By Richard Hamilton, MD, FAAEM, ABMT, Associate Professor of Emergency Medicine, Temple University Hospital and School of Medicine, Philadelphia, PA. Dr. Hamilton is Editor of Emergency Medicine Alert.
Based upon the experiences in my last few shifts in the ED, you would think that my community is being attacked by hordes of angry spiders. The mystery of this arachnid epidemic is furthered by the inability to identify spider species; most spiders are crushed beyond recognition when brought for identification, and spider identification itself is a tricky business. In fact, the only way to approach the problem of the boil of unknown origin (BUO) is to presume that the patient contracted it in a fashion consistent with the prevalent conditions in the community. This commentary will look at the differential diagnoses of boils, furuncles, and skin infections and the pitfalls of ignoring the presence of truly prevalent conditions.
Brown Recluse and Other Spider Species
The arachnid most often indicted in the BUO situation is the dreaded Loxosceles reclusa—the brown recluse or fiddleback spider. This spider has a flat 1-cm body with longer legs making its overall size slightly larger than a U.S. quarter. As its name implies, it is reclusive and decidedly nocturnal unless disturbed. Living indoors, it uses its flat body to hide in dark quiet places like closets, where it finds clothing and other comfortable items.
Most importantly, its habitat is limited to the southern Midwest United States (Nebraska, Kansas, Oklahoma, Texas, Louisiana, Arkansas, Missouri, Kentucky, Tennessee, Mississippi, Alabama, northern Georgia, and southern portions of Ohio, Indiana, Illinois, and Iowa). It has never been identified on the East or West Coast.
A brown recluse occasionally has emerged from a shipment of materials from an endemic area, but this is extremely rare.1 In fact, the spider is so reclusive that bites in infested endemic areas are unusual. Entomologists reported a home in Kansas where they collected more than 2000 L. reclusa spiders in a six-month period without any of the inhabitants ever being bitten.2
There are a number of other species of Loxosceles in the United States, but the important ones are deserta, arizonica, apachea, blanda, and devia. These latter species are found in the desert regions bordering Mexico. None of these species are found in the chaparral or coastal regions of southern California. Practitioners in endemic areas have learned to identify the subtle, bluish subcutaneous necrosis surrounding a rather unimpressive bite mark as the classic recluse bite.
There are plenty of examples of spiders that possibly can produce necrotic bites: hobo spiders (Tegenaria agrestis) from the northwestern United States; yellow sac spiders (Cheiracanthium species) and wolf spiders (Lycosidae family), found worldwide; crab spiders (Sicarius testaceus and S. lbospinosus) from South Africa; white-tailed spiders (Lampona cylindrata and murina) and black house spiders (Badumna species) from Australia. However, most of these species have proven to cause very weakly necrotic bites. For example, the hobo spider, introduced into the Pacific Northwest from Europe in the 1920s or 1930s, is presumed to be the cause of necrotic spiders bites in that region (though unproven), although the same spider is considered nontoxic in Europe.4
This fascination with spiders is not a trivial problem. In the 2001 bioterrorism attack, a pediatric case of cutaneous anthrax in New York City originally was treated as a Loxosceles envenomation.3
Beyond Spider Bites
When examining these lesions, the practitioner must rigorously consider the locale and adopt an approach that adheres to Baye's theorem: Correct empiric diagnosis presumes an awareness of the prior probability of that disease being present. For example, early Lyme disease can manifest as a lesion surrounding the tick bite that closely mimics the brown recluse spider bite. The distribution of Lyme disease—whether the vector is Ixodes scapularis or pacificus—is virtually throughout the United States. Importantly, the Northeastern coastal areas, which decidedly are not brown recluse country, are most definitely Lyme country. Better to appropriately consider Lyme disease than to inappropriately presume a brown recluse bite. In fact, the majority of furuncular lesions are simple staphylococcal skin infections. They often start as rather painful folliculitis lesions, as insect bites, or other minor trauma that becomes becomes infected secondarily. These infections traditionally respond to the wound management regimens that are appropriate for these lesions: incision and drainage when necessary, warm compresses, and anti-staphylococcal antibiotics, such as cephalexin, when there is a surrounding cellulitis. A more recent concern has been the emergence of non-hospital-associated, methicillin-resistant Staphylococcus aureus (MRSA). This organism is emerging rapidly and is resistant to cephalexin.
In 2000, a number of infectious disease surveillance experts noticed that MRSA appeared in patients who had developed staphylococcal infections not associated with the hospital—a community-associated MRSA (CA-MRSA). CA-MRSA differs from hospital-associated MRSA in that it is more likely to be susceptible to multiple antimicrobial classes (but not cephalosporins).
It does not require vancomycin for treatment. CA-MRSA seems to be most sensitive to some fluoroquinolones (levofloxacin), trimethoprim/sulfamethoxasole, tetracyclines, or rifampin (although the latter agent should not be used as a solo agent for treatment).
One interesting feature of CA-MRSA is that it encodes for genes that make Panton-Valentine leukocidins, a feature that is found rarely in hospital-associated MRSA. The Panton-Valentine leukocidins are cytotoxins that cause tissue necrosis and leukocyte destruction by forming pores in cellular membranes. Therefore, these lesions often will appear fairly necrotic and have aggressive cellulites, features that might incline the physician to consider an envenomation.5 The recent report of an outbreak of CA-MRSA in the St. Louis Rams organization highlights the degree to which this organism can spread in the local environment, another factor that might create arachnophobia.6
Causative Conditions to Consider
Other conditions to consider when evaluating a BUO include insects (infection and allergic reactions resulting from flies, mosquitoes and ticks); viral causes (herpes simplex especially herpetic whitlow); and other infectious causes (impetigo, staphylococcal and streptococcal, CA-MRSA, cutaneous anthrax, disseminated gonococcus pustules, Mycobacterium ulcerans, diabetic ulcer, necrotizing fasciitis, pyoderma gangrenosum, toxic epidermal necrolysis, cat scratch disease, rat bite fever, chancriform pyoderma, and lymphogranuloma venereum). Additional conditions are drugs/toxins (erythema nodosum, warfarin skin necrosis, erythema multiforme, Stevens-Johnson syndrome, self-inflicted wounds [drug injection sites], retained foreign bodies); topical causes (poison ivy/oak infection, hydrogen fluoride exposures); and neoplasia (papilloma, fibroma, lymphoma, osteosarcoma, fibrosarcoma, epithelial carcinoma, melanoma).
The list of possible conditions is significant. In the next patient who presents to the ED with a potential spider bite, consider your locale and how the prevalence of certain conditions makes a certain diagnosis probable or improbable, and adjust your differential accordingly.
References
1. Swanson DL, et al. Bites of brown recluse spiders and suspected necotric arachnidism. N Engl J Med 2005; 352:700-707.
2. Vetter RS, et al. Bites and stings of medically important venomous arthropods. Int J Dermatol 1998;37: 481-496.
3. Vetter RS, et al. Do hobo spider bites cause dermonecrotic injuries? Ann Emerg Med 2004;44:605-607.
4. Roche KJ, et al. Images in clinical medicine. Cutaneous anthrax infection. N Engl J Med 2001;345;1611.
5. Naimi TS, et al. Comparison of community- and health care-associated methicillin-resistant Staphylococcus aureus infection. JAMA 2003;290:2976-2984.
6. Kazakova SV, et al A clone of methicillin-resistant Staphylococcus aureus among professional football players. N Engl J Med 2005;352:468-475.
This commentary will look at the differential diagnoses of boils, furuncles, and skin infections and the pitfalls of ignoring the presence of truly prevalent conditions.
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