Spiders!  Generally, they get blamed for much more morbidity than they are actually responsible for, but occasionally they can lead to some pretty nasty problems.  In Part I, we discussed some general facts about venomous spiders relevant to humans.  We then focused on the brown recluse spider and the stages of skin lesions that it causes, from non-specific erythema to unmistakable ulceration.  Here in Part II, we’ll refine an approach to ruling out recluse spider bites, and then discuss treatment modalities for patients who are unfortunate enough to actually get bitten.

Is this cutaneous lesion caused by the bite of a spider? Why or why not? What would you tell the patient to support your opinion?

The NOT RECLUSE mnemonic 

A useful tool to consider in determining if the patient in your ED or office actually has a bite from a Loxesceles spider is this recently published mnemonic device: NOT RECLUSE. It was developed from the clinical experience of two Missouri dermatologists (who live in an endemic area) and an entomologist, Richard Vetter, whose name appears in almost every large publication on recluse spiders.

It works like this: if two or more of the following features are present, a recluse spider bite is exceedingly unlikely:

Numerous lesions: there should only be one bite.

Occurrence: the most common circumstance is disturbance of a secluded spider.

Timing: bites in North America outside of April-October are not likely.

Red center: recluse venom causes immediate destruction of the capillary bed with resulting ischemia. Thus the center of a lesion is pale, blue-white, or purple. Rarely red!

Note the pale, blue-white discoloration at the center of this wound caused by a brown recluse spider.  This is due to ischemia from capillary bed destruction.  If the center of the lesion is red, it’s probably not caused by the bite of Loxesceles!

Elevated: the lesion should be flat or sunken.

Chronic: the wounds typically heal within 3 months.  If it takes longer than this, it’s probably something else.

Large: the most dynamic recluse bites typically don’t exceed 10 cm.

Ulcerates too early:  this usually does not occur until 7-14 days after envenomation.

Ulceration is characteristic of a recluse spider bite, but typically does not occur until 1-2 weeks after envenomation.  If the lesion ulcerates earlier than this, broaden the differential diagnosis.

Swelling: usually not, except on the face.

Exudative: usually not, with the exception of blistered lesions. Pus formation = bacterial, not loxescelism!

Small pustules seen at the center of this lesion suggest it is a cutaneous abscess, probably caused by Staph or Strep rather than a brown recluse spider bite.

If it’s not a recluse spider, bite, then what is it? …your patient may ask.  Don’t get tunnel-visioned!  Maintain a broad differential diagnosis. Initiating therapy focused only on a brown recluse bite can lead to delays in important therapies for other conditions.

So here’s a few other things to consider, listed from more common to a little less so:  Staph and Strep infections, diabetic ulcers, herpes simplex or herpes zoster (with/without) superinfection, pyoderma gangrenosum, syphilis, neoplasms, and Lyme diease.


Ok, so you’ve got a suspicious lesion, you live in an endemic area, and its the middle of July, so you think you are dealing with the real deal: a bite from a brown recluse spider.  How do you actually treat this thing?

Initial treatment when you have a suspected bite includes first aid measures: ice the wound, elevate and immobilize the limb if it is on an extremity, provide local wound care, and give tetanus prophylaxis as appropriate.  Ice, by the way, is more than just a supportive care measure: the active toxin in the venom, sphingomyelinase, is inactivated by cold. So if it really was a recluse spider bite, ice can prevent a lot of the adverse affects that may follow.

Beyond this: many other treatments are described in the literature, but few are well supported by good clinical evidence. There are essentially no RCTs and probably never will be.  Potential treatments are costly, painful, and potentially toxic. Since most recluse spider bites will heal without any intervention, it’s hard to justify performing trials of these treatments.

With this said, here is a sample of some of the more common treatments that your patient may ask you about:

  • Dapsone: this antibiotic inhibits neutrophils, which are thought to play a large role in skin necrosis.  While lab studies in guinea pigs support its use, concerns for use in actual clinical practice are multiple.  It may causes hemolysis; it may lead to methemoglobinemia in patients with G6PD deficiency; and it causes side effects including headache, GI upset, liver toxicity, and agranulocytosis (check baseline CBC and LFTs before starting this drug).  On balance, it’s probably one of the safer and more accessible treatments out there, and is used commonly in Chile (though not officially recommended for use in the US).  Various other antibiotics have been tried, but are similarly not well supported.
  • Hyperbaric Oxygen: Its use has been advocated on the basis that it inactivates the sulfhydryl-containing sphingomyelinase in Loxosceles venom by oxidizing the sulfhydryl bonds. However, this has never born out to be an effective treatment in randomized trials.
  • Antihistamines are reasonable to prescribe, as they can aid in the pruritus that commonly occurs with these bites.
  • Steroids are commonly used, but unproven, except in rabbit studies.  As with other indications, they are believed to generally decrease the systemic inflammatory effect which ultimately leads to ulcer formation.
  • Topical application of Nitroglycerin has been tried experimentally.  As the pathophysiology of the bite is thought to include local vasoconstriction and platelet plugging, the idea is that a vasodilator can counteract this.  However, it’s not born out in studies.
  • Another more exotic treatment is the use of electric shocks.  The idea arose based on an observation that stun guns seem to improve insect and snake bites.   However, it’s not a recommended treatment.
  • Patients may ask you about the use of anti-venom.  This is not available in the US, but sometimes used in South America, most commonly in Brazil.  The most recent iteration of the Chilean guidelines on recluse spider bites recommends against its routine use.  efficacy and superiority to dapsone alone is not proven. Rabbit derived, most commonly used in Brazil.

Some advocate for excision of ulcers  greater than 1 cm to speed healing.  This is a surgeon’s decision, but early referral for evaluation of more significant wounds may be indicated.



As clinicians (outside of endemic areas), we will likely spend more time trying to convince our patients that their wounds are not due to the bite of a spider than ruling them in.  Still, it pays to have a working knowledge of the appearance, progression, and risk factors for recluse spider bites so we can confidently counsel our patients.  Patients will have often scoured the internet prior to seeing you and have many questions about treatments available.  A basic familiarity with what these treatments are, and how they are employed (rarely!) can go a long way in winning your patient’s trust.


  • Geruch WJ, Ennik F.  The spider Genus Loxosceles in North America, Central America, and the West Indies.  Bulletin of the American Museum of Natural History.  Volume 175: Article 3. New York: 1983.
  • Andersen RJ, Campoli J, Johar SK, Schumacher KA, Allison EJ.  Suspected Brown Recluse Envenomation: A Case Report and Review of Different Treatment Modalities.  The Journal of Emergency Medicine, Vol. 41, No. 2, pp. e31–e37, 2011.
  • Guia para el Manejo de Mordedura de Araña de los Rincones (Loxosceles laeta). Chile 2016.