El Niño is a climate phenomenon that warms up the Pacific Ocean around the equator. In the western U.S., that typically means torrential downpour; in Australia, that might mean bush fires. For the tropics, it’s associated with cycles of diseases – and now snakebites too. According to new work published in Science Advances, viper envenomation is far more prevalent in Costa Rica during both the hottest and coldest El Niño years. 

Snakes are ectothermic (not to be confused with cold-blooded), which means their body temperature depends on the temperature of their surroundings. As a result, where they go and how they eat change with the weather. Predation seems to increase with higher temperatures. To investigate, a team led by Luis Fernando Chaves from Nagasaki University Institute of Tropical Medicine examined Costa Rica’s snakebite records. Reporting these injuries is mandatory in the country, and treatment is provided for free. In total there were 6,424 snakebites reported in Costa Rica from January 2005 to December 2013. Most of these bites come from the terciopelo, Bothrops asper (pictured above and below), and they typically occurred in suburban or rural regions.

The researchers built a snakebite database using records from the Ministry of Health, and then they compared them to weather fluctuations, focusing on the El Niño Southern Oscillation (ENSO). This cyclic climatic pattern has a hot and a cold phase: While El Niño means warm ocean temperatures in the Equatorial Pacific, La Niña is characterized by unusually cold temperatures. 

They found that snakebite incidence increased by 24% for every degree Celsius above the average temperature and decreased by 1% for every 7 millimeter (0.28 inch) increase in rain above the average rainfall. And more snakebites occurred during the hot phase as well as the cold phase of ENSO. 

During the hot phase, snakebites might rise due to increased viper foraging activity with higher temperatures (that wasn’t a surprise). During the cold phase, snakebites could increase because of abrupt collapses in the abundance of prey: When there’s a shortage of rodents, snakes are forced to forage in places where they could come into contact with humans. 

Furthermore, bites occurred more frequently in poorer settings. Thanks to anti-venom, most deaths caused by snakebite envenoming are avoidable, at least in theory. Yet, of the 2.5 million people affected by snakebites a year, 85,000 will die. Snakebites, the team argues, should be included in the list of diseases and health hazards that are sensitive to environmental change, and that’s especially the case given predictions of more frequent, extreme weather events to come. 

^{Female Terciopelo, or fer de lance (the scientific name Bothrops asper). The viper snake is responsible for most snakebite envenomations in Central America. The snake in the image is from the Caribbean basin of Costa Rica.}