"Tough Mudder" adventure racers endure some pretty insane obstacles, including dumpsters full of ice water and fields of dangling live wires that can shock with 10,000 volts of electricity.
But a new report suggests a hidden risk, as well: serious bacterial illness.
The Centers for Disease Control and Prevention announced today that there was an outbreak of an infection called campylobacteriosis among 22 participants in one of the popular adventure courses, probably caused when racers inadvertently swallowed "muddy surface water contaminated with cattle or swine feces," according to the CDC release.
The races are commonly held on farmland, according to the CDC, which greatly increases the risk of getting infected by animal-borne bacteria, especially when people are crawling through the mud.
The stomach-turning symptoms
This outbreak was discovered after an October 2012 race on a cattle ranch in Beatty, Nevada.
After three patients showed up to the emergency room with bloody diarrhea, fever, and vomiting, the public health service at Nellis Air Force Base, near Las Vegas, began an investigation. Along with state and local officials, they traced the cause to a bacteria, Campylobacter coli, a common cause of diarrheal illness.
In total, they identified 22 cases of C. coli infection in and around Nellis and conducted a study to identify the cause of the illness. Of the 22 cases, 18 were considered probable, and four were confirmed by testing a stool sample.
They also checked 24 racers who hadn't gotten sick, to figure out the difference.
From what they could tell, the difference was that those infected had accidentally swallowed water while going through the obstacle course — easy to do with obstacles that involve swimming and crawling through puddles of mud as fast as possible. (Health officials found no connection between just being immersed in the water and getting sick.)
In the CDC's report, they recommend that the race planners "consider building slurry field challenges where animal fecal contamination is not likely."
All 22 participants recovered.
The dangers of mud racing
This isn't the first time that participating in an adventure race has caused an illness outbreak. Last year, in Michigan, approximately 200 racers got sick with what appeared to be norovirus, most likely after a sick participant contaminated the course.
And according to a case report in Sports Medicine, dozens of participants in a Pennsylvania Tough Mudder ended up in the emergency room, mostly with electrical injuries.
Tough Mudder is also being sued after a participant drowned during a 2013 event. The wrongful death suit accuses the company of "gross neglience."
We reached out to Tough Mudder for comment on the CDC report and will update this post if we hear back.
SEE ALSO: 10 Insane Things That Happen When A Diver Descends 400 Feet On A Single Breath