Antivenoms have the potential to cause immediate and potentially life-threating, anaphylactoid reactions. Reactions can range from mild (pruritus, rash) to severe (wheezing, hypotension, respiratory distress, cardiovascular collapse, and death). Antivenoms may also cause serum sickness. Serum sickness is a type III hypersensitivity reaction which is characterized by flu-like symptoms with or without a rash that typically occurs between 2 days and 3 weeks after antivenom administration. Serum sickness is uncomfortable but not life threatening.35

Adverse Reaction Management

Unfortunately, data on the incidence of adverse reactions to antivenoms used outside of developed nations are lacking. Medical providers should anticipate a high likelihood for serious and potentially life-threatening anaphylactoid reactions to antivenoms not approved by the US Food and Drug Administration (FDA). For this reason, antivenom should only be used in patients with moderate to severe symptoms refractory to standard medical therapies (e.g., benzodiazepines and pain medications). Epinephrine should be immediately available at the patient’s bedside prior to the administration of antivenom to ensure rapid administration if necessary.

For mild to moderate symptoms occurring during antivenom infusion (e.g., nausea, vomiting, urticaria, pruritus, chills, fever), stop the infusion immediately and treat symptoms with antiemetics, antihistamines (typically diphenhydramine), and steroids (i.e. methylprednisolone, prednisone, prednisolone). If the reaction is controlled after treatment and the patient’s condition still requires antivenom, restart the antivenom at a slower infusion rate.

For severe reactions (i.e. respiratory distress, hypotension), immediately stop the antivenom infusion and treat using a standard anaphylaxis protocol (diphenhydramine 50 mg [1 mg/kg in pediatric patients] IV, methylprednisolone 125 mg [2 mg/kg in pediatric patients] IV, and 0.3 mg [0.15 mg in pediatric patients] of 1:1000 epinephrine intramuscularly).36-39 Consider adding an H2 antihistamine such as famotidine. If necessary, intubate for airway edema not rapidly responsive to epinephrine. If antivenom is considered necessary to prevent death or disability, the antivenom may be reinitiated at a slower rate of infusion in conjunction with an epinephrine infusion.

In patients who develop serum sickness, management consists of symptomatic treatment with antihistamines and pain medications with or without a course of oral steroids.35