After Tough Mudder near San Francisco, participants report rashes and fevers from bacterial infections

More than 100 people said they contracted bacterial infections after competing in a Tough Mudder challenge on Saturday and Sunday in Sonoma, California.

The event was held at Sonoma Raceway, which is about 45 miles northeast of San Francisco, and challenged participants to run, wade and crawl through a muddy obstacle course. Three days later, the Sonoma County Department of Health Services issued a health advisory for all racers, citing multiple reports of rashes, fever, muscle pains and vomiting among the participants.

“These symptoms could be indicative of a minor illness called Swimmers’ Itch, but they can also indicate a Staph infection or other more serious bacterial infection such as Aeromonas,” the advisory said.

Swimmer’s itch, or cercarial dermatitis, is a rash caused by water parasites and doesn’t usually require medical attention. Staph infections, however, are caused by bacteria, can produce swollen, pus-filled bumps on the skin and may lead to lung infections or sepsis if left untreated. Aeromonas hydrophila bacteria, also found in water, can cause infections, with symptoms including fever, diarrhea, cramps and vomiting.

On Wednesday, 112 participants contacted Tough Mudder to report “bumpy, pus-filled, rash-like infections,” according to an email provided to NBC News. The email said it was writing on behalf of 344 participants, including the senders’ family members and friends who also raced but were not named or copied on the email.

“Most of our doctors have identified this as staph,” the email said, adding: “Many of us have these additional symptoms, as well as fever and fatigue. Several have been hospitalized for days.”

Tough Mudder did not immediately respond to a request for comment.

But in a reply to the group email, a Tough Mudder representative wrote that the organization was working to complete its own investigation, ensure that the health advisory reached all participants and assist the Sonoma County health department.

NBC News spoke to five people who said they experienced symptoms of bacterial infection after competing in one of the Tough Mudder races Sunday. 

David Long, 57, said he raced with several co-workers, then came down with a fever, chills and lesions from his ankle to his chest that night. He sought medical attention the next day and tested positive for Aeromonas on Friday, Long said.

The larger sores on his leg resembled boils and were painful, he said.

“I think there is an element of what you would call negligence,” Long said. “It doesn’t seem like it would be very difficult to test those water pools for bacteria.”

Matt Brown, a Sonoma County communications specialist, said the health department had logged seven cases of Aeromonas hydrophila infection so far, based on physician reports to the county.

“Our health department has received dozens of phone calls and emails from participants either concerned about their health or reporting symptoms,” he said.

Founded in 2009, Tough Mudder holds 5k, 10k and 15k races across the country in which participants are submerged in pits of muddy water, crawl through mud under barbed wire and scale muddy hills. 

After last year’s event in Sonoma County, a participant emailed Tough Mudder to report that she and several friends who raced had developed fevers and rashes, according to a screenshot of the message she shared with NBC News.

In the recent group email to Tough Mudder, the senders wrote that the company should have informed them about the previous complaints but did not.

Dr. Henry Chambers, chief of the Division of Infectious Diseases at San Francisco General Hospital, said mud can house thousands of species of bacteria, so crawling through muddy terrain — especially with cuts and scrapes from obstacles — carries an inherent risk of infection.

“The best way to prevent it, other than not doing the course is to immediately clean off, shower right away, so that organisms don’t have a chance to get established,” Chambers said.

Though testing the area might have detected Aeromonas bacteria before the event, he said such a course comes with myriad infection risks beyond that particular one. 

Malia Helms, 25, said she completed the 10k course with five other friends, then woke up the next morning with itchy, painful red bumps across her body. She tested positive for Aeromonas infection, she said.

“I’m still feeling sick from it, and my body is still recovering,” Helms said.

After learning of the complaints following last year’s event, Helms said she felt frustrated that the company didn’t inform this year’s participants of a potential risk.

Noa Umbaugh, 26, another member of Helms’ six-person group, said she woke up with similar bumps on her arms and legs, though she did not test positive for Aeromonas infection. After emailing Tough Mudder to report her symptoms, Umbaugh said the company told her it was looking into the matter.

Umbaugh said she wants her registration fee refunded.

“It was just supposed to be a fun thing, and now all six of us are on antibiotics,” she said.