Around 49% of brown recluse bites do not result in necrosis or systemic effects. When both types of loxoscelism do result, systemic effects may occur before necrosis, as the venom spreads throughout the body in minutes. Children, the elderly, and the debilitatingly ill may be more susceptible to systemic loxoscelism. The systemic symptoms most commonly experienced include nausea, vomiting, fever, rashes, and muscle and joint pain. Rarely, such bites can result in hemolysis, thrombocytopenia, disseminated intravascular coagulation, organ damage, and even death.[19] Most fatalities are in children under the age of seven[20] or those with a weak immune system.
While the majority of brown recluse spider bites do not result in any symptoms, cutaneous symptoms occur more frequently than systemic symptoms. In such instances, the bite forms a necrotizing ulcer that destroys soft tissue and may take months to heal, leaving deep scars. These bites usually become painful and itchy within 2 to 8 hours. Pain and other local effects worsen 12 to 36 hours after the bite, and the necrosis develops over the next few days.[21] Over time, the wound may grow to as large as 25 cm (10 inches). The damaged tissue becomes gangrenous and eventually sloughs away.
Source: Wikipedia