Low incidence of early reactions to horse-derived F(ab′)2 antivenom for snakebites in Thailand

Although antivenom is effective for systemic effects of snake envenomation, the possibility of fatal hypersensitivity reactions must be considered. The risks vary greatly among different products and cannot be excluded by negative skin test. However, clinical outcomes of patients with positive skin test remain to be determined. The medical records of 254 cases receiving antivenoms during 1997–2006 were reviewed. Most were for green pit vipers (84%) and cobras (13%). Early reactions occurred in 9 (3.5%) including 3 (1.2%) with hypotension. The incidence was more common after cobra than viper antivenoms (12.5% vs. 2.3%, respectively, p = 0.016). Skin test was negative in 7/7 tested cases. All reactions resolved. Antivenoms were re-administered in five without recurrent reactions. Overall, skin test was positive in 10/211 (4.7%). Five of them underwent desensitization. Antivenom can be given in all 10 without reactions. In conclusion, the incidence of early reactions to antivenoms was low in Thailand and skin test is not helpful at all in predicting this adverse reaction.