Tap Water Parasite Detections - A growing concern.
Waterborne parasites have occasionally been detected in the tap water of various U.S. cities over the decades. These incidents range from major outbreaks of gastrointestinal illness to isolated detections prompting health advisories.
Parasites in tap water are an ongoing concern in the United States, particularly in areas with aging infrastructure, inadequate water treatment, or poor sanitation systems.
While modern water treatment has significantly reduced the risk of parasitic infections, certain parasites like Cryptosporidium and Giardia continue to pose challenges due to their resistance to standard disinfection methods.
Persistent Parasites in Water Systems
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Cryptosporidium: This parasite is notably resistant to chlorine, a common disinfectant in water treatment. Between 2009 and 2017, the Centers for Disease Control and Prevention (CDC) reported 444 outbreaks of cryptosporidiosis in the U.S., with the number increasing by an average of 13% annually. An estimated 823,000 people contract the illness each year, though fewer than 2% of cases are reported to the CDC. This parasite can cause severe liver damage if left untreated.
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Giardia: Since 1971, Giardia has been the most commonly identified pathogen in waterborne outbreaks in the U.S., with over 130 reported incidents across 27 states. Outbreaks are often linked to inadequately treated surface water sources.
Here are the worst outbreaks that have effected American cities over the years.
Chicago, IL (Amebic Dysentery Outbreak)
Chicago experienced one of the earliest known waterborne parasite outbreaks during the 1933 World's Fair. An Entamoeba histolytica (amebic dysentery) epidemic sickened an estimated 1,400 people and caused 98 deaths.
Investigators traced the source to a hotel's water system: a backflow from a sewer-contaminated cooling tank used for making ice, which allowed the amoeba into the drinking water. This was the first recognized waterborne outbreak of amebic dysentery in a U.S. civilian population.
The incident highlighted the need for better cross-connection controls and proper plumbing to prevent sewage contamination of city water.
Boulder, CO (Giardia Outbreak)
In the summer of 1972, Giardia lamblia (also known as Giardia duodenalis) was implicated in a community-wide outbreak of giardiasis in Boulder. Hundreds of residents suffered prolonged diarrhea and gastrointestinal illness, which was eventually linked to the city's mountain water supply.
This outbreak was among the first in the U.S. to raise awareness of Giardia as a waterborne parasite. Investigations suggested that untreated surface water contaminated with animal or human fecal matter in the watershed was the likely source, as Boulder's water treatment at the time was insufficient to remove or kill Giardia cysts.
This event spurred improved water treatment standards for surface water systems nationwide.
Rome, NY (Giardia Outbreak)
Another early giardiasis outbreak struck the city of Rome, New York between late 1974 and mid-1975. Approximately 350 residents had lab-confirmed giardiasis during this period, with an attack rate of about 10% in the community.
Epidemiologists found a strong association between illness and drinking unfiltered city tap water, and a Giardia cyst was even microscopically identified in a city reservoir sample. The likely source of contamination was untreated human waste in the watershed serving Rome's water supply. This was the first time Giardia cysts were directly found in municipal tap water during an outbreak investigation, conclusively proving the water supply as the vehicle.
Public health authorities responded with boil-water advisories and eventually improvements in water treatment.
Camas, WA (Giardia Outbreak)
In March 1976, the community of Camas, Washington experienced a giardiasis outbreak caused by Giardia in the municipal water. At least 128 people had laboratory-confirmed Giardia infections, (and many more likely fell ill).
The city's water came from surface sources in the Boulder Creek and Jones Creek watersheds, which were found to be vulnerable to contamination by wild animal feces. At the time, Camas's water was chlorinated but not adequately filtered for Giardia cysts.
Health officials issued advisories and the city upgraded its water treatment processes to include filtration, given that chlorine alone was not enough to inactivate the chlorine-resistant Giardia cysts.
This outbreak underscored the importance of filtration for surface water supplies.
San Antonio (Braun Station), TX (First U.S. Cryptosporidium Outbreak)
In 1984, a suburb of San Antonio (the Braun Station community) was the site of the first documented U.S. drinking water outbreak of Cryptosporidium. In June-July 1984, over 200 people in this community suffered from cryptosporidiosis (characterized by severe diarrhea).
The outbreak was traced to a contamination of the community's well water supply by sewage, as fecal coliforms were found in the well. Stool tests identified Cryptosporidium oocysts as the cause of the gastroenteritis. This outbreak was pivotal: it alerted public health officials that Cryptosporidium, a chlorine-resistant protozoan, could infiltrate drinking water and cause widespread illness.
In response, the well was taken offline and the water system was sanitized, and the incident led to increased monitoring of groundwater sources and consideration of treatment improvements.
Carrollton, GA (Large Cryptosporidium Outbreak)
In early 1987, Carrollton in Carroll County, Georgia experienced a massive Cryptosporidium outbreak through its public water supply. Over the course of a few weeks (January-February 1987), an estimated 13,000 residents (roughly 20% of the county's population) fell ill with cryptosporidiosis.
The parasite was confirmed in tap water samples during the investigation. Officials suspected the contamination came from either cattle runoff in the river feeding the water plant or a sewage overflow near the treatment facility. Notably, the water treatment plant had been meeting then-current EPA standards, yet Cryptosporidium still passed through, highlighting that those standards were not sufficient to stop this microbe.
This outbreak - at the time the largest in the U.S. - prompted regulators to rethink drinking water regulations. Boil water advisories were issued, and eventually rules were tightened to require better filtration and disinfection targeting chlorine-resistant parasites.
Milwaukee, WI (Cryptosporidium Outbreak)
Milwaukee endured the largest waterborne disease outbreak in U.S. history in 1993, when Cryptosporidium contaminated the city's drinking water. An estimated 403,000 people fell ill with cryptosporidiosis, overwhelming hospitals and clinics.
At least 69 deaths were attributed to the outbreak, mostly among immunocompromised patients. The source was traced to the Howard Avenue Water Treatment Plant: a combination of heavy spring runoff and an ineffective filtration process allowed Cryptosporidium oocysts from Lake Michigan to slip through into the tap water. Once detected, the city issued boil-water orders and undertook immediate plant upgrades.
This 1993 Milwaukee incident led to extensive changes in water treatment regulations nationwide - including the EPA's Enhanced Surface Water Treatment Rules - to ensure filtration and disinfection practices can remove or kill parasites like Cryptosporidium. Milwaukee itself invested hundreds of millions of dollars in improvements (ozonation, better filtration, etc.) in the decades since.
Las Vegas (Clark County), NV (Cryptosporidium in Water Supply)
In early 1994, the Las Vegas area saw an outbreak of Cryptosporidium that primarily affected the immunocompromised community. Over several months, at least ~79 cases of cryptosporidiosis were confirmed, largely in patients with AIDS, and dozens of those patients died due to the infection.
Health investigators suspected the municipal water supply (from Lake Mead) as the source, since Cryptosporidium was detected in water samples and no other common exposures linked the cases. However, unlike Milwaukee, the Las Vegas outbreak did not significantly sicken the general healthy population - many may have been exposed but only those with severely weakened immune systems had serious illness.
In response, the water authority accelerated improvements: Las Vegas's treatment plants enhanced their filtration and disinfection processes (including ozone treatment) in the mid-1990s. Officials also advised vulnerable residents to boil water during the height of the outbreak.
The event underscored that even "invisible" low-level contamination in a big city's water could pose lethal risks to certain groups.
St. Bernard Parish (LA - Naegleria fowleri in Water)
St. Bernard Parish, a community just outside New Orleans, had two rare and deadly incidents involving the "brain-eating" amoeba, Naegleria fowleri, in its tap water.
In 2011, a 20-year-old resident of the parish died from primary amebic meningoencephalitis after using a neti pot with tap water infected by Naegleria. Then in 2013, a 4-year-old child visiting St. Bernard Parish also tragically died from the same amoeba, which again was linked to the household's tap water.
These cases prompted testing of the parish water system. The Naegleria fowleri amoeba was indeed found at multiple points in the parish's water lines (e.g. in Arabi and Violet areas) after the 2013 case. The likely cause was insufficient chlorine disinfectant residuals in parts of the water distribution system, which allowed the amoeba (naturally present in warm local waters) to survive in the pipes.
In response, Louisiana health officials ordered an aggressive chlorine flush ("chlorine burn") of St. Bernard's water system to raise chlorine to parasite-killing levels. They also warned residents to avoid getting tap water up their noses (since Naegleria infects via the nasal passage) and to use boiled or distilled water for sinus rinses.
These incidents led Louisiana to mandate higher minimum chlorine levels statewide and better monitoring to prevent future amoeba contamination.
DeSoto Parish, LA (Naegleria fowleri Case)
In 2011, DeSoto Parish in northwestern Louisiana also reported a fatal case of Naegleria fowleri infection from tap water. A 53-year-old woman in DeSoto Parish died after using a neti pot with contaminated tap water, similar to the St. Bernard case that year.
Subsequent testing found the amoeba present in that water system as well. This case, coupled with the St. Bernard incident, alerted officials that even treated public water (normally safe to drink) can pose a risk for amoebic infection if it enters the nasal passages. DeSoto Parish's water system underwent hyperchlorination after the incident.
Public advisories were issued about properly disinfecting water for nasal irrigation and other precautions. The Louisiana Department of Health even issued a statewide Neti Pot safety warning urging the use of boiled or sterile water for nasal rinsing.
This was one of the first times Naegleria fowleri was recognized as a tap water hazard in the U.S., leading to increased surveillance of public water systems in warm climates.
St. John the Baptist Parish, LA (Naegleria fowleri Detected)
St. John the Baptist Parish (west of New Orleans) did not have any known illnesses, but in 2015 its water system tested positive for Naegleria fowleri amoeba during state sampling. This discovery came shortly after the fatal cases in other Louisiana parishes.
The Louisiana Department of Health ordered an emergency chlorine burn of St. John's water distribution in January 2015 upon finding the amoeba. Although no one had been sickened in St. John Parish, the detection was taken very seriously because the previous year a young boy in St. Bernard Parish had died from the amoeba.
Investigators found that parts of St. John's system had fallen below required chlorine levels, which likely allowed the amoeba to persist. After weeks of flushing with elevated chlorine, follow-up tests came back clear for Naegleria.
This incident led to greater scrutiny of water utility compliance. In fact, later investigations resulted in two water system employees being indicted for allegedly falsifying chlorine testing records in St. John Parish.
The event reinforced the need for diligent maintenance of disinfectant levels to keep water safe from rare but deadly parasites.
Baker City, OR (Cryptosporidium Outbreak)
In July 2013, the small city of Baker City, Oregon experienced a significant Cryptosporidium outbreak via its municipal drinking water. An estimated 2,780 residents (over a quarter of the town's population) fell ill with cryptosporidiosis.
Health investigators confirmed the parasite in the city's water: samples of the municipal water tested positive for Cryptosporidium oocysts. The likely source was identified as livestock contamination - a specific C. parvum subtype common in cattle was found in patients, and cattle were observed grazing near the watershed that supplies Baker City.
Critically, Baker City's water treatment at the time did not include filtration; the system relied only on chlorination, which is ineffective against Cryptosporidium. Once the outbreak was recognized, the city issued boil-water notices and provided emergency drinking water. The water system was later upgraded (adding filtration and/or UV disinfection) to meet modern standards.
The Baker City incident was notable as a modern reminder that even small communities in the U.S. are vulnerable to crypto outbreaks if proper water treatments aren't in place.
Lake Jackson, TX (Naegleria fowleri in Water Supply)
In September 2020, Lake Jackson, Texas (a city of ~27,000) made national headlines when a brain-eating amoeba (Naegleria fowleri) was found in its public water system. The discovery came after a 6-year-old boy tragically died from a Naegleria infection that he likely contracted via a backyard hose or splash pad supplied by city water.
Subsequent testing of the Lake Jackson water confirmed the presence of Naegleria fowleri in the tap water. Officials immediately issued a "Do Not Use" water advisory for Lake Jackson and nearby communities, warning residents not to drink or bathe in the water until it could be disinfected. After narrowing the advisory to just Lake Jackson, a boil-water notice and nose-related precautions (like not letting water up the nose in showers) remained in effect.
The likely cause was inadequate routine chlorination: tests indicated the water system had low chlorine residuals, which allowed the amoeba to survive and proliferate. In response, the Texas Commission on Environmental Quality and city authorities flushed the system with very high chlorine levels to kill the parasite.
The incident prompted reviews of water treatment protocols in warm climates. It was a rare case of Naegleria contamination in a U.S. municipal supply, leading to heightened public awareness (and not a little alarm) about tap water safety.
Baltimore, MD (Cryptosporidium Detected in Reservoir)
In September 2023, Baltimore City officials announced that routine testing had found Cryptosporidium in the Druid Lake Reservoir, which holds finished drinking water for the city. The detected levels were low, and no illnesses were reported, but out of caution a health advisory was issued for certain areas.
Elderly, infant, and immunocompromised residents were advised to boil tap water or use bottled water due to the potential risk. The contamination was believed to have come from Baltimore's uncovered reservoirs, which can be exposed to wildlife. Indeed, experts noted that it was migratory bird season and suspected the parasite entered the treated water via droppings from birds or other animals getting into Druid Lake.
The EPA had long warned Baltimore to cover its open reservoirs to prevent such contamination. Following the Cryptosporidium detection, the city accelerated plans to install covered storage (underground tanks) for those reservoirs. Baltimore's incident illustrated how even a modern water system can detect occasional parasite intrusion; fortunately in this case, proactive monitoring caught the issue before any outbreak occurred.
The public communication stressed that risk to healthy individuals was very low, but the event reinforced the importance of source water protection and infrastructure upgrades.
Portland, OR (Ongoing Cryptosporidium Detections)
Portland is a unique case: it has an unfiltered surface water source (the Bull Run watershed) and for many years was one of the few large U.S. cities without a filtration plant. Starting in 2017, Portland's Water Bureau began detecting Cryptosporidium oocysts occasionally in routine water samples.
These detections did not result in known illnesses but did trigger regulatory requirements. Initially, Portland received variances to avoid filtration, using only disinfection and watershed protection. However, after repeated low-level Crypto detections (e.g. one oocyst on multiple days in 2017), the Oregon Health Authority notified Portland that it must install filtration.
City officials assured the public that the overall risk was low and that no boil notice was needed for general customers. They did echo EPA guidance that people with severely weakened immune systems consult their doctors about drinking the tap water.
Portland has since committed to building a full filtration plant (targeted completion by 2027). In the interim, the city has implemented additional precautions like UV treatment and frequent monitoring.
The Portland case emphasizes a proactive approach: finding parasites in water through testing even when no outbreak has occurred, and then investing in infrastructure to ensure long-term safety.
Conclusion
Parasites in tap water have become an increasingly serious concern-not just in the United States, but globally.
Despite modern advancements in water treatment, these microscopic organisms can still find their way into municipal water systems, especially in areas with aging infrastructure, inadequate filtration, or seasonal flooding.
Because of this growing threat, it's more important than ever for individuals to take responsibility for their own health by staying informed.
One of the best steps you can take is to research your local water quality reports. These are typically made available by your city or municipality and can reveal whether any contaminants, including parasites, have been found in your water supply.
Exposure to parasites in drinking water is not just an inconvenience, it can lead to severe and long-term health complications. Some of the potential effects include chronic gastrointestinal distress, nutrient malabsorption, liver damage, compromised immune function, and in some cases, even cancer.
Unfortunately, many people carry parasites for years without realizing it, mistaking symptoms for common digestive or fatigue-related issues.
To combat this hidden health risk, we strongly recommend that every adult consider doing a full-body parasite cleanse at least once every six months. Regular cleansing may help reduce the parasitic load in your body, support liver and digestive health, and improve energy levels and overall well-being.
Prevention and awareness are key-not just for those living in high-risk areas, but for anyone looking to take a proactive approach to their health in an increasingly contaminated world.