Spiders, especially brown recluses (Loxosceles reclusa) (figure 1), are frequently incriminated as causative agents in idiopathic wounds, even though diagnoses are usually made solely on the basis of dermatologic symptoms. Rarely is a spider seen inflicting the wound or captured while biting, and it is difficult to either prove or disprove spider participation in the event. The biologic distribution of the brown recluse and related recluse species indicates that many diagnoses made on cases occurring in the western United States are incorrect.¹

Figure 1 Dorsal view of the brown recluse spider, Loxosceles reclusa. The most consistent diagnostic feature for identification is the pattern of 6 eyes, grouped in pairs, with 1 anterior pair and 2 lateral pairs (most spiders have 8 eyes in 2 rows of 4). Although (more ...)

Because of my research interests in spider identification and medically important spiders, I have been avidly studying recluse spiders and their occurrence in the western United States. My experience is greatest in California, but the information presented here is pertinent to northern and western states as well. In almost a decade of research, I have found verification of fewer than 10 brown recluse specimens in California in more than 40 years of records. Sources include medical and arachnologic literature and correspondence with scores of arachnologists, county entomologists, vector control personnel, and the California Department of Food and Agriculture, which handles all exotic pest identifications in the state. Most verified specimens were individuals intercepted from facilities receiving goods from out of state. There are no populations of brown recluses in California.

Despite the lack of brown recluse spiders in California, several hundred cases of “brown recluse bites” have been reported to me in the past decade. Undoubtedly, this is only a small fraction of the total number of brown recluse bites that have been diagnosed. Although it might appear intuitive from figure 2 that these bites could be caused by the native desert recluse (Loxosceles deserta), which lives in the comparatively sparsely inhabited southeastern quadrant of California, virtually all so-called brown recluse bites have come from coastal regions with a concentrated human population. In particular, many originate from the San Francisco Bay-Sacramento area, which lies north of the native desert recluse distribution (figure 2) and has never supported a population of any recluse species. Similar scenarios have emerged from other states, cities, or regions—Vermont, New York, Pennsylvania, Florida, Chicago, South Dakota, Colorado, northern Nevada—where recluse spiders are extremely rare (<10 ever collected) or have never been documented.² There are published California accounts of 2 “brown recluse bite” case histories from a city where recluses have never been collected³ and a physician misidentification of a common spider as a brown recluse, captured while biting a child.⁴ Brown recluse bites are overdiagnosed in coastal and Northern California.

Figure 2 Endemic distributions of the brown recluse (stippling) and related recluse species (lines) in the United States, based on Gertsch and Ennik.2 Recluse populations become sporadic on either side of the demarcating range borders.

Many conditions can cause necrotic wounds that have been misdiagnosed as brown recluse bites.^5,6,7 Many of these are disease states in which an arthropod was not involved (box).

If arthropods are in fact implicated, physicians should not immediately think of accidental spider bites as the likely cause. Instead, they should consider creatures that seek mammals for blood meals—such as ticks, fleas, bedbugs, and assassin bugs—and whose bites can cause a variety of reactions.^5,6

Tickborne Lyme disease can cause a “bull's-eye wound,” which is a diagnostic symptom of brown recluse envenomation, and so Lyme disease can be misdiagnosed as a brown recluse bite.⁸ Although Lyme disease occurs at low incidence in California,⁹ because it can lead to irreversible neural and cardiac complications, its misdiagnosis as a brown recluse bite could have grave consequences.

Even in cases of verified spider bite, differentiation must be made between toxic effects of venom and secondary infections, such as incurred when pruritus leads to scratching. Furthermore, rigorous clinical investigation might reveal alternative causative agents. For example, in 1 case report, a large spider bit an Australian woman, and therapy for the spider bite was ineffective. Subsequent screening determined that the “necrotic arachnidism” was actually a fungal infection called sporotrichosis,¹⁰ another condition misdiagnosed in the United States as brown recluse bite.⁵

Since the early 1980s, a European native, the hobo spider (Tegenaria agrestis), has been implicated in necrotic lesions.¹¹ Although it lives in the Pacific Northwest, east to Montana and south into northern Utah, hobo spider bite diagnoses have already been made in California, even though it has never been documented in the state. The bite of the yellow sac spider (Cheiracanthium mildei), another European native, is usually minor but can cause mild necrosis.¹² It was uncommon in the United States before 1950 but has since spread coast to coast, and it is often found in homes.

Physicians in nonendemic brown recluse regions (figure 2) should be cautious in implicating brown recluses in idiopathic necrotic wounds. The medical community needs to be aware of the many causes of necrotic wounds that can be misdiagnosed as recluse bites. Culture of specimens taken from wounds might accurately determine the plethora of causative agents of so-called brown recluse bites, which can vary from being arthropod, bacterial, viral, or fungal in nature or due to underlying disease states.