Experimental evaluation of ovine antisera to Thai cobra (Naja kaouthia) venom and its α-neurotoxin

Abstract

Conventional treatment of Naja kaouthia (Thai cobra) envenoming requires large volumes (up to 600 ml) of equine antivenom, which results in a high incidence of serum reactions. The inefficiency of the antivenom is assumed to be related to the high percentage (approx. 20%) of α-neurotoxin, a relatively weak and highly toxic immunogen, present in the native venom. First, antibodies to N. kaouthia venom were raised in sheep, which protected mice against challenge with whole venom. Second, ovine antibodies to the purified neurotoxin and to three different neurotoxin conjugates were developed and their neutralising abilities against either whole venom or neurotoxin were compared using murine ed50 tests. High titre antibodies, assessed by enzyme immunoassay and Western blot, were obtained from all four neurotoxin immunisation regimens. Neurotoxin conjugated to rabbit anti-sheep IgG produced the highest titres against both neurotoxin and whole venom. This antiserum provided protection against neurotoxin challenge but failed to protect against whole venom. Furthermore, the addition of neurotoxin antibodies to whole venom antiserum did not enhance the neutralisation efficacy of the latter. These findings raise the possibility that in mice other toxins apart from the neurotoxin may significantly contribute to the lethal effect of N. kaouthia venom.