The most important toxin isolated from Loxosceles venom appears to be sphingomyelinase. This toxin causes dermonecrosis, platelet aggregation, and complement-related hemolysis in vitro and is thought to be responsible for dermonecrosis and systemic effects in humans.¹⁰

The initial bite is generally painless, although a stinging sensation is sometimes noted. After 2 to 8 hours, mild to severe pain develops, which is thought to result from vasospasm causing ischemia.⁷ There may be two small puncta and transient erythema, with itching, swelling, and mild to moderate tenderness. The bite site then pales, with the surrounding tissue becoming red and swollen. In lesions that progress, a characteristic bulls-eye wound develops, with a red erythematous center, a white ring of induration, and an outer blue cyanotic ring.⁵ The central area can then form an eschar and slough off over the next 5 to 7 days, leaving an ulcer that heals in varying amounts of time (6 to 8 weeks). Skin grafting has been required in some cases.¹¹

Systemic effects include fevers, chills, malaise, weakness, and nausea and vomiting, along with a generalized pruritic, morbilliform, or petechial rash.⁷ Rare effects (<1%)⁵ include hemolysis, disseminated intravascular coagulation, and rhabdomyolysis with resulting renal failure.

CRITICAL DECISION

What treatment should a patient be given who presents with findings concerning for a brown recluse spider bite?

Therapy for patients suspected of having a recluse bite is generally supportive, although various treatments have been tried without significant clinical improvement. Elevation, immobilization, application of ice, local wound care, and tetanus prophylaxis are reasonable for initial therapy. Excision may be required in severe cases, although there is controversy over the timing, with more recent recommendations advising delaying surgery until 6 to 8 weeks after the bite.⁵

Tarantulas

Tarantulas are large, hairy, mygalomorph spiders that are found primarily in the Desert Southwest but can be found as far east as the Mississippi River and as far north as Arkansas.6 In addition, tarantulas are sold as pets throughout the United States. Tarantula bites usually will cause mild stinging with minimal surrounding inflammatory reaction, no dermonecrosis, and no serious systemic toxicity.¹² Treatment for tarantula bites is generally supportive, with local wound care, tetanus prophylaxis, and oral analgesics as needed.

CRITICAL DECISION

Dermatitis and blurry vision in a patient who has recently handled a tarantula should prompt consideration of what condition?

In addition to their bites, New World tarantulas have defensive urticating hairs located on their dorsal abdomen and, when threatened, they will rise on their back legs and use leg vibrations to flick a volley of hairs at their attackers.¹³ These hairs can produce a persistent papular dermatitis. Even more concerning, the hairs can penetrate the cornea, causing a foreign body keratoconjunctivitis or ophthalmia nodosa. Ophthalmia nodosa is a chronic, granulomatous, nodular reaction that occurs as a reaction to vegetable or arthropod hairs.¹⁴ Treatment of ophthalmia nodosa requires referral to an ophthalmologist for slit lamp examination and possible surgical removal of the hairs. Topical corticosteroids have also been helpful in managing this complication.¹³

Insect Envenomations

Ants

There are more than 6,000 different species of ants, and most of them are capable of both biting and stinging. Fire ants (Solenopsis sp.) are responsible for most injuries, including anaphylaxis. Since their introduction to the United States, the red imported fire ant is thought to have been responsible for more than 80 deaths.¹⁵ Fire ant venom contains piperidines, alkaloid compounds that are neurotoxic, cytotoxic, and hemotoxic and create an intense burning sensation.¹⁵ The fire ant grabs with its jaws and then stings in a circle around the bite that develops into a necrotic lesion known as a sterile pustule. The pustule may last for several days and is pathognomic for fire ant stings.¹⁶ A large local reaction may occur similar to those with other stinging insects with pruritic edema, induration, and erythema that can affect the entire limb.¹⁶ Anaphylaxis can occur hours after a sting.