[Nuisance Pests] Wrongly Incriminated

In North America, spiders are frequently blamed for causing skin lesions when there is no known cause.
Frequently, a person will have a lesion and immediately blame the brown recluse spider. If that person doesn’t live where brown recluses are found (the south-central Midwest), then a recluse bite is highly improbable.
Having spider “tunnel-vision,” people then incriminate the hobo spider. Unless they live in the northwestern quadrant of the United States, or in British Columbia, Canada, these people don’t have hobo spiders. (Even so, venom toxicologists are questioning whether hobo spiders are actually dangerous.)
Finally, in a determined vendetta against spiders after eliminating recluses and hobo spiders, the ubiquitous yellow sac spider (genus Cheiracanthium) is then blamed. However, recent research is showing that this spider doesn’t cause necrotic (i.e., rotting flesh) skin lesions as is widely thought.

A BAD RAP. Recently, venom toxicologists have been re-examining many of the lesser-known spiders that have been incriminated in medically important events. Instead of using circumstantial evidence as was common in the past, researchers have been relying on the current gold standard for spider envenomation: verified bites in humans. What has been unfolding is that most of these marginally implicated spiders, including the yellow sac spider, have been wrongly blamed for causing skin lesions. The bites still may be painful and cause discomfort for a day or two, but they don’t cause skin lesions.
The basis for blaming yellow sac spiders is due to experimental research in the northeastern United States in 1970 where these spiders were pushed into guinea pig flesh, with bites resulting in necrotic skin lesions combined with the finding of yellow sac spiders in homes where people developed similar lesions. However, in none of the human cases was a spider caught in the act of biting.
Corroborative research from South Africa in a series of papers from the 1980s steadfastly blamed endemic yellow sac spiders for similar necrotic lesions, again with no verified culprit. All accusations toward the spider were made from circumstantial evidence, unsubstantiated case histories and extrapolation from studies with test animals.
In fact, in the North America study, the researchers actually did a purposeful bite in a human subject with minor effects that healed the next day with no necrosis. In the South African studies, the spiders were said to leave a characteristic 6-mm to 8-mm distance between fang marks. However, it would appear to be an anatomic impossibility for a spider of such small size to spread its fangs half its body length and still inflict a bite.
In comparison to these non-definitive studies, a recent paper has been published on the lack of necrotic skin lesions in verified yellow sac spider bites. Twenty verified bites by these spiders, 10 each from the United States and Australia, showed the following as the typical response: 1) hurts like a bee sting when it happens; 2) can cause pain, swelling, redness, itching; and 3) the symptoms last an average of about two hours but can extend up to two days. There was no necrosis nor allergic reaction.
A search of the worldwide literature found 39 cases of yellow sac spider envenomation sufficiently convincing to be deemed verified bites (a spider was found or described in some manner); in only one case, a mild pea-sized necrotic skin lesion developed, with the rest having only mild effects. An additional 64 purported bites lacked sufficient evidence or were described solely from the symptoms in the patient and therefore were excluded as non-verified bites. Interestingly, all of the serious necrotic skin lesions allegedly caused by yellow sac spiders were in this latter dubious group.
Therefore, 59 verified bites were assembled with only one case of minor necrosis. This is sufficient evidence that yellow sac spiders should not be considered to commonly cause necrotic skin lesions.

EXAMINE OTHER CAUSES. Instead of yellow sac spiders, people need to go back to the list of other medical conditions that are far more likely to cause skin lesions.
One newly-emerging disease that is receiving more recognition as causing skin lesions in people housed in close quarters (e.g., prisons, nursing homes, sports camps, military barracks) is a contagious bacterial infection resistant to many antibiotics. This is methicillin-resistant Staphylococcus aureus, or MRSA, and was covered in a previous PCT article (“[They’re Not] Spider Bites,” April 2005). This disease is turning up with great regularity in many places such that PCOs need to be acquainted with the conditions that encourage the spread and maintenance of this infection.
In addition to MRSA, there are many medical conditions that result in necrotic skin lesions, many of which are far more serious than typical spider bites and in some cases can lead to fatalities if diagnosis or treatment is incorrect or delayed.
In past decades, spiders were accused of creating medical havoc based largely on circumstantial evidence because common consensus held that it was rare to catch a spider in the act of biting. However, it is now apparent that one reason spiders were not often caught in the act is because they didn’t actually cause most of the medical mischief attributed to them. Spiders were often elevated to medical significance based on unconfirmed speculation that was eventually accepted as factual based on the repetitive telling of the story, not through solid evidence. Documentation of verified spider bites and their effects is proving many spiders innocent long thought guilty. The yellow sac spider is one of these wrongly incriminated creatures. Its bites do cause pain on inception and minor discomfort for a short time afterward but are not a cause of rotting flesh skin lesions in humans. They should be considered no greater danger than typical house spiders.

The author is a staff research associate in the department of entomology at the University of California-Riverside and can be reached at rvetter@giemedia.com.

References
Vetter, R. 2005. [They’re not] spider bites. PCT 33(4): 86-88.
Vetter, R.S., G.K. Isbister, S.P. Bush and L.J. Boutin. 2006. Verified bites by Cheiracanthium spiders in the United States and Australia: where is the necrosis? Amer. J. Trop. Med. Hyg. 74: 1043-1048.