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Loxoscelism is a condition produced by the bite of the recluse spiders (genus Loxosceles). It is the only proven cause of arachnogenic necrosis in humans.[n 1] While there is no known therapy effective for loxoscelism, there has been research on potential antivenoms and vaccines. Due to the number of diseases that may mimic loxoscelism, it is frequently misdiagnosed by physicians.[n 2]
The first documented case of loxoscelism in the United States occurred in 1879 in Tennessee. Although there are up to thirteen different Loxosceles species in North America (eleven native and two nonnative), Loxosceles reclusa is the species most often involved in serious envenomation. 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.
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. Although the majority of Loxosceles bites result in minor skin irritation, severe envenomation, while rare, may produce painful ulcerative lesions up to 15.75 inches (40 cm) across. Such lesions often heal within 6 to 8 weeks, but can leave lasting scars.
- Viscerocutaneous loxoscelism refers to the systemic manifestations that occur (infrequently) after Loxosceles bites. Symptoms include nausea and vomiting, malaise, fever, hemolytic anemia, and thrombocytopenia.:455 Disseminated intravascular coagulation may appear in as many as 16% of patients, most often children. Occasionally, acute renal failure may develop from myonecrosis and rhabdomyolysis, leading to coma and eventual death.
Loxosceles toxins 
Loxosceles venom has several toxins, the most important of which is the tissue-destroying agent sphingomyelinase D, in all recluse species to varying degrees. Only one other known spider genus (Sicarius) and several pathogenic bacteria are known to contain this enzyme.
Many necrotic lesions are erroneously attributed to the bite of the Brown Recluse, especially in areas outside of its natural habitat. Diagnosis can be difficult because it is usually necessary to retrieve the envenomating spider. There is no known concise chemical test to determine if the venom in a patient is from a Brown Recluse. The bite itself is not usually painful, and is in many cases not felt. Bite victims may delay seeking medical treatment for up to a week. The diagnosis is further complicated by the fact that the brown recluse does not have a remarkable physical appearance. Because of this, other, non-necrotic species are frequently mistakenly identified as a brown recluse. Only a certified arachnologist is able to positively identify a brown recluse specimen as such.
One possible explanation for the disproportionate amount of misdiagnosed bites is the Tegenaria agrestis, also known as the Hobo spider. The T. agrestis may also have a necrotic bite, though these claims are disputed. The matter is being more diligently researched because of some strong circumstantial evidence that the T. agrestis is not necrotic. The area with the highest frequency of questionable brown recluse diagnoses, which is the North West United States, is not within the brown recluse’s range. It is, however, the main area where T. agrestis may be found.
Despite being the most dangerous type of spider bite in many areas, there is no established treatment for the bite of a Loxosceles spider. In many cases the body can heal itself, and the only treatment is to wait. There are, however, some remedies currently being researched.
Anti-venoms can be effective in controlling the symptoms of a necrotic 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. The effectiveness of such anti-venoms is very time-dependent. If 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 most anti-venoms are largely ineffective at this point. Because of this, anti-venoms are perhaps more effective in theory than in practice. They have, however, been proven to be very effective if administered in a timely manner and should not be discarded as a legitimate technique when circumstances permit.
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 spread of necrosis is a great enough threat that it needs to be removed. If the necrosis is allowed to spread far enough, sometimes it is necessary to amputate a limb, or part of a limb, to prevent potentially deadly spread of necrosis.
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 Brown Recluse is the most commonly encountered by humans. The range of the brown recluse spider extends from southeastern Nebraska to southernmost Ohio and south into Georgia and most of Texas. It can be distinguished by violin shaped markings on its back, and this is the commonly known identifying feature. A more conclusive way to identify a brown recluse is by the number of eyes. The brown recluse has six eyes, an uncommon number among arachnids. However, many lesser known species of the Loxosceles genus are believed to have similar venoms. L. reclusa is a very non-aggressive species. 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 of its fairly extensive range, its bite is the most common 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. 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 
L. deserta is found in the southwest corner of the 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 significant due to the high likelihood of human-to-spider encounters.
Tegenaria agrestis (Hobo or Aggressive House Spider) 
Many necrotic lesions in the northwestern United States have been attributed to the bite of the brown recluse spider. Most of these diagnoses are probably erroneous, however, because this area is outside of the natural range of the brown recluse. In a significant number of the homes of bite victims in the northwest, there have been large populations of T. agrestis. This fact has led many to believe that the Hobo Spider is also necrotic.
See also 
- List of cutaneous conditions
- List of spiders associated with cutaneous reactions
- The recluse spiders are the only genus definitively shown to cause necrotic bites in humans. 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.
- 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.
- Swanson, David L.; Vetter, Richard S. (2006). "Loxoscelism". Clinics in Dermatology 24 (3): 213–21. doi:10.1016/j.clindermatol.2005.11.006. PMID 16714202. Retrieved 12 April 2011.
- 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. ISSN 1824-307X. Retrieved 12 April 2011.
- James, William D.; Berger, Timothy G. (2006). Andrews' Diseases of the Skin: clinical Dermatology. Saunders Elsevier. ISBN 0-7216-2921-0.
- Vetter, Richard S.; Bush, Sean P. (2002). "Reports of Presumptive Brown Recluse Spider Bites Reinforce Improbable Diagnosis in Regions of North America Where the Spider Is Not Endemic". Clinical Infectious Diseases 35 (4): 442–446. doi:10.1086/341244.
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- 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. ISSN 0093-0415. PMC 1022570. PMID 8053187.
- 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. doi:10.1590/S0037-86822003000300019. PMID 12908048.
- Baird, Craig R.; Stoltz, Robert L. (2005). Range Expansion of the Hobo Spider, Tenegaria agrestis, in the Northwestern United States (Araneae, Agelenidae).