Black Widow Latrodectus

Latrodectus or "widow" spiders are found throughout the United States. Of the estimated 30,000 species of spiders, the black widow is probably the most well known, although of the five species found commonly in the United States, only three (L. mactans, L. variolus, and L. hesperus) are actually black. Other varieties may be predominantly brown or red (L. geometricus, or brown widow, and L. bishopi, or red widow). The classic orange-red hourglass-shaped marking is noted only in L. mactans. Female spiders are relatively large, with a body size ranging up to 1.5 cm in length and leg spans of 4 to 5 cm. The male spider is approximately one-third the size of the female, lighter in color, and his bite can not penetrate human skin. Black widow spiders are found most often in wood piles, basements, garages, or sheds. Latrodectus will aggressively defend her web, particularly when guarding her eggs. Most black widow bites occur between April and October and usually are seen on the hands and forearms.

The black widow spider injures its victim by secretion of one of the most potent neurotoxic venoms produced by any animal, producing release of acetylcholine, norepinephrine, and other neurotransmitters at the neurosynaptic junction.

CLINICAL FEATURES Most Latrodectus bites are almost immediately mild to moderately painful. The pain often begins as a pinprick sensation at the bite site but may spread quickly to include the entire bitten extremity.12 Erythema appears approximately 20 to 60 min after the bite. In over 50 percent of cases, the initial erythema evolves into a lesion resembling a "target," with a diameter of about 1 to 2 cm ( Fig 188-6). Victims frequently complain of muscle cramp-like spasms in large muscle groups, although physical examination of the "cramping" extremity rarely exhibits rigidity, and serum creatine kinase concentrations do not appear to be significantly elevated. The pain often increases progressively, becomes generalized, and can involve the trunk, back, and abdomen. Involvement of the abdominal wall musculature with severe pain and cramping has been mistaken for peritonitis.

Latrodectus bite victims may experience severe pain for 24 h or more that can be intermittent in course. Rarely, pain may persist for several days, whereas muscle weakness may continue for weeks to months. The most serious complications include hypertension, which is reported in 10 to 30 percent of envenomations and is likely the result of catecholamine release. In addition, severe envenomation also may cause shock, coma, and respiratory failure secondary to muscle paralysis.

DIAGNOSIS AND TREATMENT Since an immediate pinprick sensation is almost always reported with Latrodectus bites, it is rare for victims not to see the offending spider. There are no confirmatory laboratory tests, but the presence of the typical target lesion along with pain, muscle spasms, and toxic appearance is virtually pathognomonic of the diagnosis.

The initial therapy is supportive care of the airway, breathing, and circulation. Wound care should consist of routine cleansing of the bite site and tetanus prophylaxis, if indicated.

Patients should be given appropriate opioid analgesics and benzodiazepines for pain relief and muscle relaxation. Intravenous calcium gluconate has been advocated to relieve pain and muscle spasms from the black widow bite, although controlled data are lacking, and a review of 163 patients with Latrodectus envenomation found ineffective relief of muscle spasm and pain with calcium treatment.12 Most severe bites will require parenteral opioids for extended periods, necessitating admission for observation. Antivenom is reserved only for severe envenomation refractory to the measures indicated earlier. 12 The usual dose is one to two vials infused over 20 to 30 min. Since antivenom is a horse-serum preparation, anaphylaxis can occur, particularly in patients with a history of allergy to horse-serum products or those who have been sensitized by prior antivenom treatment.12 Skin testing should be performed before antivenom administration. Serum sickness, while a theoretical possibility, was not observed in 58 patients treated with 1 or 2 vials of antivenom.12

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