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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 into the skin, 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. Some spider bites do leave a large enough wound that infection may be a concern. 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 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 that 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 that is sufficiently toxic (in the quantities delivered) to require medical attention and, of those that do, fatalities are exceedingly rare. (For details that substantiate these claims, see the remainder of this article.)

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.[1][2][3]

Necrotic venom

Spiders known to have necrotic venom occur most notoriously in the family Sicariidae, which includes both the recluse spiders and the six-eyed sand spiders. Spiders in this family possess a known dermonecrotic agent sphingomyelinase D,[6][7] which is otherwise found only in a few pathogenic bacteria.[8][9] 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 sicariid spiders such as the brown recluse.[10] 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.[11] Even in the absence of systemic effects, serious bites from sicariid 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 10 inches (25 cm) in extreme cases. Bites usually become painful and itchy within two to eight hours, pain and other local effects worsen 12 to 36 hours after the bite, and then necrosis will develop over the next few days.[12]

Systemic effects are unusual but include Mild symptoms such nausea, vomiting, fever, rashes, and muscle and joint pain. Rarely, more severe symptoms occur including red blood cell destruction (hemolysis), low platelets (thrombocytopenia), and loss of clotting factors (disseminated intravascular coagulation).[13] Children may be more susceptible to systemic loxoscelism effects. Deaths have been reported for both the brown recluse and the related South American species Loxosceles laeta and Loxosceles intermedia related to hemolysis and the injury that results to the kidney. However, some deaths have occurred where no brown recluse live leading to perception of misdiagnosis.

Numerous other spiders have been associated with necrotic bites. Examples include the Hobo spider, a member of the usually inoffensive family Agelenidae and various Sac spiders, members of the family Miturgidae. However, the bites from these spiders do not conclusively cause skin necrosis and none are known to produce the severe symptoms that rarely follow from a recluse spider bite.[14][15] 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 whether danger is 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.[16][17]

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:

  • Firstly, 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.
  • Secondly, there have been reports of allergic reactions to spider contact but only one of anaphylactic shock,[18] 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).[19]
  • Thirdly, 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 serious bites develop symptoms quickly, within the hour. While a serious medical condition may result (see latrodectims and loxocelism) fatalities are exceedingly rare. Appropriate medical treatment can improve speed of recovery . The scenario given in movies such as Arachnophobia, where bite victims die within minutes, does not occur. Small children are considered an exception because 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 death occurred before the development of an antivenom. Since the antivenom was developed there have been no fatalities due to this species.

The spider-bites documented as the most dangerous to humans are those of the Sydney funnel-web spider and the Brazilian wandering spider. These spiders are potentially more dangerous than widow spiders because they have longer fangs and can inject greater quantities of 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. Only 1 out of 200 bites is serious [20]Atrax robustus has approximately 1.7 mg of venom. Bites of Six-eyed sand spiders have been described as dangerous to humans, but there is a lack of proof for this.[21]: p.25 

The genus Latrodectus (of which the black widow spider is the most notorious) have been credited with killing more people per year, worldwide, than any other spider. Because they are not very large, they are much harder to detect than a large Brazilian wandering spider or a tarantula. Their venom is extremely potent. 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 (0.04 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). 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. Widow spiders venom may rarely cause serious complications in people. Fatalities had been reported as high as 5% of bites and as low as 0.2% of bites.[22]

Measurements

The LD-50 (median lethal dose) figures have limited utility since the effects of venoms differ widely from species to species. Before an antivenom was developed, deaths from Atrax and Hadronyche did occur, but the frequency of non-lethal bites is unknown. 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. 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, so data on which to base valid conclusions is lacking.

Genus Species Common name Body length Venom amount LD-50 Alternate LD-50 Deaths reported
Atrax A. robustus Sydney funnel-web spider 24–32 mm.[19] 0.25 mg (F) and 0.81 mg (M) [23] 2 mg [24] 0.16 mg/kg [25] unknown 13 attributed deaths from 1927 to 1980[26]
Hadronyche H. formidabilis Northern tree funnel-web spider 23–45 mm. 1 death.[27] High rates of severe envenoming.[19][28]
Hadronyche H. cerberea Southern tree funnel-web spider High rates of severe envenoming.[28]
Latrodectus L. mactans Black widow 8–15 mm [19] 0.02–.03 mg.[29][30] 0.002 mg/kg [29]* 0.9 mg/kg 36 deaths recorded from 1965 to 1990 in the U.S.
5% of reported bites prior to antivenom availability [19]
Latrodectus L. tredecimguttatus Malmignatte (approx. same) (approx. same) 0.68 μg/kg [31] 16.25 μg/kg [31] possibility of deaths in Southern Europe first attributed to the brown recluse, suggesting larger frequency of the bites.
Loxosceles L. reclusa Brown recluse 1.2 cm (0.75 in) [19] 6–10 mm [19] .13–.27 mg.[31][32] necrosis and amputation of limbs more common, deaths rare[19]
Loxosceles L. intermedia 0.48 mg/kg [33]
Loxosceles L. laeta Chilean recluse 1.45 mg/kg [33]
Loxosceles L. gaucho 0.74 mg/kg [33]
Phoneutria P. bahiensis Brazilian wandering spider 30 mm 1.079 mg [34] .00061–.00157 mg/kg [34] cardiac failure reported in 5 out of 12 bitten
Phoneutria P. boliviensis Brazilian wandering spider 30 mm 1.079 mg.[34] .00061–.00157 mg/kg [34]
Phoneutria P. fera Brazilian wandering spider 30 mm [19] 1.079 mg [34] .00061–.00157 mg/kg [34] disputed effectiveness of the antivenin - 4 deaths out of 7 administered [19]
Phoneutria P. nigriventer Brazilian wandering spider 3–5 cm (1.25–2 in) [35] 2.15 mg [32] 1.079 mg.[34] 15.20 ng/mg.[32] 00061–.00157 mg/kg [34] 200 µg/kg (0.2 ng/mg) [32] severe cardiac failure, signs of priapism and irreversible damage to the central nervous system recorded.
18 deaths in Brazil alone from 2007 to 2010 [19]
Phoneutria P. reidyi Brazilian wandering spider 30 mm .00061–.00157 mg/kg [34] 0.3 mg/kg
Sicarius spp. Six-eyed sand spider 17 mm large necrotic lesions
Haplopelma H. huwenum (previously Selenocosmia huwena) Chinese bird spider 0.70 mg/kg [36] 1 death reported of a 5-year-old child suffocated, possibly caused by allergens to the venom.
Poecilotheria P. ornata Fringed ornamental tarantula Instances of coma reported.[37][unreliable source?]
Poecilotheria P. fasciata ** Sri Lankan ornamental tarantula Instances of cardiac failure reported [37][dead link][unreliable source?]
Cheiracanthium spp. Yellow Sac spider 6–10 mm one case of irreversible damage to the skin reported [19]
Cheiracanthium C. japonicum Japanese sac spider 6–10 mm
Macrothele M. holsti, M. gigas, M. taiwanensis [30] Primitive burrowing spiders No deaths reported in Taiwan.[38]
Steatoda S. grossa Cupboard spider Mild widow-like symptoms reported, no severe consequences
Study suggests its venom can be effective in treating widow bites because of their similarity.

* 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.

References

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