Brown Recluse Spiders: Ominous but Not Necessarily a Threat in Arizona

Tuesday, March 29, 2005

SCOTTSDALE, Ariz. — They have six eyes, arranged in pairs. Their legs are long and their bodies flat. They show up at night and hide out, in stealth fashion, in clothing, bed sheets or towels left on the floor.

They are brown recluse spiders - members of a family of arachnids known to spawn fear and loathing among sufferers of arachnophobia. Their human victims, if they insist they've been bitten, often show up at hospital emergency rooms, certain of a nasty fate.

The reality is far less ominous. Bites from brown recluse spiders occur much less commonly that the public believes, according to a study by David Swanson, M.D., Department of Dermatology, Mayo Clinic in Arizona. More seriously, according to Dr. Swanson, conditions such as infections, herpes, diabetic ulcers, chemical burns, syphilis and Lyme disease can be misdiagnosed as being brown recluse spider bites. Misdiagnosing a skin ulcer as a spider bite could delay proper treatment and actually put patients at risk, cautions Dr. Swanson. "Physicians need to be skeptical of any undocumented reports of spider bites and should entertain a broad differential diagnosis before concluding skin lesions are from spiders."

Dr. Swanson's findings, published in the New England Journal of Medicine, offer a more tempered view of this member of the genus Loxosceles. The Loxosceles spiders are found in areas ranging from southern California to western Arizona to Texas, Alabama and Kentucky, and it is rare that bites would occur outside of the spider's natural habitat.

The spiders should actually be "caught in the act" of biting its victim before confirming a diagnosis, Dr. Swanson notes.

"Verbal history from the patient is often unreliable," says Dr. Swanson. In a study of 600 patients with suspected spider bites who sought care at the University of Arizona or Los Angeles County Hospital, 80 percent of the bites were proved to be caused by ticks, fleas, bedbugs, mites, crickets or even grasshoppers. Curiously, in more than 90 percent of cases where the patient insisted he or she was bitten by the brown recluse, the actual arachnid was unavailable for identification.

The good news is that brown recluse spider bites usually heal without medical intervention and rarely cause skin necrosis and scarring. Treatment should include routine first aid - elevation, immobilization, ice, and, if warranted, a tetanus shot. Even in rare cases, where the bites do cause necrosis and scaring, it is very unlikely to be life-threatening.

The upshot of Dr. Swanson's research, in partnership with a researcher from the Department of Entomology at the University of California, Riverside, is that brown recluse spiders are not plentiful in Arizona and that "bites" are rare and often mask symptoms of more serious conditions.

"For some people, it's exotic to consider being bitten by a spider, and they speculate in their medical history that a spider caused their skin problem," says Dr. Swanson. "Doctors tend to listen to their patients and take their word for it. The danger is that the real diagnosis will be missed, and the patient isn't getting good care as a result."

Arizona is not at great risk for the brown recluse spiders, the physician says, because only their cousins reside here and those cousins tend to stay away from the cities. "Diagnoses of bites outside of their endemic areas are highly suspect," Dr. Swanson adds.

Mayo Clinic is a private group practice of medicine dedicated to providing diagnosis and treatment of patient illnesses through a systematic focus on individual patient needs. As a leading academic medical center in the Southwest, Mayo Clinic focuses on providing specialty and surgical care in more than 65 disciplines at its outpatient facility in north Scottsdale and at Mayo Clinic Hospital. The 208-licensed bed hospital is located at 56th Street and Mayo Boulevard (north of Bell Road) in northeast Phoenix, and provides inpatient care to support the medical and surgical specialties of the clinic, which is located at 134th Street and Shea Boulevard in Scottsdale.

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