Pit Viper Antivenin
Antivenin (Crotalidae) Polyvalent is a critical, life-saving **antidote** indicated for the treatment of envenomation from North, Central, and South American pit vipers (e.g., rattlesnakes, copperheads, cottonmouths, fer-de-lance). * **Formulations**: Available as an **equine-origin** product (whole IgG antibodies) and an **ovine-origin** product (Fab fragments, e.g., CroFab®). * **Risks**: Up to 50% of the veterinary-labeled equine product contains equine albumin and other proteins, carrying a significant risk of hypersensitivity reactions (anaphylaxis). * **Clinical Pearl**: While the ovine Fab product (CroFab) is human-labeled, it is frequently used off-label in veterinary medicine. Fab fragments generally have a lower risk of anaphylaxis compared to whole-IgG equine products, though they have a shorter half-life and may require repeated dosing to prevent venom recurrence. * Treatment requires aggressive supportive care and can be highly expensive.
Mechanism: Antivenins provide **passive immunity** to the patient. * They contain concentrated serum globulins (or Fab fragments) obtained from animals (horses or sheep) hyperimmunized with pit viper venom. * **Mechanism**: The immunoglobulins directly bind to and neutralize the complex toxic proteins and enzymes present in the venom. * **Effect** → Rapidly reverses venom-related systemic signs and coagulation abnormalities. * *Note*: Venom-induced thrombocytopenia, particularly from Timber Rattlesnakes, may be resistant to antivenin treatment.
Dosing by species
- Pit viper envenomation · 1-2 vials of antivenin. Initially, give one vial, by diluting to 100-250 mL of crystalloid fluids and initially administer by slow IV · IV · prn · Adjust infusion volume in smaller patients to prevent fluid overload.
- Systemic effects of envenomation · 1-2 vials slowly IV diluted in 250-500 mL saline or lactated Ringer's · IV · prn · Use only if necessary to treat systemic effects. Administer antihistamines; corticosteroids are contraindicated.
- Pit viper envenomation · 1-2 vials of antivenin. Initially, give one vial, by diluting to 100-250 mL of crystalloid fluids and initially administer by slow IV · IV · prn · Adjust infusion volume in smaller patients to prevent fluid overload. Intravascular bites or bites to torso/tongue require prompt, aggressive administration.
- Pit viper envenomation · 1-5 rehydrated vials (10-50 mL) IV depending on severity of symptoms, duration of time after the bite, snake size, patient size. Additional doses may be given every 2 hours as required. If unable to give IV, may administer IM as close to bite as practical. · IV/IM · q2h as required · The smaller the victim, the larger the dose (venom amount/kg is higher).
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Known severe hypersensitivity to equine or ovine proteins (relative contraindication in life-threatening envenomation)
Adverse effects
- Anaphylaxis (Type I hypersensitivity)
- Anaphylactoid reactions
- Serum sickness (Type III hypersensitivity, delayed)
- Nausea
- Pruritus
- Hyperemia of the inner pinna
Drug interactions
- Analgesics/Sedatives · May mask the clinical signs associated with the venom; use with caution initially.
- Antihistamines · Theoretical risk of potentiating venom, though diphenhydramine is routinely and safely used by clinicians for pre-treatment.
- Beta-blockers · May mask the early signs associated with anaphylaxis.
- Corticosteroids · Generally out of favor for treating snakebite envenomation, but may be useful to treat secondary anaphylaxis. Contraindicated in horses per some references.
- Heparin · Reportedly not effective in treating the thrombin-like enzymes found in rattlesnake venom.
Monitoring
- Signs associated with an allergic response (anaphylaxis, anaphylactoid-reactions, serum sickness)
- CBC with platelets
- Coagulation parameters
- Biochemical profile
- Hydration status
- ECG
Overdose
Specific overdose information is not provided, but fluid overload is a significant risk in smaller patients receiving large volumes of crystalloid fluids used to dilute the antivenin.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.