They may look like harmless blobs under a microscope, but minuscule critters living in some of Yellowstone National Park's most popular swimming holes could be very dangerous.
Park officials recently had their long-held suspicions confirmed that an amoeba and a bacterium, both known to cause illness and even death in some cases, are lurking in certain thermal waters throughout Yellowstone.
A yearlong University of Montana study showed that the amoeba Naegleria fowleri and the bacterium Legionella existed in 23 warm-water areas in Yellowstone and Grand Teton national parks, including at the Boiling River between Mammoth and Gardiner, a popular soaking spot that more than 28,000 people use every year.
Although park officials figure that the two species have been in the water for a long time, there are no confirmed cases of anyone dying or even becoming seriously ill from exposure in the thermal waters.
For years, signs at swimming holes have warned visitors about the possibility of such tiny creatures in the water. The signs are being changed to say that Naegleria fowleri and Legionella do, in fact, live in those waters.
"This is an opportunity to educate the public about the risks associated with swimming (in the thermal waters) and to give them an opportunity to make an informed choice about whether they want to swim or not," said Cheryl Matthews, a Yellowstone spokeswoman.
She said she didn't know of any plans to limit public access to the park's waterways because of the recent discovery.
Both organisms were found in waters with temperatures between 77 and 122 degrees Fahrenheit.
Naegleria fowleri is a single-celled organism found in soil and water around the world, especially warm and shallow water.
The amoeba is known to cause primary amoebic meningoenchephalitis (PAM), a rare disease that causes inflammation of the lining of the brain and spinal cord. The often-fatal disease can take hold when water containing the organism gets into the nose or mouth and moves through the nasal passage to the brain.
Those infected usually die in seven to 10 days but there is treatment if the disease is found early enough. Initial symptoms include headache, fever, nausea and vomiting, stiff neck, confusion, loss of balance and bodily control, seizures and coma as symptoms worsen.
Aside from the Boiling River, the amoeba has been confirmed at Nymph Creek, Hillside Springs, Seismic Geyser, Mallard Lake Trail, Madison Campground, Terrace Springs and Bathtub Spring. However, not all hot springs were surveyed, so park officials say all thermal waters should be considered suspect.
Park officials also say that although there's no guarantee of safety for those who swim in the thermal waters, a few things can be done to reduce risk, including not letting any water touch your head or face.
While researches were looking for Naegleria fowleri in the thermal waters, they also found the first evidence of Legionella in Yellowstone.
The bacterium is found in human-built water systems, such as evaporative air coolers, and natural systems, including warm-water environments.
Legionellosis is an infection caused by the bacterium Legionella pneumophila, which is an aquatic organism that's been shown to cause Legionnaires' disease, a kind of pneumonia. A less severe form of the species causes Pontiac fever, which is an acute-onset, flu-like, non-pneumonic illness.
Legionnaires' disease has been shown to be fatal in 5 to 15 percent of cases, according to park officials.
Researchers understand that people can become sick when they breathe in Legionella-containing mists from air coolers but it's not clear how the Legionella spreads in a natural environment.
Matthews said park managers are hoping to conduct further research about the existence of Legionella in the air and mist at some of Yellowstone's thermal areas.