On Spiders and Spider Bites

 
        Little Miss Muffet
        Sat on a tuffet,
        Eating her curds and whey;
        Along came a spider,
        Who sat down beside her
        And frightened Miss Muffet away.



Spiders having medically significant venom exist in all but the coldest parts of the world. There is general agreement on which spiders give bites that may produce lasting damage or death, but not such general agreement on how one might sort spiders identified by genus and species in order of their threat to humans.

The following types of spider are known to have medically significant bites, with symptoms ranging from localized pain all the way to severe tissue destruction and potential death. Spiders whose bites have caused fatalities which are well documented in the scientific literature are so indicated in the section headers. Only four genera (Phoneutria, Atrax, Latrodectus, and Loxosceles) are known to have killed humans; three other genera (Hadronyche, Missulena, and Sicarius) possess venom which toxicology studies have shown have lethal potential (being similar to Atrax and Loxosceles venom in composition). There are suspected but unconfirmed deaths reported in the literature from species in Tegenaria and Haplopelma.

A spider bite is an injury resulting from a spider's forced interaction with other than prey organisms that can lead to medically significant complications. The most often seen cases of spider bites occur in humans and domesticated animals because of the cosmopolitan coexistence of both. About half the spiders encountered in everyday life possess chelicerae strong enough to penetrate human skin. And although 98-99% of the bites thereby inflicted are harmless, more rarely, the symptoms can include necrotic wounds, systemic toxicity and, in some cases, death. Four genera are known to have potentially lethal bites.

In almost all cases of biting, the chief concern is the spider's venom, although in some rare cases medically non-significant spiders can transmit infectious diseases (such as the West Nile virus) from the previous food remains. Spiders regarded as dangerous possess venom that is toxic to humans, in quantities that can be delivered by a single bite.

Only three genera of spiders are known to be non-venomous, i.e. lacking venom glands or any proper way to deliver it. They include the families Uloboridae, Holarchaeidae and Mesothelae. These spiders, however, do possess fangs and can deliver sharp, unpleasant bites if disturbed. In addition, the fangs of Mesothelae can often inflict infections spread through the skin, mostly due to their big size, which in theory could represent more danger than the bite of a known, but not lethal venomous spider species.

Experts on spider bites have noted that misdiagnoses of bites by both the general public and the medical community are quite common; many other conditions and diseases are confused with spider bites, sometimes preventing or delaying proper remedy, which can lead to deleterious outcomes. For example, there are numerous documented infectious and non-infectious conditions (including pyoderma gangrenosum, bacterial infections by Staphylococcus (including MRSA) and Streptococcus, herpes, diabetic ulcer, fungal infections, chemical burns, toxicodendron dermatitis, squamous cell carcinoma, localized vasculitis, syphilis, toxic epidermal necrolysis, sporotrichosis, and Lyme disease) exhibiting lesions that have been initially misdiagnosed as brown recluse spider bites by medical professionals. Many of these conditions are far more common and more likely to be the source of mysterious necrotic wounds, even in areas where recluses are present.

source:http://en.wikipedia.org/wiki/Spider_bite