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Title: Loxoscelism  
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Subject: Spider bite, Latrodectism, Bee sting, Bristleworm sting, Hydroid dermatitis
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Classification and external resources
Dermonecrosis, a hallmark of loxoscelism
ICD-10 T63.3
ICD-9 989.5
Scars left over from a healed recluse bite.

Loxoscelism is a condition occasionally produced by the bite of the recluse spiders (genus Loxosceles). The area becomes dusky and becomes a deep open sore as the skin around the bite dies (necrosis). It is the only proven type of necrotic arachnidism in humans.[1][n 1] While there is no known therapy effective for loxoscelism, there has been research on antibiotics, surgical timing, hyperbaric oxygen, potential antivenoms and vaccines.[1] Because of the number of diseases that may mimic loxoscelism, it is frequently misdiagnosed by physicians.[n 2]

Loxoscelism was first described in the United States in 1879 in Tennessee.[2] Although there are up to 13 different Loxosceles species in North America (11 native and two nonnative), Loxosceles reclusa is the species most often involved in serious envenomation. Loxosceles reclusa has a limited habitat that includes the Southeast United States. In South America, L. laeta, L. intermedia (found in Brazil and Argentina), and L. gaucho (Brazil) are the three species most often reported to cause necrotic bites.[1]


Loxoscelism may present with local and whole-body symptoms:

  • Necrotic cutaneous loxoscelism is the medical term for the reaction most common in loxoscelism. It is characterized by a localized gangrenous slough at the site of bite. The majority of Loxosceles bites result in minor skin irritation that heals in one week. Severe reactions, while rare, may produce painful ulcerative lesions up to 15.75 inches (40 cm) across.[1] Such lesions often heal within 6 to 8 weeks, and can leave lasting scars.
  • Viscerocutaneous loxoscelism refers to the combination of local and systemic manifestations that occur infrequently after Loxosceles bites. Symptoms include low energy, nausea and vomiting, and fever. Destruction of blood cells (hemolytic anemia) may require transfusion and injure the kidney.[3]:455 Consumption of clotting factors (so called Disseminated intravascular coagulation) and destruction of platelets (thrombocytopenia) is reported most often in children. DIC may lead to dangerous bleeding. Occasionally, acute renal failure may develop from myonecrosis and rhabdomyolysis, leading to coma.[1]

Loxosceles toxins

Loxosceles venom has several toxins, the most important for necrotic arachnidism is the tissue-destroying agent sphingomyelinase D. It is present in all recluse species to varying degrees and not all recluse are equivalent. This toxin is present in only one other known spider genus (Sicarius).[1] The toxin converts the structural components of the cell membrane into strange ring forms that perhaps act as a trigger for cellular self-destruction.[4]


The spider biting apparatus is short and bites are only possible in experimental animals with pressure on the spider's back. Thus many bites occur when a spider is trapped in a shirt or pant sleeve. Diagnosis should not be difficult because the envenomating spider was trapped. There is no commercial chemical test to determine if the venom is from a Brown Recluse. The bite itself is not usually painful. Many necrotic lesions are erroneously attributed to the bite of the Brown Recluse. (See Note). Skin wounds are common and infections will lead to necrotic wounds. Thus many terrible skin infections are attributed falsely to brown recluse.[5] Many suspected bites occurred in areas outside of its natural habitat.[6] A wound found one week later may be misattributed to spider. The diagnosis is further complicated by the fact that no attempt is made to positively identify the suspected spider. Because of this, other, non-necrotic species are frequently mistakenly identified as a brown recluse.[7] Several certified arachnologist are able to positively identify a brown recluse specimen on request.[8]

Reports of presumptive Brown Recluse spider bites reinforce improbable diagnoses in regions of North America where the spider is not endemic such as Florida, Pennsylvania, and California.


Despite being one of the few medically important spider bite, there is no established treatment for the bite of a Loxosceles spider. Physicians help the body to heal itself, and the only treatment is to wait. There are, however, some remedies currently being researched.


Anti-venoms are commercially prepared antibodies to toxins in animal bites. They are specific for each bite. There are several anti-venoms commercially available in Brazil, which have been shown to be effective in controlling the spread of necrosis in rabbits.[9] When administered immediately, they can almost entirely neutralize any ill effects. If too much time is allowed to pass, the treatment becomes ineffective. Most victims do not seek medical attention within the first twelve hours of being bitten, and these anti-venoms are largely ineffective after this point. Because of this, anti-venoms are not being developed more widely. They have, however, been proven to be very effective if administered in a timely manner and could be utilized in Brazil as a legitimate technique.

Surgical treatment

In cases where a large dermonecrotic lesion has developed, sometimes it is most effective to surgically remove the dead tissue. This is not ideal, since it will usually leave a large open sore behind, but in certain cases, the dead tissue becomes an infectious threat and needs to be removed.

Other species

It is suspected that most if not all species of the Loxosceles genus have necrotic venom. Over fifty species have been identified in the genus, but significant research has only been conducted on species living in close proximity to humans.

Loxosceles reclusa (Brown Recluse Spider)

Among the spiders bearing necrotic venom, the L. reclusa is a very non-aggressive species.[10] There have been documented cases where a house has a very large population for many years without any of the human inhabitants being bitten. For this reason, L. reclusa bites are relatively rare, but, because its range is overlaps human habitation, its bite is the cause of loxoscelism in North America.

Loxosceles laeta (Chilean Recluse Spider)

Loxosceles laeta, commonly known as the Chilean Recluse Spider, is generally considered to be one of the most toxic species in the Loxosceles genus. It has a very wide range, having populations in Guatemala, Panama, Curaçao, Trinidad, Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile and Argentina in South and Central America. In North America, there are populations in Vancouver, Canada, Massachusetts, California, Kansas and Florida. L. laeta can also be found in Finland and Australia. L. laeta has been documented at elevations between 200m and 2340m.[11] This range can probably be attributed to the species ability to last long periods of time without food or water. The laeta is cryptozoic, meaning it lives in dark concealed places. This can often mean piles of wood or brick for the laeta, facilitating more transportation of the species into new areas. Another reason for the laeta’s strong populations is the high fertility rate among its females. Each female can produce up to fifteen egg sacs in its life, with between fifty and one hundred and fifty eggs in each. Loxosceles laeta eggs have a high egg fertility index.

Loxosceles deserta (Desert recluse)

L. deserta is found in the Southwest United States. Human interactions with it are rare, because it usually is only found in native vegetation. It is not usually found within heavily populated areas, but its range does come near these areas. It is considered medically unimportant due to the low likelihood of human-to-spider encounters.

Cheiracanthium inclusum (Yellow Sac Spider)

Cheiracanthium inclusum, also known as the Black-Footed Yellow Sac spider, has been implicated in necrotic skin lesions. The C. inclusum's venom may be weakly necrotic. Arachnologists contest this assertion [12] This spider can be found all over North, Central, and South America, as well as in The West Indies. It is often encountered by people indoors and outdoors alike.

Tegenaria agrestis (Hobo Spider)

Many necrotic lesions in the northwestern United States have been attributed to spider bite. The Centers for Disease Control made a survey[13] as brown recluse are not in the Pacific Northwest. However, there is a large populations of T. agrestis.[14] This fact has led many to believe that the bite of Hobo Spider is also necrotic. Critics note that this evidence is only circumstantial [5] The species itself is actually of European origins and known to have never caused such effects over the hundreds of years that it has been known, recorded, interacted and bitten people. Their medically significant bite should be regarded as a myth.[15][16]

See also


  1. ^ The recluse spiders are the only genus definitively shown to cause necrotic bites in humans. The layers of skin die and slough away leaving an ulcer. Since at least 1872, the blanket term necrotic arachnidism has been used in the medical literature, often erroneously implicating spiders that do not cause dermonecrosis. Spider species blamed for necrosis in the past have included wolf spiders, white-tailed spiders, black house spiders, yellow sac spiders, orb weavers, and funnel-weaving spiders such as the hobo spider.[1]
  2. ^ Diseases that may cause symptoms similar to loxoscelism include: streptococcal or staphylococcal infection (particularly by methicillin-resistant Staphylococcus aureus), herpes simplex, herpes zoster, diabetic ulcer, fungal infection, pyoderma gangrenosum, lymphomatoid papulosis, chemical burn, Toxicodendron dermatitis, squamous cell carcinoma, neoplasia, localized vasculitis, syphilis, Stevens-Johnson syndrome, toxic epidermal necrolysis, erythema nodosum, erythema multiforme, gonococcemia, purpura fulminans, sporotrichosis, Lyme disease, cowpox, and anthrax.[1]


  1. ^ a b c d e f g h Swanson, David L.; Vetter, Richard S. (2006). "Loxoscelism". Clinics in Dermatology 24 (3): 213–21.  
  2. ^ Appel, MH; Bertoni da Silveira, R; Gremksi, W; Veiga, SS (2005). "Insights into brown spider and loxoscelism". Invertebrate Survival Journal (University of Modena and Reggio Emilia) 2 (2): 152–158.  
  3. ^ James, William D.; Berger, Timothy G. (2006). Andrews' Diseases of the Skin: clinical Dermatology. Saunders Elsevier.  
  4. ^
  5. ^ a b Vetter, Richard S. "Medical Myth: Myth: idiopathic wounds are often due to brown recluse or other spider bites throughout the United States." Western Journal of Medicine 173.5 (2000): 357.
  6. ^ Vetter, Richard S., G. B. Edwards, and Louis F. James. "Reports of envenomation by brown recluse spiders (Araneae: Sicariidae) outnumber verifications of Loxosceles spiders in Florida." Journal of medical entomology 41.4 (2004): 593-597.
  7. ^ Vetter, Richard S. "Arachnids misidentified as brown recluse spiders by medical personnel and other authorities in North America." Toxicon 54.4 (2009): 545-547.
  8. ^
  9. ^ Barbaro, K.C.; Knysak, I.; Martins, R.; Hogan, C.; Winkel, K. (2005). "Enzymatic Characterization, Antigenic Cross-Reactivity And Neutralization Of Dermonecrotic Activity Of Five Loxosceles Spider Venoms Of Medical Importance In The Americas". Toxicon 45 (4): 489–99.  
  10. ^ Fisher, R. G.; Kelly, P.; Krober, M. S.; Weir, M. R.; Jones, R. (1994). "Necrotic Arachnidism". The Western Journal of Medicine 160 (6): 570–2.  
  11. ^ Gonçalves-de-Andrade, Rute M.; Tambourgi, Denise V. (2003). "First Record On Loxosceles Laeta (Nicolet, 1849) (Araneae, Sicariidae) In The West Zone Of São Paulo City, São Paulo, Brazil, And Considerations Regarding Its Geographic Distribution". Revista da Sociedade Brasileira de Medicina Tropical 36 (3): 425–6.  
  12. ^ Vetter, Richard S., et al. "Verified bites by yellow sac spiders (genus Cheiracanthium) in the United States and Australia: Where is the necrosis?." American Journal of Tropical Medicine and Hygiene 74.6 (2006): 1043.
  13. ^
  14. ^ Baird, Craig R.; Stoltz, Robert L. (2005). "Range Expansion of the Hobo Spider, Tenegaria agrestis, in the Northwestern United States (Araneae, Agelenidae)". 
  15. ^ Diaz, James H. "Most necrotic ulcers are not spider bites." The American journal of tropical medicine and hygiene 72.4 (2005): 364-367.
  16. ^ Bennett, Robert G., and Richard S. Vetter. "An approach to spider bites. Erroneous attribution of dermonecrotic lesions to brown recluse or hobo spider bites in Canada." Canadian family physician 50.8 (2004): 1098-1101.
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