The risk of snakebites is rising globally as reptiles shift their habitats to cope with rising temperatures and growing human pressures, according to a new study of venomous snakes led by the World Health Organization. Spitting cobras in Africa, vipers in Europe and South America, cottonmouth moccasins in North America, and kraits in Asia are increasingly crossing paths with humans due to climate disruption and landscape changes.
The trend is expected to worsen over the coming decades as snakes - like many species - adjust their range to escape hotter conditions. While most snake species will suffer habitat loss, a significant number of the deadliest snakes are likely to spread more widely, taking them into areas where they haven't been seen before and potentially affecting billions of people. “The overlap between humans and venomous snakes will be greater,” said 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 occur in remote areas and go unreported, but the paper estimates about 4 million cases annually, mostly in the tropics. While the vast majority aren't dangerous, there are 138,000 deaths and 400,000 disabilities each year - almost half in South Asia. Until now, risk distribution was understood locally or nationally, with little analysis of how climate and demographic trends could alter it.
The study, published Thursday in PLOS Neglected Tropical Diseases, aims to fill that gap. Using public and private databases, citizen science platforms, museum records, scientific literature, and expert observations, researchers mapped the distributions of all 508 medically important snake species across the planet to a granularity of 1 square kilometer. They then projected how rising temperatures would alter their overlap with human populations by 2050 and 2090.
The greatest risk, they found, is 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 due to hotter weather and conversion of forests, wetlands, and grasslands into ranches, monocultures, and towns. Some could be pushed closer to extinction. Others are likely to move. The black mamba, for instance, is expected to retreat from Kenya’s coast and many areas of Ethiopia, Eritrea, Congo, and Djibouti, expanding in South Africa and parts of Nigeria and Somalia.
In some cases, range shifts will take venomous snakes into places where human populations are unaccustomed to such threats. Cottonmouth moccasins in the US are forecast to head as far north as New York. Kraits in Asia could migrate from Myanmar’s forests and China’s Yunnan province to densely populated central and northern Chinese cities. The European viper, found in the UK, is expected to have more human encounters, though other viper types 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 south to north, where populations are larger.
“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 encounters in farmyards, near water sources, and even near playgrounds or running tracks. Dangers are amplified in poor, remote areas where people work barefoot in fields with little healthcare access. Wealthier countries like Australia have many venomous snakes but very low mortality rates because farm workers wear boots, use tractors, and live near clinics with antivenoms.
The researchers say the study should help health authorities target resources on high-risk areas and prepare for changes 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.