The risk of snakebites is increasing across the world as reptiles shift their habitats to cope with rising temperatures and growing human pressures, a study of venomous snakes has found.
Spitting cobras in Africa, vipers in Europe and South America, cottonmouth moccasins in North America and kraits in Asia are coming into greater contact with people as a result of climate disruption and landscape change, according to the research, which was led by the World Health Organization.
This trend is forecast to become more pronounced in the coming decades as snakes – like many other species – adjust their range to escape hotter conditions.
Most species will suffer a decline of habitat, but a significant number of the deadliest snakes are likely to spread more widely, taking them into areas where they have not been seen before and potentially affecting billions of people.
“The overlap between humans and venomous snakes will be greater,” said one of the authors, David Williams of the WHO and the University of Melbourne. “You could consider this a risk of walking out of the back door, stumbling and getting bitten.”
Snakebite statistics are sketchy because many happen in remote areas and go unreported. But the authors of the new paper say there are about 4m cases every year, mostly in the tropics. The vast majority are not dangerous, but there are 138,000 deaths and 400,000 disabilities annually – almost half of which occur in south Asia. Until now the distribution of risk was understood at a local or national level, with little analysis of how this could alter in the future as a result of climate and demographic trends.
The study, published in PLOS Neglected Tropical Diseases on Thursday, aims to fill that knowledge gap. Using public and private databases, citizen science platforms, museum records, scientific literature and expert observations, the researchers mapped the distributions of all 508 medically important snake species across the planet to a granularity of 1 sq km. They then projected how rising temperatures would alter their overlap with human populations by 2050 and 2090.
They found the greatest risk was to the snakes themselves. Most species, including puff adders in Africa, coral snakes in the Amazon and copperheads in Papua New Guinea and Australia, will struggle as a result of hotter weather and the conversion of forests, wetland and grasslands into ranches, monocultures and towns. Some could be pushed closer to extinction.
Others are likely to move. The black mamba, for example, is expected to retreat from the coast of Kenya and many areas of Ethiopia, Eritrea, Congo and Djibouti and expand in South Africa and parts of Nigeria and Somalia.
The black mamba could expand in South Africa and parts of Nigeria and Somalia. Photograph: blickwinkel/Alamy
In some cases, the shift of range is likely to take venomous snakes into places where human populations are unaccustomed to such a threat. Cottonmouth moccasins in the US are forecast to head as far north as New York. Kraits in Asia could migrate from the forests of Myanmar and the Chinese province of Yunnan to the densely populated central and northern cities of China. The European viper, which is found in the UK, is expected to have more human encounters, though other types of viper may decline. In India, which registers about 60,000 snakebite deaths each year, the deadliest snakes – including common cobras, Russell’s vipers and kraits – are projected to move from the south to the north, which has more people.
“In 50 years, species will appear where they have not been found before, putting them into contact with people who have not been used to this particular problem in the past,” Williams said. He predicted that in some countries there would be encounters in farmyards or near water sources. In others, near playgrounds or running tracks.
Dangers are amplified in poor, remote areas where people work barefoot in fields and have little access to healthcare. Wealthier countries such as Australia have many venomous species of snake but very low mortality rates because farm workers are more likely to wear boots, use tractors and live close to clinics with antivenoms.
The researchers say the new study should help health authorities to target resources on areas at high risk and to prepare for the changes that lie ahead – for both humans and snakes.
“Our predictions can be used to decide where to stockpile which antivenom, how to ensure adequate capacity of individual health facilities, how to improve healthcare accessibility of remote at-risk communities, and where to focus conservation efforts for threatened snake species,” the authors said in a statement.
