Brown Recluse Spiders

Eleven species of Loxosceles are indigenous to the continental United States, four of which are known to be harmful to humans.
Brown Recluse Spiders - Articles

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Loxosceles reclusa male. Photo by Richard Vetter

Sicariidae—Sixeyed Sicariid Spiders

(Loxosceles reclusa and other Loxosceles species)

Eleven species of Loxosceles are indigenous to the continental United States, four of which are known to be harmful to humans. Brown recluse spiders are established in sixteen states: Alabama, Arkansas, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, Ohio, Oklahoma, Tennessee, and Texas. In addition, isolated occurrences have been reported in Arizona, California, the District of Columbia, Florida, North Carolina, New Jersey, Pennsylvania, Washington, and Wyoming. Brown recluse spiders are rarely encountered in Pennsylvania, but they may be transported in boxes and similar items from a locale where the spiders normally occur.

The Mediterranean recluse, Loxosceles rufescens, is a closely related species that was introduced from southern Europe. It is established in the steam tunnels of Penn State and in other locations in the Northeast. These spiders are not known to have bitten any employees or students, despite their long sojourn in the Penn State steam tunnels. The bites of these spiders do not produce the severe reactions typically associated with the brown recluse spider.


Native ranges of Loxosceles species in the United States. Recluse spiders outside these areas are rare and almost exclusively restricted to buildings. Image by Richard Vetter, used with permission

Description

These spiders are chocolate brown in color, and their bodies are about 9 millimeters in length with long legs. They have three pairs of eyes, arranged in a triad, and have a violin-shaped marking on the cephalothorax. The body of the “violin” is near the eyes and the neck of the “violin” extends backward, ending before the abdomen. Males are similar to females in appearance.

Life History


Brown recluse eye pattern. Photo by Steven Jacobs, Penn State Extension

After mating in June or July, the female will deposit twenty to fifty eggs in a spherical case. She can produce two to five such batches of eggs during her lifetime. Laboratory-raised individuals can live for two to three years. The young require about one year to mature. The brown recluse, L. reclusa, in its normal range, prefers to inhabit gaps under rocks, boards, and the bark of dead trees and logs. In structures, it will live inside cracks in walls and boards and behind and under any number of items in storage. The brown recluse prefers nesting sites that are warm and dry. In contrast, L. rufescens prefers to nest in cooler, more humid places.

Medical Importance

Multiple studies have found that “brown recluse bites” are overdiagnosed and the majority of “bites” are misdiagnoses of other issues, including poison ivy, chemical burns, and diabetic ulcers. Within their native range, brown recluse can be common and abundant in homes, yet confirmed bites are rarely reported (one study reported a home in which more than 2,000 spiders were collected over a six-month period, yet the residents hadn’t been bitten in six years). When bites do occur, it is usually because a spider is trapped against the skin and feels threatened, such as when someone puts on shoes that were left out overnight or rolls over a spider while sleeping.

Outside their native range (which includes Pennsylvania), brown recluse are restricted to buildings and are almost exclusively brought in by humans (such as when someone moves from an area where brown recluse are native). They are therefore extremely rare and localized. They are not found outdoors, and the risk of being bitten is virtually nonexistent.

Brown recluse bites are commonly thought to cause large, necrotic lesions. While this can occur, they are rare. Approximately 90 percent of brown recluse bites result in no reaction or a small (~5 millimeter), red papule that heals on its own. Approximately 10 percent of bites result in dermonecrotic lesions. These lesions develop over the course of two weeks, during which the skin surrounding the bite turns black, dries out, and eventually sloughs off. These bites take two to four months to fully heal. Dermonecrosis is exaggerated in obese victims because of the increased destruction of poorly vascularized adipose tissue. Less than 1 percent of bites result in systemic symptoms that include hemolytic anemia and acute kidney injury. These symptoms are most common in children and can be fatal in 12–36 hours, so they do represent a true medical emergency.

In summary, brown recluse bites are commonly reported, but they are actually relatively rare where brown recluse natively occur, and virtually nonexistent where they are not native. When bites do occur, the vast majority cause no or minor symptoms. However, serious medical issues, such as dermonecrotic lesions and potentially fatal systemic symptoms, can occur, so confirmed bites should be monitored and treated with caution.

Reference

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