• 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 ]

• between-treatment groups difference in incidence of treatment failures (absence of pain relief)
• between-treatment groups difference in incidence of drug-related adverse reactions.

The purpose of this study is to compare the safety and effectiveness of an investigational antivenom and the current standard of care (pain management with opioid analgesics) for treating patients with a widow spider bite. The working hypotheses are as follows:

1. the investigational antivenom is more promptly effective at alleviating the pain associated with a widow spider bite than routine management with opioid pain medication
2. the investigational antivenom is as safe a treatment as opioid pain medication in treating patients with a widow spider bite.

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.

• Arachnidism
• Latrodectism

• Biological: widow spider antivenom
• Biological: Placebo

• Experimental: Patients with moderate or severe pain due to Black Widow envenomation
• Placebo Comparator: Patients with moderate to severe pain due to Black Widow envenomation

Inclusion Criteria:

• patient presents for treatment within 72 hours from time of symptoms onset
• clinical diagnosis of widow spider envenomation
• patient has moderate to severe pain intensity

Exclusion Criteria:

• history of significant cardiac, respiratory, hepatic, or renal disease
• distracting injury or chronic pain syndrome that would obscure pain intensity assessment
• history of asthma or known sensitivity to fentanyl, morphine, diazepam, or equine serum
• pregnant or lactating