2,000+ brown recluse spiders in a Kansas home inflict no bites in the occupants, UCR study notes

December 02, 2002

RIVERSIDE, Calif. -- Have you had a skin wound lately and did a physician tell you a brown recluse spider was the culprit?

A UC Riverside study, focusing on 2,055 brown recluse spiders in a Kansas home, notes that many skin lesions are misdiagnosed by doctors as "brown recluse spider bites." The study finds that even where brown recluses can be very common, bites from these spiders are uncommon. Moreover, the study finds that in non-endemic areas, there aren't enough brown recluses to account for skin conditions diagnosed as "brown recluse bites."

"Doctors throughout the United States diagnose bites as brown recluse bites, yet arachnologists in many of their states are unable to find reliable populations of the spiders to account for these bites," said Rick Vetter, staff research associate in the department of entomology at UC Riverside and an internationally known expert on spiders. "Yet, the general public is fully convinced that the spiders are causing skin lesions despite the virtual absence of brown recluses in the entire state. Typically, the finding of one brown recluse in a non-endemic area results in a newspaper story that starts a wave of arachnophobia, spider-stompings and recluse bite diagnoses. This is what motivated our study."

Vetter and Diane Barger report their findings in a paper published in the November 2002 issue of the Journal of Medical Entomology. The study focused on Barger's home built in the 19th century in Lenexa, Kansas. Barger collected 2,055 brown recluse spiders for six months, from June 2001 to November 2001, and shipped them to Vetter for recluse verification and study. Despite the abundance of spiders in the Barger home, none in the Barger family of four members received bites from the spiders.

"So we have a family living in the midst of thousands of brown recluses and none of the family members was bitten," said Vetter. "We need to ask what then are the chances of being bitten by a brown recluse in states that are not known to have these spiders? What does it say about the validity of 'brown recluse bite' diagnoses from these areas?"

The paper's findings are important because factions of both the medical community and the general public remain convinced that brown recluses are not only ubiquitous but are also biting people. "If these brown recluse bite diagnoses in western, northern and eastern seaboard states were correct," said Vetter, "then we would be finding thousands of brown recluses. Instead we find only one every year or so; yet hundreds of brown recluse bites are made every year. The actual causes of these wounds may be infections, bug bites, diabetes, bed sores. But some of the conditions, such as lyme disease, anthrax, and necrotizing bacteria, that are misdiagnosed as brown recluse bites can kill you if they aren't treated in time.

"When one brown recluse is found, or even allegedly found, in an area that is not known to have them," Vetter said, "it often results in a hyperbolic media response, an overzealous public response, and the generation of many additional bite diagnoses by the general public. What this study shows is that this reaction to only one spider is far out of proportion to the actual threat the spiders pose and that until you can reliably find dozens of brown recluses in an area, there is no reason to consider them as being responsible for the skin lesions."

Vetter explained that brown recluse spiders are shy and rarely cause envenomations in humans. "In areas of North America where the brown recluse is not native, typically, there are no populations of the spider and verified finds of it are indeed extremely rare. Yet, the medical community in non-native recluse areas diagnose many more bites from these non-existent spiders every year than the historical finding of spiders in the same area.

"A good number of skin lesions get diagnosed as brown recluse spider bites," Vetter said. "But if the spiders were the cause, then arachnologists, home owners, pest control people, and others, would have no trouble finding them. It is actually extremely rare for anyone to find a brown recluse in California, the northern U.S. or the eastern seaboard. Yet, curiously, hundreds of brown recluse diagnoses are made in these areas each year by physicians."

In 1990 in South Carolina, 940 physicians responding to a survey reported 478 brown recluse spider bites in the state for that year. But South Carolina is outside of the native range of the brown recluse. An arachnologist in South Carolina looked for brown recluses intensively for five years and never found a brown recluse spider or had anyone bring him one.

In 2000, in the 21 counties under the jurisdiction of the Tampa Poison Control Center, 95 brown recluse bite diagnoses were made. But according to Florida's arachnologists, no brown recluses have ever been found in these 21 counties.

Vetter has been recording all the brown recluse bites he has heard of in California for the last three years. He has also scoured the records and talked to hundreds of people about brown recluses. He was able to find evidence of only eight brown recluses found in California. But he was informed of 120 California bite diagnoses in three years.

"Considering the fact that a family of four in Kansas can live in this house with over 2,000 brown recluses and not be bitten," said Vetter, "how many thousands and millions of brown recluses would have to live in South Carolina, Florida and California for these bite diagnoses to be correct?"

The University of California, Riverside offers undergraduate and graduate education to nearly 16,000 students and has a projected enrollment of 21,000 students by 2010. It is the fastest growing and most ethnically diverse campus of the preeminent ten-campus University of California system, the largest public research university system in the world. The picturesque 1,200-acre campus is located at the foot of the Box Springs Mountains near downtown Riverside in Southern California. More information about UC Riverside is available at http://www.ucr.edu or by calling 909-787-5185.

For a listing of faculty experts on a variety of topics, please visit http://mmr.ucr.edu/experts/.

University of California - Riverside

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