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What Happened at Tough Mudder Sonoma: Hundreds Get Sick With Possible Bacterial Infection

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a dozen people, dressed in athletic gear, climb in and out of a muddy pit in a field
Competitors take part in the Tough Mudder event in Sonoma in 2022. (Tayfun Coskun/Anadolu Agency via Getty Images)

More than 100 athletes who participated last weekend in the Tough Mudder obstacle course race at Sonoma Raceway have since reported experiencing fevers, vomiting, muscle pain and a distinct rash that includes boils.

Sonoma County public health officials on Wednesday released a health advisory — urging anyone with symptoms to see their doctor or go to a local emergency room, and notify the county via a form on their website. An email and text message from event organizers also went out to participants, with Tough Mudder noting it was working with local health officials.

A Tough Mudder event involves hundreds of people climbing over and crawling through mud and muddy water, which is likely how participants picked up the infection, officials said.

“Most affected persons have pustular rash, fever, myalgias, and headache. These symptoms could be indicative of a minor illness called ‘swimmer’s itch,’ but they can also indicate a staph infection or other more serious bacterial infection such as Aeromonas,” reads the health advisory.

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The total number of people with symptoms is likely greater than what’s been so far officially reported, as thousands of people participated in the event over the two days and reports have been accumulating on social media and in a Reddit thread.

As of Friday afternoon, a handful of people with symptoms reported tests from doctors indicating the presence of the bacteria Aeromonas, which is naturally occurring in mud and warm water.

“I’m not surprised this would happen once in a while,” said Ian Young, an infectious disease expert and associate professor with Toronto Metropolitan University who specializes in recreational waterborne illnesses. If you’re crawling through mud and water, you’ll be exposed to bacteria in that dirt and water. But, he emphasized, given the number of events like this that happen every weekend, an outbreak of this kind is rare.

Confused about why people would subject themselves to crawling through mud and dirty water? Tough Mudder is actually a very popular race, held at dozens of venues around the country and attracting hundreds of thousands of people.

What is Tough Mudder?

Tough Mudder is a type of obstacle course race (commonly called “OCR” among athletes who race them frequently). Over a distance of anywhere from 5 kilometers to 15 miles, depending on the event, participants complete a range of “obstacles” that can include anything from crawling through the mud under barbed wire to climbing over walls and logs to traversing through complicated rope courses and monkey bars.

Tough Mudder, which was founded in 2010, is one of several companies that organize these kinds of obstacle course races. Spartan Race, which acquired Tough Mudder in 2020, is the other major race organizer and specializes in more competitive events.

While there are elite athletes who compete for prizes in some of these races, Tough Mudder is known more for its camaraderie and experiential vibe. Times are generally not recorded, and athletes are expected to help each other over walls and through obstacles. Tough Mudder is known more for pushing a person’s comfort boundaries than for its physical challenges. For example, the final obstacle is always a field of lightly electrified dangling wires participants have to run through to reach the finish line. (In my experience, having completed a few of these, it’s best not to think too much about it and just go.)

Most of the people who did the Tough Mudder in Sonoma are recreational athletes, who do it with friends, and mix it in with marathons or CrossFit classes.

Michelle Spinosa said this year’s event was her second one. “I got two soccer moms to do this with me and we did the full 9-mile event,” she said, joking that they were over 10 years older than most of the men around them.

Because Spinosa works in public health, she said she had signed up for the first time-slot at the start of the day — thinking it would simply be less gross than going after the sun beat down on thousands of bodies crawling through mud. (A tactic I also adhere to.)

“I knew that could happen,” Spinosa said of the infection risk, although she emphasized that despite the unintended outcome it still seemed like a well-run event. “I don’t think this was any more gross than any other of these events.”

A big part of the appeal, she said, is the camaraderie as you work with other people to overcome obstacles, some of which by design cannot be completed without help. “You’re working with strangers to accomplish something that’s gross and hard.”

Wait, what?! Why do people do Tough Mudder and obstacle course races?

Because jumping into mud pits is fun!

“There’s something really empowering, too,” about knowing you can do these challenging things, Spinosa said.

Tough Mudder, historically, has billed itself as a way to push people’s boundaries and help them find out what they’re capable of. There’s a whole shelf of research (CGI) that’s been hotly discussed about why this particular brand of what’s known as Type II fun is especially popular among white-collar workers. But it doesn’t take much theorizing to realize there’s some innate appeal to getting muddy with your friends and climbing over a log.

“There are two main reasons people do these,” said Erin Beresini, author of the book Off Course: Inside the Mad, Muddy World of Obstacle Course Racing. One is the physical challenge, she said. “Where everything in life has gotten so easy, there’s this primal need to be challenged physically that they’re not getting in their every day.”

The second reason is the camaraderie. Bonds formed in that challenge, she said, tend to be stronger and form faster.

Personally, having done a half-dozen OCRs, I’ve always found the Spartan Races more interesting, because they’re less about inflicting pain and more about the actual physical obstacles. But, as the sport has developed, they’ve also become more challenging and require more training and skill, compared to Tough Mudder races. It doesn’t take too much training to crawl through mud — hence their appeal.

What if I have an infection?

Sonoma County health officials are urging anyone with symptoms to see a doctor or, if you don’t have a primary care provider, to go to your local emergency department.

Testing and investigations are still being done to figure out whether the outbreak was caused by staph — which can be carried by other humans — or a bacteria, like Aeromonas, found in the mud and water. There are lots of different kinds of possible bacteria in muddy water, noted Young, the infectious disease expert, and runoff from nearby cow pastures in Sonoma could certainly contribute.

But Aeromonas, which has already been detected in a number of participants, is believed to be the main culprit.

“These bacterial infections can develop when skin is exposed to soils and mud,” county health officials said in its letter to participants. “If untreated, serious illness and sepsis can develop.”

What is Aeromonas and how common is it?

Aeromonas hydrophila is a type of bacteria that’s found naturally in mud and warm fresh and brackish water — meaning it isn’t necessarily the result of contamination or human influence. It’s found in North America, as well as in other parts of the world, said Young — especially warmer areas, like in Florida — and may be emerging as climate change warms waters.

It’s relatively ubiquitous in fish, reptiles and mammals, and can lead to gastrointestinal distress. But an outbreak like this is pretty uncommon. “It’s just bad luck,” Young said, while noting there is always a risk, especially when people may have open cuts or have to submerge themselves.

As with almost all forms of bacteria, there are a few documented cases of incredibly rare severe drug-resistant infections.

Are there any regulations on these kinds of races?

Yes, and no. Largely, the extent of the regulations depend both on where the race is held and what the event’s insurance provider requires. Florida, for example, allowed one of the first COVID-era mass participation races to come back with a 1,000-person Spartan Race in June 2020 — at a time when most states and municipalities were still refraining from permitting events of that nature.

Regulations from the insurance providers and industry itself have, historically, focused on the safety of cold-water obstacles and preventing medical risks like heart attacks or drownings.

“There’s an expectation of controlled danger,” said Beresini — meaning that participants sign a waiver and typically understand they could be hurt or injured, but expect reasonable precautions to be taken and immediate medical attention to be provided if injured.

Typically, as was the case in Sonoma, permits are granted by county and city officials who require certain standards and safety plans. However, while organizers of events like triathlons and open-water swims are often required to test water quality beforehand, doing so is not required for crawling through mud or simply jumping into water pits.

“You can’t really test the mud,” said Young, “it’s not practical.”

In an email, Tough Mudder CEO Giles Chater told participants his company is working closely with local health officials and “completing our own thorough investigation, which includes extensive sampling, as we attempt to identify the cause.”

There have also been reports of some similar illnesses after last year’s Sonoma Tough Mudder event — though nothing close to this scale. Spinosa said her friend, who raced with her, had a slight rash after participating last year.

But, it’s not clear what could necessarily be done to prevent this, said Young, other than moving the event (if it was caused by bacteria specific to that area or impacted by nearby cow pastures), and alerting participants of the risk of bacterial exposure (especially when conditions are warmer and more people have already gone through the course). Young’s main advice: Wash your hands, clean off and shower after the event. And during the race, try not to swallow any water or mud or get it in open wounds.

But, he added, “There’s only so much you can do.”

KQED’s Billy Cruz contributed reporting to this story.

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