Along Came a Spider...

Lesions are often blamed on spider bites, but only the black widow and brown recluse present envenomation threats


      In 2006, there were more than 82,000 calls to U.S. poison control centers reporting spider bites.1 Maybe you ran a few of those calls. What species is the most common in your neighborhood? Have you seen the dreaded Springfield recluse? How about the Philadelphia or Sacramento recluses?

   These are all closely related to the Los Angeles, Chicago and New York recluses. Related, of course, because they don't exist. Ever since purveyors of urban myth discovered e-mail and YouTube, recluse spiders (also known as brown recluses, brown spiders or Loxosceles spiders) are blamed for almost all lesions of unknown origin.2 We even get calls to 9-1-1 for "spider bites" from patients who are, in reality, suffering from some sort of inflammation that has nothing to do with spiders.

   Spider bites dwell more in the realm of the emergency department than the back of an ambulance, but acute envenomation does happen and can be quite painful. There is a possibility of death in some poisonous spider bites, but as a group they are much more likely to be colossally annoying than fatal.

SPIDER MYTHS AND REALITIES

   Almost all spiders are poisonous.3 However, in most cases, the venom is either too weak, in too small a quantity or too specific to insects to be toxic to humans. Even in species that have toxic venom, their fangs and other mouth parts may not be big or strong enough to bite humans.4

   In spiders, size doesn't matter—some of the biggest are the least dangerous. Those hairy tarantula-size monsters that crawled around on Indiana Jones are teddy bears compared to a black widow.4

   There are known to be only two medically significant species of spider in the United States. Most widespread is the black widow, common throughout the entire country. Cousins of the black widow live in all parts of the world. The infamous brown recluse, known best for necrotic arachnidism—a fancy term for ulcerated lesions caused by spider bites—is found only in the southeastern region of the U.S.2 There are a few other Loxosceles spiders found in the south, but none with the reputation of the brown recluse.

   All Loxosceles spiders have essentially the same toxic venom. For our purposes, they should be considered the same. Definitively identifying a brown recluse is too difficult for the average healthcare provider and should be left to the professional arachnologist.

   Despite its well-documented and very limited habitat, the brown recluse is blamed for lesions presenting in emergency departments from California to Maine. The Internet is full of brown recluse bite "victims" vigorously defending their homegrown diagnoses without suitable specimens to identify. The desire to know what causes such lesions is so strong, it concocts recluse spiders where none exist.

   In recent years, that need for answers and the well-known lack of brown recluse habitat in most of the country brought a new spider villain to the forefront: The hobo spider (Tegenaria agrestis) is notoriously hard to identify—harder even than the brown recluse—and is gaining ground as a supposed producer of necrotic arachnidism.5

   Physicians are not immune to the hype. There have been misdiagnoses of lesions as spider bites, even when the spider in question didn't live in the area. For instance, while several brown recluse bites have been diagnosed in the San Francisco Bay area, no recluse or hobo spiders have been found there.6

BLACK WIDOWS

   Black widow spiders (of the genus Latrodectus) are different than recluse spiders because their venom affects the body systemically rather than locally.4 There is a real danger of systemic reaction and death from black widow bites. However, though death is possible, it's unlikely.

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