[Tech Talk] Spider Bites: The Inside Story

Through my years in the industry, I have heard countless people talk about being bitten by spiders. But how often does this actually occur? Recently, I decided to find out.

My instinct was that spiders don’t bite people all that often — but what documented evidence was there to support this theory? After a quick search, I found some documentation on bite rates of spiders and people.

But before we get to the documentation, let’s look at spiders and why they bite. Most varieties of spiders produce venom, and some spiders will expel digestive juices into their prey. The venom is used to subdue prey, and in some cases, to begin the digestion of their prey. Producing venom takes a lot of the spider’s resources and time — for instance, Cupiennius salei takes anywhere from eight to 16 days to refill its venom glands after use. Venom is a valuable resource to a spider and needs to be used prudently; random, unprovoked biting of non-prey animals, like people, is not a prudent use of a valuable resource.

Spiders’ venom glands are surrounded by muscles, enabling spiders to control the amount of venom injected. One published study demonstrated that individual spiders inject more venom when dealing with larger, more active and difficult to subdue prey while injecting less into smaller, less active prey.

So why bite? Humans are not a food source for spiders. Spiders only bite people when they are threatened. One published study reports that for huntsman spiders, 75 percent of verified bites occurred when the spiders were handled. Because of this, 82 percent of the bites were found on the hands and feet. Most spiders only bite people as a last ditch defense response.

In another study, 2,055 brown recluse spiders were captured over a six-month period in a Kansas home — 400 of those spiders were big enough to inflict envenomation in humans. Despite the number of capable spiders, no one was bitten.

For a spider bite with a reaction to occur, several conditions must be met. The spider must have a reason to bite, and as we have seen, most spiders are not overly aggressive and only bite in self-defense.

Spiders must have fangs capable of penetrating human skin — many adult spiders do not have fangs of sufficient length to do that.

Even though most spiders produce venom, not all venoms trigger a response in humans. Many responses to a spider bite will be localized reactions of the immune system to the foreign proteins injected by the spider – the classic swelling, redness and itching that occurs with many biting insects.

The clinical definition of a spider bite is very specific, and it is suspected that many lesions or skin conditions are misidentified as spider bites. A clinical definition of a spider bite consists of three components: pain at the site of the bite immediately following the bite; collection of the spider at the time of, or immediately after the bite; and identification of the spider by an expert to verify it is capable of producing symptoms.

Only then, when all of these conditions are met, is it scientifically a spider bite.

Something else? Based on all of the available information, I believe it is reasonable to conclude that random, unprovoked spider bites are rare. The black widow spider can cause severe, systemic reactions in people who are bitten; in areas where black widow spiders are common, there are few verified bites.

One current phenomena surrounding spider bites is the reported occurrence of necrotic ulcers or lesions caused by spider bites, particularly the brown recluse, yellow sac spider and others (including the hobo spider and even wolf spiders). This phenomenon is called “necrotic arachnidism” and is common in the United States. The ability of hobo spiders to cause these ulcerations was first reported in the Northwest United States in the late 1980s — however, through venom analysis, that is now a disputed claim.

Brown recluse spider bites are diagnosed throughout the country while the range of these spiders is limited to very localized areas. As reported in the Journal of Medical Entomology, the Florida State poison control center reported medical diagnosis of 124 brown recluse bites over a four-year period while only 11 verified brown recluses have been collected in Florida over a 100-year period. Clearly, other medical conditions are being misdiagnosed as spider bites.

While there are a handful of spiders whose bites can cause serious reactions in people, spider bites are not that common. Even dangerous spiders are not aggressive and prefer to retreat and escape rather than bite. Random, unprovoked bites are extremely rare. Pest management professionals should be wary of bite reports and certainly should not exaggerate the frequency of the bites with customers, as this could lead to legal and ethical issues in our industry.

Being a generalist predator, spiders are beneficial organisms. However, many people harbor a fear of spiders, and their webbing causes aesthetic concerns, so don’t fear: there will be a need to provide control services for these customers.

As a Board Certified Entomologist with 40 years of pest management experience, Jeff Weier, BCE, is director of technical services and training for Sprague Pest Solutions, Tacoma, Wash. An accomplished speaker, trainer and author on pest prevention, stored food pests, mating disruption and sanitation, he is a member of the Copesan Technical Committee.

Copesan is an alliance of pest management companies with locations throughout North America. To learn more, visit www.copesan.com.

References

Binford, G. J. An Analysis of Geographic and Intersexual Chemical Variation in Venoms of the Spider Tegenaria agrestis (Agelenidae). Toxicon 39, 955-968. 2001.

Foelix, R. F. 2011. Biology of Spiders, 3rd ed. Oxford University Press, New York.

Gaver-Wainwright, M.M., R.S. Zack, M.J. Foradori, and L.C. Lavine.

Misdiagnosis of Spider Bites: Bacterial Associates, Mechanical Pathogen Transfer and Hemolytic Potential of Venom From the Hobo Spider, Tegenaria agrestis (Araneae: Agelenidae). J.Med.Entomol. 48[2], 382-388. 2011.

Ishbister, G.K. and J. White. Clinical Consequences of Spider Bites: Recent Advances in Our Understanding. Toxicon 43, 477-492. 2004.

Kisbister, G. Necrotic Arachnidism: The Mythology of a Modern Plague. Lancet 364, 549-553. 2004.

Vetter, R.S. and D.K. Barger. An Infestation of 2055 Brown Recluse Spiders (Araneae: Sicariidae) and No Envenomations in a Kansas Home: Implications for Bite Diagnosis in Nonendemic Areas. J.Med.Entomol. 39[6], 948-951. 2002.

Vetter, R.S., G.B. Edwards, and L.F. James. Reports of Envenomation by Brown Recluse Spiders (Araneae: Sicariidae) Outnumber Verifications of Loxosceles Spiders in Florida. J.Med.Entomol. 41[4], 593-597. 2004.

Wigger, E., L. Kuhn-Nentwig, and W. Nentwig. The Venom Optimisation Hypothesis: A Spider Injects Large Venom Quantities Only Into Difficult Prey Types. Toxicon 40, 749-752. 2002.