicting reports with some supporting its efficacy and others have suggested it should no longer be used routinely, if at all.

Use of antivenom for severe spider bites may be indicated, especially in the case of neurotoxic venoms. Effective antivenoms exist for Latrodectus, Atrax, and Phoneutria venom. Antivenom in the United States is in intravenous form but is rarely used, as anaphylactic reaction to the antivenom has resulted in deaths. In Australia, antivenom in intramuscular form was once commonly used, but use has declined. In 2014 some doubt as to antivenom effectiveness has been raised.

Estimating the number of spider bites that occur is difficult as the spider involvement may not be confirmed or identified. Several researchers recommend only evaluating verified bites: those that have an eyewitness to the bite, the spider is brought in, and identified by expert. With "suspected arachnidism" the diagnosis came without a spider positively identified.

Bites by the redbacks (Latrodectus hasselti) number a few thousand yearly throughout the country. Antivenom use is frequent but declining. Children may have fewer complications from bites. Funnel web spider bites are few 30–40 per year and 10% requiring intervention. The Sydney funnel web and related species are only on the east coast of Australia. During epidemics of latrodectism from the European black widow upwards of 150 bites a year were documented.

The American Association of Poison Control Centers reported that they received calls regarding nearly 10,000 spider bites in 1994. and black widow spiders with 1,800 bites.

Numerous spider bites are recorded in Brazil with 5000+ annually. Loxosceles species are responsible for the majority of reports. Accidents are concentrated in the southern state of Parana with rates as high as 1 per 1,000 people. Bite from Phoneutria (Brazilian wandering spider) number in the thousands with most being mild. Severe effects are noted in 0.5% of cases, mostly in children.

Recorded treatment from the 1890s for spider bites in general was rubbing in tobacco juice to the bitten skin, similar to some of the traditional uses of the tobacco plant for various bites and stings from Central and South America. Wild dancing and music was the cure for tarantism—the frenzy was believed to arise from the bite of a spider. An antivenom was developed against wolf spiders in Brazil and used for decades. Wolf spiders have since been exonerated—they never caused illness.