The pathophysiology of a spider bite is due to the effect of its

venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a st ...

. A spider envenomation occurs whenever a spider injects

into the skin. Not all spider bites inject venom – a dry bite, 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 An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...

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. 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 toxic to humans (in the quantities delivered) to require medical attention. Of those that do, fatal outcomes are exceedingly rare. Spider venoms work on one of two fundamental principles; they are either neurotoxic (impairing the nervous system) or necrotic (dissolving tissues surrounding the bite). In some cases, the venom targets vital organs and systems.

Neurotoxic venom

Spiders paralyze prey with

neurotoxic Neurotoxicity is a form of toxicity in which a biological, chemical, or physical agent produces an adverse effect on the structure or function of the central and/or peripheral nervous system. It occurs when exposure to a substance – specificall ...

venom of some sort. A few have a venom that cross reacts with mammalian nervous system, though the specific manner in which the

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...

is attacked varies from spider to spider. When motor neurons are excited the symptoms include muscle spasms, cramps, and twitching. When autonomic nerves are involved sweating, drooling, gooseflesh. In the extreme unstable blood pressure and heart rate can result. *

Widow spider ''Latrodectus'' is a broadly distributed genus of spiders with several species that are commonly known as the true widows. This group is composed of those often loosely called black widow spiders, brown widow spiders, and similar spiders. Howeve ...

venom contains components known as

latrotoxin A latrotoxin is a high-molecular mass neurotoxin found in the venom of spiders of the genus ''Latrodectus'' (widow spiders) as well as at least one species of another genus in the same family, '' Steatoda nobilis''. Latrotoxins are the main activ ...

s, which cause the massive release of the

neurotransmitters A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neurot ...

causing muscle contractions, sweating, and gooseflesh. This can affect the body in several ways, including causing painful abdominal cramps * The atracotoxins of Australian funnel-web spiders work by opening

sodium channel Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the chann ...

s, causing excessive neural activity including strange sensations (paresthesias), muscle contractions, unstable blood pressure (hypertension or hypotension). The venom may cause fluid to accumulate in the lungs (pulmonary edema) that can be fatal. * The venom of

Brazilian wandering spider ''Phoneutria'' is a genus of spiders in the family (biology), family Ctenidae. Phoneutria venom is potentially medically significant to humans. They are mainly found in northern South America, with one species in Central America. Members of t ...

s is also a potent neurotoxin, which attacks multiple types of ion channels. Principally generating severe pain that travels up the limb, autonomic effects, including painful erections, occur with moderate envenomation. With severe envenomation heart and lung failure can result in death. In addition, the venom contains high levels of serotonin, making an envenomation by this species particularly painful.

Necrotic venom

Spiders known to have

necrotic Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...

venom occur most notoriously in the family

Sicariidae Sicariidae is a family of six-eyed venomous spiders known for their potentially necrotic bites. The family consists of three genera and about 160 species. Well known spiders in this family include the brown recluse spider and the six-eyed sand ...

, which includes both the recluse spiders and the six-eyed sand spiders in the genus ''

Hexophthalma ''Hexophthalma'' is a genus of spiders in the family Sicariidae. Although the genus was originally erected in 1878 (then with the name ''Hexomma''), it was merged into the genus '' Sicarius'' in the 1890s, and remained unused until revived in 2 ...

'' and '' Sicarius''. Spiders in this family possess a known dermonecrotic agent

sphingomyelinase D Sphingomyelin phosphodiesterase D (EC 3.1.4.41, sphingomyelinase D) is an enzyme of the sphingomyelin phosphodiesterase family with systematic name sphingomyelin ceramide-phosphohydrolase. These enzymes catalyse the hydrolysis of sphingomyelin, res ...

, which is otherwise found only in a few pathogenic bacteria. 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 Kidney failure, also known as end-stage kidney disease, is a medical condition in which the kidneys can no longer adequately filter waste products from the blood, functioning at less than 15% of normal levels. Kidney failure is classified as eit ...

, and in some cases, death. Even in the absence of systemic effects, serious bites from sicariid spiders may form a necrotising

ulcer An ulcer is a discontinuity or break in a bodily membrane that impedes normal function of the affected organ. According to Robbins's pathology, "ulcer is the breach of the continuity of skin, epithelium or mucous membrane caused by sloughing o ...

that destroys soft tissue and may take months and very rarely years to heal, leaving deep

scar A scar (or scar tissue) is an area of fibrous tissue that replaces normal skin after an injury. Scars result from the biological process of wound repair in the skin, as well as in other organs, and tissues of the body. Thus, scarring is a n ...

s. The damaged tissue may become

gangrenous Gangrene is a type of tissue death caused by a lack of blood supply. Symptoms may include a change in skin color to red or black, numbness, swelling, pain, skin breakdown, and coolness. The feet and hands are most commonly affected. If the ga ...

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. Systemic effects are unusual but include mild

nausea Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of the ...

vomiting Vomiting (also known as emesis and throwing up) is the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting can be the result of ailments like food poisoning, gastroenteri ...

fever Fever, also referred to as pyrexia, is defined as having a temperature above the normal range due to an increase in the body's temperature set point. There is not a single agreed-upon upper limit for normal temperature with sources using val ...

rash A rash is a change of the human skin which affects its color, appearance, or texture. A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, c ...

es, and muscle and joint pain. Rarely, more severe symptoms occur including red blood cell destruction (

hemolysis Hemolysis or haemolysis (), also known by several other names, is the rupturing ( lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo ...

), low platelets (

thrombocytopenia Thrombocytopenia is a condition characterized by abnormally low levels of platelets, also known as thrombocytes, in the blood. It is the most common coagulation disorder among intensive care patients and is seen in a fifth of medical patients a ...

), and loss of clotting factors ( disseminated intravascular coagulation). Children may be more susceptible to systemic

loxoscelism Loxoscelism () is a condition occasionally produced by the bite of the recluse spiders (genus ''Loxosceles''). The area becomes dusky and a shallow open sore forms as the skin around the bite dies ( necrosis). It is the only proven type of nec ...

effects. Deaths have been reported for both the brown recluse and the related South American species ''

Loxosceles laeta The Chilean recluse spider, ''Loxosceles laeta'', is a highly venomous spider of the family Sicariidae. In Spanish, it (and other South American recluse spiders) is known as ', or "corner spider"; in Brazilian Portuguese, as ' or "brown spider". ...

'' and '' Loxosceles intermedia'' related to hemolysis and the injury that results to the kidney. Deaths attributed to brown recluse where no brown recluse live, highlight misdiagnosis and misconception Numerous other spiders have been associated with necrotic bites. The white tailed spider (''Lampona'' spp.) had been suspected in necrotic lesions for decades only to be exonerated by the first extensive review. An early report Sac spider causing necrosis has been frequently referenced. Recent surveys doubt the incidence of necrosis. Necrosis from Hobo spider, a member grass spider family Agelenidae, bite is under the same debate and doubt.

Differential diagnosis

The skin manifestations of recluse venom are thought to arise from

Sphingomyelinase Sphingomyelin phosphodiesterase (EC 3.1.4.12, also known as neutral sphingomyelinase, sphingomyelinase, or SMase; systematic name sphingomyelin cholinephosphohydrolase) is a hydrolase enzyme that is involved in sphingolipid metabolism reactions. ...

D. The enzyme acts on cell membranes. The action is therefore limited as the venom can only spread through a set area. The originally red swollen area becomes a dry black ulcer. Skin infections, in particular the widespread methicillin-resistant Staphylococcal aureus, remain swollen and red. Pus forms and the lesion often drains. It can continue to spread and expand as the bacteria grow. Other skin lesions and infections are much more common than spider bites. Physicians have reported brown recluse spider bites where no brown recluse exist. ''Ed: in 100 pictures retrieved online only 3 were consistent with Sphingomyelinase pathophysiology.''

Incidence of severe envenomation

Despite public concern about spider bites, severe envenomation is rare. Few spiders have toxins in sufficient volume to harm people. Of those that do, some have limited habitats. Spider behavior may be caused by limited human interaction. Spider defense against predators include camouflage, and escape by falling or running. Biting is a last resort and the amount of venom injected varies greatly. Spider venom toxicity can be evaluated in experimental animals, or reported from accidental bites. Different experimental animals have different reactions to the same venom. The dose that is lethal to half of the animals poisoned is the LD50 and those values for mice is below. The LD50 of many poisons is known for humans but not that of spider venom. 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 The Sydney funnel-web spider (''Atrax robustus'') is a species of venomous mygalomorph spider native to eastern Australia, usually found within a radius of Sydney. It is a member of a group of spiders known as Australian funnel-web spiders. I ...

; that death occurred before the development of an

antivenom Antivenom, also known as antivenin, venom antiserum, and antivenom immunoglobulin, is a specific treatment for envenomation. It is composed of antibodies and used to treat certain venomous bites and stings. Antivenoms are recommended only if th ...

. Since the antivenom was developed there have been no fatalities due to this species. The neurotoxic venoms of the

and the

are both known to have lethal complications. For the Brazilian wandering spider only 1 out of 200 bites is serious, in part because they appear to be capable of biting without injecting venom. ''Atrax robustus'' has a limited distribution and few bites are reported yearly. The geographical range of the widow spiders is very great. As a result, far more people are exposed, worldwide, to widow bites than any other spider. Widow spiders bites are most often mild but 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. Loxosceles live in areas of South America and the southern United States. There are populations of desert and Arizona recluse in the Western deserts of the United States, but bites are rarely reported from these species. In established areas many spiders may populate the home. Even still the "reclusive" nature of the spider limits true bites. More bites had been reported in Florida than recluses ever found in the area.

Clinical presentation

{, class="wikitable" , - align="left" style="background-color: #cccccc;" ! Genus ! Species ! Common name ! Body length ! Venom amount ! LD-50 ! Alternate LD-50 ! Deaths reported , - , ''

Atrax ''Atrax'' is a genus of venomous Australian funnel web spiders that was first described by O. Pickard-Cambridge in 1877 from the type species ''Atrax robustus''. it contains only three species: ''A. robustus'', '' A. sutherlandi'', and '' A. ...

'' , ''A. robustus'' ,

, 24–32 mm. , 0.25 mg (F) and 0.81 mg (M) 2 mg , 0.16 mg/kg , unknown , 13 attributed deaths from 1927 to 1980 , - , '' Hadronyche'' , ''H. formidabilis'' , Northern tree funnel-web spider , 23–45 mm. , , , , 1 death. High rates of severe envenoming. , - , '' Hadronyche'' , ''H. cerberea'' , Southern tree funnel-web spider , , , , , High rates of severe envenoming. , - , ''

Latrodectus ''Latrodectus'' is a broadly distributed genus of spiders with several species that are commonly known as the true widows. This group is composed of those often loosely called black widow spiders, brown widow spiders, and similar spiders. Howeve ...

'' , '' L. mactans'' , Black widow , 8–15 mm , 0.02–.03 mg.http://www.thudiv.com/variety/spider/spider1.htm) (Tung Hai University, Taiwan, article in Chinese broken link) , 0.002 mg/kg * , 0.9 mg/kg , 36 deaths recorded from 1965 to 1990 in the U.S.
5% of reported bites prior to

availability , - , ''

'' , ''L. tredecimguttatus'' , Malmignatte , (approx. same) , (approx. same) , 0.68 μg/kg , 16.25 μg/kg , possibility of deaths in Southern Europe first attributed to the brown recluse, suggesting larger frequency of the bites. , - , ''

Loxosceles The recluse spiders (''Loxosceles'' (), also known as brown spiders, fiddle-backs, violin spiders, and reapers, is a genus of spiders that was first described by R. T. Lowe in 1832. They are venomous spiders known for their bite, which sometimes ...

'' , ''L. reclusa'' , Brown recluse , 1.2 cm (0.75 in) 6–10 mm , .13–.27 mg. , , , necrosis and amputation of limbs more common, deaths rare , - , ''Loxosceles'' , ''L. intermedia'' , , , , 0.48 mg/kg 0.34 mg/kg (SC) , , , - , ''Loxosceles'' , ''L. laeta'' ,

Chilean recluse The Chilean recluse spider, ''Loxosceles laeta'', is a highly venomous spider of the family Sicariidae. In Spanish, it (and other South American recluse spiders) is known as ', or "corner spider"; in Brazilian Portuguese, as ' or "brown spider". ...

, , , 1.45 mg/kg , , High rates of severe envenomation. , - , ''Loxosceles'' , ''L. gaucho'' , , , , 0.74 mg/kg , , , - , ''

Phoneutria ''Phoneutria'' is a genus of spiders in the family Ctenidae. Phoneutria venom is potentially medically significant to humans. They are mainly found in northern South America, with one species in Central America. Members of the genus are commo ...

'' , ''P. bahiensis'' ,

, 30 mm , 1.079 mg , .00061–.00157 mg/kg , , cardiac failure reported in 5 out of 12 bitten , - , ''Phoneutria'' , ''P. boliviensis'' ,

, 30 mm , 1.079 mg. , .00061–.00157 mg/kg , , , - , ''Phoneutria'' , ''P. fera'' ,

, 30 mm , 1.079 mg , .00061–.00157 mg/kg , , disputed effectiveness of the antivenin – 4 deaths out of 7 administered , - , ''Phoneutria'' , ''P. nigriventer'' ,

, 3–5 cm (1.25–2 in) , 2.15 mg 1.079 mg. , 15.20 ng/mg. 00061–.00157 mg/kg , 200 µg/kg (0.2 ng/mg) , severe cardiac failure, signs of

priapism Priapism is a condition in which a penis remains erect for hours in the absence of stimulation or after stimulation has ended. There are three types: ischemic (low-flow), nonischemic (high-flow), and recurrent ischemic (intermittent). Most cases ...

and irreversible damage to the central nervous system recorded.
18 deaths in Brazil alone from 2007 to 2010 , - , ''Phoneutria'' , ''P. reidyi'' ,

, 30 mm , , .00061–.00157 mg/kg , 0.3 mg/kg , , - , ''

'' , spp. , Six-eyed sand spiders , 17 mm , , , , large necrotic lesions , - , '' Haplopelma'' , '' H. schmidti'' (previously ''H. huwenum'', ''Selenocosmia huwena'') , Chinese bird spider , , 0.70 mg/kg , , 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.http://www.spidertalk.net/SpiderTalk/post.php?action=reply&fid=1&tid=2165&repquote=16279 , - , ''Poecilotheria'' , ''P. fasciata'' ** , Sri Lankan ornamental tarantula , , , , , Instances of cardiac failure reported , - , '' Cheiracanthium'' , spp. , Yellow Sac spider , 6–10 mm , , , , one case of irreversible damage to the skin reported , - , ''Cheiracanthium'' , ''C. japonicum'' , Japanese sac spider , 6–10 mm , , , , , - , ''

Macrothele ''Macrothele'' is a genus of mygalomorph spiders in the family Macrothelidae, and was first described by A. Ausserer in 1871. It is the only genus in the family Macrothelidae, and most species occur in Asia, from India to Japan, and Java, with ...

'' , ''M. holsti'', ''M. gigas'', ''M. taiwanensis'' , Primitive burrowing spiders , , , , , No deaths reported in Taiwan.{{cite journal , author1=Hung, Shin-Wen , author2=Wong, Tzong-Leun , title=Arachnid Envenomation in Taiwan , journal=Ann. Disaster Med. , volume=3 , issue=Suppl. 1 , pages=S12–S17 , url=http://www.disaster.org.tw/chinese/annmed/Vol3supp1/3.pdf , - , '' 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

Parasitic infestations, stings, and bites of the skin spider bites Toxic effects of venomous animals