Spider bite

  Spiders occasionally bite humans. Although 98-99% of spider bites are harmless,^[1] more rarely, the symptoms of their bites can include necrotic wounds, systemic toxicity, and in some cases, death. Four genera are known to have potentially lethal bites.^[2]

In almost all cases of spider bite, the chief concern is the spider's venom. Spiders regarded as dangerous possess venom which is toxic to humans in the quantities which can be delivered by a single bite.

The use of the terms "poison" or "poisonous" in the context of spider bites 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 sometimes consumed as food.^[3]

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Biology and ecology of spider bites

  Spiders are predatory animals that consume other animals (including other spiders) for food. For the vast majority of spider species, biting (and injection of venom) is the way the spiders subdue their prey; the spiders will use their venom to paralyze or kill their victims, often consuming them later. Spiders also use biting as a defensive mechanism, though the primary purpose of spider venom is to capture food.

Spider mouthparts

      Spiders do not have teeth. Instead, they have two chelicerae, each with two segments, the fang and the basal portion. The fang, the organic functional equivalent to a hypodermic needle, is what penetrates the skin, fur, or exoskeleton of the spider's target — spider mouthparts are primarily intended for envenoming a spider's prey in most species, typically insects and other small arthropods; not for biting humans. The basal portion includes all or part of the spider's venom glands, which can be squeezed under voluntary control by the spider to force venom out of the glands and into the wound.^[4]

The images in this section show the fang of an immature Psalmopoeus cambridgei at various levels of magnification, with various commercially available needles shown for comparison.

When a spider bites, the two parts of each chelicera come together like a folding knife, and when making a threat display or actually preparing to bite, the spider will also open the angle of the fangs with the basal portion of chelicerae and also open the angle of the basal portion with the cephalothorax. In the tarantulas and other Mygalomorphae, the horizontal separation of the tips of the fangs does not change much, but in the other spiders the tips of the fangs move apart from each other as well as elevating. In more precise, scientific terms, the bite mechanism is called either "co-axial" or "bi-axial". The Mygalomorphea (Tarantulas, Trapdoor spiders etc.) offer a co-axial bite while the Labidognatha sub order, comprising all other spiders such as Lycosids, Ctenids and most web spiders, etc. deliver the more common bi-axial bite.

Even the tips of the fangs of the rather large spider shown above are quite sharp, and the spider's body is well adapted to driving the fangs into flesh. Some spider bites, such as those of the Sydney funnel-web spider, are reported to have penetrated toe nails and soft leather shoes.  

Types of bites

Spiders have the capability to control how much venom (if any) is injected into a target, and adjust the dosage given according to circumstances. As venom costs the spider energy to produce, it is advantageous to the spider not to waste it. Spider bites are characterized as either attacking or defensive, depending on whether the spider is attempting to subdue prey, or to repel a perceived threat. When capturing prey, spiders will typically adjust the amount of venom delivered based on the size of the meal; when defending itself, a spider's only goal is to secure relief from being squeezed or otherwise injured. A bite in which little or no venom is injected is frequently referred to as a dry bite.

As spiders do not prey on humans (or other large mammals), spiders do not "attack" people. It is rare for spiders to mistake humans for prey. Almost all bites that humans receive are defensive bites, and frequently the spider drives the human off just by the mechanical pain of its bite, thus it is not unusual for humans to receive dry bites or partial envenomations. Thus, most spiders are unlikely to otherwise bite humans because they do not identify humans as prey. Some spiders (including highly venomous species like the Brazilian wandering spider) will behave aggressively towards large animals (including people) that the spider perceives as a threat. Even in those cases, however, they will first make a determined threat display and will bite only if they have no other choice. Spiders of the genus Phoneutria have extremely toxic venom and approximately ten times as much venom as other spiders that are known to produce fatalities in adult humans^[5] , but serious consequences from their bites are not common, as many bites received by humans are believed to be dry or of low volume.^[6]

Many bites occur when a person steps on a spider, or inserts a limb into an article of clothing that the spider occupies. Even small spiders may deliver painful bites to people when pinched. For instance, Phidippus audax – a common jumping spider, which may grow to be approximately 3/8 inch (1 cm) long – is capable of inflicting a bite that is about as painful as a bee sting when pinched between the folds of a human's palm or otherwise seriously annoyed.

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. ^[7] 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 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. ^[2]
• 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 ^[8] 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^[9]. 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.^[10] 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.^[11]

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.^[12] 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. A partial list includes the hobo spider, the white-tailed 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.^[13][14] 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. ^[15][16]

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^[17]. 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 Isodine® 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.)^[18] First aid may also involve the application of an ice pack to control inflammation, the application of aloe vera to soothe and help control the pain, 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.^{[citation needed]}

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.^[19]

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.^[20]

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.^[21]

• Wound infection is rare. Antibiotics are not recommended unless there is a credible diagnosis of infection.^[22]

• Studies have shown surgical intervention is ineffective and may worsen outcome. Excision may delay wound healing, cause abscesses, and lead to objectional scarring.^[23]

• Anecdotal evidence suggests that application of nitroglycerin patches may be effective in treating recluse bites.^[24] 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.^[25]

• 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.^[26] 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.

Types of spiders with medically significant venom

Spiders having medically significant venom exist in almost all parts of the world except those that are coldest. 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 spiders 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 bite has 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 positively 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.

Brazilian wandering spiders (Phoneutria--confirmed deaths)

Main article: Brazilian wandering spider

The Brazilian wandering spider (a ctenid spider) is a large, brown spider rather like a North American Wolf spider in appearance. However, it has a highly toxic venom (one of the most neurologically active), and is regarded (along with the Australian venomous funnel-web spiders below) as among the most dangerous spiders in the world.^[27] It, like several other more harmless spiders, may hitch a ride in clusters of bananas. As a result, any large spider appearing in a bunch of bananas should be treated with due care. Oddly, many of the bites of this species are alleged to be dry bites (in which no venom is released), but because of the margin of error when identifying the precise subspecies involved (assuming the spider body is saved or captured), this claim is not definitive. In either case, the bite is at minimum mechanically painful due to the large size of the chelicerae (fangs) and considering the high levels of serotonin contained in the venom, the bite can be one of the most excruciating of all spider envenomations. The spiders are as large as some small tarantulas and, as already mentioned, have fairly long fangs. While venom from either spider can be deadly to children and the infirm, since the development of antivenom to the venoms of both were developed (the funnel web spider in the mid-1980s and the wandering spider in 1996), no human deaths from their bites have been recorded. Nevertheless, any large spider which makes a threat display (raising front legs, rearing back to display fangs) when encountered should be treated with caution - especially in areas where these two types of spiders may be present.

Australian venomous funnel-web spiders (Atrax, Hadronyche--confirmed deaths)

Main article: Australian venomous funnel-web spiders

  The Australian venomous funnel-web spiders, such as the Sydney funnel-web spider (a mygalomorph only distantly related to the araneomorph funnel-web spiders) frequently bite people and are regarded as among the most dangerous in the world. They are quite aggressive spiders, and are prone to biting when confronted, rather than running away. The Sydney funnel-web spider, a large, bulky, black spider, is restricted to a relatively small area around Sydney, Australia. Its venom contains a compound known as robustotoxin which is highly toxic to primates. Unlike the Brazilian wandering spider, which is alleged to occasionally deliver dry bites, these spiders typically deliver a full envenomation when they bite.   There are other dangerous species of Atrax and Hadronyche related to this spider in surrounding parts of Australia, including 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.

One other genus in the Hexathelidae family has been reported to cause severe symptoms in humans. The genus Macrothele in Taiwan has been attributed to severe bites, but no fatalities.^[28] There are no known deaths attributed in the literature to any funnel-web species other than A. robustus

Tangle-web spiders (Theridiidae)

Two genera of the tangle web spiders have venom which is known to be medically significant. One genus, the widow spiders of genus Latrodectus, has caused more human fatalities than any other. The other genus, the false widow spiders of Steatoda, has a far less serious bite.

Widow spiders (Latrodectus--confirmed deaths)

Main article: Widow spider

The widow spiders (genus Latrodectus), such as the black widow and red-back spider, are spiders that carry a neurotoxic venom [1] which can cause a set of symptoms known as Latrodectism. Like many spiders, widows have very poor vision, and they move with difficulty when not on their web. Widow spiders are large, strong-looking house spiders (but still have relatively spindly legs and deep, globular abdomens). The abdomen is dark and shiny, and has one or several red spots, either above or below. The spots may take the form of an hourglass, or two triangles, point-to-point. Male widows, like most spiders, are much smaller than the females, and may have a variety of streaks and spots on a browner, less globular abdomen. The males are generally considered to be much less dangerous (if at all) than the females. Widows tend to be quite non-aggressive, but will bite if the web is disturbed and the spider feels threatened. The venom, although rarely life-threatening, produces very painful effects including muscle spasms and 'tetanus-like' contractions. A serious bite will often require a short hospital stay. Children, elderly, and ill individuals are at most risk of serious effects.

False black widows (Steatoda)

Main article: Steatoda

  The False black widow spiders (also known as false katipo, false button spider, cupboard spider, and in Australia, brown house spider) are spiders of the genus Steatoda. They resemble widow spiders in size and physical form, which is not surprising since they are members of the same Family. While the bite of Steatoda spiders are nowhere near as serious as that of true widow spiders, several of these spiders do have medically significant bites. The bite of Steatoda grossa, commonly known as the cupboard spider, is known to cause symptoms which have been described as a very minor widow bite; the medical community now refers to the symptoms of Steatoda bites as steatodism. Other spiders in this genus known to be problem biters include two chiefly European varieties, S. paykulliana and S. nobilis, and a species found mainly in New Zealand and South Africa, S. capensis

Use of widow spider antivenom has been shown effective in treating steatodism.^[29] The genera Steatoda and Latrodectus are biologically close cousins; both belong to the family Theridiidae. There are over 100 species in this genus, but only several species have been associated with medically significant bites.

Members of this genus are characterized by the "D" shape of the cephalothorax, and the way the relatively straight line thus formed is mirrored by the blunt forward surface of the abdomen.They look something like this: Ə Other genera in this family generally have cephalothoraxes that are more oval in shape or even rather round, and that give the appearance of two body parts that are joined by a small connector.

Sicariidae spiders

The family Sicariidae includes two genera, both of which have highly dangerous and necrotoxic bites. One genus, Loxosceles, are the well-known recluse spiders, a genus which is distributed worldwide (but is most commonly found in the Americas). The other genus, Sicarius, is far less known; being found only in the Southern Hemisphere. Spiders in both genera have venom containing the dermonecrotic compound sphingomyelinase D.

Recluse spiders (Loxosceles--confirmed deaths)

Main article: Recluse spider

Recluse spiders (Loxosceles spp.), such as the brown recluse spider, also known as "violin spiders" or "fiddlers" from the dark violin-shaped marking on the cephalothorax, are slow-moving, retiring spiders which wander about in dim areas and under things, and so are more easily trapped against one's skin by clothing, bed sheets, etc. The spiders will often creep along at a very slow pace and then make a sudden dart for a couple of inches, then return to the previous languid pace. Recluses are extremely venomous. Most encounters with this spider occur from moving boxes or rooting about in closets or under beds. The range of the brown recluse, L. reclusa in the US is approximately the southern 2/3 of the country by the eastern 3/4 of the country. A number of related recluse spiders (some non-native introductions) are found in southern California and nearby areas, as well.

Most recluse spider bites are minor with no necrosis. However, a small number of bites produce severe dermonecrotic lesions, and, sometimes, severe systemic symptoms, including organ damage. Rarely the bite may also produce the systemic condition with occasional fatalities.

A minority of bites form a necrotizing ulcer that destroys soft tissue and may take months and, on very rare occasions, years to heal, leaving deep scars. The damaged tissue will become gangrenous and eventually slough away. The initial bite frequently cannot be felt and there may be no pain, 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.^[11]

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.^[12] 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.

Even more dangerous is the Chilean recluse, a species native to South America and found in many parts of the world, including in southern California and other southwestern states. Bites of this spider have been known to cause systemic reactions in 15% of reported cases, and fatalities in 3-4% of cases. ^[30]

Six eyed sand spiders (Sicarius)

Main article: Six-eyed sand spider

  The six-eyed sand spider, of southern Africa (and other spiders in the genus Sicarius), is considered by some to be the world's most venomous spider. Assays of its venom have led some to consider this spider's bite as the most dangerous on record; and currently no antivenom exists for its bite. Fortunately, this specimen rarely interacts with humans, and is seldom known to bite; recorded envenomations by this spider are rare. A cousin of the recluse spider (and possessing the same toxic compound as found in recluse venom), this spider buries itself in the sand and strikes from ambush at prey that wanders too closely. Sand particles adhere to cuticles on its abdomen, thus acting as a natural camouflage if uncovered. If disturbed, it will run a short distance and bury itself again.

Little is known about the bite of other Sicarius species; however numerous other species have also been found to possess venom containing sphingomyelinase D.

Mouse spiders (Missulena)

Main article: mouse spider

The mouse spiders of the genus Missulena are a type of primitive burrowing spider found primarily in Australia. Several species of this genus are known to possess a venom which contains compounds similar to robustotoxin, the substance in funnel-web venom which is deadly to humans, and there have been several recorded bites by this spider producing severe symptoms requiring emergency medical treatment. However, unlike the funnel-web spiders, which have resulted in at least 13 deaths in the last 100 years,^[31] there are no recorded human fatalities due to mouse spider bites, and many bites by this spider result in no serious complications. It is suspected that unlike Atrax and Hadronyche, which typically deliver full envenomations when they bite, that mouse spiders often give "dry" bites. When severe envenomation does occur, funnel-web antivenom has been shown to be effective.^[32]

True tarantulas (Theraphosidae)

Main article: Tarantula

The true tarantulas, of the family Theraphosidae, are fearsome looking spiders with somewhat notorious reputations. As large spiders, they have very powerful fangs and are capable of delivering a sizable quantity of venom. However, many species of tarantula are known to be relatively harmless to humans. Tarantulas are typically divided between New World and Old World types; depending on what part of the world the spider in question comes from.

New world tarantulas

  New World tarantulas--those indigenous to the Americas--have bites that generally pose little threat to humans (other than causing localized pain). The primary means of defense for these spiders are urticating hairs, which can cause irritation and other typical symptoms in humans.

Old world tarantulas

  Old World tarantulas, especially those indigenous to Asia, are another matter. These species lack urticating hairs, and use biting as a defensive mechanism (as well for subduing prey). In addition, these spiders are far less docile; and more likely to try and bite an adversary (including humans) if provoked. The effect of Old World tarantula venoms is not well studied, for the most part; however much anecdotal evidence suggests they have stronger venom than their New World counterparts.

There are many anecdotal reports from individuals in the tarantula pet trade of significant bite from Poecilotheria spp., occasionally resulting in hospitalization. Symptoms include localized pain and swelling, exhaustion, moderate to severe muscle cramping, labored breathing and fever, sometimes delayed days after the initial bite.^{[33][34][35][36]}

One species whose venom has been studied extensively is the Chinese bird spider (Haplopelma spp.), a tarantula of the subfamily Ornithoctoninae. The venom has been found to contain numerous novel toxins, is effective at killing mice, and has been blamed for at least one fatality in China. However, there is little documented clinical evidence of the effects of this spiders' bite in humans; so firm conclusions about the level of danger posed by this spider cannot be drawn.

Yellow sac spiders (Chiracanthum)

Main article: Yellow sac spider

  The yellow sac spiders, Chiracanthum sp., take shelter in silk tubes during the daytime and generally come out to hunt at night. These pale yellow or whitish spiders are often found in houses at the top of walls, or wandering across ceilings. They are also commonly found outdoors on foliage. The draglines they leave while hunting are one of the most common "cobwebs" that are removed with broom and vacuum cleaner. People may unintentionally make contact with them in the dark and so be bitten. However, many people will live their entire lives in close proximity to them and never suffer a bite. Nevertheless, the spider's bite is considered toxic.

Huntsman spiders (Heteropoda)

Main article: Huntsman spider

  The huntsman spiders have a worldwide reputation for scaring people. They are large, defend their nests, and may move toward people and make threat displays. They frequently enter houses and hunt over the walls and ceilings where they may run rapidly for long distances without pausing. When they actually do bite people, the bites are very unpleasant, but these spiders are not regarded as dangerous. They are quite common in parts of Australia. Australian huntsman spiders are typically non-aggressive except when defending their nests or their young.

There is one spider in California and Japan, probably a huntsman (tentatively identified as a member of the Sparassidae family, Heteropoda venatoria), which might run over and bite your finger if you touch the wall that it is clambering over. That behavior may well occur because its eyesight is good enough to see movement and general shape, but not sufficient to avoid mistaking something else for its natural prey. In general, however, members of this genus scramble wildly to escape when they become aware of a human moving into their vicinity.

Redback jumping spiders (Phidippus johnsoni)

Main article: Redback jumping spider

  Some people have reported being bitten by redback jumping spiders, one of the visually most prominent species among the genus Phidippus. Many reports come from California, although their range is much wider, and people elsewhere may have unpleasant contacts with them. These relatively large, alert but slow-moving jumping spiders have bright red abdomens (the females have a black stripe), and should be clearly visible. It is unclear how bites to humans occur. Accidental contact seems rather unlikely since jumping spiders have excellent vision and can easily avoid being brushed by a human hand. It is also unlikely that they would mistake a human finger for their natural prey. One source suggests that, since they are quite attractive, children may try to pick them up and in that way elicit a defensive bite. Since these spiders are quite large, their body length being around 12 mm (1/2 inch), the volume of their available venom is accordingly rather large. Fortunately, however, the worst consequences reported have been three to four days of discomfort, with no permanent damage. Like most of the larger spiders, the consequences of a bite seem little different from those of a wasp or bee sting. Since they do not frequent human habitations it should ordinarily be easy to avoid unpleasant contact with them. Even when encountered the ability of jumping spider to detect and track human movement should prevent most potential bite situations.

Comparative analysis

It is often asked which type of spider is the most "dangerous" in the world. There isn't a simple answer to this question, as there are many things which must be taken into account when considering the amount of danger posed by spider bites:

• First, it is often the case that a spider bite is "dry" – the skin may be pierced, but little or no venom is injected into the victim. In such an instance, little or none of the spider's dangerous potential for harm is manifested.
• Second, there have been reports of spider bites (by spiders considered otherwise harmless) causing allergic reactions in some individuals, up to and including anaphylactic shock, a life-threatening condition (much the same as a sting from an ant, bee, or wasp may produce a harmful effect apart from the toxic quality of its venom).^{[citation needed][37]}
• Third, many spiders listed as dangerous are seldom encountered, or have dispositions that make them unlikely to bite despite the high toxicity of their venom.
• Finally, little is known about the toxicity of many spiders, due to their infrequent encounters with humans; the list of venomous spiders is limited to those that are linked to medical events in humans or who otherwise have been extensively studied.

It should also be noted that, for healthy adults, a bite by even the most toxic spiders on the list may require hours before death ensues; if timely appropriate emergency medical treatment is administered, victims may be expected to recover. The scenario given in movies such as Arachnophobia, where bite victims die within minutes, does not occur. One exception to this picture occurs because in the case of very small children the amount of venom dispersed throughout the body is many times the concentration in an adult. There is at least one recorded case of a small child dying within 15 minutes of a bite from a Sydney funnel-web spider; that event occurred before the development of an antivenom. Since the antivenin was developed there have been no fatalities due to this species.

The spiders believed to be most dangerous to humans, in terms of the risk posed by a bite, are the Sydney funnel-web spiders and Brazilian wandering spider. These spiders are potentially more dangerous than widow spiders because they have longer fangs and possess greater quantities of venom, thus they are capable of injecting far more venom to greater depths. Phoneutria nigriventer has approximately 2 mg of venom, but frequently gives dry bites or at least does not deliver all of its available venom. Atrax robustus has approximately 1.7 mg of venom. Bites of six-eyed sand spiders are thought likely to be even more dangerous to humans than any of the others, but fortunately there have been very few bites. Two instances of human envenomation are known; one was fatal, and the other involved the loss of an arm. This, along with venom testing in rabbits, leads some experts to believe that Sicarius may be the most harmful spider envenomation known, as no other species has an observed 50% lethality rate.

By general agreement, spiders of the genus Latrodectus (of which the Black Widow spider is the most notorious) kill more people per year, worldwide, than any other spider.^{[citation needed]} Because they are so small, they are much harder to detect than a large Brazilian Wandering Spider or a Tarantula. Though their venom is extremely potent, these spiders are not especially large. Compared to many other species of spiders, their chelicerae are not very large. In the case of a mature female, the hollow, needle shaped part of each chelicera, the part that penetrates the skin, is approximately 1 mm (approx. .04 in) long, sufficiently long to inject the venom to a dangerous depth. The males, being much smaller, can inject far less venom and inject it far less deeply. The actual amount injected, even by a mature female, is very small in physical volume (.02–.03 mg). When this small amount of venom is diffused throughout the body of a healthy, mature human, it usually does not amount to a fatal dose. Deaths in healthy adults from Latrodectus bites are rare in terms of the number of bites per thousand people. Only sixty-three deaths were reported in the United States between 1950 and 1989 (Miller, 1992). On the other hand, the geographical range of the widow spiders is very great. As a result, far more people are exposed, worldwide, to widow bites than are exposed to bites of more dangerous spiders, so the highest number of deaths worldwide are caused by members of the genus Latrodectus. Widow spiders have more potent venom than most if not all spiders, and prior to the development of antivenom, 5% of bites resulted in fatalities.^[38]

Measurements

The LD-50 figures have limited utility since the effects of venoms differ widely from species to species. The University of California at Riverside reports that prior to the development of an antivenom, 5% of victims of widow spiders would die, but comparable figures are not available for the other species. Before an antivenom was developed, deaths from Atrax and Hadronyche were very common. Some deaths from Phoneutria bites are reported, but much of their range is in the Amazon so reporting of bites may not be very complete.

Most LD-50 figures are based on experiments with laboratory mice. There are great differences in the sensitivities of various kinds of organisms to various kinds of venom. The relative sensitivities of mice to various venoms may not allow prediction of the exact degree of human sensitivity. So most of these figures can only give a rough approximation of the medical consequences of various spider bites to humans. Nevertheless, any venom capable of killing other organisms in small doses should be avoided by humans. A case in point are the Sicarius spp. The venom of these spiders is extremely active in laboratory animals, but there are few if any documented reports of Sicarius bites in humans.

* This value is based on experience with human exposures.
** Several other kinds of tarantulas in the pet trade are regarded as giving non-trivial bites. Tarantulas are typically far larger than spiders with the most toxic kinds of venom. However, the sheer volume of the venom may compensate for its lesser toxicity. The effects of a full envenomation are probably unknown for many species of tarantulas, so due caution is advisable.

Spiders and similar creatures with unsupported reputations

There are several species of spider (and a few other arachnids which are not spiders, but are frequently confused with them), who have had unsupported reputations for being harmful to humans. In some cases, that the species is now considered harmless is a settled matter for arachnologists and other professionals; in other cases (such as the hobo spider), prior scientific belief that a spider is harmful to humans is now being questioned.

Hobo spiders (Tegenaria agrestis)

Main article: Hobo spider

The hobo spider, Tegenaria agrestis, may wander away from its web, especially in the fall, and thus come into contact with people and potentially bite. This spider is found in the northwestern United States, western Canada and throughout much of Europe. Studies performed by arachnologist Darwin Vest reported that this spider's venom caused significant necrotic effects in laboratory animals^[51][52], and medical authorities in the Pacific Northwest have blamed this spider for at least one fatality. Many agricultural authorities have published the advice that this species is potentially harmful, and medical personnel in the western United States and Canada have been advised to consider hobo spider bites when patients present with necrotic wounds. Many brown recluse bites have been reported in the U.S. west coast states (Washington, Oregon, and northern California) where populations of brown recluse spiders have not been found; some of these alleged bites have been attributed to hobo spiders instead.^[53]

However, in Europe, where the spider originates, the species is considered a harmless outdoor relative of the common house spider (Tegenaria domestica), and no other spider in the genus Tegenaria is considered to be harmful to people. Attempts to replicate Vest's study that reported necrotic effects of the venom have failed, thus casting the "dangerous" status of this spider into doubt. In addition, Vest's methodolgies have been questioned; he has been accused of incorrectly identifying symptoms to hobo spider bites when no positive identification of the spider was made. The one fatality attributed to the spider by medical authorities has also been questioned, and there are no documented cases where an otherwise-healthy person has developed a necrotic lesion from a positively-identified hobo spider bite. Many scientists now question whether or not the spider is harmful at all. ^[54][55]

Lycosa tarantula

  Lycosa tarantula, a species of wolf spider which is found near Taranto, Italy (and the origin of the name tarantula, which today refers to a completely different kind of spider), was once blamed for a condition known as tarantism. Workers in the fields would suffer bites, and observe large, conspicuous, hairy spiders in the area. That spider, L. tarentula, was blamed for the pain and suffering (and occasional death) associated with tarantism. It is known that the bite of L. tarentula, while sometimes painful, has no serious medical consequences for humans. It is also suspected that the real culprit was another spider, Latrodectus tredecimguttatus, a type of widow spider, and one which is now known to be very dangerous to people.

White-tailed spider

The white-tailed spider, a species indigenous to Australia, has long been blamed for a necrotic bite, producing symptoms similar to a brown recluse. However, recent studies into this spider have led many to believe that its bite produces no serious effects in humans; in particular, necrotic ulcers were not observed. White-tail bites do cause localized pain and lesions. ^[15] Nonetheless, the white-tailed spider still is rather infamous in Australia, and frequently referred to as a dangerous spider.

Harvestman (Daddy-long-legs)

  The spider-like arachnids known as Opiliones (also known as harvestmen or daddy long-legs), are a species often handled by humans. They are the subject of an urban legend which claims that they possess venom which is deadly to humans, but have fangs which are unable to penetrate human skin, and thus are essentially harmless. In truth, these arachnids are completely harmless to humans, as they have no venom whatsoever. In addition, incidents of opiliones biting people are rare, and no reported bites by these species have had any lasting effects.

The term "daddy long-legs" also can refer to the similar-looking cellar spider. This species (a true spider) can bite humans, but its venom is not known to have any effects beyond mild discomfort at the site of the bite.

"Sun spiders"

  The arachnids of the order Solifugae, also known as wind scorpions or sun spiders, are neither spiders nor scorpions. In the Middle East, it is common belief among some American soldiers stationed there that Solifugae will feed on living human flesh. The story goes that the creature will inject some anesthetizing venom into the exposed skin of its sleeping victim, then feed voraciously, leaving the victim to awaken with a gaping wound. Solifugae, however, do not produce such an anesthetic, and do not attack prey larger than themselves unless threatened.

Further, Solifugae are known to not possess any venom (other than one species in India, which may possess venom according to one study ^[56]). However, due to the large size of their jaws, bites by Solifugae can cause significant wounds, which should be treated accordingly to avoid infection.^[7]

References