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Is It Really a Brown Recluse Spider Bite?

Roberts, James R. MD

doi: 10.1097/01.EEM.0000558189.99735.09



Most individuals and many clinicians overestimate the incidence of symptomatic spider bites. Any lesion without a definitive cause is frequently considered to be a spider envenomation. A symptomatic bite from any arachnoid is rather unusual, however, and the classic skin lesions associated with brown recluse spider envenomation are even rarer.

The term loxoscelism is used to describe the syndrome associated with envenomation from the brown recluse spider (Loxosceles reclusa). Because such bites are uncommon, there is little formal research into the specifics of envenomation and even fewer data about the most effective treatments. Unless the spider is caught or the victim is an entomologist, it's impossible to confirm the specific culprit with certainty.

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Suspected Brown Recluse Envenomation: A Case Report and Review of Different Treatment Modalities

Andersen RJ, Campoli J, et al.

J Emerg Med.


This is an interesting case report of a suspected brown recluse spider envenomation that includes a rather detailed analysis of potential treatments. A 72-year-old man came to the ED with back pain, weakness, and diarrhea. He had a large wound on his upper back that he thought was from a bug bite. The wound had progressed and was the size of an egg when he presented to the ED. The lesion was painful to the touch, pruritic, and not relieved with any intervention. The patient said he did not have fever, chills, night sweats, nausea, or vomiting. He had a past history of coronary artery disease, hyperlipidemia, and hypertension.

He appeared to be in pain. A 7x7 cm ulcer on his right upper back demonstrated erythema and induration, and it had a 3x3 cm necrotic center. The ulcer was reddish-blue, and the necrotic center contained a dark eschar. Laboratory tests were initially normal. Because of the wound's necrotizing nature, the patient underwent wide debridement.

Post-operatively he demonstrated a significant anemia and an increased INR. He was treated with blood transfusions and taken to the operating room where a large hematoma was removed. The wound was closed with sutures about a week later. A spider was never seen, and the clinicians could not prove the exact etiology, but they concluded it was a brown recluse spider bite based on his clinical course and the characteristics of the lesion.

Brown recluse spider bite is almost always a clinical diagnosis because there is no way to prove which, if any, spider was the culprit. The brown recluse bite has a classic pattern of pruritus, pain, erythema, and an irregular erythematous ring around the bite. A hemorrhagic vessel develops in about three days, and an ulcer with a dark necrotic eschar later forms. Early mild systemic symptoms, as described by this patient, are not uncommon.

The authors stated that most brown recluse spider bites heal without aggressive medical treatment, and the best clinical approach is simply basic wound care. Antihistamines may be used to control itching, and tetanus status should be updated. Many treatments are suggested in the literature, but no intervention has proven beneficial other than local wound care and surgical debridement. The problem with all reports is that the diagnosis is a clinical call that could be incorrect.

Dapsone has long been suggested as a treatment for brown recluse spider bites. Theoretically, its leukocyte-inhibiting effects limit the inflammatory response at the wound site. Unfortunately, many adverse effects are attributed to dapsone, including hemolysis, sore throat, aplastic anemia, jaundice, methemoglobinemia, peripheral neuropathy, and hyperbilirubinemia. Dapsone should not be used in children who can develop more aggressive loxoscelism. Many of the side effects of dapsone overlap with the signs and symptoms associated with a spider bite. The few studies that suggest dapsone is beneficial have conflicting data and problems with study design. Dapsone intervention is still of questionable value and not recommended by most authors.

Other investigators have suggested using hyperbaric oxygen (HBO) to treat loxoscelism. The results are conflicting, though the physiology of HBO suggests it could be helpful. The side effects of HBO are well known, including sinus and ear problems and potential oxygen toxicity. It's also an expensive intervention. The authors of this report review a number of studies involving HBO for brown recluse spider bites, but they were small, not blinded, and had no control groups. HBO may have some benefit, but it's questionable if it is clinically significant. Combining dapsone and HBO has likewise not been proven effective.

Using nitroglycerine for treating brown recluse spider envenomations has been suggested with anecdotal reports of efficacy. Local vasodilation is thought to be helpful. These authors said the literature does not support nitroglycerine for brown recluse spider bites. Interestingly, electroshock therapy has been suggested, but no well-done studies suggest a benefit for spider or snake bites.

Some authors recommend early surgical debridement, and surgical techniques vary. The consensus is that surgery should not be performed for at least six to eight weeks until the lesion has been stabilized. Early bite site excision is not supported.

Most brown recluse spider lesions heal without treatment. These authors found that no treatment significantly altered the natural outcome for these bites. These authors do not recommend dapsone, HBO, nitroglycerine, electric shock therapy, or early surgical excision.

Comment: Many lesions attributed to Loxosceles reclusa envenomation are actually from other causes, making the reported incidence falsely high. The only way to prove a brown recluse spider bite is to identify the spider. This is rarely possible because the bite is not initially symptomatic or can be occult, and if the spider is seen, it's usually squashed beyond recognition. The brown recluse spider does not live in the northern states, and bites are most often reported in the South, Southwest, and Midwest. Serious complications of brown recluse spider bites include systemic toxicity and multisystem failure. The extent of systemic loxoscelism does not correlate with the size or characteristics of the lesion.

It was found a number of years ago that CA-MRSA infections were frequently diagnosed as spider bites. An outbreak of CA-MRSA skin infections in a prison prompted the inmates to sue because of lack of bug control and treatment for spider bites.

The brown recluse is a small, rather nondescript spider. It is difficult to identify even when it is not squashed. Authors have stated that the appearance of the spider's eyes is diagnostic, but how would anyone be able to analyze the eyes of the spider? There is a violin marking on the anterior thorax, but this is also not a reliable way to identify this spider. Similar to its name, these spiders are very much recluses, inhabiting quiet areas in homes that are rarely disturbed, like attics and basements, and outside in dead trees but not vegetation. You would not, for example, get a brown recluse spider bite from weeding your garden or lawn. Reports of large series of patients who have been diagnosed with loxoscelism had been proven to be correct in only seven to 14 percent of cases, emphasizing the rarity and overdiagnosis of these bites.



The spider's venom contains a large number of clinically active enzymes, including sphingomyelinase, the enzyme responsible for skin necrosis. These spiders are not aggressive, and it is believed that they bite only as the last line of defense. Rolling over in bed on the spider or putting on spider-containing footwear are common possibilities for envenomation.

The initial bite is relatively painless, causing only a minor burning sensation. The characteristics of the bite site rapidly change, making a correct diagnosis even more challenging. Initially the site of the bite may show two small cutaneous puncture wounds, occasionally with surrounding erythema. Blisters can develop around the bite. Most bites resolve without significant skin involvement or complications, but in some patients a lesion with a dark depressed center and a central eschar can develop. The incidence of necrosis has been estimated to be only about 10 percent. Lesions progress and are usually at their maximum in 10 to 14 days. It may take months for ulcers to heal fully.

Systemic effects can occasionally occur and sometimes be life-threatening. Early minor systemic complaints without further progression are common. It seems reasonable to order some basic blood tests on anyone with systemic complaints, but systemic complications are not correlated with the degree of skin involvement. Renal failure, hemolysis, rhabdomyolysis, coma, and death are rarely reported. Children seem to be more susceptible to systemic symptoms, likely due to their size. A number of characteristics and certain color changes of the site have been attributed to brown recluse spider bites, but the appearance of the bite is quite variable, and wound appearance cannot prove envenomation.

It's tempting to intervene, but only local skin care is suggested for these bites. Patients should be told that a spider bite can progress and be associated with systemic findings. Some recommend early antibiotics. Because MRSA is more common and cannot be differentiated from a spider bite, I would prescribe doxycycline or sulfamethoxazole-trimethoprim for five to seven days.

A presumed spider bite can also be erythema migrans, the skin lesion of Lyme disease, or early presentation of herpes simplex or zoster. Of course, there are numerous bugs, ants, fleas, and ticks that can cause skin lesions. A few other articles about these envenomations can be found in the EMN online archive. Enter “brown recluse spider” in the search box on

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Dr. Robertsis a professor of emergency medicine and toxicology at the Drexel University College of Medicine in Philadelphia. Read the Procedural Pause, a blog by Dr. Roberts and his daughter, Martha Roberts, ACNP, PNP, at, and read his past columns at

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