• between-treatment groups difference in pain intensity (pre- and post-treatment) [ Time Frame: within first two hours ] [ Designated as safety issue: No ]
  

• between-treatment groups difference in proportion of treatment failures (absence of pain relief or return to hospital) [ Time Frame: within 24 hours after discharge from emergency department ] [ Designated as safety issue: No ]
  
• between-treatment groups difference in incidence of drug-related adverse events. [ Time Frame: onset of adverse events within 21 days ] [ Designated as safety issue: Yes ]
  

The purpose of this randomized, double-blind, multi-center phase II trial is to examine the safety and efficacy of Aracmyn® [Antivenin Latrodectus (Black Widow) Equine Immune F(ab)2], a new antivenom, for treatment of patients envenomed by the widow spider. Twenty-four subjects will be randomized to receive either the study drug or control (normal saline) through intravenous (IV) infusion. Following infusion, subjects will be monitored for changes in pain and clinical signs during a two-hour observation period. A standard dose of IV fentanyl will be offered to all subjects as pain medication at specific intervals following treatment. Vital signs, blood samples, and pain intensity scores (using a visual analog scale, VAS) will be collected before and after the infusion, as well as during the observation period. This study uses a treatment failure protocol, which involves administration of Merck Antivenin to subjects who do not obtain adequate pain relief from the study drug or control. Primary efficacy and safety endpoints for this 12-month trial will be improvement in pain intensity and clinical signs and incidence of adverse events, respectively.