Spider Bites

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 Liphistiidae. These spiders, however, do possess fangs and can deliver sharp, unpleasant bites if disturbed. In addition, the fangs of Liphistiidae 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.

One subject that deserves in-depth study is the nature of the infections associated with spider bites. In many cases of suspected necrotising bites the symptoms are consistent with bacterial colonisation of the moribund tissue, where the persistence of the festering is caused by bacterial action preventing healing that otherwise would have cleared up the problem a few days. Untreated, such wounds sometimes fester for months, though, as noted above, it does not follow that every alarming case really is caused by a spider bite. Appropriate treatment for genuine necrotising bites however, is aggressive antibiotic injection at the first sign of infection, where antivenin might be unavailable or even hazardous.

The use of the terms "poison" or "poisonous" in the context of spider bite is discouraged, as poison generally refers to substances which are harmful if absorbed through epithelial linings (e.g., eaten, or absorbed through the skin). The effect of eating spiders is, in general, unknown, but some spiders (such as tarantulas) are consumed as food.

Spider venom

The chief concern with the bite of medically significant spiders is the effect of the spider's venom. A spider envenomation occurs whenever a spider bites someone and chooses to inject venom into the wound. Not all spider bites involve injection of venom into the wound, and the amount of venom injected can vary based on the type of spider and the circumstances of the encounter. With very few exceptions, such as the so-called camel spider (which is not a true spider), the mechanical injury from a spider bite is not a serious concern for humans. Some spider bites do leave a large enough wound that infection may be a concern, and other species are known to consume prey which is already dead, which also may pose a risk for transmission of infectious bacteria from a bite. However, it is generally the toxicity of spider venom which poses the most risk to human beings; several spiders are known to have venom which can be fatal to humans in the amounts that a spider will typically inject when biting.

All spiders are capable of producing venom, with the exception of the hackled orb-weavers, the Holarchaeidae, and the primitive Mesothelae. (Other arachnids often confused with spiders, such as the harvestman and sun spiders, also do not produce venom). Nonetheless, only a small percentage of species have bites which pose a danger to people. Many spiders do not have mouthparts capable of penetrating human skin. While venoms are by definition toxic substances, most spiders do not have venom which is sufficiently toxic (in the quantities delivered) to require medical attention and, of those that do, only a few are known to produce fatalities.

Spider venoms work on one of two fundamental principles; they are either neurotoxic (attacking the nervous system) or necrotic (attacking tissues surrounding the bite, and in some cases, attacking vital organs and systems).

Neurotoxic venom

The majority of spiders with serious bites possess a neurotoxic venom of some sort, though the specific manner in which the nervous system is attacked varies from spider to spider.

• Widow spider venom contains components known as latrotoxins, which cause the release of the neurotransmitter acetylcholine, stimulating muscle contractions. This can affect the body in several ways, including causing painful abdominal cramps, as well as interfering with respiration, and causing other systemic effects.
• The venom of Australasian funnel-web spiders and mouse spiders works by opening sodium channels, causing excessive neural activity which interferes with normal bodily function.
• The venom of Brazilian wandering spiders is also a potent neurotoxin, which attacks multiple types of ion channels. In addition, the venom contains high levels of serotonin, making an envenomation by this species particularly painful.

Necrotic venom

Spiders known to have necrotic venom are found in the family Sicariidae, a family which includes both the recluse spiders and the six-eyed sand spiders. Spiders in this family possess a known dermonecrotic agent sphingomyelinase D, which is otherwise found only in a few pathogenic bacteria. Some species in this family are more venomous than others; according to one study, the venom of the Chilean recluse and several species of six-eyed sand spider indigenous to southern Africa, contains an order of magnitude more of this substance than do other Sicariidae spiders such as the brown recluse. Bites by spiders in this family can produce symptoms ranging from minor localized effects, to severe dermonecrotic lesions, up to and including severe systemic reactions including renal failure, and in some cases, death. Even in the absence of systemic effects, serious bites from Sicariidae spiders may form a necrotising ulcer that destroys soft tissue and may take months and very rarely years to heal, leaving deep scars. The damaged tissue may become gangrenous and eventually slough away. Initially there may be no pain from a bite, but over time the wound may grow to as large as 10 inches (25 cm) in extreme cases. Bites usually become painful and itchy within 2 to 8 hours, pain and other local effects worsen 12 to 36 hours after the bite with the necrosis developing over the next few days.

Serious systemic effects may occur before this time, as the venom spreads throughout the body in minutes. Mild symptoms include nausea, vomiting, fever, rashes, and muscle and joint pain. Rarely more severe symptoms occur including hemolysis, thrombocytopenia, and disseminated intravascular coagulation. Debilitated patients, the elderly, and children may be more susceptible to systemic loxoscelism. Deaths have been reported for both the brown recluse and the related South American species L. laeta and L. intermedia.

Numerous other spiders have been associated with necrotic bites in the medical literature. Examples include the Hobo spider and the Yellow Sac spider. However, the bites from these spiders are not known to produce the severe symptoms that often follow from a recluse spider bite, and the level of danger posed by each has been called into question. So far, no known necrotoxins have been isolated from the venom of any of these spiders, and some arachnologists have disputed the accuracy of many spider identifications carried out by bite victims, family members, medical responders, and other non-experts in arachnology. There have been several studies questioning danger posed by some of these spiders. In these studies, scientists examined case studies of bites in which the spider in question was positively identified by an expert, and found that the incidence of necrotic injury diminished significantly when "questionable" identifications were excluded from the sample set.

Treatment

Treatment for bites depends on the type of spider in question. Most spider bites are harmless, and will require no first aid. If you experience major discomfort and require medical treatment, and a spider was observed in the act of biting, then a spider expert may be needed to determine the species of spider that has bitten you—identification of the spider's species might determine the proper course of treatment. For this reason it is preferable to capture the spider—either alive, or in a well-preserved condition. Spiders which have been flattened, or which are allowed to desiccate or decay, may not be useful in achieving a positive identification. Most medical responders are not trained to identify spiders, and few hospitals have spider experts on staff. Contrary to media reports, it is not (in general) possible to identify the type of spider responsible for a bite solely from observed symptoms.

Unless a spider is observed in the act of biting, it should not be assumed that a spider bite has occurred (or that a wound, injury, or illness was caused by a spider). Assumption that a reported injury was caused by a spider is the most common source of false reports, which in some cases have often led to misdiagnosis and mistreatment, with potentially life-threatening consequences. Many spider bites, including those by some dangerous species, are relatively painless at first and may go unnoticed if not directly observed. These bites may only be noticed later if serious symptoms appear, in such cases the spider is usually no longer present.

Treatments for more minor bites should be as for any puncture wound. The wound should first be encouraged to bleed to wash out any foreign material and debris. (Many wounds will not bleed because they are so small in diameter that they seal immediately.) Topical antiseptics such as povidone-iodine should be applied on the off chance that the bite introduced some virus or microbe beneath the skin level and that the antiseptic can penetrate to that depth. The bite should be observed for a couple of days so that medical attention can be sought if signs of infection appear. (It is obviously difficult to get antiseptic to penetrate to the bottom of such a puncture.) First aid may also involve the application of an ice pack to control inflammation, the application of aloe vera to soothe, and if serious symptoms appear, prompt medical care.

In the case of bites by widow spiders, Australian venomous funnel-web spiders, or Brazilian wandering spiders, prompt medical attention should be sought; in some cases the bites of these spiders may develop into a medical emergency. Medical attention should also be sought if a severe allergic reaction occurs.

Necrotic bites

There is no established treatment for necrosis. Routine treatment should include elevation and immobilization of the affected limb, application of ice, local wound care, and tetanus prophylaxis. Many other therapies have been used with varying degrees of success including hyperbaric oxygen, dapsone, antihistamines (e.g., cyproheptadine), antibiotics, dextran, glucocorticoids, vasodilators, heparin, nitroglycerin, electric shock, curettage, surgical excision, and antivenom. None of these treatments have been subjected to controlled, randomized trials to conclusively show benefit. In almost all cases, bites are self-limited and typically heal without any medical intervention.

Occasionally, infections of Methicillin-resistant Staphylococcus aureus (MRSA) are misdiagnosed as necrotic spider bites; this can have severe consequences as a MRSA infection is frequently a medical emergency.

Specific treatments

Some specific courses of treatment may be indicated to deal with severe symptoms:

• Dapsone is commonly used in the USA and Brazil for the treatment of necrosis. There have been conflicting reports about its efficacy and some have suggested it should no longer be used routinely, if at all.
• Wound infection is rare. Antibiotics are not recommended unless there is a credible diagnosis of infection.
• Studies have shown surgical intervention is ineffective and may worsen outcome. Excision may delay wound healing, cause abscesses, and lead to objectional scarring.
• Anecdotal evidence suggests that application of nitroglycerin patches may be effective in treating recluse bites. Recluse venom is a vasoconstrictor, and nitroglycerin causes vasodilation, allowing the venom to be diluted into the bloodstream, and fresh blood to flow to the wound. Theoretically this prevents necrosis, as vasoconstriction may contribute to necrosis. However, one scientific animal study found no benefit in preventing necrosis, with results showing it increased inflammation and it caused symptoms of systemic envenoming. The authors concluded the results of the study did not support the use of topical nitroglycerin in brown recluse envenoming.
• Use of antivenom for severe spider bites is frequently indicated, especially in the case of neurotoxic venoms. Effective antivenoms exist for Latrodectus, Atrax, and Phoneutria venom. Recluse bites are now treatable by antivenom; an antivenom for Loxosceles bites is now available in South America, and it appears antivenom may be the most promising therapy. However, the recluse antivenom is most effective if given early, and because of the relatively painless bite delivered by recluses, patients do not often present until 24 or more hours after the event, possibly limiting the effect of this intervention. Due to the risk of serum sickness, use of antivenom is generally not indicated unless serious symptoms are present, and/or the patient fails to respond to other forms of treatment.