Life-threatening anaphylaxis following antivenom administration in two patients with horse dander allergy

Antivenom is used to treat systemic envenomation and is immunoglobulin G raised from animals, mainly horses, inoculated by venom.

We report two cases of life-threatening anaphylaxis following the administration of antivenom in patients with known horse dander allergy.

A 14-year-old boy presented with redback spider envenomation. He had a known horse dander allergy. He received antivenom to treat severe pain and developed immediate severe anaphylaxis requiring endotracheal intubation and inotropic support. A 52-year-old man with horse dander allergy presented with suspected snakebite and received antivenom. He developed immediate profound anaphylaxis and suffered a prolonged cardiac arrest. He later died from multiorgan failure secondary to hypoxia.

Severe anaphylaxis following antivenom administration in Australia is uncommon. In the setting of known horse dander allergy there is likely to be an increased risk of severe anaphylaxis due to probable cross-reactivity of shared allergens between horse dander and serum.

Dander allergy to the same animal antivenom is raised from, likely confers an increased risk of life-threatening anaphylaxis. Asking someone specifically about the relevant animal dander allergy prior to antivenom administration is important to better weigh the risk-benefit analysis of antivenom administration.