The accurate diagnosis and treatment of tegenarism (hobo spider poisoning) presents a relatively new challenge to physicians and other health care workers in the northwestern United States and southwestern Canada. Necrotic arachnidism was virtually unknown in these regions until the late 1960s, but now has become commonplace. In 1994 a disproportionate number of spider bites (10.9% of the national total) were reported to poison control centers in Idaho, Oregon and Washington, states which comprise only 4% of the U.S. population. With the rapid spread of the hobo spider, and its increasing contact with humans in its North American range, this trend is expected to continue. It is important therefore, that clinicians become familiar with tegenarism, and adopt appropriate protocols for dealing with bites.

Spider bites are placed into three separate categories; possible, probable, and proven. Possible spider bite applies to those cases in which the physician feels could be a bite by a spider, but which lack sufficient clinical or circumstantial evidence to support a firm diagnosis. Probable spider bite applies to those cases in which the clinical and/or circumstantial evidence support the likelihood of spider bite, but the biting spider was not recovered and positively identified. Proven spider bite applies to those cases in which the clinical and circumstantial evidence support the diagnosis of spider bite, and the biting spider was captured and positively identified. Approximately 80% of suspected spider bites seen in U.S. clinics turn out to be the result of other causes in the final diagnosis. Conversely, a large number of spider bites are misdiagnosed when first evaluated.

Upon admission to the clinic or emergency room, the first diagnostic consideration (when presented with possible or probable spider bites) should be to rule out or confirm other possible causes. Delayed type hypersensitivity (DTH) and immediate type hypersensitivity (ITH) reactions to the bites of parasitic arthropods are the lesions most commonly mistaken for "spider bite". These two types of reactions may closely resemble the very early stages of necrotic spider bite lesions (the DTH reaction may occasionally exhibit very limited centralized necrosis), but neither will produce deep, persistent dermal necrosis. Suspect "spider bites" that appear in rows or patches are almost always bites by parasitic arthropods, which often bite and feed at one site, then move a centimeter or two and bite again. Spider bites are usually single, unless the biting spider was caught between tissue and clothing and could not escape.

Mycoinfections, particularly tinea corporis, are another common differential diagnosis of spider bite in the northwestern United States. Bacterial infections of staphylococcal origin, particularly bacterial cellulitis and impetigo may exhibit both erythematous swelling and/or open cutaneous ulcers which resemble stages of necrotic arachnidism. Cat scratch disease has also been implicated in an occasional "spider bite" case. Clostridium perfringens has been the actual causative agent of some severe local reactions blamed on spider bite. In the northwestern United States, particularly in desert regions, tularemia may be confused with spider bite: Tularemia may produce both systemic manifestations and a local lesion which very closely resemble severe tegenarism. Tularemia should be suspected in any possible or probable "spider bite" case which exhibits pneumonia in conjunction with a local ulcerative lesion and systemic illness. Whenever possible, samples of lesion exudate should be obtained and cultured for bacterial growth: Spider bite lesions are generally sterile, and only occasionally develop secondary infection. Any "spider bite" that responds to antibiotic therapy is probably not a spider bite.

Viral infections with cutaneous manifestations may also be considered a differential diagnosis for suspected spider bite. Herpes zoster has been the actual causative agent in a number of suspected "spider bite" cases in the United States. Lyme disease should be considered as a possible differential diagnosis in areas where either the deer tick, Ixodes dammini, or the western black-footed tick, Ixodes pacificus, occur. Other possible differential diagnoses of "spider bite" include erythema (chronica migrans, multiforme and nodosum), scalded skin syndrome, and Stevens-Johnson syndrome. At least a dozen other disease states which produce cutaneous manifestations have been implicated in "spider bite" cases. Probable necrotic spider bite cases which occur outside of the known ranges of the brown recluse or the hobo spider are best diagnosed as probable necrotic arachnidism, rather than applying the name of a suspected biter.

In proven cases, where the patient actually brings the biting spider (or its remains) to the clinic, it is important to seek a positive identification of the specimen at once. While a tentative identification can be made at the clinic, positive identification, as well as age and sex determination by a qualified arachnologist is important; all of these factors can effect the severity and course of a hobo spider envenomation. In the northwestern United States, spiders can be positively identified by

ROD CRAWFORD, Curator of Arachnids

Burke Museum

University of Washington

Box 353010

Seattle, WA 98195-3010 U.S.A.

Telephone (206)543-9853


Spiders submitted for positive identification should always be accompanied with basic collection information, including the date, the exact location (street address, etc.), the name of the collector(s) and any other information about the spider that is available. Questions directed to Rod Crawford should concern spiders and/or spider identification, not questions regarding bites or envenomation.

Once the diagnosis of proven or probable hobo spider bite is established, a determination can be made as to the severity of the case, and treatment protocols can be implemented. Most patients will present exhibiting local effects only, and can be treated as outpatients. Tetanus prophylaxis is indicated, and the patient should be advised to clean the wound frequently, and (if on an extremity) to keep it immobilized and rested. Diphenhydramine (Benadryl) may be prescribed to help control itching. Prophylactic antibiotic therapy is not warranted, and antibiotics should be used as needed only to control secondary infection. Laboratory studies, including a CBC with platelets, Hb, Hct, BUN, creatinine and urinalysis are advisable in all cases of tegenarism. The patient should be scheduled with a followup visit, and should be told to watch the lesion closely for signs of darkening, etc. It is good practice to inform the patient of the typical healing time-table (see Hobo Spider Poisoning): It is also good to inform the patient (at each visit) that lesions of necrotic spider bite are often difficult to predict, and can sometimes get progressively worse.

Subjects that are bitten in areas which contain large amounts of adipose tissue, particularly if the victim is obese, are at high risk for the development of deep, slow healing lesions. In such cases excisional therapy is not indicated, and may exacerbate the condition of the lesion. While natural healing by slow granulation is preferred, split thickness skin grafts may be considered after the necrotizing process is completed (8 weeks); The initial graft attempt is usually not successful.

Patients exhibiting systemic effects of tegenarism should have the above mentioned laboratory screen performed immediately. If evidence of any significant abnormalities (except a moderate leukocytosis) are discovered, the patient should be given a regimen of a prednisolone based corticosteroid, and should be hospitalized. Typically, a tapered dose starting with the equivalent of 4 mg of Decadron is recommended. Some physicians experienced with tegenarism prefer to administer corticosteroids whether or not systemic effects are present; considering the insidious nature of some effects of hobo spider poisoning, this practice may have merit. Antianxiety medications such as hydroxyzine (Atarax) have proven beneficial to systemically poisoned hobo spider bite victims, and may provide some comfort to nervous or emotionally upset victims of any necrotic spider bite.

Tegenarism is a relatively new disease state and much is yet to be learned from it. An excellent clinical reference on necrotic arachnidism (published largely on the bite of the brown recluse spider, before the hobo entered the scene), is Wasserman, G.S. and Anderson, P.C. (1983-84): Loxoscelism and necrotic arachnidism. J. Toxocol.-Clin. Toxicol., 21(4 &5), pp. 451-472. Physicians that have treated hobo spider bite cases are encouraged to publish case histories and observations on this topic. Hyrum the hobo spider, 1997 Darwin K. Vest, Eagle Rock Research

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1999 Darwin K. Vest, Eagle Rock Research