Snake weather

Climate study shows uptick in bites after rainy seasons and drop following droughts

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If you’re terrified of being bitten by a snake in California, you might not want to plan a hike just after a stretch of rainy weather. That could be tough to swallow considering it’s commonly believed that snakes are more active in dry, hot weather. But a study led by Grant Lipman, MD, clinical associate professor of emergency medicine at Stanford Medicine, proved otherwise.

Lipman routinely treats people with venomous snakebites in the emergency room. But his inspiration for finding out how climate affects bite numbers didn’t strike at work; rather, it came when he saw a 3-foot-long rattlesnake on a trail where he was running in the brown hills near the Stanford campus during a severe drought. “I wondered if there are more snakebites during droughts,” he said.

So he and two other researchers — one from the University of Colorado-Boulder and one from the University of California-San Francisco — examined 20 years of snakebite data from every phone call made to the California Poison Control System from 1997 to 2017. Details included dates, times and sites where the calls were made; patient ages and genders; where the bites occurred on the body; treatments; and medical outcomes.

In all, 5,365 snakebites — five of them fatal — were reported, all from rattlesnakes. The median patient age was 37, and the patients were mostly male.

Looking at reports of the wettest and driest years during the 20-year period, researchers found that precipitation was a strong predictor of bites, with the number peaking following the heavy precipitation years of 2006 and 2011. But the number of bites dropped during two periods of extreme drought between 2002 and 2005, and from 2007 to 2010. From 2015 to 2016, the most severe drought on record in California, the number of bites reached their lowest, according to the study, published Sept. 5 in Clinical Toxicology.

“We can predict a big snakebite season because of prior wet winters and have antivenom in places where there are a lot of hikers or trail runners,” Lipman said. “It’s important information for people who work and play in California.”

The researchers theorized that the reason for more bites after rain is that wet weather results in more shrub growth and, with that, an increase in rodents — the snakes’ primary food source.

“More food, more snakes, more snakebites,” Lipman said.

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Tracie White

Tracie White is a science writer in the Office of Communications. Email her at

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