CME

An approach to spider bites

Erroneous attribution of dermonecrotic lesions to brown recluse or hobo spider bites in Canada

Robert G. Bennett, MSC, PHD    Richard S. Vetter, MSC

This article has been peer reviewed. Cet article a fait l’objet d’une évaluation externe. Can Fam Physician 2004;50:1098-1101.

Dr Bennett and Mr Vetter are professional entomologists who have each been studying spiders and other arachnids for about 25 years. Dr Bennett works as an insect pest management specialist with the British Columbia Ministry of Forests. Mr Vetter is an entomologist with the University of California at Riverside and the San Bernardino County Museum.

Case

Consider the following reported cases of alleged bites from brown recluse spiders (Loxosceles reclusa) in Canada.

• After camping in northwestern British Columbia, a young girl is hospitalized for 5 weeks with a necrotic lesion on her leg that requires débridement and excision to the deep fascia and subsequent skin grafts.1

• A teenager sleeping in a tent in BC’s Okanagan Valley develops a painful dermonecrotic lesion on one buttock and is hospitalized for 3 days.1

• A physician sleeping outdoors overnight in Algonquin Park in Ontario develops an idiopathic lesion.2

• A labourer in Winnipeg, Man, feels something in his work glove and the following day presents to an emergency ward with a necrotic lesion on his finger.3

In each of these cases “loxoscelism” (the cutaneous and systemic effects of a bite from a brown recluse spider4) was diagnosed although no brown recluse or other spider was caught in the act of biting, captured, or identified by a qualified expert.

In North America, many medical conditions that cause dermonecrosis have been misdiagnosed as the effects of bites from brown recluse or other spiders,5-17 and fully 80% of spider-bite diagnoses are erroneous.13 In general, spiders are erroneously blamed around the world for causing dermonecrotic lesions when the evidence points in many other directions.5,14,18-22 Such misdiagnoses can lead to serious medical complications (or possibly litigation) if diagnosis is delayed or treatment is inappropriate or ineffective.8,23

Sources of information

Because most studies are retrospective and few data on spider bites have been verified, the literature on spider envenomation is often based on inference from animal model toxicology; circumstantial evidence; poorly designed or otherwise inferior clinical studies; and unfortunately, considerable hyperbole. For example, in South America, experimentally induced necrosis and poor clinical reporting put blame on a harmless wolf spider (family Lycosidae) for causing skin lesions.24 Wolf spider antivenin was developed and used for decades. Subsequent tracking of 515 documented wolf-spider bites in humans with no necrosis showed the attribution to be erroneous and the treatment unwarranted.24 In another example, based mostly on shreds of unsubstantiated evidence, white-tailed spiders (Lampona species) were erroneously named the etiologic agents of human dermonecrosis in Australia. This led to bogus case histories, unnecessary calls for research funding, and hyperbole in the popular and medical press.22 A recent prospective report of 130 verified white-tailed spider bites, none of which caused necrosis,21 might put this myth to rest. Closer to home, several American states each report dozens to hundreds of loxoscelism diagnoses annually even though brown recluse spiders are extremely rare or have never been found in those states or regions.14,17,25

We have substantial personal knowledge and experience of the facts and myths about spider bites. We are professional entomologists and have worked extensively with medical professionals and the public in Canada and the United States on loxoscelism and other issues around spider bites.

Main message

Brown recluse spiders. Loxoscelism is rare even where brown recluse spiders are an abundant native species. Homes in areas where brown recluse spiders are endemic can support large populations of these spiders without anyone who lives in them being envenomated. For example, one Kansas family collected 2055 brown recluse spiders in their home over 6 months without incident.26

No brown recluse spiders and only three specimens of any type of recluse spider (all belonging to a well-known vagrant South American species) have ever been found in Canada.5 Despite this, loxoscelism continues to be diagnosed in this country. The unsubstantiated belief that brown recluse spiders are being transported outside their natural range in sufficient numbers to cause necrotic lesions has led to erroneous diagnosis of many more brown recluse spider bites in non-endemic areas, such as Canada, than these spiders could possibly cause.5,14,17,25

Even if brown recluse spiders occasionally found their way into Canada, the statistical probability of people being bitten by immigrant spiders is effectively zero.17 If loxoscelism truly occurred in Canada, brown recluse spiders would be sufficiently common that specimens would regularly be turned in for identification by homeowners and pest control personnel. To our knowledge, this has never happened.

Hobo spiders. In recent years, in southwestern Canada and adjacent regions of the United States, hobo spiders have been implicated in relatively minor necrotic lesions among humans.27,28 Hobo spiders exist in Canada only in rare, localized populations in extreme southern British Columbia29 and, contrary to popular belief, are timid and unlikely to bite. We know of no authentic Canadian report of hobo spider envenomation. Also, new research suggests that hobo spiders have been falsely accused,30 and researchers are reevaluating whether hobo spider venom is even toxicologically active.

Other spiders. Physicians might lean away from diagnoses of brown recluse or hobo spider bites but continue to blame other spiders as the cause of idiopathic necrosis. A recent prospective study of 750 verified bites from a variety of spider species in Australia demonstrated, however, that not one bite from any spider resulted in a necrotic lesion.20 Some of the spiders discussed are found around the world, so this study could be applicable worldwide.

Differential diagnoses. There are many diagnoses more reasonable than “spider bite” or loxoscelism for idiopathic necrotic lesions. Various infections (bacterial, viral, fungal) and cancers (basal cell carcinoma, lymphoma); topical (poison ivy or oak, burns), arthropod-vectored (Lyme disease, tularemia), and vascular disorders; and conditions of miscellaneous or idiopathic etiology (eg, pressure ulcers, pyoderma gangrenosum) are more realistic and credible causes of such lesions.5,17,31 All have been misdiagnosed as resulting from spider bites.

The criterion standard for spider-bite diagnosis should be a spider caught in the act of biting or otherwise reliably associated with a lesion (and properly identified by a qualified arachnologist). Unless this standard is met, a working diagnosis of spider bite should not be considered. Proper identification is critical; in our experience, the general public and the medical community identify many harmless spiders as brown recluse32,33 or, more recently, hobo spiders.

Much of the literature on spider envenomation is deficient in definitive, prospective case histories.20,22 Reports of alleged spider bites should be supported by solid proof of spider involvement. Not providing this proof entrenches the mistaken belief that spider bites are a common cause of necrosis and leads to the proliferation of misdiagnoses. Decreased reliance on diagnoses of loxoscelism or other spider bite necrosis will result in improved health care through reduction of misdiagnoses, incorrect remedies, patient anxiety or arachnophobia, and potential litigation.

Conclusion

From available case information, it is currently impossible to propose accurate diagnoses for the four cases reported in our introduction. Priority consideration should be given to one or more of the many alternative and more probable causes of necrosis.

In Canada, little evidence suggests that dermonecrotic lesions can be attributed to the effects of spider envenomation. Reports of alleged effects of spider bites should not be published unless spider involvement is conclusively proven. To do otherwise creates a confused clinical picture and throws up obstacles to appropriate and improved health care.

Competing interests

None declared

Correspondence to:Robert G. Bennett, British Columbia Ministry of Forests, 7380 Puckle Rd, Saanichton, BC V8M 1W4; telephone (250) 652-6593; fax (250) 652-4204; e-mail Robb.Bennett@gems6.gov.bc.ca

References

1. Baldwin GA, Smith DF, Douglas S. Loxoscelism in Canada. Can Med Assoc J 1988;138:521-2.

2. Nelson J. Letter to the editor. Can Med Assoc J 1988;138:888-9.

3. Rose G, Ross LL, Palatnick W, Embil JM. Photo quiz. Clin Infect Dis 2001;32:595, 636-7.

4. Atkins JA, Wingo CW, Sodeman WA. Probable cause of necrotic spider bite in the midwest. Science 1957;126:73.

5. Vetter RS, Bush SP. Reports of presumptive brown recluse spider bites reinforce improbable diagnosis in regions of North America where the spider is not endemic. Clin Infect Dis 2002;35:442-5.

6. Chow RK, Ho VC. Treatment of pyoderma gangrenosum. J Am Acad Dermatol 1996;34:1047-60.

7. Freedman A, Afonja O, Chang MW, Mostashari F, Blaser M, Perez-Perez G, et al. Cutaneous anthrax associated with microangiopathic hemolytic anemia and coagulopathy in a 7-month-old infant. JAMA 2002;287:869-74.

8. Kunkel DB. The myth of the brown recluse spider. Emerg Med 1985;17(5):124-8.

9. Osterhoudt KC, Zaoutis T, Zorc JJ. Lyme disease masquerading as brown recluse spider bite. Ann Emerg Med 2002;39:558-61.

10. Roche KJ, Chang MW, Lazarus H. Cutaneous anthrax infection. N Engl J Med 2001;345:1611.

11. Rosenstein ED, Kramer N. Lyme disease misdiagnosed as a brown recluse spider bite. Ann Intern Med 1987;107:782.

12. Russell FE. A confusion of spiders. Emerg Med 1986;18(11):8-9, 13.

13. Russell FE, Gertsch WJ. For those who treat spider or suspected spider bites [letter]. Toxicon 1983;21:337-9.

14. Vetter RS. Myth: idiopathic wounds are often due to brown recluse or other spider bites throughout the United States. West J Med 2000;173:357-8.

15. Vetter RS, Bush SP. The diagnosis of brown recluse spider bite is overused for dermonecrotic wounds of uncertain etiology. Ann Emerg Med 2002;39:544-6.

16. Vetter RS, Bush SP. Chemical burn misdiagnosed as brown recluse spider bite. Am J Emerg Med 2002;20:68-9.

17. Vetter RS, Cushing PE, Crawford RL, Royce LA. Diagnoses of brown recluse spider bites (loxoscelism) greatly outnumber actual verifications of the spider in four western American states. Toxicon 2003;42:413-8.

18. Isbister GK. Necrotic arachnidism in Australia. Toxicon 2001;39:1941-2.

19. Isbister GK. Spider mythology across the world. West J Med 2001;175:86-7.

20. Isbister GK, Gray MR. A prospective study of 750 definite spider bites with expert spider identification. Q J Med 2002;95:723-31.

21. Isbister GK, Gray MR. A prospective study of 130 definite bites by Lampona species. Med J Aust 2003;179:199-202.

22. White J. Debunking spider bite myths—necrotising arachnidism should be a diagnosis of last resort. Med J Aust 2003;179:180-1.

23. Vetter RS. Brown recluse spider bite diagnoses and lawsuits. Pediatr Emerg Care 2003;19:291-2.

24. Ribeiro LA, Jorge MT, Piesco RV, Nishioka SA. Wolf spider bites in Sao Paulo, Brazil: a clinical and epidemiological study of 515 cases. Toxicon 1990;28:715-7.

25. Vetter RS, Edwards GB, James LF. Reports of envenomation by brown recluse spiders (Araneae: Sicariidae) outnumber verifications of Loxosceles spiders in Florida. J Med Entomol. In press.

26. Vetter RS, Barger DK. An infestation of 2,055 brown recluse spiders (Araneae: Sicariidae) and no envenomations in a Kansas home: implications for bite diagnoses in nonendemic areas. J Med Entomol 2002;39:948-51.

27. Vest DK. Necrotic arachnidism in the Northwest United States and its probable relationship to Tegenaria agrestis (Walckenaer) spiders. Toxicon 1987;25:175-84.

28. Vest DK, Keene WE, Heumann M. Necrotic arachnidism—Pacific Northwest. MMWR Morb Mortal Wkly Rep 1996;45(21):433-6.

29. Vetter RS, Roe AH, Bennett RG, Baird CR, Royce LA, Lanier WT, et al. Distribution of the medically-implicated hobo spider (Araneae: Agelenidae) and a benign congener, Tegenaria duellica, in the United States and Canada. J Med Entomol 2003;40:159-64.

30. Binford G. An analysis of geographic and intersexual chemical variation in venoms of the spider Tegenaria agrestis (Agelenidae). Toxicon 2001;39:955-68.

31. Isbister GK, Whyte IM. Suspected white-tail spider bite and necrotic ulcers. Intern Med J 2004;34(1-2):38-44.

32. Vetter RS. Envenomation by an agelenid spider, Agelenopsis aperta, previously considered harmless. Ann Emerg Med 1998;32:739-41.

33. Vetter RS. Identifying and misidentifying the brown recluse spider. Dermatol Online 1999;5(2):7. Available at: http://matrix.ucdavis.edu/DOJvol5num2/special/recluse.html. Accessed 2004 May 5.