Nineteen people fell ill with a diarrheal disease in Montana last year after drinking untreated water that many believed to be from a natural spring but which was, in fact, just creek drainage brimming with pathogenic bacteria.

One person was hospitalized in the outbreak, which ended only after authorities diverted the water source, local health officials reported Thursday in Morbidity and Mortality Weekly.

The outbreak follows a trend that sprang up in the US several years ago of drinking so-called "raw water." That is untreated, unfiltered water collected directly from freshwater sources that is often claimed—without evidence—to have health benefits.

Proponents have argued that raw water avoids undesirable components of municipal water, which they identify as disinfectants, fluoride, imaginary "mind-control" drugs, traces of pharmaceuticals, and heavy metals, such as lead from pipes. They also suggest, without evidence, that raw water can contain unique probiotics and other "natural" minerals and compounds that can improve health.

Health officials have pointed out that untreated, unfiltered water is a clear health risk, given the likelihood of pathogenic bacteria, viruses, and parasites, as well as naturally occurring contaminants, such as radionuclides and mineral deposits.

In the case of the Montana outbreak, the contaminant was Campylobacter jejuni, a pathogenic bacterium that can spread from carrier animals, such as birds. In humans, Campylobacter bacteria causes diarrhea, which is often bloody, as well as stomach cramps, nausea, and vomiting.

Pandora’s watery box

In May 2022, health officials in northwestern Sanders County reported six Campylobacter cases to state officials, raising immediate concern and spurring an investigation. The average annual number of cases is just five. All six of those first cases reported drinking water from the same source, which many community members believed to be a natural spring.

But, it wasn't—it was a concrete box that poured out water from a creek near Paradise, Montana. The concrete box was situated on railroad property near a track. It was likely built in the early 1900s to prevent the creek from eroding the track bed, the local officials reported. Moreover, when officials investigated the box, they found an empty bird's nest inside—which was the likely source of Campylobacter. Amid further investigation, health authorities confirmed Campylobacter was present in the water pouring out of the box, and whole genome sequencing showed the bacteria contaminating the water were a close match to those in the sickened people.

In the six weeks following the initial case report, 13 more cases occurred, despite signs posted around the box indicating that the water wasn't safe to drink. On June 28, the Montana Department of Transportation permanently cut off the water source by rerouting the creek water so that it remained underground. No other cases have been identified since.

"Persons drinking water from outdoor sources, including creeks, rivers, and streams, should always treat the water before drinking it," the health officials concluded. "Boiling water is the most reliable way to kill germs, but treatment including filtration will also reduce the risk of illness from drinking water from outdoor sources."