[Focus On Spiders] Along Came a Spider

Features - PCT News

What's the liklihood of living in a home with thousands of spiders and never getting bit? Well, it's more than you think.

July 8, 2003

How many brown recluse spiders would constitute an infestation in the average home? Probably one would be too many for most people, although many homes in the central Midwestern and southern United States have them. How many recluses would constitute a medical liability? If you believe the hyperbole kicked around by the general public and news media, it would only take a handful before your body parts would fall off. Despite the tales regarding the dangers of brown recluses, a neglected aspect of recluse biology is that people in individual homes live with dozens to hundreds of them yet envenomations are rare. At the least, bites are far less common than expected given spider abundance.

HOUSE OF RECLUSE HELL. In the Journal of Medical Entomology (2002, vol. 39, pp. 948-951), I published an article with Diane Barger regarding her Kansas home and the tremendous number of brown recluse spiders therein (see related story on page 86). The Barger residence was built in the 1850s and reputedly was once the residence of Wild Bill Hickok. Diane holds three college degrees and would probably be a doctor if health problems hadn’t undermined career plans. However, she never lost interest in biology nor her fascination with the scientific process. The Bargers saw many spiders in the five+ years in their home but in June 2001, daughter Brenna caught two spiders and suspected that they were brown recluses. Specimens were confirmed at the University of Kansas. Diane then collected recluses every night, attempting to rid her home of potentially dangerous spiders. After several weeks of collecting (and tallying recluses into small, medium and large size categories), she contacted me and from there we devised a research project.

Diane (and saintly tolerant husband, Dale) collected recluses every night in their home until mid-September, initially with cumbersome flashlights and then with headlamps and aspirators as recluse fervor increased. Although they snatched as many recluses as possible, some were smashed into oblivion to prevent escape. Additionally, flatbed sticky traps were deployed throughout the house. In mid-September, the school year started, non-spidery commitments called and collecting diminished from daily to weekly. In early November, the project was terminated, recluses were sent to me and data tallied.

Diane hand-collected or smashed 1,213 recluses and another 842 were passively intercepted by sticky traps, for a total of 2,055 brown recluse spiders in one house in six months. Many were spiderlings, too small to be of medical concern, but we estimated about 450 (about 22 percent of the total) were large enough to cause envenomation. Yet despite all these recluses, the Bargers have lived there for seven years and no family member has ever shown evidence of a bite.

Of course, they also found recluses when not looking for them. Ninety brown recluses were discovered during routine activities like moving books or newspapers, shoving stored bedding into a washing machine, pulling a paper towel off the rack with a recluse riding on the back side before being obliterated and one even crawling on Diane while in bed. And still no bites.

Because the Kansas home was obviously extraordinary, a secondary part of this study examined two lesser-infested homes near St. Louis, Mo., and Tulsa, Okla., which yielded 45 and 30 brown recluses, respectively. Four people lived in each home for two and four years, respectively, and again, no one has shown evidence of a bite.

SIGNIFICANCE OF THIS RESEARCH. Although this research might seem like an attempt to be listed in the Guinness Book of World Records its significance relates to the implications for bite diagnoses in nonendemic areas. Even though brown recluse spiders are endemic to a large area of the United States (southeastern Nebraska to southernmost Ohio, south through Texas and northern Georgia), they are blamed for causing dermatological wounds throughout North America including states with zero to just a handful of historically verified specimens. It is extremely rare for this spider to establish a population outside its endemic range.

Despite this, thousands of brown recluse bites are diagnosed annually throughout North America, including such improbable recluse habitats as Alaska and Canada. (Although the equally poisonous, non-native Mediterranean recluse is sparsely scattered throughout the country, infestations typically are limited to a few buildings, they are not a major health risk and proven bites are non-existent or extremely rare at best.) Why is it that Midwest families can live with plentiful recluses and not get bit, yet in regions with no or rare, verifiable recluse populations, bite diagnoses are commonplace?

Answer: physician overdiagnoses abound. In 1990, a survey of 940 South Carolina physicians reported 478 brown recluse bite diagnoses. Although pest management professionals have found one or two recluse infestations, South Carolina is outside the endemic brown recluse range; an arachnologist collecting intensively there (five+ years, 300+ spider species) never found one recluse nor had one submitted to him by the public. In 2000, 95 brown recluse bites were recorded from the 21 counties under the jurisdiction of the Tampa poison control center; Florida’s most prominent arachnologist declares that no brown recluses have ever been collected in those counties. A Colorado medical paper states that brown recluse spiders cause "a significant medical problem in Colorado in the autumn of every year." An extensive statewide survey by the Denver Museum arachnologist involving 500 volunteers yielded 24,000+ spiders and not one recluse. These scenarios are replicated in other non-endemic states.

The data from the three Midwest homes demonstrates that you can live amidst dozens to thousands of brown recluse spiders and not get bit. The Barger home has possibly produced more brown recluse spiders in six months than have been historically found in all non-endemic states, constituting more than half the continental United States area. Therefore, in non-endemic recluse areas like Colorado, central Florida and most of South Carolina, the chances of finding an actual recluse spider is extremely low and the probability of getting bit is statistically almost zero. At the least, annual physician bite diagnoses far outnumber historical verifications of recluses. Recluse bite diagnoses in non-endemic recluse areas should not be considered unless the spiders can be found at the immediate scene of the alleged envenomation and identified by qualified experts. Non-arachnologists (doctors, members of the pest management community, health officials, "bite" victims) frequently mis-identify harmless spiders as brown recluses, reinforcing their misconceptions.

Although many conditions that doctors misdiagnose as brown recluse bites heal with general care, some afflictions can be detrimental if misdiagnosed and given incorrect remedy. Several "brown recluse bite" victims from Wisconsin, California and the Northeastern U.S. actually had Lyme disease, which is treatable with common antibiotics but possibly fatal or permanently debilitating if misdiagnosed and mistreated. Additional brown recluse bite diagnoses have actually been cancer, necrotizing bacteria, diabetic ulcer or chemical burn, among other afflictions.

Many pest management professionals have been using PCT articles to combat the brown recluse overdiagnosis issue in North America but physicians do not want to be educated by a pest control journal. Therefore, listed below are several medical journal references for dissemination to physicians in non-endemic recluse areas to educate them that their brown recluse diagnoses have low probability of accuracy. To further educate the medical community, I am currently collaborating on manuscripts with two Harvard medical school professors on this overdiagnosis issue.

The author is a staff research associate in the Entomology Department at the University of California — Riverside. He has 14 publications in medical journals regarding venomous arthropods, eight of which pertain to the overdiagnosis of brown recluse bites in North America.


Bennett, R. G. and R. S. Vetter. 2003. Erroneous attribution of necrotic lesions to brown recluse, hobo or other spiders in Canada. Canadian Family Physician.

Isbister, G. K. 2001. Spider mythology across the world. Western J. Medicine 175:86-87.

Masters, E. J. and L. E. King, Jr. 1994. Differentiating loxoscelism from Lyme disease. Emergency Medicine 26 (10): 47-49.

Osterhoudt, K. C., T. Zaoutis and J. J. Zorc. 2002. Lyme disease masquerading as brown recluse spider bite. Annals of Emergency Medicine 39:558-561.

Vetter, R. S. 1999. Identifying and misidenti-fying the brown recluse spider. Dermatol. Online 5 (2): http://matrix.ucdavis.edu/DOJvol 5num2 special/recluse.html.

Vetter, R. S. 2000. Myth: idiopathic wounds are often due to brown recluse or other spider bites throughout the United States. Western J. Medicine 173:357-358.

Vetter, R. S. and D. K. Barger. 2002. An infestation of 2,055 brown recluse spiders (Araneae: Sicariidae) and no envenomations in a Kansas home: implications for bite diagnoses in non-endemic areas. J. Medical Entomol. 39:948-951.

Vetter, R. S. and S. P. Bush. 2002. The diagnosis of brown recluse spider bite is overused for dermonecrotic wounds of uncertain etiology. Annals of Emergency Medicine 39: 544-546.

Vetter, R. S. and S. P. Bush. 2002. Reports of presumptive brown recluse spider bites reinforce improbable diagnosis in regions of North America where the spider is not endemic. Clinical Infectious Diseases 35:442-445.

Vetter, R. S. and S. P. Bush. 2002. Chemical burn misdiagnosed as brown recluse spider bite. Amer. J. Emergency Medicine 20: 68-69.

Vetter, R. S. and P. K. Visscher. 1998. Bites and stings of medically important venomous arthropods. Internat. J. Dermatol. 37:481-496.