When it rains, snake bites soar

Hikers and trail runners be warned: Rattlesnakes and other venomous reptiles may bite more people during rainy years than in seasons wracked by drought, a new study shows.

The research, which was led by Caleb Phillips of the University of Colorado Boulder and Grant Lipman of the Stanford University School of Medicine, examined 20 years of snakebite data from across California. Their findings contradict a popular theory among many wilderness health professionals that drought might increase snake bites by pushing the reptiles into the open where they are more likely to run into people.

Instead, the group discovered that for every 10 percent increase in rainfall over the previous 18 months, cases of snake bites spiked by 3.9 percent in California’s 58 counties.

The results could have implications for efforts to prevent and treat dangerous encounters between humans and snakes, especially as climate patterns shift across the western United States.

“This study shows a possible unexpected, secondary result of climate change,” said Phillips, an adjunct assistant professor in CU Boulder’s Department of Computer Science. “We probably need to take climatological changes into account when we coordinate systems that may seem unrelated like planning how we distribute antivenin supplies or funding poison control centers.”

Phillips and his colleagues suspect that the reason for the surge in snake bites during wet years may come down to snake food. Mice and other rodents, the prime meals for rattlesnakes, flourish in rainy years — and that might give snakes a boost.

Phillips said that he’d be eager to find out if the same trends appear outside of California. Colorado is home to three closely-related species of venomous reptiles: prairie, western and massasauga rattlesnakes. Bites from these animals rarely kill humans, but tragedy can strike. Colorado triathlete Dan Hohs, for example, died from a rattlesnake bite in 2017.

The researchers wondered how climate change might influence the frequency of such encounters. They pored through 5,365 cases of rattlesnake bites reported to the California Poison Control System between 1997 and 2017. The team compared those cases to a range of other information, including climate data from NASA and drought records from the National Drought Mitigation Center.

What the group found was surprising: When California counties experienced drought, recorded cases of snake bites dropped off. Those incidents hit record low levels statewide in 2015 and 2016 when California was in the middle of a historic dry spell.

The researchers published their findings today in the journal Clinical Toxicology.

Research in Colorado and other parts of the United States suggests that the impact of warming temperatures on rainfall patterns will be a mixed bag — with some regions experiencing more severe storms and others seeing drier weather.

Phillips, an avid trail runner and trained wilderness first responder, urges outdoor enthusiasts like him to stay calm. “If you encounter a rattlesnake,” Phillips said, “don’t pick a fight with it, and it won’t pick a fight with you.”

Other co-authors of the study include Derrick Lung and Hallam Gugelman of the University of California, San Francisco School of Medicine and Katie Doering of the Stanford School of Medicine.

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