In fact, many wounds that are necrotic and diagnosed as brown recluse bites can actually be methicillin-resistant Staphylococcus aureus (MRSA)[10] or simple staphylococcus infections. Other causes include skin cancer, Lyme disease, and other infected insect bites and skin lesions. Brown recluse bites may produce a range of symptoms known as loxoscelism. There are two types of loxoscelism: cutaneous (skin) and systemic (viscerocutaneous).
Most bites are minor with no necrosis. However, a small number of bites produce severe dermonecrotic lesions, and, sometimes, severe systemic symptoms. These symptoms can include organ damage, and occasionally even death; most fatalities are in children under 7[11] or those with a weaker-than-normal immune system. (For a comparison of the toxicity of several kinds of spider bites, see the list of spiders having medically significant venom.) In one study of brown recluse bites, the incidence of skin necrosis was 37% and the incidence of systemic illness was 14%. [12]
A minority of brown recluse spider bites form a necrotizing ulcer that destroys soft tissue and may take months to heal, leaving deep scars. The damaged tissue will become gangrenous and eventually slough away. The initial bite frequently cannot be felt and there may be no pain, but over time the wound may grow to as large as 25 cm (10 inches) in extreme cases. Bites usually become painful and itchy within 2 to 8 hours; pain and other local effects worsen 12 to 36 hours after the bite with the necrosis developing over the next few days