Immediate immune-mediated (non-IgE) hypersensitivity reactions

Immediate immune-mediated (non-IgE) hypersensitivity reactions to antivenoms occur in about 25% of patients in Australia. The majority of these are urticaria or rash, and severe anaphylaxis only occurs in 2 to 3% of patients. Hypotension is the most common manifestation of severe anaphylaxis. The pathophysiology of immediate immune-mediated (non-IgE) hypersensitivity reactions remains unclear.

True IgE-mediated hypersensitivity reactions are rare. They may occur in snake handlers who have had previous exposure to venom and the reaction is anaphylaxis to the venom, rather than the antivenom. Do not withhold antivenom in these patients.

Treatment for hypersensitivity reactions includes:

• temporarily ceasing the antivenom
• administering intramuscular adrenaline (epinephrine) if the reaction is severe.

See also Anaphylaxis for management.

Premedication before antivenom is not recommended in Australia, though it may be recommended in other countries. Evidence suggests adrenaline may be an effective premedication for antivenoms with higher reaction rates (eg antivenoms used in other countries).