LATRODACTISM. Latrodectus mactans are small, dark spiders also called "black widows." They have a black or brown underside with a red, orange, or white hourglass marking on the back. They are commonly found in fields, under stones, and in outhouses. Their venom is neurotoxic, and they bite usually on the genitalia or buttocks. Pain develops within about an hour, with accompanying reddening and swelling. Systemic symptoms include muscle cramping, rigidity, and later weakness, sweating, bradycardia, hypothermia, and hypotension. The mortality rate is about 5% in children. Treatment alternatives include intravenous 10% calcium gluconate and corticosteroids; however, the most effective therapy is systemic antivenom.

LOXOSCELISM. (Fig 35-10C-D, Fig 35-11)- Loxo-sceles reclusus is found in the United States, and Loxosceles laeta is found in Central and South America. The

Loxosceles spiders are light-brown to chocolate in color, with nocturnal habits, and are commonly found seeking warmth in discarded clothing. Usually the affected areas include the arm and thigh in adults or the face in children. Pain develops 2 to 8 hours after the bite. The lesion becomes indurated and red, showing a central blister and necrosis that can be quite large. The necrotic area eventually becomes mummified; around the 14th day the resulting eschar may slough off. Rare symptoms include fever, chills, vomiting, and petechiae on skin, as well as thrombocytopenia and hemolytic anemia, especially in children. Treatment is with corticosteroids and dapsone, which may be effective in limiting the size and extension of the necrosis.

Figure 35-11. Loxoceles.

Disease Caused by Chiggers

Chiggers Petechiae
Figure 35-12.Chigger bites.

These mites, also known as harvest mites, are the cause of the infestation known as trombiculiasis. It is seen worldwide, although most frequently in tropical areas. The disease is acquired while walking through vegetation, and the affected area is usually on exposed skin depending on the clothing used. The offending chigger is the larval stage of the mite, 0.25 to 0.4 mm in diameter, orange to red in color, with three pairs of legs. It gets fixed to the skin by its buccal apparatus and starts a process of liquefying and sucking the skin elements. As a consequence it produces a type of papular urticaria: multiple red, itchy papules which are extremely pruritic. Topical treatment is with steroids and antipruritic lotions; occasionally this condition requires systemic antibiotic and steroid therapy.

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