Cryptosporidium in Drinking Water: Why Municipalities Still Struggle with Removal

It all began in Milwaukee, Wisconsin, in the early spring of 1993. Everything seemed normal— residents embraced the warmer weather, and families hydrated with water from their taps. But little did they know something perilous had been lurking in their pipes.

For two weeks, calls about severe stomach issues poured into the city’s health department. Emergency rooms brimmed with weary patients, while schools and businesses faced major staffing issues after scores of employees called out sick.

Many blamed it on a bad case of food poisoning or something in the air. However, it later turned out that a tiny parasite known as Cryptosporidium had contaminated the city’s public water supply, setting off the largest-ever waterborne disease outbreak in U.S. history.

Today, many U.S. water systems are still vulnerable to contamination from this nasty parasite. Before we explore the reason behind this, let’s first discuss what Cryptosporidium is and how it gets into drinking water.

What is Cryptosporidium?

Cryptosporidium, commonly known as “Crypto,” is a microscopic protozoan parasite that infects the gastrointestinal tract of humans and animals. At just 3-4 microns in size on average, it is tiny yet mighty—life-threatening even. When ingested, usually through contaminated food or water, this parasite increases the risk of a highly contagious diarrheal disease called cryptosporidiosis.

How Does Cryptosporidium Get into Drinking Water?

Cryptosporidium enters surface water sources through the feces of infected wild and domestic animals and livestock. Depending on the weather, it can spread rapidly when water levels rise.

During heavy rains, sewer overflows can discharge diluted raw sewage containing Cryptosporidium-laced feces into nearby waterways. Runoff from urban and agricultural areas can also wash animal waste from land into rivers, streams, reservoirs, etc.

Public water systems that get water from these sources can contain the parasites, ultimately sending the contaminated water to homes and businesses—like what triggered the Milwaukee Cryptosporidium crisis in 1993.

The 1993 Milwaukee Cryptosporidium Outbreak: A Closer Look

Before 1993, little was known about the risks of Cryptosporidium in drinking water—at least in the U.S. However, that changed after the parasite invaded Milwaukee’s municipal drinking water system.

Oocyst levels in the city’s water supply peaked over two weeks, but unfortunately, its Howard Avenue Water Treatment Plant failed to remove Cryptosporidium adequately during the drinking water treatment process.

Nearby rivers rose significantly from snowmelt, flushing parasite-laden runoff into Lake Michigan. At the time, Milwaukee didn’t have enough barriers to safeguard the treated water under extreme weather conditions.

The aftermath was scary—at least 403,000 of the city’s 1.6 million residents reported symptoms, including diarrhea, vomiting, fever, chills, and body aches. Pharmacies ran out of over-the-counter gastrointestinal medicines. About 4,000 people were admitted to local hospitals.

A local infectious disease physician later linked the symptoms to cryptosporidiosis, an infection with the parasitic protist Cryptosporidium. Health officials began testing stool samples for this organism and found many more cases. They realized the parasite had been lurking in residents’ pipes. But get this: for the past two weeks, the Milwaukee Water Works received dozens of telephone complaints about local tap water appearing cloudy.

Health officials attributed at least 100 deaths to exposure to the parasite, primarily impacting AIDS patients and other immuno-compromised people. Drug therapy provided little relief for these vulnerable groups as there was no fully effective treatment against the infection.

Today, the outbreak remains the largest waterborne disease epidemic in documented U.S. history. As a result, Cryptosporidium was thrust into the spotlight as a rising threat to safe drinking water in America and the world.

Why Are Many U.S. Water Systems Still Vulnerable to Cryptosporidium Contamination Today?

After over three decades since the Milwaukee Cryptosporidium outbreak, you’d think all U.S. water systems would be fully equipped to combat the parasite in drinking water. Regrettably, many still haven’t defeated this stubborn parasite—and we’ll tell you why.

While most infectious pathogens die quickly outside a host, Cryptosporidium is a unique exception. It’s what you’d call a true survivor. When oocysts (the parasite’s infectious life stage) leave the body, their complex cell walls form a nearly impenetrable outer shell to protect the organism. This tough protective armor makes the parasite highly resistant to standard disinfection techniques, such as chlorination. Therefore, it can be challenging for many drinking water systems to remove Cryptosporidium effectively.

Furthermore, oocysts are typically small enough to slip through many filters designed to remove larger organisms. As a result, oocysts can then persist in water and other damp environments for months, fully primed to attack their next victim. Unless municipal water providers implement methods to destroy and remove these parasites, they can quickly end up in “treated” water sent to our taps.

What Are the Health Risks Associated with Cryptosporidium in Drinking Water?

Once oocysts are ingested, their shells crack, releasing Cryptosporidium into the host’s intestines, where as little as ten parasites can trigger an infection. Just three to four days after infection, a person can release as many as one billion oocysts in diarrhea in a single day. This shedding often continues for an average of 18 days.

According to the CDC, the most reported symptom of cryptosporidiosis is watery diarrhea, but infected folks may also experience others, including:

  • Stomach cramps
  • Nausea
  • Dehydration
  • Vomiting
  • Fever
  • Weight loss

Healthy people typically recover without treatment after a week or two, but the illness can become chronic and life-threatening for the following populations:

  • Those with inherited diseases that affect the immune system.
  • Those with HIV/AIDS.
  • Cancer and transplant patients who are taking certain immunosuppressive drugs.
  • Elderly people
  • Children

What Regulations are in Place to Prevent or Limit Cryptosporidium Contamination in Drinking Water?

In light of the 1993 Cryptosporidium crisis in Milwaukee and other ensuing outbreaks, the U.S. Environmental Protection Agency (EPA) and other water industry players established drinking water standards for the parasite in drinking water. These regulatory actions include:

  • The Information Collection Rule (ICR): Through the ICR (adopted in May 1996), the EPA began requiring large public water systems serving 100,000 people or more to monitor their source water for microbial contaminants, including Cryptosporidium, for 18 consecutive months. This allowed water providers to determine if a surface water source is vulnerable to contamination.
  • Interim Enhanced Surface Water Treatment Rule (IESWTR): Armed with data collected through the ICR, the EPA published its first IESWTR in 1998. According to the EPA, “IESWTR specifies treatment requirements to address Cryptosporidium and other microbial contaminants in public water systems serving 10,000 or more persons.” The EPA estimated that this rule would “reduce the likelihood of the occurrence of outbreaks of cryptosporidiosis.”
  • The Long Term 1 Enhanced Surface Water Treatment Rule (LT1ESWTR): The EPA later split its surface water treatment rules and introduced the LT1ESWTR in 2002. The primary purpose of this rule is to improve control of waterborne microbes, especially Cryptosporidium, specifically in larger, higher-risk systems. It also mandates extra treatment based on the mean Cryptosporidium concentrations and additional turbidity (cloudiness) improvements. The rule also requires systems not providing filtration treatment to develop watershed protection programs to address Cryptosporidium.
  • The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR): In 2008, the LT2ESWTR extended LT1ESWTR provisions to smaller utilities. Under this rule, smaller water providers must monitor their water supplies and conduct monthly sampling for Cryptosporidium for the initial two years. Monitoring “crypto” isn’t cheap, so as a cost-cutting measure, the EPA requires smaller filtered water systems (serving fewer than 10,000) to first monitor for E. coli—a bacterium that is less expensive to analyze than Cryptosporidium—and then watch for Cryptosporidium if their E. coli results exceeded specified concentration levels.

Currently, the Maximum Contaminant Level Goal (MCLG) for Cryptosporidium is set at zero, indicating that no detectable amount of the organism is considered acceptable in drinking water. Compliance with these regulations helps protect public health by minimizing the risk of waterborne diseases like cryptosporidiosis.

Related: Why Federal Regulations Are Vital for Clean Drinking Water, But Not Sufficient

How To Test Your Drinking Water for Cryptosporidium

So, you’ve learned what Cryptosporidium is, how it gets into drinking water, its health effects, and how it’s regulated. Now, you might be wondering if it’s lurking in your drinking water supply. There are a few ways to find out—some more reliable than others. Let’s look at a couple of them.

  • Check with your local water provider. Most utilities regularly test for Cryptosporidium as part of their water quality monitoring programs. They should be able to tell you if cysts have been detected in the public water supply, even in very small numbers. Many utilities also provide water quality reports detailing contaminant testing history, so reach out to your local water utility and ask for a copy of the latest report or search for it online.
  • Determine your water source. Surface water supplies like rivers and lakes are more vulnerable to Cryptosporidium contamination from animal waste runoff. Well-protected and filtered groundwater sources tend to be lower risk. Your utility can clarify water sources and treatment used.
  • Look at local public health alerts. If Cryptosporidium levels spike in your municipal supply, utilities are required to notify local health departments. Increased reports of gastrointestinal illness can also signal issues. Stay updated on these vital public notifications.
  • Test your water. At-home test kits can screen for Cryptosporidium using methods like reverse osmosis filtration and immunological detection. However, accuracy depends on proper sample collection protocol, so check your results with utility data. Laboratory testing is usually more thorough and accurate than at-home testing, however. It typically involves purchasing a water test kit from a state-certified lab, sending a water sample to the lab, and awaiting the results.

Related: 10 Common Problems a Water Test Can Detect in Tap Water | What is a Boil Water Advisory? – And What to Do If One is Issued in Your Area

How Do I Get Rid of Cryptosporidium in Drinking Water?

Not every U.S. city tests its water for Cryptosporidium, let alone remove it. However, it continues to wreak havoc in communities every year. That means you’ll need to take matters into your own hands to protect your household from this vicious parasite.


The CDC recommends bringing the untreated water to a rolling boil for one minute (3 minutes at altitudes greater than 6,562 feet) to kill Cryptosporidium. The heat damages the oocysts’ protective shell and inactivates the organisms so they can’t reproduce. This method is beneficial when treating small amounts of water. However, it may be inconvenient, time-consuming, and even expensive if you constantly need to treat water for a large household.

Ultraviolet (UV) Purification

Another option is exposing the contaminated water to UV light. UV filters use UV radiation to inactivate Cryptosporidium oocysts and render the parasite noninfectious. The Springwell UV Water Purification System, in particular, kills up to 99.9% of microscopic pathogens, so you never have to worry about a boil water advisory again. It targets and destroys everything from bacteria and viruses to parasites like Cryptosporidium and Giardia. This helps protect against waterborne illnesses such as cryptosporidiosis, gastroenteritis, flu, typhoid, cholera, and hepatitis.


Some specialized microfilters have filters with specific pore sizes that enable them to remove various microbes from water. A perfect example is our Lead & Cyst Water Filter System. This system has a 0.5-micron nominal filter that doesn’t just remove soluble and insoluble lead but more than 99.95% of Cryptosporidium oocysts. These organisms are usually approximately 3 to 4 micrometers in size. That means many microfilters can effectively remove them from water—though not all will filter out smaller microbes like viruses. Essentially, microfilters have tiny holes that allow water molecules to pass through while blocking elements with dimensions larger than the filter’s pore size.

Learn more: How Do UV Systems Work and Why You Need One for Your Home

Final Thoughts

We’ve come a long way since the 1993 Cryptosporidium outbreak in Milwaukee, Wisconsin. But even with today’s advanced treatment technologies, many U.S. drinking water systems still struggle to combat this chlorine-resistant parasite. But look on the bright side: The crisis helped introduce rules and regulations that would push water plants to keep hunting for weaknesses and close many safety gaps in their water treatment processes that left us vulnerable before.

There’s no guarantee we can escape this microscopic troublemaker, as it doesn’t only strike in drinking water. What we can do, however, is take steps to reduce our exposure to it by boiling our water before consuming it or installing a UV Water Purification System or lead water filter to remove the parasite from our water supplies.