|
Most snakebites are caused by non-venomous snakes. Of the roughly 3,000 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. Three hundred and twenty five 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 5.4 million snakebites, 2.5 million envenomings, resulting in perhaps 125,000 deaths.〔 Others estimate 1.2 to 5.5 million snakebites, 421,000 to 1.8 million envenomings, and 20,000 to 94,000 deaths.〔 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.〔 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.〔〔 Victims are typically male and between 17 and 27 years of age.〔〔 Children and the elderly are the most likely to die.〔 == 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 inflict more bites in North African countries, where the puff adder is typically not found. The black mamba, although responsible for far fewer snakebite incidents, is the species which has the highest mortality rate in Africa and in the world.〔 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 cobra species include the forest 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 puff adder species 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 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 number of snakebites. It is also the species which has the highest mortality rate worldwide. 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Epidemiology of snakebites」の詳細全文を読む スポンサード リンク
|