Most snakebites are caused by non-venomous snakes. Of the roughly 3,700 known species of snake found worldwide, only 15% are considered dangerous to humans. Snakes are found on every continent except Antarctica. There are two major families of venomous snakes, Elapidae and Viperidae. 325 species in 61 genera are recognized in the family Elapidae and 224 species in 22 genera are recognized in the family Viperidae, In addition, the most diverse and widely distributed snake family, the colubrids, has approximately 700 venomous species, but only five genera—boomslangs, twig snakes, keelback snakes, green snakes, and slender snakes—have caused human fatalities.
Since reporting is not mandatory in many regions of the world, snakebites often go unreported. Consequently, no accurate study has ever been conducted to determine the frequency of snakebites on the international level. However, some estimates put the number at 1.2 to 5.5 million snakebites, 421,000 envenomings, resulting in perhaps 20,000 deaths, but the actual number of deaths may be as high as 94,000. Many people who survive bites nevertheless suffer from permanent tissue damage caused by venom, leading to disability. Most snake envenomings and fatalities occur in South Asia, Southeast Asia, and sub-Saharan Africa, with India reporting the most snakebite deaths of any country. An analysis cross-referencing exposure to venomous snakes and accessibility of medical treatment identified that 93 million people worldwide are highly vulnerable to dying from snakebite.
Available evidence on the effect of climate change on the epidemiology of snakebite is limited but it is expected that there will be a geographic shift in risk of snakebite: northwards in North America and southwards in South America and in Mozambique, and increase in incidence of bite in Sri Lanka.
Worldwide, snakebites occur most frequently in the summer season when snakes are active and humans are outdoors. Agricultural and tropical regions report more snakebites than anywhere else. In North America, the victims are typically male and between 17 and 27 years of age. Children and the elderly are the most likely to die.
+ Number of Snakebite Envenomings per Year RegionLow EstimateHigh Estimate Asia237,3791,184,550 Australasia1,0991,260 Caribbean1,0988,039 Europe3,9619,902 Latin America80,329129,084 North Africa / Middle East3,01780,191 North America2,6833,858 Oceania3614,635 Sub-Saharan Africa90,622419,639 Total: 420,549 1,841,158
+ Number of Deaths from Envenoming per Year RegionLow EstimateHigh Estimate Asia15,38557,636 Australasia24 Caribbean1071,161 Europe48128 Latin America5402,298 North Africa / Middle East4378 North America57 Oceania227516 Sub-Saharan Africa3,52932,117 Total: 19,886 93,945
Africa
Although Africa is home to four venomous snake families—Atractaspididae, Colubridae, Elapidae, and Viperidae—approximately 60% of all bites are caused by vipers alone. In drier regions of the continent, such as sahels and savannas, the saw-scaled vipers inflict up to 90% of all bites. The puff adder is responsible for the most fatalities overall, although saw-scaled vipers (Echis spp) inflict more bites in North African countries, where the puff adder is typically not found. The black mamba (Dendroaspis polylepis) is classified as the snake of highest medical importance in Africa by the World Health Organization, and although it is responsible for fewer snakebite incidents than snakes like puff adders, it is the species with the highest mortality rate in Africa and in the world (per capita) if left untreated. Before the advent of a polyvalent antivenom produced by the South African Institute for Medical Research, black mamba envenomation was certain to cause fatality (100% rate).
Most bites occur in industrial plantations, which attract many types of snake prey. Banana plantations are associated with vipers such as night adders, while rubber and palm tree plantations attract elapids, including cobras and black mambas. Important African cobra species include the Cape cobra and the black-necked spitting cobra. In forested areas of Guinea, cobras may inflict more than 30% of all venomous bites.
Mambas, cobras, and some larger viper species from the genus Bitis may claim territories of up to , which are used by the snakes for hunting and reproduction. These snakes are large enough to defend their territory from intruders, including humans. The black mamba, in particular, is among the most venomous snakes in the world and one of the most aggressive. It is the longest venomous snake on the continent and is able to move at , making it unusually dangerous. Although black mambas cause only 0.5-1% of snakebites in South Africa, they produce the highest mortality rate and the species is responsible for many snake bite fatalities. The black mamba is the species with the highest mortality rate in every single nation in which it occurs, despite not being responsible for the most numerous snakebites. It is also the species which has the highest mortality rate worldwide. Although antivenom saves many lives, mortality due to black mamba envenomation is still at 14%, even with antivenom treatment. In addition to antivenom treatment, endotracheal intubation and mechanical ventilation are required for supportive therapy.
There are also venomous colubrids in Africa, although of these only two arboreal genera, the boomslang and the twig snakes, are likely to inflict life-threatening bites. Of the Atractaspididae, Atractaspis is the species involved in the majority of bites. Since these snakes are nocturnal and fossorial, living in burrows underground, bites remain rare, peaking at 1 to 3% in certain areas of the Sudanian savanna. However, there is no antivenom or other effective therapy for Atractaspis envenomation, and the case fatality rate remains approximately 10%, with death typically occurring quickly.
Snakebites in Africa are most common in the sub-Saharan countries (scorpion stings are more common in Northern Africa), and typically occur during the rainy season when snakes are more active. Agricultural plantations and pastoral areas are involved in approximately 50 to 60% of reported snakebites, while outdoor activities such as collecting firewood and water account for another 20% of bites. Approximately 1 million snakebites occur in sub-Saharan Africa each year, resulting in up to 500,000 envenomations, 25,000 deaths and another 25,000 permanent disabilities.
Because there is no reliable reporting system in place and because most victims never report their injury to health care facilities, these numbers are uncertain. One comprehensive study estimates that 91,000 to 420,000 snake envenomings occur in sub-Saharan Africa annually, resulting in anywhere from 4,000 to 30,000 deaths. The same study estimates that 3,000 to 80,000 envenomations occur in North Africa, although far fewer people, less than 100, die each year. The variability of these estimates is indicative of the difficulty in accurately assessing snakebite morbidity and mortality.
In sub-Saharan Africa, over 50% of snakebite injuries are not appropriately treated. Between 40 and 80% of victims, depending on the country, exclusively rely on traditional medicine for treatment. Most victims who receive treatment by health care professionals have nevertheless delayed seeking medical attention for over 24 hours, and often up to 1 to 2 weeks. In many sub-Saharan countries, poor availability of expensive antivenom contributes to morbidity, and snakebites continue to remain a neglected health problem.
Asia In Bangladesh, the incidence of snakebites is very high, like other tropical countries of Southeast Asia. Here most snakebite victims dwelling in rural areas are farmers, fishermen, and hunters and also there are a high number of snakebite occurrences that happened at their homes as most of the snakes are nocturnal animals and poor people have the practice of sleeping on the floor. An epidemiological study estimated about 8000 snakebites per year with 22% mortality which has been identified to be one of the highest in the world. Nonetheless, there are approximately 80 species of snakes found in Bangladesh; among them only few are venomous. These are cobra, krait, Russell's viper, sawscaled viper, green snakes, and sea snakes. However, most of the bites are reported by nonvenomous snakes and even as many as 40% bites inflicted by venomous snakes do not produce signs of envenoming.
On the Indian subcontinent, almost all snakebite deaths have traditionally been attributed to the Big Four, consisting of the Russell's viper, Indian cobra, saw-scaled viper, and the common krait. However, studies have shown that the hump-nosed viper, previously considered essentially harmless and misidentified as the saw-scaled viper, is capable of delivering a fatal bite. In regions of Kerala, India, it may be responsible for nearly 10% of venomous bites. Commonly used antivenoms in India do not appear to be effective against hump-nosed viper bites. According to the most conservative estimates, at least 81,000 snake envenomings and 11,000 fatalities occur in India each year, making it the most heavily affected country in the world. The Malayan pit viper and banded krait are two other species involved in a significant number of venomous bites.
In Myanmar (Burma), approximately 70 to 80% of bites are inflicted by the Russell's viper, which is frequently found in rice fields. In Japan, the majority of venomous bites are inflicted by the Asian pit vipers. In Sri Lanka, approximately 40% of bites are caused by the Russell's viper, while 35% are caused by the Indian cobra. In Thailand, the monocled cobra is responsible for the majority of snakebite fatalities. Tea plantations are sometimes associated with elapids such as the common cobras and the king cobra.
Reports of large man-eating pythons are common in Southeast Asia, although the threat is typically exaggerated. However, in the Philippines, more than a quarter of Aeta men (a modern forest-dwelling hunter-gatherer group) have reported surviving a reticulated python attack. Pythons are nonvenomous ambush predators, and both the Aeta and pythons hunt deer, wild pigs, and monkeys, making them competitors and prey.
Throughout Western Asia, the species responsible for the majority of bites tend to be more venomous than European snakes, but deaths are infrequent. Studies estimate that perhaps 100 fatal bites occur each year. The Palestine viper and Lebetine viper are the most important species. While larger and more venomous elapids, such as the Egyptian cobra, are also found throughout the Middle East, these species inflict fewer bites.
Australasia
The vast majority of venomous snakebites in Australasia occur outside of Australia, in the surrounding islands. At least 300 envenomations occur each year in Papua New Guinea, 30 in the Solomon Islands, and 10 Vanuatu. The majority of bites in New Guinea are caused by the death adders, and a smaller number by the endemic small-eyed snake and other local species. Most of the Pacific Islands are free of terrestrial snakes, although sea snakes are common in coral reefs. In the Oceanian realm, only Micronesia and Tonga, where at least 10 envenomations occur annually, face an appreciable burden of snakebite.
In Australia, there are many more annual cases of spiderbite and jellyfish envenomation than snakebite; however, when considering the number of fatal cases, snakebite is the most serious type of envenoming. Australia is unique in that it is the only continent where venomous snakes constitute the majority of species. Nearly all venomous species in Australia are elapids, as vipers are absent from the continent.