Several people fall ill with mysterious rashes, fever after Tough Mudder event

California health officials have issued an advisory after several people who competed in a Tough Mudder run and obstacle course competition last weekend fell ill with as-yet unexplained symptoms.

The Sonoma County Department of Health Services (SDHS) said it has received multiple reports of rashes, fever, muscle pain, nausea and vomiting among those who participated in the event at the Sonoma Raceway, a car racetrack 30 miles north of San Francisco, on Aug. 19 and 20.

"The Tough Mudder race involved extensive skin exposure to mud. Most affected persons have pustular rash, fever, myalgias, and headache," read the health advisory, published Wednesday.

According to the SDHS, the symptoms could be due to "a minor illness called Swimmer's Itch," also known as cercarial dermatitis, which is caused by an allergic reaction to certain microscopic parasites, according to the Centers for Disease Control and Prevention.

The parasites live in some species of birds and mammals. If their eggs find their way into water and hatch, the larvae can infect a certain species of aquatic snail that in turn release microscopic larvae that can burrow in the skin of humans and cause symptoms, including rashes.

However, the SDHS said the people may be experiencing a staph infection, or a more serious infection from a bacterium known as Aeromonas hydrophila.

Staph infections are caused by staphylococcus bacteria and usually only cause minor skin infections, according to the Mayo Clinic, though the infections can become more serious if the bacteria enters the bloodstream and travels to the host's bones, heart or lungs.

Aeromonas hydrophila, however, is typically found in fresh water or sewage and, while rare, it can infect humans, with reports of complications including diarrhea, kidney disease, hemolytic uremic syndrome – which occurs when the small blood vessels in the kidneys become damaged and inflamed, potentially leading to kidney failure – meningitis, and sepsis, according to the Public Health Agency of Canada.

"If you participated in the race and have a rash with fever or other symptoms, please see your medical provider or, if you do not have a medical provider, your local emergency department. You may wish to take this Advisory with you. Incubation period is 12 to 48 hours," the SDHS health advisory states.

The SDHS did not immediately respond to ABC News' request for comment, nor did organizers of the Tough Mudder event.

Tough Mudder participant Nicole Villagran told local ABC affiliate KGO TV that she did not participate in all of the obstacle course but she still found bumps on her body the next day.

"You wake up the next day and you're like, 'What is all this on my arm? Like, what is going on here?'" she said. "And it's on both arms. That's where I was digging and doing army crawls and it's on the inside of my knees where I was pushing off of as well."

Fellow participant Curtis Vollmar told KGO TV that 12 of the 13 people he did the Tough Mudder event with also experienced the symptoms, though they are fading.

"Pretty much gone," he said in reference to his arms. "You can't really see much of it but from the elbows up and knees down, I was just covered in these, like, pimply red blister-type things."