Tales of gory wounds have made this spider feared in areas where it’s not even found.
The brown recluse spider, Loxosceles reclusa, is one of the few North American spiders well known to the public because of its sinister reputation for causing skin lesions. Most bites from the brown recluse are of minor concern: They don’t develop serious skin injuries and they heal nicely without medical intervention. In rare cases, though, bites can cause severe rotting flesh lesions, taking several months to heal and leaving a disfiguring scar.
Despite what many members of the medical community and the public mistakenly believe, however, the brown recluse is not a common spider throughout the U.S. It’s only widespread in the Midwestern region, in an area bordered roughly from southeastern Nebraska to southwestern Ohio, south into Texas and to northern Georgia.
In the middle of its range — Missouri, Oklahoma, Kansas and Arkansas — the brown recluse is extremely common in homes. Massive populations can be found with little effort in many structures. For example, one study found these spiders in 22 of 25 Kansas homes surveyed, averaging 83 spiders per house, with a population range of one to 526.
In another Kansas incident, a family collected 2,055 brown recluses in their house over a six-month period. Researchers in Oklahoma collected more than 1,100 brown recluses in one barn in three nights.
Toward the periphery of the brown recluse range — for example, northern Georgia, the central parts of Illinois and Indiana and the eastern areas of Tennessee and Kentucky — brown recluse populations start to diminish. However, its infamy has led to both the public and the medical community believing that the spider is ubiquitous throughout North America.
In areas that don’t have brown recluses, a multitude of dermal wounds have been mistaken as brown recluse bites, when the actual causes have nothing to do with spiders. However, all recluse spider species tested so far carry the dangerous venom enzyme that causes skin lesions, so even untested species should be considered of potential medical importance. There are 10 other native Loxosceles species in the U.S., all living in the Southwestern region. Five species have widespread distribution.
There are two non-native species in North America. The Mediterranean recluse is a worldwide tramp species. It has been reported in many locations around the country; however, a typical infestation is large in number yet restricted to one to several buildings. The exception is in central Washington DC, where they’re fairly common.
Finding a population in one building doesn’t necessarily guarantee they’ll be found in nearby structures, unless the structures are connected by underground passageways such as pipe conduits or steam tunnels.
Except on very rare occasions, Mediterranean recluses have never been found outside buildings in the U.S. These spiders look very similar to brown recluses, and are frequently misidentified as such.
Another species of recluse spider from South America has stable populations in a few cities in urban southern California — and a few museum basements elsewhere — but its is neither widespread nor known to bite people, despite residing in heavily populated areas of southern California for more than 70 years.
The brown recluse is a fairly nondescript, medium-sized spider, about 3/8-in. in body length. For many years, publications almost uniformly mentioned that the best way to identify a brown recluse spider is by a violin pattern on the dorsal surface of the cephalothorax (the body part to which the legs attach). Although this is basically true, because there are several other harmless house spiders that have similar markings, non-arachnologists frequently misidentify almost any brown spider found around a home as a brown recluse.
A more accurate method to identify a recluse spider is to count the eyes. Most spiders have eight eyes in two rows of four (or something approximating this). By contrast, recluse spiders have six eyes: a pair in front and a pair on either side, separated by a space. Without a magnifying lens, they will appear as three dots. Spitting spiders, genusScytodes, also have this eye pattern but typically look very different from recluses.
Another reason that the eye pattern should be used over the violin pattern is that the violin pattern is faint or missing in young or newly molted brown recluses — as well as in several other North American recluse species. Therefore, it’s likely for an actual recluse to be misidentified as harmless when, in fact, it could be dangerous.
The brown recluse spider is an urban pest. It is similar to pigeons, rats and cockroaches in that its numbers increase in association with humans. However, for unknown reasons, the other species found in the Southwestern deserts are not urban pests. They typically only infest homes that are surrounded by native vegetation, like cactus and creosote.
AS BAD AS ITS BITE?
The main reason for the hoopla regarding the brown recluse is its ability to sometimes cause skin lesions from envenomations. A bite typically occurs when the spider is trapped between exposed flesh and some object, so people are typically bitten while dressing or when rolling over in bed at night.
However, one fact that’s often overlooked is that about 90 percent of brown recluse bites heal without serious consequences, and also heal nicely without medical intervention.
In the few bites that develop serious lesions, the initial bite is barely perceptible. Pain develops within three to eight hours. In some cases, a bull’s eye lesion will form — but this is also a symptom of Lyme disease, so it’s not diagnostic solely for recluse bites. A hardened ulcer may form during the course of a week, which later will fall away from the wound exposing soft undertissue. Healing may be slow, requiring two to three months. However, even serious lesions heal nicely in most cases.
Although there are no proven deaths from brown recluse spider bites — a spider found in the act of biting, properly identified and the bite victim died — there have been several recorded child deaths where a recluse bite is the most probable causative agent. Nonetheless, even these presumptive deaths are extremely rare. There is about one reported every decade, which pales in comparison to the injury and death risks of riding in a car, for example.
One of the most important aspects of recluse bite medicine in the last decade or so is the awareness that there are more than 40 different medical conditions that result in skin lesions that physicians could misdiagnose as brown recluse spider bites. Many misdiagnoses are occurring throughout North America, in states and provinces where no populations of brown recluses are known — and in many cases, where no specimens have ever been found.
Some of these conditions are far worse than any recluse bite would ever be, and in some cases, could be deadly if not treated properly. This is especially true when a condition is misdiagnosed; the wrong remedy is given and the condition continues to run rampant.
Although brown recluse spiders can be difficult to control, with a combination of sticky traps and persistent chemical application, their numbers can be greatly reduced to the point where they’re found in low enough numbers that a homeowner or business might consider acceptable.
It’s been general procedure that pest management professionals (PMPs) don’t guarantee eradication of brown recluses, but instead reduction of the population to such low numbers that it will significantly reduce the spider’s potential for causing damage. Yet for some people, even one brown recluse spider is one too many.
The first step is to verify the presence of brown recluse spiders in a home by using sticky traps placed next to walls and other vertical surfaces. If a house has recluses, it should have plenty of them — and they should be easily trapped. Recluse spiders leave behind a shed skin in an asterisk pattern, which will allow a PMP to determine whether recluses had been present at one point, even if no spiders can be found. (See photo.)
Older homes seem more susceptible for infestation because of numerous cracks and crevices, but even in Missouri or eastern Kansas, not every home has recluse spiders.
One Midwestern company’s threshold is to treat if more than six brown recluses are captured in traps. Treating baseboards, dusting wall voids and crevices with micro-injectors has been effective, with follow-up visits each month for the first three months. Service then becomes quarterly.
Preventing spider entrance into the home may also be effective by sealing and caulking holes and cracks, and by power spraying the vegetation on the house. Physically removing old silk and shed skins allows you to see whether there is additional activity on the next visit. However, many homes cannot be made impenetrable.
In addition to actual treatment, there are a couple of things you can advise your clients to do that can reduce or minimize the chances of encountering brown recluse spiders:
- In the bedroom, pull the bed away from the wall, remove all ruffles or bed skirts and all clutter from under the bed such that the only way a brown recluse could climb on the bed is up one of the four legs. Some people put sticky traps under the foot of each bed leg, which can be done in homes without small children or inquisitive pets.
- In the garage or attic, store clothing in spider-proof clothing bags or zipper-closure plastic bags. This is important for items that are not used very often, such as baseball mitts, sports shoes, gardening gloves and apparel.
Vetter, an entomologist with the University of California-Riverside, can be reached by e-mail at firstname.lastname@example.org.