Production of highly potent horse antivenom against the Thai cobra (Naja kaouthia).

Naja kaouthia (NK) causes the highest fatality due to snake venom poisoning in Thailand. The specific antivenom produced is of low potency and in short supply. The aim of this study was to improve the antivenom potency. Bentonite and complete Freund's adjuvants (CFA) and various immunogens were compared. Six groups of three to five horses were immunized as follows: Group 1, NK venom adsorbed on bentonite; Group 2, NK venom in CFA; Group 3, NK venom in CFA in multi-emulsion formulation; Group 4, NK venom in 25% CFA; Group 5, NK neurotoxin 3 (NK3) conjugated with tetanus toxoid (NK3-TT) in CFA; Group 6, NK3 conjugated with diphtheria toxoid (NK3-DT) in CFA. Horses in Group 2-6 produced antivenom of very high neutralizing activity, up to four times higher than that of horses of Group 1. CFA (100 or 25%) or as a multi-emulsion formulation, induced comparable neutralizing antibody production with all three immunogens. All horses showed normal weight gain during the course of immunization. Group 1 horses exhibited minimal local reactions while horses in the other five groups had mild and comparable local reactions at the injection sites. No significant differences in the reactions caused by CFA in different formulations or different immunogens were observed. The production of highly potent antivenom against N. kaouthia from these horses should help solve problems associated with the currently available antivenom.