U.S. Drinking-Water Systems Still Haven’t Defeated This Nasty Parasite

The U.S.’s largest-ever outbreak of waterborne illness—cryptosporidiosis—hit Milwaukee 30 years ago. Why are many other water systems still vulnerable to the same parasite today?

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Thirty years ago a tiny parasite in the water supply in Milwaukee, Wis., touched off the largest waterborne disease outbreak in U.S. history. Although that city’s water is now renowned for its high quality, public health departments across the country are still battling the same diarrhea-inducing organism. What makes it so tough?

Reports of gastrointestinal illnesses throughout the Milwaukee area began pouring into the city’s health department in April 1993. A local infectious disease physician eventually identified a case of cryptosporidiosis, an infection with the parasitic protist Cryptosporidium. When health officials began testing stool samples for this organism, they found many more cases. The parasite, they realized, was lurking in the pipes: for the past two weeks the Milwaukee Water Works had been receiving dozens of telephone complaints about local tap water appearing cloudy.

At a hastily called late-night meeting on April 7 of that year, Milwaukee’s mayor John Norquist asked the late Jeff B. Davis, an epidemiologist at the Wisconsin Division of Public Health, “Would you drink the water?” Davis’s answer, “No, I wouldn’t,” shocked the mayor. Within an hour, Norquist arranged a press conference and declared Milwaukee’s drinking water unsafe for consumption unless it was boiled. Television news anchors scrambled to report the mayor’s “boil order” for water, and newspaper editors reworked their front pages.

Over the next eight days Milwaukee cleaned and disinfected its water treatment plants, state and federal officials declared the supply safe for consumption, and the boil order was rescinded. But by that time more than 400,000 local residents—approximately half of the 800,000 people served by Milwaukee’s water-distribution system—had reported cryptosporidiosis symptoms, including diarrhea, vomiting, fever, chills and body aches. Pharmacy shelves ran out of over-the-counter gastrointestinal medicines. More than 4,000 people were admitted to local hospitals. By the time the crisis subsided, at least 100 people had died from exposure to the parasite.

Cryptosporidium remains a serious health problem today. The Centers for Disease Control and Prevention reported 444 outbreaks of cryptosporidiosis in the U.S. between 2009 and 2017, and the number has increased by an average of 13 percent each year. A 2019 CDC report estimates that 823,000 people get the illness each year and that fewer than two percent of cases are reported to the CDC.

These outbreaks occur across the country and beyond. In late September 2023 the Baltimore Department of Public Works announced that Cryptosporidium had been detected in samples from a large drinking-water reservoir. The city issued a boil-water order for people with health conditions that could make them more vulnerable. Recent outbreaks have also been reported in North Carolina and Oregon. The U.K. and New Zealand have also battled severe outbreaks in the last few months.

What makes cryptosporidiosis such a nasty and stubborn health problem? First reported in humans in 1976, this extremely contagious disease spreads when people drink water contaminated with Cryptosporidium. In the water supply the parasite remains in a life stage called an oocyst, which is four to five micrometers in diameter and shielded by a protective outer shell. This helps the organism resist pathogen-killing processes traditionally used by water treatment facilities.

Once the oocysts are ingested, the shells crack—releasing Cryptosporidium into the host’s intestines, where as few as 10 of the parasites can cause an infection. These parasites reproduce at an incredible speed: Just three to four days after infection, a person can shed as many as one billion oocysts in diarrhea in a single day. And this shedding continues for an average of 18 days.

“Cryptosporidium has a long incubation period,” says CDC epidemiologist Michele Hlavsa. “From the point when you’re exposed to the pathogen to the point where you develop symptoms, the time frame could be a week or more. Then these people have to be sick enough to see a doctor and get tested.”

Cryptosporidiosis can cause one to two weeks of nausea, stomach cramps, vomiting, dehydration and fever, but the most commonly reported symptom is watery diarrhea. Although such claims might sound hyperbolic, Hlavsa says infected people have reported up to 40 episodes of watery stools per day.

But diarrhea is a symptom of many illnesses, and most laboratories do not routinely test stool samples for Cryptosporidium. Because Cryptosporidium is hard to detect and infected people can be contagious for several weeks, epidemiologists assume that many cases may be unreported and that outbreaks may be more widespread than they appear to be. Some experts estimate that only one percent of confirmed Cryptosporidium infections are officially documented.

Scientists do know how to prevent Cryptosporidium outbreaks: kill or filter out the parasites in public drinking water before it gets to the tap. The Environmental Protection Agency’s Interim Enhanced Surface Water Treatment Rule (IESWTR) requires large water systems to remove 99 percent of Cryptosporidium from drinking water. In 1998 the EPA estimated that implementing this rule would “reduce the likelihood of the occurrence of outbreaks of cryptosporidiosis.”

Yet removing these parasites from public drinking water is an extremely challenging process. The hard-shelled oocytes are resistant to the chlorine disinfectants used by many municipal water treatment plants. Fortunately, there are other options.

Advanced technologies such as ozonation have proved effective in removing oocysts. In this process a device called an ozone generator runs a stream of oxygen through a high-voltage electric field, which breaks down some of the oxygen molecules, whose atoms combine with other oxygen molecules to produce ozone. The resulting oxygen-ozone mixture is pumped into holding tanks, where the highly corrosive ozone destroys the cell walls of any microorganisms in the water—rendering parasites such as Cryptosporidium inert—before breaking down naturally. The water then moves through several more filtration and treatment processes before reaching household taps.

Another option is exposing water to ultraviolet (UV) light, which inactivates Cryptosporidium oocysts and renders the parasite noninfectious. “UV is an interesting concept—basically irradiating the water as it passes through a UV reactor—but the process doesn’t necessarily destroy the organism. The process just renders it so that the parasite can’t reproduce,” says Dan Welk, water plants manager at the Milwaukee Water Works.

After Milwaukee’s Cryptosporidium outbreak, the city invested more than $500 million in upgrading its water treatment plant facilities; it has since garnered industry awards for the quality of its drinking water. Milwaukee’s treatment process starts with ozonation and moves through a series of steps designed to remove Cryptosporidium. And the city is open to doing more. “We’re always looking to see if there are other treatment techniques that we could potentially add to the plant to address an emerging concern,” Welk says.

Not every U.S. city tests its drinking water for Cryptosporidium, however, and it continues to strike every year. According to the EPA’s Drinking Water Infrastructure Needs Survey and Assessment (DWINSA) released in September 2023, the U.S. needs to invest $625 billion over the next 20 years to upgrade its drinking-water infrastructure.

In the meantime public health experts are working to improve diagnostic testing and reporting tools, which help them track outbreaks. But the CDC says accurate Cryptosporidium reporting is still several years away—meaning there is still the threat of another widespread outbreak such as the one that occurred in Milwaukee. “Cryptosporidium isn’t just spreading locally. It’s spreading over multiple jurisdictions—and we might not be picking up these outbreaks,” Hlavsa says. “An infection could start in one spot and move quickly to five different states.”