View source for Brown recluse spider

{{Short description|Species of spider with venomous bite native to US}}
{{About||the spider ranging from southeastern California to Arizona|desert recluse|the spider inhabiting Hawaii|Mediterranean recluse spider}}
{{Use dmy dates|date=February 2021}}
{{Speciesbox
| name = Brown recluse
| image = Brown Recluse.jpg
| genus = Loxosceles
| species = reclusa
| authority = [[Willis J. Gertsch|Gertsch]] & [[Stanley B. Mulaik|Mulaik]], 1940<ref name=WSC_s34303>{{citation |title=Taxon details ''Loxosceles reclusa'' Gertsch & Mulaik, 1940 |work=World Spider Catalog |publisher=Natural History Museum Bern |url=http://www.wsc.nmbe.ch/species/34303 |access-date=December 17, 2016 }}</ref>| range_map = Loxosceles reclusa range.png
| synonyms = {{Specieslist|Loxosceles reclusus|orthographic variant}}
| synonyms_ref =<ref name=WSC_s34303/>
}}

The '''brown recluse''' ('''''Loxosceles reclusa''''', [[Sicariidae]], formerly placed in a family "Loxoscelidae") is a [[recluse spider]] with [[Necrosis|necrotic]] venom. Similar to those of other recluse spiders, their bites sometimes require medical attention. The brown recluse is one of two spiders in North America with dangerous venom, the other being the [[Latrodectus|black widow]].

Brown recluse spiders are usually between {{convert|6| and |20|mm}}, but may grow larger. While typically light to medium brown, they range in color from whitish to dark brown or blackish gray. The [[cephalothorax]] and abdomen are not necessarily the same color. These spiders usually have markings on the [[Dorsum (biology)|dorsal]] side of their [[cephalothorax]], with a black line coming from it that looks like a violin with the neck of the violin pointing to the rear of the spider, resulting in the nicknames '''fiddleback spider''', '''brown fiddler''', or '''violin spider'''.<ref name=Yp>{{cite web |url=https://www.yikespest.com/ |title=Brown recluse in Newburgh Indiana | access-date=5 Apr 2021 |website=Yikespest |df=dmy-all }}</ref>

==Description==
[[File:Loxosceles reclusa adult male 4.jpg|thumb|The brown recluse has three pairs of eyes, unlike most spiders with four pairs of eyes.]]

The violin pattern is not a definitive identifier, as other spiders can have similar markings (e.g. [[Pholcidae|cellar spiders]] and [[pirate spider]]s). Instead, while most spiders have eight eyes, recluse spiders have six eyes arranged in pairs (dyads) with one [[Medial (anatomy)|median]] pair and two [[lateral (anatomy)|lateral]] pairs. Only a few other spiders have three pairs of eyes arranged in this way (e.g., [[Scytodidae|scytodids]]). Recluses have no obvious coloration patterns on the abdomen or legs, and the legs lack spines.<ref name=DermatolOnlineJ1999-Vetter>
{{cite journal
 | last1 = Vetter    | first1 = R.
 | last2 = Shay      | first2 = M.
 | last3 = Bitterman | first3 = O.
 | year = 1999
 | title = Identifying and misidentifying the brown recluse spider
 | journal = [[Dermatology Online Journal]]
 | volume = 5 | issue = 2 | page = 7
 | pmid = 10673460
 | doi = 10.2340/00015555-0082 | doi-access = free 
 | url=http://dermatology.cdlib.org/DOJvol5num2/special/recluse.html
}}
</ref>
The violin marking can vary in intensity depending on the age of the brown recluse spider, with mature spiders typically having dark violin shapes.<ref>
{{cite web
 |first=Michael F. |last=Potter
 |date=12 July 2018
 |title=Brown recluse spider
 |series=Department of Entomology / College of Agriculture
 |publisher=[[University of Kentucky]]
 |url=https://entomology.ca.uky.edu/ef631
 |access-date=15 August 2022
}}
</ref><ref>
{{cite web
 |first=Laura |last=Geggel
 |date=18 February 2022
 |title=Brown recluse spiders: Facts, bites & symptoms
 |website=Livescience
 |url=https://www.livescience.com/39996-brown-recluse-spiders.html
 |access-date=15 August 2022
 |lang=en
}}
</ref>

==Distribution==
[[File:Brown-recluse-coin-edit.jpg|right|thumb|A large brown recluse compared to a [[Penny (United States coin)|US penny]] (diameter {{convert|0.75|in|mm|disp=or}})]]
The documented range of this species lies roughly south of a line from southeastern [[Nebraska]] through southern [[Iowa]], [[Illinois]], and [[Indiana]] to southwestern [[Ohio]]. In the southern states, it is native from central [[Texas]] to western [[Georgia (U.S. state)|Georgia]] and north to [[Kentucky]].<ref>
{{cite web
 | last=Jone  | first=S.C.
 | title=Brown recluse spider
 | department = Fact sheet
 | publisher=[[Ohio State University]]
 | url=http://ohioline.osu.edu/hyg-fact/2000/2061.html
 | url-status=dead  | access-date=February 18, 2006
 | archive-url=https://archive.today/20120716133101/http://ohioline.osu.edu/hyg-fact/2000/2061.html
 | archive-date=July 16, 2012
}}
</ref><ref name=NEJM2005-Vetter>
{{cite journal
 | vauthors = Swanson D, Vetter R
 | year = 2005
 | title = Bites of brown recluse spiders and suspected necrotic arachnidism
 | journal = [[New England Journal of Medicine]]
 | volume = 352 | issue = 7 | pages = 700–707
 | pmid = 15716564   | s2cid = 27932931
 | doi = 10.1056/NEJMra041184
}}
</ref>

Despite rumors to the contrary, the brown recluse spider has not established itself in California or anywhere outside its native range.<ref name=VetterMyth>{{cite web | last=Vetter |first=Rick | title=Myth of the brown recluse: Fact, fear, and loathing |website=spiders.ucr.edu |publisher=[[University of California, Riverside]] |place=Riverside, CA | url=https://spiders.ucr.edu/myth-brown-recluse-fact-fear-and-loathing | access-date=26 October 2022 }}</ref> There are other species of the genus ''[[Loxosceles]]'' native to the southwestern part of the United States, including California, which may resemble the brown recluse, but interactions between humans and the recluse species in California and the region are rare because those species' native ranges lie outside of dense human populations.<ref name=VetterMyth/> The number of "false positive" reports based on misidentifications is considerable; in a nationwide study where people submitted spiders that they thought were brown recluses, of 581 from California only one was a brown recluse—submitted by a family that moved from Missouri and brought it with them (compared to specimens submitted from Missouri, Kansas, and Oklahoma, where between 75% and 90% were recluses).<ref>
{{cite journal
 | last = Vetter | first = R.S.
 | year = 2005
 | title = Arachnids submitted as suspected brown recluse spiders (Araneae: Sicariidae): ''Loxosceles'' species are virtually restricted to their known distributions but public perception is that they exist throughout the United States
 | journal = [[Journal of Medical Entomology]]
 | volume = 42 | issue = 4 | pages = 512–521
 | doi = 10.1093/jmedent/42.4.512
 | pmid = 16119538 | s2cid = 22023570 | issn = 0022-2585
}}
</ref>
From this study, the most common spider submitted from California as a brown recluse was in the genus ''[[Titiotus]]'', whose bite is deemed harmless. A similar study documented that various arachnids were routinely misidentified by physicians, pest control operators, and other non-expert authorities, who told their patients or clients that the spider they had was a brown recluse when in fact it was not.<ref>
{{cite journal
 | last = Vetter | first = R.S.
 | year = 2009
 | title = Arachnids misidentified as brown recluse spiders by medical personnel and other authorities in North America
 | journal = Toxicon
 | volume = 54 | issue = 4| pages = 545–547
 | pmid = 19446575
 | doi = 10.1016/j.toxicon.2009.04.021
| bibcode = 2009Txcn...54..545V
 }}
</ref>
Despite the absence of brown recluses from the Western U.S., physicians in the region commonly diagnose "brown recluse bites", leading to the popular misconception that the spiders inhabit those areas.<ref>
{{cite journal
 | last1 = Vetter   | first1 = R.S.       | last2 = Cushing | first2 = P.E.
 | last3 = Crawford | first3 = R.L.       | last4 = Royce   | first4 = L.A.
 | year = 2003
 | title = Diagnoses of brown recluse spider bites (loxoscelism) greatly outnumber actual verifications of the spider's presence in four western American states
 | journal = Toxicon
 | volume = 42 | issue = 4 | pages = 413–418
 | pmid = 14505942
 | doi = 10.1016/S0041-0101(03)00173-9
}}
</ref>

Over the last century, spiders have occasionally been intercepted in locations where they have no known established populations; these spiders may be transported fairly easily, though the lack of ''established'' populations well outside the natural range also indicates that such movement has not led to the colonization of new areas, after decades of opportunities.<ref>
{{cite web
 |last=Edwards |first=G.B.
 |year=2018
 |title=brown recluse spider
 |department=Fact sheet
 |series=Institute of Food and Agricultural Sciences
 |website=entomology.ifas.ufl.edu
 |place=Gainesville, FL 
 |publisher=[[University of Florida]]
 |url=http://entomology.ifas.ufl.edu/creatures/urban/spiders/brown_recluse_spider.htm
}}
</ref><ref>
{{cite press release
 |last=Palmer |first=Diane
 |date=September 2007
 |title=Brown recluse spiders blamed for more wounds than they inflict, study suggests
 |publisher=[[Clemson University]]
 |series=Public service activities
 |url=http://www.clemson.edu/newsroom/articles/2007/september/brownreclusestudy.php5
 |url-status=dead  |access-date=12 September 2007
 |archive-url=https://web.archive.org/web/20090516233122/http://www.clemson.edu/newsroom/articles/2007/september/brownreclusestudy.php5
 |archive-date=16 May 2009
}}
</ref> Note that the occurrence of brown recluses in a single building (such as a warehouse) outside of the native range is not considered as successful colonization; such single-building populations can occur (e.g., in several such cases in Florida),<ref>
{{cite journal
 | last1 = Vetter | first1 = R.S.
 | last2 = Edwards | first2 = G.B.
 | last3 = James | first3 = L.F.
 | year = 2004
 | title = Reports of envenomation by brown recluse spiders (Araneae: Sicariidae) outnumber verifications of ''Loxosceles'' spiders in Florida
 | journal = [[Journal of Medical Entomology]]
 | volume = 41 | issue = 4| pages = 593–597
 | issn = 0022-2585 | pmid = 15311449
 | doi = 10.1603/0022-2585-41.4.593 | doi-access = free
}}
</ref>
but do not spread, and can be easily eradicated.<ref>
{{cite report
 |last=Edwards |first=G.B.
 |year=2001
 |title=The present status and a review of the brown recluse and related spiders, ''Loxosceles'' spp. (Araneae: Sicariidae), in Florida
 |series=Entomology Circular |volume=406
 |publisher=Division of Plant Industry, [[Florida Department of Agriculture and Consumer Services]]
}}
</ref>
The spider has also received numerous sensationalized media reports of bites occurring where these spiders are absent (and no specimens were found), such as a 2014 report from Thailand, where a man was claimed to have died from a brown recluse bite.<ref>
{{cite news
 |first=Taweesak |last=Sukkasem
 |date=28 July 2014
 |title=1st Thai death from recluse spider
 |newspaper=[[Bangkok Post]]
 |place=Thailand
 |url=https://www.bangkokpost.com/thailand/politics/422905/1st-thai-death-from-recluse-spider
 |access-date=5 October 2024
}}
</ref>
Many misidentifications and erroneous geographic records stem from the similarity between ''L. reclusa'' and a related introduced species, the [[Mediterranean recluse spider|Mediterranean recluse]] (''Loxosceles rufescens''), which is found worldwide, including numerous sightings throughout the United States; the two species are superficially almost indistinguishable, and misidentifications are common, making it difficult to distinguish which reports of recluses refer to which species.<ref>
{{cite journal
 | last1 = Greene   | first1 = Albert     | last2 = Breisch | first2 = Nancy L.
 | last3 = Boardman | first3 = Thomas     | last4 = Pagac   | first4 = Benedict B.
 | last5 = Kunickis | first5 = Edward     | last6 = Howes   | first6 = Randall K.
 | last7 = Brown    | first7 = Paul V.
 | year = 2009
 | title = The Mediterranean recluse spider, ''Loxosceles rufescens'' {{small|(Dufour)}}: An abundant but cryptic inhabitant of deep infrastructure in the Washington, D.C. area (Arachnida: Araneae: Sicariidae)
 | journal = [[American Entomologist]]
 | volume = 55 | issue = 3 | pages = 158–169
 | doi = 10.1093/ae/55.3.158 | doi-access = free
}}
</ref>

==Life cycle==
Adult brown recluse spiders often live about one to two years. Each female produces several egg sacs over a period of two to three months, from May to July, with approximately 50 eggs in each sac. The eggs hatch in about one month. The spiderlings take about one year to grow to adulthood. The brown recluse spider is resilient and can tolerate up to six months of extreme drought and scarcity or absence of food. On one occasion, a brown recluse survived in controlled captivity for over five seasons without any food at all.<ref name=lowrie>
{{cite journal
 |vauthors=Horner NV, Stewart KW
 |year=1967
 |title=Life history of the brown spider, ''Loxosceles reclusa'' ({{small|Gertsch & Muliak}})
 |journal=[[Texas Journal of Science]]
 |volume=19 |page=333
}}
</ref>

As part of the [[haplogynae]], brown recluses do not [[Ballooning (spider)|balloon]], which limits their ability to widely [[Biological dispersal|disperse]] geographically.<ref name=Vetter-2015>
{{cite book
 |last=Vetter |first=Richard S.
 |year=2015
 |title=The Brown Recluse Spider |edition=1st
 |publisher=Cornell University Press
 |location=Ithaca, NY
 |isbn=978-0-8014-7985-4
 |pages=43–44, 57, 63, 67
 |url=https://www.cornellpress.cornell.edu/book/9780801479854/the-brown-recluse-spider/#bookTabs=1
 |access-date=1 January 2020
}}
</ref>{{rp|style=ama|p= 63}}
The brown recluse will, though not habitually, [[Cannibalism|cannibalize]] another if food becomes scarce; especially during its typical [[mating]] season from June to September or when an unreceptive female encounters an aggressive male.<ref name=Vetter-2015/>{{rp|style=ama|pp= 43–44, 67}}

==Behavior==
{{one source|section|date=May 2024}}
A brown recluse's stance on a flat surface is usually with all legs radially extended. When alarmed it may lower its body, withdraw the forward two legs straight rearward into a defensive position, withdraw the rearmost pair of legs into a position for lunging forward, and stand motionless with [[pedipalps]] raised. The pedipalps in mature specimens are dark and quite prominent and are normally held horizontally forward. When threatened it usually flees, seemingly to avoid a conflict, and if detained may further avoid contact with quick horizontal rotating movements or even resort to assuming a lifeless pose ([[Apparent death|playing dead]]). The spider does not usually jump unless touched brusquely, and even then its avoidance movement is more of a horizontal lunge rather than a vaulting of itself entirely off the surface. When running, the brown recluse does not leave a silk line behind, which would make it more easily tracked when it is being pursued. Movement at virtually any speed is an evenly paced gait with legs extended.

The brown recluse spider displays [[Autotomy#Spiders|autotomy]] as a [[Anti-predator adaptation|defense mechanism]] against physical, [[Predation|predatory]] attack to a leg as well as to prevent [[Predation|predatory]], [[venom]] injections from spreading to the rest of the body.<ref name=Vetter-2015/>{{rp|style=ama|p= 57}} "Once a leg is lost, a recluse spider does not [[Regeneration (biology)|regenerate]] a new one with subsequent [[Moulting|molts]]",<ref name=Vetter-2015/>{{rp|style=ama|p= 57}} unlike the [[huntsman spider]], which does [[Regeneration (biology)|regenerate]] autotomized [[leg]]s. With each time that a leg is autotomized, the recluse "changes its [[Animal locomotion|gait]] to [[Compensation (psychology)|compensate]] for the loss."<ref name=Vetter-2015/>{{rp|style=ama|p= 57}}

==Habitat==
Brown recluse spiders build asymmetrical (irregular) webs that frequently include a shelter consisting of disorderly threads. They frequently build their webs in woodpiles and sheds, closets, garages, [[plenum space]]s, cellars, and other places that are dry and generally undisturbed. When dwelling in human residences they seem to favor cardboard, possibly because it mimics the rotting tree bark which they inhabit naturally.<ref>
{{cite web
 |title=Brown recluse spider
 |department=Control information, bites, & more
 |series=Pest guide
 |website=pestworld.org
 |url=https://www.pestworld.org/pest-guide/spiders/brown-recluse-spiders/
 |access-date=18 August 2020
}}
</ref> Human-recluse contact often occurs when such isolated spaces are disturbed and the spider feels threatened. Unlike most web weavers, they leave these lairs at night to hunt. Males move around more when hunting than the females, which tend to remain nearer to their webs.

==Bite==
{{Main|Loxoscelism}}
Like all members of the ''Loxosceles'' genus, the brown recluse has potent [[cytotoxicity|tissue-destroying]] [[venoms]] containing the dermonecrotic agent [[Sphingomyelin phosphodiesterase D|sphingomyelinase D]].<ref name=STDB>
{{cite web
 |title=Sphingomyelinase&nbsp;D (LiSicTox-{{mvar|α}}IA2ai)
 |series=Spider toxin database
 |website=ArachnoServer.org
 |publisher=[[QFAB Bioinformatics|Queensland Facility for Advanced Bioinformatics]]
 |url=http://www.arachnoserver.org/toxincard.html?id=138
 |url-status=dead  |access-date=15 July 2021
 | archive-url = https://web.archive.org/web/20161010220451/http://www.arachnoserver.org/toxincard.html?id=138
 | archive-date=10 October 2016
}}
</ref>
Most bites are minor with no dermonecrosis, but a small number of brown recluse bites produce [[loxoscelism]],<ref>
{{cite journal
 |last=Vetter |first=Richard S.
 |year=2006
 |title=Loxoscelism
 |journal=Clinics in Dermatology
 |volume=24 |issue=3 |pages=213–221
 |doi=10.1016/j.clindermatol.2005.11.006
 |pmid=16714202
 }}
</ref> a condition where the skin around the bite dies. While loxoscelism usually manifests as a skin condition (cutaneous loxoscelism), it can also include systemic symptoms like fever, nausea, and vomiting (viscerocutaneous loxoscelism). In very rare cases, bites can even cause [[hemolysis]] – the bursting of [[red blood cells]].<ref>
{{cite magazine
 |title=Greta Binford 1997–
 |year=2008
 |magazine=Biography Today
 |series=General Series  |volume=17  |issue=1  |page=14
 |issn=1058-2347  |isbn=978-0-7808-1016-7
 |quote=American biologist, arachnologist, and pioneering researcher [o]n spider venom
 |url=https://archive.org/details/biographytodayge0000unse_y5i7
 |via=archive.org  |url-access=registration
}}
</ref>
In one study of clinically diagnosed brown recluse bites, skin necrosis occurred 37% of the time, while systemic illness occurred 14% of the time.<ref name=Leach>
{{cite journal
 |vauthors=Leach J, Bassichis B, Itani K
 |date=July 2004
 |title=Brown recluse spider bites to the head: Three cases and a review
 |journal= Ear, Nose & Throat Journal 
 |volume=83  |issue=7  |pages=465–470
 |doi=10.1177/014556130408300712 |doi-access=free
 |pmid=15372917
 |url=http://www.thefreelibrary.com/Brown+recluse+spider+bites+to+the+head%3A+three+cases+and+a+review-a0124261634
|url-access=subscription
 }}, in turn citing:<br/>
{{cite journal
 | last1 = Wright | first1 = S.W.       | last2 = Wrenn | first2 = K.D.
 | last3 = Murray | first3 = L.         | last4 = Seger | first4 = D.
 | year = 1997
 | title = Clinical presentation and outcome of brown recluse spider bite
 | journal = Annals of Emergency Medicine
 | volume = 30 | issue = 1 | pages = 28–32
 | doi = 10.1016/s0196-0644(97)70106-9
 | pmid = 9209221
}}</ref>

As suggested by its [[specific name (zoology)|specific name]] ''reclusa'' (recluse), the brown recluse spider is rarely aggressive, and bites from the species are uncommon. In 2001, more than 2,000&nbsp;brown recluse spiders were removed from a heavily infested home in Kansas, yet the four residents who had lived there for years were never harmed by the spiders, despite many encounters with them.<ref>
{{cite journal
 | last1 = Vetter | first1 = Richard S.
 | last2 = Barger | first2 = Diane K.
 | date=November 2002
 | title = An infestation of 2,055&nbsp;brown recluse spiders (Araneae: Sicariidae) and no envenomations in a Kansas home: Implications for bite diagnoses in nonendemic areas
 | journal = Journal of Medical Entomology
 | volume = 39 | issue = 6 | pages = 948–951
 | doi=10.1603/0022-2585-39.6.948 | doi-access = free
 | pmid=12495200 | s2cid = 18535207
}}
</ref><ref name=rgj>
{{cite web
 |last=Robison |first=Mark
 |date=28 June 2011
 |title=Is that a brown recluse in your bathtub?
 |department=Fact checker
 |newspaper=[[Reno Gazette-Journal]]
 |place=Reno, NV
 |url=http://www.rgj.com/article/20110628/NEWS20/106260379/Fact-Checker-brown-recluse-your-bathtub-?nclick_check=1
 |url-status=dead |access-date=22 May 2013
 |archive-url=https://archive.today/20130615192631/http://www.rgj.com/article/20110628/NEWS20/106260379/Fact-Checker-brown-recluse-your-bathtub-?nclick_check=1
 |archive-date=15 June 2013
}}
</ref>
The spider usually bites only when pressed against the skin, such as when tangled within clothes, shoes, towels, bedding, inside work gloves, etc. Many human victims report having been bitten after putting on clothes or shoes that had not been worn recently or had been left for many days undisturbed on the floor. The fangs of the brown recluse are not large enough to penetrate most fabric.<ref name=Sandidge2009>
{{cite report
 |last=Sandidge |first=J.S. <!-- |author-link=Jamél S. Sandidge -->
 |year=2009
 |title=Brown recluse spiders: A knowledge based guide to control and elimination
 |publisher= BRS Pest Control 
 |location=[[McLouth, Kansas|McLouth]], KS
}}
</ref>

When both types of loxoscelism do result, systemic effects may occur before necrosis, as the venom spreads throughout the body in minutes. Children, the elderly, and the debilitatingly ill may be more susceptible to systemic loxoscelism. The systemic symptoms most commonly experienced include nausea, vomiting, fever, rashes, and muscle and joint pain. Rarely, such bites can result in hemolysis, [[thrombocytopenia|low platelet levels]], [[disseminated intravascular coagulation|blood clots throughout the body]], organ damage, and even death.<ref name=NEJM2005-Wasserman>
{{cite journal
 | last = Wasserman | first = G.
 | year = 2005
 | title = Bites of the brown recluse spider
 | journal = [[New England Journal of Medicine]]
 | volume = 352 | issue = 19 | pages = 2029–2030; author reply 2029–2030
 | pmid = 15892198
 | doi = 10.1056/NEJM200505123521922
}}
</ref>
Most fatalities are in children under the age of seven<ref>
{{cite news
 |title=Tenn. boy ruled killed by spider bite
 |date=8 October 2004
 |agency=[[Associated Press]]
 |website=MSNBC.com
 |url=http://www.msnbc.msn.com/id/6209004/
 |url-status=dead
 |archive-url=https://web.archive.org/web/20070313202521/http://www.msnbc.msn.com/id/6209004/
 |archive-date=13 March 2007
}}
</ref>
or those with a weak immune system.

While the majority of brown recluse spider bites do not result in any symptoms, [[skin|cutaneous]] symptoms occur more frequently than systemic symptoms. In such instances, the bite forms a necrotizing [[Ulcer (dermatology)|ulcer]] as the result of [[soft tissue]] destruction and may take months to heal, leaving deep [[scar]]s. These bites usually become painful and itchy within 2–8&nbsp;hours. Pain and other local effects worsen 12–36&nbsp;hours after the bite, and the necrosis develops over the next few days.<ref name=ClinTox-Wasserman>
{{cite journal
 | vauthors = Wasserman G, Anderson P
 | year = 1983–1984
 | title = Loxoscelism and necrotic arachnidism
 | journal = [[Journal of Toxicology: Clinical Toxicology]]
 | volume = 21 | issue = 4–5 | pages = 451–72
 | pmid = 6381752
 | doi = 10.3109/15563658308990434
}}
</ref>
Over time, the wound may grow to as large as 25&nbsp;cm (10&nbsp;inches). The damaged tissue becomes [[gangrene|gangrenous]] and eventually sloughs away. ''L.&nbsp;reclusa'' can produce slightly more than 0.1&nbsp;[[μL]] of venom, though the average yield is less.<ref>
{{cite journal
 |first=Charles  |last=Kristensen
 |date=10 October 2005
 |title=Comments on the natural expression and artificial extraction of venom gland components from spiders
 |journal=[[Toxin Reviews]]
 |volume=24  |issue=3–4  |pages=257–270 
 |publisher=Taylor & Francis
 |lccn=2005245307 |issn=1556-9543
 |doi=10.1080/07313830500236168
 |lang=en
 |quote=Milking or gland extraction can be used to harvest ... toxins ... from spiders that are suitable for a wide variety of studies, but the milked venom and extracts should not be assumed to be equivalent to naturally expressed venoms&nbsp;... 
}}
</ref>

===Misdiagnosis===
There is an [[ELISA]]-based test for brown recluse venom that can determine whether a wound is a brown recluse bite, although it is not commercially available and not in routine clinical use. Clinical diagnoses often use [[Occam's razor|Occam's razor principle]] in diagnosing bites based on what spiders the patient likely encountered and previous similar diagnoses.<ref name=NEJM2005-Vetter/><ref name=Leach/><ref>
{{cite journal
 | vauthors = Gomez H, Krywko D, Stoecker W
 | year = 2002
 | title = A new assay for the detection of ''Loxosceles'' species (brown recluse) spider venom
 | journal = [[Annals of Emergency Medicine]]
 | volume = 39 | issue = 5 | pages = 469–474
 | pmid = 11973553 | pmc = 3201721
 | doi = 10.1067/mem.2002.122914
}}
</ref>

{{harvp|Stoecker|Vetter|Dyer|2017}} suggested the [[mnemonic]] "NOT RECLUSE", shown below, as a memory device to help laymen and medical professionals more objectively screen and diagnose potential cases of loxoscelism.<ref name=Evans-2017-02-22>
{{cite news
 |first=John |last=Evans
 |date=22 February 2017
 |title=NOT RECLUSE – New mnemonic to prevent misdiagnosis of skin conditions as spider bites
 |website=derm.city
 |url=https://www.derm.city/post/2017/02/22/not-recluse-new-mnemonic-to-prevent-misdiagnosis-of-skin-conditions-as-spider-bites
}} — free access news article gives the full mnemonic
</ref><ref>
{{cite journal
 |last1=Stoecker |first1=William V.
 |last2=Vetter   |first2=Richard S.
 |last3=Dyer     |first3=Jonathan A.
 |year=2017
 |title=NOT RECLUSE – A mnemonic device to avoid false diagnoses of brown recluse spider bites
 |journal=JAMA Dermatology
 |volume=153 |issue=5 |pages=377–378
 |doi=10.1001/jamadermatol.2016.5665
 |pmid=28199453
}}
</ref>
:{| style="text-align:left;vertical-align:top;"
|+ '''NOT RECLUSE''' mnemonic<ref name=Evans-2017-02-22/>
|-
|colspan=2 align=center|''Note that these are '''contrary''' criteria: Any '''one''' being true indicates that the injury is '''not''' a brown recluse bite.''
|- style="vertical-align:top;"
| {{big|'''N'''}}umerous
| {{big|&nbsp;}}More than one wound.
|- style="vertical-align:top;"
| {{big|'''O'''}}ccurrence&nbsp;
| {{big|&nbsp;}}The injury did not occur in a place where brown recluses are likely to be found:<br/>{{big|&nbsp;}}Either outside of the spider's geographic territory ... or not in an enclosed space<br/>{{big|&nbsp;}}like a box, closet, or attic.{{big|&nbsp;}}
|- style="vertical-align:top;"
| {{big|'''T'''}}iming
| {{big|&nbsp;}}The wound arose sometime between November and March.
|- style="vertical-align:top;"
| {{big|'''R'''}}ed center
| {{big|&nbsp;}}The center of the wound is red.
|- style="vertical-align:top;"
| {{big|'''E'''}}levated
| {{big|&nbsp;}}The middle of the wound is elevated, not sunken.
|- style="vertical-align:top;"
| {{big|'''C'''}}hronic
| {{big|&nbsp;}}The wound has persisted for more than three months.
|- style="vertical-align:top;"
| {{big|'''L'''}}arge
| {{big|&nbsp;}}The wound is more than {{convert|10|cm|in|0|abbr=off}} wide.
|- style="vertical-align:top;"
| {{big|'''U'''}}lcerates<br/> too early
| {{big|&nbsp;}}The wound gets crusty within the first week.
|- style="vertical-align:top;"
| {{big|'''S'''}}wollen
| {{big|&nbsp;}}The wound swells up if it's below the neck.
|- style="vertical-align:top;"
| {{big|'''E'''}}xudative
| {{big|&nbsp;}}The wound is "wet" – oozing pus or clear fluid.
|}

There are numerous documented infectious and noninfectious conditions that produce wounds that have been initially misdiagnosed as recluse spider bites by medical professionals, including:
{{div col|colwidth=15em}}
* ''[[Pyoderma gangrenosum]]''
* Infection by ''[[Staphylococcus]]''
* Infection by ''[[Streptococcus]]''
* Herpes
* Diabetic ulcers
* Fungal infection
* Chemical burns
* [[Urushiol-induced contact dermatitis|''Toxicodendron dermatitis'']]
* [[Squamous cell carcinoma]]
* Localized [[vasculitis]]
* [[Syphilis]]
* [[Toxic epidermal necrolysis]]
* [[Sporotrichosis]]
* [[Lyme disease]]<ref>
{{cite journal
 |vauthors=Osterhoudt KC, Zaoutis T, Zorc JJ
 |year=2002
 |title=Lyme disease masquerading as brown recluse spider bite
 |journal=[[Annals of Emergency Medicine]]
 |volume=39 |issue=5 |pages=558–561
 |pmid=11973566  |doi = 10.1067/mem.2002.119509
}}
</ref>
{{div col end}}

Many of these conditions are far more common and more likely to be the source of necrotic wounds, even in areas where brown recluse spiders actually occur.<ref name=NEJM2005-Vetter/> The most important of these is [[methicillin-resistant Staphylococcus aureus|methicillin-resistant ''Staphylococcus aureus'']] (MRSA), a bacterium whose necrotic lesions are very similar to those induced by recluse bites, and which can be lethal if left untreated.<ref>
{{cite news
 |url=http://www.ktla.com/news/landing/ktla-spider-bite-death,0,5285653.story
 |title=Spider bite: Coroner's office says Pomona teen died of staph infection, not spider bite
 |website=[[KTLA]].com <!-- Bot generated title -->
 |archive-url=https://web.archive.org/web/20120318102617/http://www.ktla.com/news/landing/ktla-spider-bite-death%2C0%2C5285653.story
 |archive-date=18 March 2012
}}
</ref> Misdiagnosis of MRSA as spider bites is extremely common (nearly 30% of patients with MRSA reported that they initially suspected a spider bite), and can have fatal consequences.<ref>
{{cite journal
 | last1 = Moran | first1 = G.J.
 | display-authors = etal
 | year = 2006
 | title = Methicillin-resistant ''S. aureus'' infections among patients in the emergency department
 | journal = [[New England Journal of Medicine]]
 | volume = 355 | issue = 7 | pages = 666–674
 | doi=10.1016/0304-3940(90)90780-d | doi-access = free
 | pmid = 1691470
}}
</ref>

Reported cases of brown recluse bites occur primarily in [[Arkansas]], [[Colorado]], [[Kansas]], [[Missouri]], [[Nebraska]], [[Oklahoma]], and [[Texas]].
There have been many reports of brown recluse bites in California – though a few related species of spiders may be found there, none of the related spiders in California is known to bite humans.<ref name=VetterMyth/>

To date, the reports of bites from areas outside of the spider's native range have been either unverified or, if verified, the spiders have been moved to those locations by travelers or commerce.

==== Other spiders ====
: ''For a comparison of the toxicity of several kinds of spider''
: ''bites, see the [[list of medically significant spider bites]]''

Many arachnologists believe that a large number of bites attributed to the brown recluse in the [[West Coast of the United States|U.S. West Coast]] are either from other spider species or not spider bites at all. Other spiders in western states that might possibly cause necrotic injuries are the [[hobo spider]], [[Loxosceles deserta|desert recluse spider]], and the [[yellow sac spider]].

For example, the venom of the [[hobo spider]], a common European species now established in the northwestern United States and southern [[British Columbia]], has been reported to produce similar symptoms as the brown recluse bite when injected into laboratory rabbits. However, the toxicity of hobo spider venom has been called into question: Actual bites (rather than syringe injections) have not been shown to cause necrosis, and no necrotic hobo spider bites have ever been reported where it is native.<ref name=AnnEmergMed2004-Isbister>
{{cite journal
 | vauthors = Vetter R, Isbister G
 | year = 2004
 | title = Do hobo spider bites cause dermonecrotic injuries?
 | journal = [[Annals of Emergency Medicine]]
 | volume = 44 | issue = 6 | pages = 605–607
 | pmid = 15573036
 | doi = 10.1016/j.annemergmed.2004.03.016
}}
</ref>

Numerous other spiders have been associated with necrotic bites in the medical literature. Other recluse species, such as the [[Loxosceles deserta|desert recluse]] (found in the deserts of southwestern United States), are reported to have caused necrotic bite wounds, though only rarely.<ref name = Vetter2008>
{{cite journal
 | last = Vetter | first = R.S.
 | year = 2008
 | title = Spiders of the genus ''Loxosceles'' (Araneae, Sicariidae): A review of biological, medical, and psychological aspects regarding envenomations
 | journal = [[Journal of Arachnology]]
 | volume = 36 | pages = 150–163
 | s2cid = 7746032
 | doi = 10.1636/RSt08-06.1
 | url = https://www.biodiversitylibrary.org/part/229051
}}
</ref>
The hobo spider and the [[yellow sac spider]] have also been reported to cause necrotic bites. However, the bites from these spiders are not known to produce the severe symptoms that can follow from a recluse spider bite, and the level of danger posed by these has been called into question.<ref>
{{cite journal
 |vauthors=Bennett RG, Vetter RS
 |date=August 2004
 |title=An approach to spider bites. Erroneous attribution of dermonecrotic lesions to brown recluse or hobo spider bites in Canada
 |journal=Canadian Family Physician
 |volume=50  |issue=8  |pages=1098–1101
 |pmid=15455808 |pmc=2214648
 |url=http://www.cfp.ca/cgi/reprint/50/8/1098.pdf
}}
</ref><ref>
{{cite journal
 |first=James H. |last=Diaz
 |date=April 2005
 |title=Most necrotic ulcers are not spider bites
 |journal=American Journal of Tropical Medicine and Hygiene
 |volume=72 |issue=4 |pages=364–367
 |doi=10.4269/ajtmh.2005.72.364 |doi-access=free
}}
</ref>

So far, no known necrotoxins have been isolated from the venom of any of these spiders, and some arachnologists have disputed the accuracy of spider identifications carried out by bite victims, family members, medical responders, and other non-experts in arachnology.
There have been several studies questioning the danger posed by some of the other spiders mentioned: In these studies, scientists examined case studies of bites in which the spider in question was identified by an expert, and found that the incidence of necrotic injury diminished significantly when "questionable" identifications were excluded from the sample set.<ref name=WhiteTail>
{{cite journal
 |vauthors=Isbister GK, Gray MR
 |date=August 2003
 |title=White-tail spider bite: A prospective study of 130 definite bites by ''Lampona'' species
 |journal=The Medical Journal of Australia
 |volume=179 |issue=4 |pages=199–202
 |pmid=12914510 |s2cid=46155627
 |doi=10.5694/j.1326-5377.2003.tb05499.x
 |url=http://www.mja.com.au/public/issues/179_04_180803/isb10785_fm.html
|url-access=subscription
 }}
</ref><ref>
{{cite journal
 |vauthors=Isbister GK, Hirst D
 |date=August 2003
 |title=A prospective study of definite bites by spiders of the family Sparassidae (huntsmen spiders) with identification to species level
 |journal=Toxicon
 |volume=42 |issue=2 |pages=163–171
 |pmid=12906887
 |doi= 10.1016/S0041-0101(03)00129-6
|bibcode=2003Txcn...42..163I
 }}
</ref>

===Bite treatment===
First aid involves the application of an ice pack to control [[inflammation]] and prompt medical care. If it can be easily captured, the spider should be brought with the patient in a clear, tightly closed container so it may be identified by an arachnologist; if there is no specimen at all, then confirmation by an expert is impossible.<ref name=AnnEmergMed2002-Vetter/>

Routine treatment should include 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]], [[surgery|surgical excision]], and [[antivenom]].<ref>
{{cite journal
 |vauthors=Maynor ML, Moon RE, Klitzman B, Fracica PJ, Canada A
 |date=March 1997
 |title=Brown recluse spider envenomation: A prospective trial of hyperbaric oxygen therapy
 |journal=[[Academic Emergency Medicine]]
 |volume=4 |issue=3 |pages=184–192
 |pmid=9063544
 |doi= 10.1111/j.1553-2712.1997.tb03738.x |doi-access=free
}}
</ref><ref>
{{cite journal
 |vauthors=Maynor ML, Abt JL, Osborne PD
 |year=1992
 |title=Brown recluse spider bites: Beneficial effects of hyperbaric oxygen
 |journal=Journal of Hyperbaric Medicine
 |volume=7 |issue=2 |pages=89–102
 |issn=0884-1225
 |url=http://archive.rubicon-foundation.org/4477
 |url-status=usurped |access-date=July 22, 2008
 |archive-url=https://web.archive.org/web/20110727224716/http://archive.rubicon-foundation.org/4477
 |archive-date=27 July 2011
}}
</ref>
In almost all cases, bites are self-limited and typically heal without any medical intervention.<ref name=NEJM2005-Vetter/>

[[Outpatient care|Outpatient]] [[palliative care]] following discharge often consists of a weak or moderate strength [[opioid]] (e.g. [[codeine]] or [[tramadol]], respectively) depending on pain scores, an [[anti-inflammatory]] agent (e.g. [[naproxen]], [[cortisone]]), and an [[antispasmodic]] (e.g. [[cyclobenzaprine]], [[diazepam]]), for a few days to a week. If the pain or spasms have not resolved by this time, a second medical evaluation is generally advised, and [[differential diagnosis|differential diagnoses]] may be considered.{{medical citation needed|date=August 2022}}

====Specific treatments====
In presumed cases of recluse bites, [[dapsone]] is often used for the treatment of necrosis, but controlled clinical trials have yet to demonstrate efficacy.<ref>
{{cite journal
 | last1 = Elston | first1 = D.M.     | last2 = Miller  | first2 = S.D.
 | last3 = Young  | first3 = R.J.     | last4 = Eggers  | first4 = J.
 | last5 = McGlasson | first5 = D.    | last6 = Schmidt | first6 = W.H.
 | last7 = Bush   | first7 = A.
 | year = 2005
 | title = Comparison of colchicine, dapsone, triamcinolone, and diphenhydramine therapy for the treatment of brown recluse spider envenomation: A double-blind, controlled study in a rabbit model
 | journal = [[Archives of Dermatology]]
 | volume = 141  | issue = 5  | pages = 595–597
 | doi=10.1001/archderm.141.5.595  | pmid=15897381
}}
</ref>
However, dapsone may be effective in treating many "spider bites" because many such cases are actually misdiagnosed microbial infections.<ref name=AnnEmergMed2002-Vetter>
{{cite journal
 | vauthors = Vetter R, Bush S
 | year = 2002
 | title = The diagnosis of brown recluse spider bite is overused for dermonecrotic wounds of uncertain etiology
 | journal = [[Annals of Emergency Medicine]]
 | volume = 39 | issue = 5 | pages = 544–546
 | pmid = 11973562 | doi = 10.1067/mem.2002.123594
}}
</ref>
There have been conflicting reports about its efficacy in treating brown recluse bites, and some have suggested it should no longer be used routinely, if at all.<ref name=AmJEmergMed2003-Bryant>
{{cite journal
 | vauthors = Bryant S, Pittman L
 | year = 2003
 | title = Dapsone use in ''Loxosceles reclusa'' envenomation: is there an indication?
 | journal = [[American Journal of Emergency Medicine]]
 | volume = 21  | issue = 1  | pages = 89–90
 | pmid = 12563594  | doi = 10.1053/ajem.2003.50021
}}
</ref>

Wound infection is rare. Antibiotics are not recommended unless there is a credible diagnosis of infection.<ref name=MoMed1998-Anderson>
{{cite journal
 | last = Anderson | first = P.
 | year = 1998
 | title = Missouri brown recluse spider: A review and update
 | journal = Missouri Medicine
 | volume = 95 | issue = 7 | pages = 318–322
 | pmid = 9666677
}}
</ref>

Studies have shown that surgical intervention is ineffective and may worsen outcomes. Excision may delay wound healing, cause [[abscesses]], and lead to scarring.<ref name=AnnSurg1985-Rees>
{{cite journal
 | vauthors = Rees R, Altenbern D, Lynch J, King L
 | year = 1985
 | title = Brown recluse spider bites: A comparison of early surgical excision versus dapsone and delayed surgical excision
 | journal = [[Annals of Surgery]]
 | volume = 202 | issue = 5 | pages = 659–663
 | pmid = 4051613 | pmc = 1250983
 | doi = 10.1097/00000658-198511000-00020
}}
</ref>

Purportedly application of [[Glyceryl trinitrate (pharmacology)|nitroglycerin]] stopped necrosis.<ref>
{{cite web
 |last=Burton |first=K.
 |year=2021
 |title=The brown recluse spider: Finally stopped in its tracks
 |series=Yosemite forest
 |website=geocities.com
 |url=http://www.geocities.com/Yosemite/Forest/2021/recluse/intro.html
 |access-date=September 2, 2006
 |archive-url=https://web.archive.org/web/20060420183312/http://www.geocities.com/Yosemite/Forest/2021/recluse/intro.html
 |archive-date=April 20, 2006
}}
</ref> However, one scientific animal study found no benefit in preventing necrosis, with the study's results showing it increased inflammation and caused symptoms of [[Loxoscelism|systemic envenoming]]. The authors concluded the results of the study did not support the use of topical nitroglycerin in brown recluse envenoming. <!--
Are there medical experiments with leeches? Yes, there are many, they're even used in surgery sometimes, but not specifically recorded in cases of brown recluse evenoming.
--><ref name=AnnEmergMed2001-Lowry>
{{cite journal
 | vauthors = Lowry B, Bradfield J, Carroll R, Brewer K, Meggs W
 | year = 2001
 | title = A controlled trial of topical nitroglycerin in a New Zealand white rabbit model of brown recluse spider envenomation
 | journal = Annals of Emergency Medicine
 | volume = 37 | issue = 2 | pages = 161–165
 | pmid = 11174233  | doi = 10.1067/mem.2001.113031
}}
</ref>

Antivenom is available in South America for the venom of related species of recluse spiders. However, the bites, often being painless, usually do not present symptoms until 24 or more hours after the event, possibly limiting the effect of this intervention.<ref name=ClinTox2003-Isbister>
{{cite journal
 | vauthors = Isbister G, Graudins A, White J, Warrell D
 | year = 2003
 | title = Antivenom treatment in arachnidism
 | journal = Journal of Toxicology: Clinical Toxicology
 | volume = 41  | issue = 3  | pages = 291–300
 | pmid = 12807312  | s2cid = 37946164
 | doi = 10.1081/CLT-120021114
}}
</ref>

====Spider population control====
Due to increased fear of these spiders prompted by greater public awareness of their presence in recent years, the extermination of domestic brown recluses is performed frequently in the lower midwestern United States. Brown recluse spiders possess a variety of adaptive abilities, including the ability to survive up to 10 months with no food or water.<ref name=Sandidge>
{{cite journal
 |vauthors=Sandidge JS, Hopwood JL
 |year=2005
 |title=Brown recluse spiders: A review of biology, life history and pest management
 |journal=[[Transactions of the Kansas Academy of Science]]
 |volume=108 |issue=3–4 |pages=99–108
 |doi=10.1660/0022-8443(2005)108[0099:BRSARO]2.0.CO;2
 |s2cid=84863537 |jstor=20058665
}}
</ref> Additionally, these spiders survive significantly longer in a relatively cool, thermally stable environment.<ref name=Elzinga>
{{cite journal
 |last=Elzinga |first=R.J.
 |year=1977
 |title=Observations on the longevity of the brown recluse spider, ''Loxosceles reclusa'' {{small|(Gertsch & Mulaik)}}
 |journal=[[Journal of the Kansas Entomological Society]]
 |volume=50 |issue=2 |pages=187–188
 |jstor=25082920
}}
</ref>

==References==
{{reflist|25em}}

==External links==
{{refbegin|25em|small=yes}}
* {{cite web
 |first=Ken |last=Cramer Brown
 |year=2007
 |title=Illinois and Iowa brown recluse project
 |publisher=[[Monmouth College]]
 |place=Illinois
 |url=http://department.monm.edu/biology/recluse-project/index.htm
 |url-status=dead |access-date=5 October 2024
 |archive-url=https://web.archive.org/web/20120226212258/http://department.monm.edu/biology/recluse-project/index.htm
 |archive-date=2012-02-26  |df=dmy-all
}}
* {{cite web
 |title=Brown recluse spider
 |department=Featured creatures
 |series=Institute of Food and Agricultural Sciences
 |website=entomology.ifas.ufl.edu
 |place=Gainesville
 |publisher=[[University of Florida]]
 |url=http://entomology.ifas.ufl.edu/creatures/urban/spiders/brown_recluse_spider.htm
}}
{{refend}}

{{Commons|Loxosceles reclusa}}
{{Wikispecies|Loxosceles reclusa}}

{{Taxonbar|from=Q284352}}
{{Authority control}}

[[Category:Sicariidae]]
[[Category:Endemic spiders of the United States]]
[[Category:Taxa named by Willis J. Gertsch]]
[[Category:Spiders described in 1940]]
[[Category:Venomous arthropods]]