The black widow spider (Latrodectus mactans) is the prototype for the genus Latrodectus, several members of which cause human disease. The black widow spider is not particularly aggressive but will defend her web, which is often found in woodpiles, basements, and garages. Most envenomations occur between April and October, with bites most commonly located on the hand and forearm. The clinical presentation of severe and sustained muscle spasm is produced by a neurotoxic protein, which causes the release of acetylcholine and norepinephrine at the presynaptic neuromuscular junction. The initial bite may be mild to moderately painful but is often missed. Within approximately 1 hour, local erythema and muscle cramping begin, followed by generalized cramping involving large muscle groups such as the thighs, shoulders, abdomen, and back. Associated clinical features can include fasciculations, weakness, fever, salivation, vomiting, diaphoresis, localized sweating at the envenomation site, and a characteristic pattern of facial swelling called Latrodectus facies. Rare cases of seizures, uncontrolled hypertension, and respiratory arrest have occurred.
Management and Disposition
Treatment of the local wound should include cleansing and tetanus prophylaxis. Severe pain and spasm may require intravenous benzodiazepines and opiates. Calcium gluconate infusion has long been recommended to reduce symptoms, although evidence for its efficacy is controversial. Antivenom exists but carries the same risk as all horse serum products. Antivenom should be considered only in cases of respiratory arrest, seizures, uncontrolled hypertension, and pregnancy.
Black Widow Spider (With Offspring). Latrodectus mactans, with characteristic hourglass marking on its abdomen. (Photo contributor: Lawrence B. Stack, MD.)
Black Widow Spider Bite. The bite of the black widow spider is clinically subtle. Local reaction is usually trivial, as in this confirmed bite with a small patch of mild erythema. (Photo contributor: Gerald O’Malley, DO.)
Black Widow Facies. A pattern of facial swelling, known as Latrodectus facies, may occur several hours after envenomation. (Photo contributor: Gerald O’Malley, DO.)
Of the five Latrodectus species indigenous to the United States, only three are black and only one has the orange-red hourglass marking.
Envenomations by L mactans can mimic an acute abdomen and should be considered in the differential diagnosis of patients presenting with severe acute abdominal pain.