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INTRODUCTION

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Centruroides exilicauda (formerly known as Centruroides sculpturatus) is the only scorpion of medical importance in the United States. It is indigenous to the deserts of Arizona but also can be found in Texas, New Mexico, California, and Nevada.11 Occasionally, envenomations occur in nonindigenous areas of the country from “stowaway” scorpions in travelers’ luggage.32

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HISTORY

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Arizona poison centers receive several thousand calls annually for scorpion stings. Although the incidence of morbidity with these envenomations is significant, no deaths associated with the toxic effects of scorpion venom have been reported in the United States for almost 50 years. However, deaths are associated with anaphylactic reactions to scorpion venom.7 Antivenom for the Centruroides spp. was produced in horses in Mexico as early as the 1930s.11 In 1947, antivenom was produced from rabbits and cats immunized with C. sculpturatus and Centruroides gertschi.29 The Antivenom Production Laboratory at Arizona State University (APL-ASU) began producing antivenom to C. sculpturatus in goats in 1965. This antivenom was used for treatment of scorpion stings in Arizona until 2004, when production ceased and stockpiles expired. In June 2000, Silanes Laboratory received orphan drug status for a Centruroides scorpion antivenom, an equine-derived F(ab′)2 from Centruroides limpidus, Centruroides noxius, Centruroides suffusus suffusus, and Centruroides meisei (formerly known as Centruroides elegans) manufactured by Instituto Bioclon of Mexico, and referred to as Anascorp. In August 2011, Anascorp was approved by the US Food and Drug Administration (FDA) for the treatment of Centruroides exilicauda envenomations.15

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PHARMACOLOGY

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Antivenom for scorpions is prepared in the same manner as other anti­venom products (Antidotes in Depth: A34 and A37).

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Pharmacokinetics and Pharmacodynamics

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In a study of 8 healthy patients (6 males and 2 females, age 17–26 years) without prior scorpion envenomation, a bolus intravenous dose of 47.5 mg of Anascorp was administered. Serial blood samples were collected over 21 days. Measured pharmacokinetic parameters (mean ± standard deviation) included: area under the curve (AUC), 706 ± 352 μh/mL; clearance, 83.5 ± 38.4 mL/h; half-life, 159 ± 57 hours; and steady state volume of distribution (Vdss), 13.6 ± 5.4 L.33

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ROLE IN CENTRUROIDES SPECIES

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One vial of Anascorp antivenom contains sufficient F(ab′)2 to neutralize 150 mouse LD50 of Centruroides venom.25 Safety and efficacy are documented in both animals and humans.1,8,9,10 In the FDA review and subsequent approval of Anascorp, six trials were submitted for consideration. Only one study was prospective, randomized, double-blinded, and controlled and demonstrated a clear benefit in reduction of signs and symptoms of scorpion envenomation by 4 hours postadministration (8/8 vs. 1/7), reduction in quantity of sedative administered (mean midazolam dose of 0.1 mg/kg (0.0–0.2 mg/kg) versus 4.6 mg/kg (0.1–16.7 mg/kg), and absence of free serum venom ...

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