LEVENS, France, June 29 (UPI) -- Mud run participants might have to start worrying about more than sprained ankles or scraped knees after more than 1,000 people became sick to their stomach after participating in an event last week in France.
The regional health agency in southern France said 1,000 of the 8,400 participants involved in the Levens Mud Day on June 20 reported symptoms of acute gastroenteritis. The agency said the stomach ailments -- mostly diarrhea, vomiting and fever -- were caused by norovirus, a highly infectious disease that causes an inflammation of the stomach or intestines.
The U.S. Centers for Disease Control and Prevention says norovirus is transmitted through contaminated water or food, or by touching contaminated surfaces.
Health officials say it's unclear how the runners contracted the virus, but some doctors said they diagnosed patients who said they didn't eat any food at the event, The Local.fr reported.
It's possible the runners may have ingested mud contaminated with the virus along the race course, possibly through the presence of feces in the mud. Some runners took to Mud Day's Facebook page complaining that sections of the course smelled distinctly of horse manure.
A report from the CDC in May 2014 found 22 people who participated in a long-distance obstacle run -- which included sections in mud and water -- in October 2012 became ill after inadvertently swallowing muddy water during the race. The runners later became sick with campylobacteriosis, which has symptoms of bloody diarrhea, nausea and vomiting.
"Primary and emergency care providers, as well as public health professionals, should be aware that obstacle adventure race events could pose a heightened risk for outbreaks from inadvertent ingestion of contaminated water or mud and might consider outreach to educate participants on the health risks from oral contact with contaminated surface water or mud," the CDC report said.
The Mud Day organizers in Levens said they are making "every effort to determine what could be causing stomach upset" in some of the run's participants.