Interspecific and intraspecific variability in venom composition of Naja naja and Naja kaouthia (Reptilia: Elapidae) populations from different habitats in Bangladesh.

Abstract

The spectacled cobra (Naja naja) and monocled cobra (Naja kaouthia), widespread venomous snakes in South and Southeast Asia, occur in diverse habitats and cause neurotoxic envenoming. Despite reported venom variability of these two cobras across their range, no comparative study has been conducted from the interconnected but distinct habitats of Bangladesh. Using venomics and antivenomics, we analysed 26 individual venom samples of N. kaouthia and 17 of N. naja from Bangladesh across age groups and locations, respectively. Significant interspecific and intraspecific venom variability was observed, with geographically connected populations showing minimal divergence, while isolated populations (separated by river barriers or distinct ecosystems) exhibited pronounced compositional differences. Ontogenetic differences in venom composition between adult N. kaouthia and their juvenile offspring were detected. Commercially available Incepta polyvalent antivenom, produced against India's "Big Four" (including southern Indian N. naja), demonstrated poor efficacy against Bangladeshi cobra venoms. Collectively, our analyses demonstrate the existence of multi-dimensional variation in cobra venoms of Bangladesh that is influenced by biotic and abiotic factors. We emphasize the urgent need for region-specific antivenoms incorporating venom from ecologically distinct populations and age groups of both species across South Asia to improve snakebite treatment efficacy as well pre-clinical assessments to address biogeographic and ontogenetic venom diversity. SIGNIFICANCE: Snakebite envenoming is a major neglected tropical disease and a leading occupational health hazard especially for rural populations in many low-and middle-income countries. As differences in snake venom composition between and within species can greatly affect the clinical course of envenoming and the efficacy of treatment, detailed knowledge of this variability is highly important for public health planning and the development of better antidotes. In Bangladesh, the monocled cobra (Naja kaouthia) and the spectacled cobra (Naja naja) belong to the medically most important and most widely distributed common snake species, but data on the variability of their venoms in this country has been limited and its relation to climatic and other environmental factors remained unexplored. Here we report on the analysis of 43 individual venom samples from 26 N. kaouthia and 17 N. naja from different age groups and geographical localities in Bangladesh, using venomics and antivenomics methods. Our findings show that the venoms of these cobras are highly diverse qualitatively and quantitatively, with significant inter- and intraspecific, geographic and ontogenetic variability and differences in their reactivity with a commercial antivenom. The observed geographical variability appears to be influenced by climatic and other environmental variables of different habitats in Bangladesh. When designing improved antivenoms, geographically appropriate and more diverse venom samples, also from different age groups of the snakes, should be included to cover this variability and ensure that the clinically significant toxins of all cobra venom varieties in Bangladesh are well neutralized by the antivenoms.