Cobras (genus Naja) are widely distributed over Asia and Africa, and cobra envenomation is responsible for a large number of mortality and morbidity on these continents. Like other elapid venoms, cobra venoms are neurotoxic in nature; however, they also exhibit local cytotoxic effects at the envenomed site, and the extent of cytotoxicity may vary from species to species. Cobra venoms are predominated by the non-enzymatic three-finger toxin family which constitutes about 60-75% of the total venom. Cytotoxins (CTXs), an essential class of the non-enzymatic three-finger toxin family, are ubiquitously present in cobra venoms. These low-molecular-mass toxins, contributing to about 40 to 60% of the cobra venom proteome, play a significant role in cobra venom-induced toxicity, more prominently in dermonecrosis (local effects).
1. Epidemiology of Cobra Bites in the World
Snake envenoming is a devastating health issue that affects millions of individuals across the globe, particularly in the developing countries of tropical and subtropical regions. Global epidemiological data suggest 81,000–138,000 snakebite-associated deaths from 1.8–2.7 million cases of envenoming yearly, thereby highlighting the urgency of research into this neglected public health concern
[1][2]. Moreover, region-wise distribution of snakebite data suggests most of the cases of snake envenoming are concentrated in the rural areas of South Asia, Southeast Asia, and East Sub-Saharan African countries
[3].
The majority of the medically critical venomous species of snakes belong to the Viperidae (340 species), Elapidae (360 species), and Atractaspidae (69 species) families
[4]. Accordingly, the World Health Organization (WHO) has recognized venomous snakes under categories 1 and 2 depending upon their distribution, venom lethality, and incidences of envenoming and deaths. Category 1 medically essential species are of the highest medical importance, and the WHO has recognized at least 109 species under this category. Furthermore, it has been observed that most medically relevant species are represented by only a few genera. For instance, about 25 Category 1 medically important snake species in Africa belong to only five genera—
Naja,
Dendroaspis,
Echis,
Bitis, and
Cerastes [5].
Cobras, represented by the genus
Naja (nāgá, meaning ‘snake’ in Sanskrit), belong to the Elapidae family of snakes, and most of their species are classified under Category 1 by the WHO. There are nearly 30 species of cobra (
Naja) (
Table 1) that are widely distributed in Africa and Asia
[6][7], and they are responsible for a considerable number of snake envenoming cases on these continents. For various reasons, most global epidemiological studies have not discretely recorded the exact estimates of cobra envenoming in Africa and Asia. While neurotoxicity is one of the vital clinical manifestations to distinguish cobra envenomation, other elapid snakes, for example, mambas (
Dendroaspis sp.), kraits (
Bungarus sp.), or even some viperids, for example, berg adders (
Bitis atropos)
[8], and Russell’s vipers (
Daboia russelii) from southern India and Sri Lanka
[9][10][11] have also been reported to inflict neurological disorders in patients. Therefore, identifying the inflicting species based solely on clinical manifestations might not always be accurate.
Snake Species |
Common Name |
Medical Importance/ Category |
Region of Distribution |
Number of Countries |
Human Population (2020) in This Species’ Range |
N. anchietae |
Anchieta’s cobra |
Highest, Secondary |
Africa |
6 |
19,008,230 |
N. annulata |
Banded water cobra |
Highest, Secondary |
Africa |
10 |
114,642,902 |
N. annulifera |
Snouted cobra |
Highest |
Africa |
8 |
70,731,878 |
N. ashei |
Ashe’s spitting cobra |
Highest |
Africa |
6 |
32,513,269 |
N. atra |
Chinese cobra |
Highest |
Asia and Australasia |
5 |
570,266,425 |
N. christyi |
Christy’s water cobra |
Secondary |
Africa |
3 |
15,111,896 |
N. guineensis |
Black forest cobra |
Highest, Secondary |
Africa |
7 |
55,106,930 |
N. haje |
Egyptian cobra |
Highest, Secondary |
Africa |
21 |
443,884,351 |
N. kaouthia |
Monocled cobra, Thai cobra |
Highest, Secondary |
Asia |
11 |
976,884,863 |
N. katiensis |
Mali cobra, West Africa brown spitting cobra |
Highest, Secondary |
Africa |
12 |
123,542,818 |
N. mandalayensis |
Mandalay spitting cobra |
Highest |
Asia and Australasia |
1 |
14,774,047 |
N. melanoleuca |
Black and white cobra, Forest cobra |
Highest, Secondary |
Africa |
11 |
244,375,176 |
N. mossambica |
Mozambique spitting cobra |
Highest, Secondary |
Africa |
10 |
130,049,980 |
N. naja |
Indian cobra, Spectacled cobra |
Highest, Secondary |
Asia and Australasia |
5 |
1,656,817,409 |
N. nigricincta |
Western barred spitting cobra, Zebra cobra |
Highest, Secondary |
Africa |
4 |
11,381,021 |
N. nigricollis |
Black-necked spitting cobra |
Highest, Secondary |
Africa |
33 |
727,256,279 |
N. nivea |
Cape cobra |
Highest |
Africa |
4 |
17,651,152 |
N. nubiae |
Nubian spitting cobra |
Secondary |
Africa |
5 |
39,843,095 |
N. oxiana |
Central Asian cobra, Transcaspian cobra |
Highest, Secondary |
Asia and Australasia, Middle East |
8 |
242,127,307 |
N. pallida |
Red spitting cobra |
Secondary |
Africa |
6 |
59,847,176 |
N. peroescobari |
Sao Tome cobra |
Highest |
Africa |
1 |
189,185 |
N. philippinesis |
Northern Philippine cobra |
Highest |
Asia and Australasia |
1 |
592,982,107 |
N. sagittifera |
Andaman cobra |
Secondary |
Asia and Australasia |
1 |
373,959 |
N. samarensis |
Southern Philippine cobra, Visayan cobra |
Highest |
Asia and Australasia |
1 |
30,350,207 |
N. savannula |
West African banded cobra |
Highest, Secondary |
Africa |
16 |
151,894,138 |
N. senegalensis |
Senegalese cobra |
Highest, Secondary |
Africa |
13 |
84,781,768 |
N. siamensis |
Indochinese spitting cobra, Siamese spitting cobra |
Highest, Secondary |
Asia and Australasia |
5 |
119,240,121 |
N. sputatrix |
Southern Indonesian spitting cobra |
Highest, Secondary |
Asia and Australasia |
2 |
167,089,984 |
N. subfulva |
Brown forest cobra |
Highest, Secondary |
Africa |
22 |
377,545,129 |
N. sumatrana |
Equatorial spitting cobra |
Highest, Secondary |
Asia and Australasia |
6 |
124,654,470 |
Compositionally, cobra venoms are predominated by low-molecular-mass (<20 kDa) enzymatic and non-enzymatic toxins. In addition, among the non-enzymatic class, the three-finger toxin (3FTX) is the prominent protein (toxin) family that significantly contributes to cobra venom-induced pathophysiology and toxicity
[10][19][20][21][22][23][24][25][26]. Notably, among the various sub-classes of 3FTXs, the proportion of cytotoxins (CTX) and post-synaptic neurotoxins (NTX) dictates the lethality of cobra venoms
[24][27].
2. Composition of Cobra Venom: A Summary
Biochemical and pharmacological analyses of crude
Naja venom and their purified toxins are crucial in highlighting the variation in venom composition owing to the different geographical locations of these species
[28][29]. However, these conventional methods are not apt for the characterization of non-enzymatic proteins, and with cobra venoms being primarily dominated by non-enzymatic toxins, a comprehensive cobra venom composition has yet to be unraveled. Recent developments in mass spectrometry-based snake venom toxin profiling have enabled thorough analysis of cobra venom composition. Proteomic findings have suggested that cobra venoms are predominated by enzymatic phospholipase A
2 (PLA
2) (~13–15 kDa) and non-enzymatic 3FTXs (~6–9 kDa); together, they constitute about 90% of the total cobra venom. Other enzymatic proteins in cobra venoms include L-amino acid oxidase, serine proteases, metalloproteases, phosphodiesterases, 5′-nucleotidases, cholinesterases, phospholipase B, hyaluronidases, and aminopeptidases. In addition, the minor non-enzymatic classes of toxins in cobra venoms include Cobra venom factors, Kunitz-type serine protease inhibitors, nerve growth factors, cystatin, natriuretic peptides, cysteine-rich secretory proteins, ohanin-like proteins or vespryns, vascular endothelial growth factors, and C-type lectins or snaclecs (reviewed by
[4][30][31]).
Notably, the proportion of cobra venom CTXs was found to vary dramatically across different
Naja species; it was ~13% in Taiwanese
N. kaouthia venom, while it constitutes ~73% of
N. nigricollis venom (
Figure 1). In general, venoms from African spitting cobras have a higher proportion of CTXs than the Asiatic cobra ones, indicating geographical variation in snake venom composition. Interestingly, while the abundance of neurotoxins corresponds well to the severity of neurotoxicity in envenomed patients, the extent of local tissue-damaging effects of cobra venom do not correlate well with the proportions of CTXs, thereby suggesting variable cytotoxicity of the toxin isoforms as well as a contribution by other cobra venom toxins that can inflict local effects on their own or in complex with CTXs
[32].
Figure 1. Proteomics analyses determined the relative abundance of CTXs in cobra venoms from diverse geographical locations (Source Kalita et al
[33]). The mean relative abundance of CTX in African cobra venoms (61.1 ± 12.6%) is significantly higher as compared to Asian cobra venoms (38.8 ± 17.2%); p-value = 0.001.