Water is essential to life, but not all water is safe to drink or swim in. Despite stringent public health regulations and modern sanitation systems, waterborne diseases continue to pose a public health concern in the United States. These illnesses can occur when individuals consume, inhale mist from, or come into contact with water contaminated by pathogenic microorganisms — bacteria, viruses, or parasites. Among these, one particular microbial agent stands out as the most frequently identified cause of waterborne disease outbreaks in the U.S.
This article explores the most common microbial agent responsible for waterborne illness in the United States, how it spreads, symptoms, risk factors, and effective prevention strategies.
Understanding Waterborne Diseases in the United States
Waterborne diseases are illnesses caused by microorganisms that are transmitted through contaminated water. These can include drinking water, recreational water (such as swimming pools or lakes), and water used in food preparation. Contamination occurs when pathogens from human or animal waste, improperly treated sewage, or agricultural runoff enter water sources.
While the U.S. has robust water treatment systems, occasional lapses in infrastructure, natural disasters, or contamination events can compromise water quality and lead to outbreaks. Public health departments track these outbreaks through systems like the CDC Waterborne Disease and Outbreak Surveillance System (WBDOSS), which monitors occurrences of waterborne illness linked to contaminated water exposure.
Since 2009, the most commonly reported cause of waterborne disease outbreaks in the U.S. is not a bacteria or a virus, but a microscopic parasite — Cryptosporidium, the causative agent of cryptosporidiosis.
Cryptosporidium: The Leading Waterborne Microbial Agent in the U.S.
What Is Cryptosporidium?
Cryptosporidium is a protozoan parasite that lives in the intestines of infected humans and animals. It is passed through the stool in the form of oocysts — hardy, environmentally resistant stages that can survive for long periods outside a host. These oocysts can contaminate water sources, leading to widespread human infections when water is inadequately treated or when recreational water is contaminated.
There are numerous species of Cryptosporidium, but the most common species infecting humans are Cryptosporidium parvum and Cryptosporidium hominis.
Why Cryptosporidium Is a Major Public Health Threat
Cryptosporidium presents a major challenge due to its high resistance to chlorine, the most common disinfectant used in water treatment. While chlorine effectively kills many bacteria and viruses, it has limited ability to inactivate Cryptosporidium oocysts. This resistance means that even well-maintained water systems can experience contamination unless additional filtration or ultraviolet (UV) disinfection methods are employed.
Outbreaks are commonly associated with:
- Contaminated drinking water sources, particularly from surface water supplies like lakes and rivers
- Poorly maintained swimming pools, splash pads, and water parks
- Food contaminated by infected food handlers
Due to these transmission routes, cryptosporidiosis often occurs in community-wide outbreaks linked to recreational water use. According to data from the Centers for Disease Control and Prevention (CDC), during the 2016–2017 outbreak season, Cryptosporidium was the most frequently identified cause of waterborne disease outbreaks in the United States.
How Cryptosporidium Causes Illness
Transmission Pathways
Cryptosporidium spreads primarily through the fecal-oral route. Transmission occurs in several ways:
- Waterborne transmission: Consumption or ingestion of contaminated water.
- Person-to-person transmission: Especially in childcare centers or healthcare settings.
- Animal-to-person transmission: Contact with feces from infected animals, especially calves and livestock.
- Foodborne transmission: Ingesting food contaminated by infected individuals or agricultural runoffs.
Chlorine Resistance and Water Systems
One of the reasons Cryptosporidium is so persistent in waterborne disease surveillance is due to its resistance to standard chlorine-based disinfection. In properly functioning public water systems, filters and UV light are more effective in removing Cryptosporidium than chlorine alone. However, smaller community systems, especially those relying on unfiltered surface water, may be more vulnerable to contamination.
In recreational water environments like swimming pools and water parks, Cryptosporidium outbreaks often occur when a swimmer who is shedding the parasite contaminates the water. Because small amounts of water can be swallowed incidentally during swimming, these outbreaks are particularly common in children.
Incubation Period and Symptom Onset
After exposure to Cryptosporidium, the incubation period typically lasts between 2 to 10 days, with symptoms often appearing about 7 days after infection. The most common symptoms of cryptosporidiosis include:
- Watery diarrhea
- Stomach cramps or pain
- Nausea and vomiting
- Fever and dehydration
In otherwise healthy adults and children, the illness usually resolves on its own within 1 to 2 weeks. However, for individuals with weakened immune systems — such as those with HIV/AIDS, cancer patients undergoing chemotherapy, or organ transplant recipients — the infection can be severe, prolonged, and even life-threatening.
Historical Waterborne Outbreaks Involving Cryptosporidium
Milwaukee Cryptosporidiosis Outbreak (1993)
One of the most notable cryptosporidiosis outbreaks occurred in Milwaukee, Wisconsin, in 1993. It is considered the largest waterborne disease outbreak in U.S. history. It affected an estimated 403,000 people and led to at least 100 deaths, primarily among people with weakened immune systems. The outbreak was linked to a water treatment plant that was unable to effectively remove Cryptosporidium oocysts from the city’s drinking water supply.
This event spurred significant changes in water treatment practices across the United States, including improvements in filtration methods and monitoring programs, and catalyzed the development of the Long Term 2 Enhanced Surface Water Treatment Rule (LT2 Rule) by the Environmental Protection Agency (EPA).
Recent U.S. Cryptosporidiosis Outbreaks
More recent outbreaks in states like Ohio, New York, and Oregon have been tied to recreational water use, especially in swimming pools and splash parks. For example, in 2022, a multi-state outbreak affected several hundred individuals, predominantly children, after exposure to inadequately chlorinated recreational water facilities.
These recurring outbreaks highlight the need for improved prevention measures, particularly in community water and recreational water environments.
Who Is Most at Risk?
While cryptosporidiosis can affect anyone, certain populations are at higher risk for severe illness and poor outcomes:
Children in Daycare or Swimming Environments
Young children, particularly those under the age of five, are at increased risk due to their frequent contact with recreational water and less-developed hygiene habits. Daycare centers and swimming programs can act as amplification points for transmission.
People with Weakened Immune Systems
As mentioned, immunocompromised individuals — including those receiving immunosuppressive drugs and those with HIV/AIDS — can experience chronic, severe diarrhea that leads to weight loss and malnutrition. In severe cases, Cryptosporidium can disseminate beyond the gastrointestinal tract and involve other organs, such as the biliary system or respiratory tract.
International Travelers
In addition to domestic waterborne outbreaks, international travelers, especially to regions with poor sanitation, are at higher risk for exposure. Contaminated water sources abroad can compound the risk of infection and contribute to the spread of new Cryptosporidium subtypes into the U.S.
Diagnosing and Treating Cryptosporidiosis
Diagnosis
Diagnosing cryptosporidiosis involves identifying the parasite in stool samples. Techniques include:
- Microscopy after acid-fast staining
- Antigen detection immunoassays
- Molecular methods like polymerase chain reaction (PCR)
More advanced diagnostic methods can identify the specific species of Cryptosporidium and determine its source or track outbreak patterns.
Treatment
There is no uniformly effective cure for cryptosporidiosis, particularly in immunocompromised patients. However, supportive care typically involves:
- Staying hydrated — especially important due to fluid loss from diarrhea
- Nutritional support — maintaining adequate nutrition can help the body fight the infection
- Antimicrobial therapy — drugs such as nitazoxanide may shorten the duration of symptoms in immunocompetent patients
- In immunocompromised adults, the FDA-approved drug nitazoxanide and other investigational agents under research may provide some relief
Currently, the U.S. Food and Drug Administration (FDA) has approved a new medication — CryptosporiGAM — as an immune globulin for treating immunocompromised individuals with chronic cryptosporidiosis.
Prevention and Control Measures
Drinking Water Safety and Advanced Treatment Methods
Because Cryptosporidium oocysts are chlorine-resistant, effective removal requires advanced water treatment strategies, including:
| Water Treatment Method | Effectiveness Against Cryptosporidium |
|---|---|
| Conventional Filtration | Moderate |
| Membrane Filtration | High |
| Ultraviolet (UV) Disinfection | Very High |
| Ozonation | Effective at high doses |
Public and private water systems must comply with EPA’s LT2 Rule, which focuses on monitoring and improving filtration effectiveness for Cryptosporidium removal from surface water sources.
Recreational Water Safety: Swimming with Health in Mind
The CDC recommends several water safety practices for swimming pool operators and the public:
- Avoid entering pools if experiencing diarrhea
- Shower before swimming
- Encourage bathroom breaks for children every hour during swimming
- Ensure proper disinfection and pH monitoring in pools and splash pads
Hygiene, Education, and Community Awareness
Promoting good personal hygiene remains a cornerstone of outbreak control. Public awareness campaigns that educate communities about the importance of handwashing, avoiding water contamination, and early medical care for persistent diarrhea can significantly reduce transmission rates.
Looking Ahead: Future Challenges and Research Needs
Despite being recognized as a major waterborne pathogen for over three decades, Cryptosporidium remains challenging to control in part due to the lack of a consistently effective drug or vaccine. Research is currently focusing on:
- Developing new antiparasitic drugs that act specifically on Cryptosporidium
- Improving detection methods to enable faster outbreak identification
- Creating vaccines, particularly for vulnerable populations in developing countries and within the U.S. healthcare system
Ongoing investment in water infrastructure and improved recreational water guidelines will also be essential in reducing the future burden of cryptosporidiosis in the United States.
Conclusion
Among the array of microbial agents that cause waterborne illness in the U.S., Cryptosporidium has emerged as the most common and challenging pathogen, frequently linked to outbreaks in both drinking and recreational water sources. Its chlorine resistance, ease of transmission, and potential severity in immunocompromised people make it a serious public health threat. However, with enhanced water treatment technologies, vigilant monitoring, education, and behavioral interventions, the spread of cryptosporidiosis can be controlled.
Understanding Cryptosporidium’s risks and adopting preventive measures can help keep our water — and our communities — healthy and safe.
References
- Centers for Disease Control and Prevention (CDC). “Waterborne Disease & Outbreak Response.” https://www.cdc.gov/healthywater/surveillance/waterborne-disease-surveillance.html
- United States Environmental Protection Agency (EPA). “LT2 Rule Overview.” https://www.epa.gov/dwreginfo/lt2-enhanced-surface-water-treatment-rule
- Yoder, J. S., et al. “Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water – United States, 2013–2014.” MMWR Morbidity and Mortality Weekly Report, 2018.
- Checkley, W., et al. “Burden of Cryptosporidiosis and Pathogenesis.” The Lancet Infectious Diseases, 2015.
What is the most common waterborne disease in the United States?
The most common waterborne disease in the United States is caused by the parasite Cryptosporidium, which leads to an illness known as cryptosporidiosis. This microscopic organism lives in the intestines of infected humans and animals and is passed through feces into the environment. Due to its ability to survive outside the host for extended periods, it can contaminate water supplies and lead to widespread outbreaks.
Cryptosporidium is particularly concerning because it is resistant to chlorine and other common disinfectants used in water treatment. This resistance means that even treated water can harbor the parasite if it is not properly filtered or if the treatment process has any lapses. The disease typically causes symptoms such as watery diarrhea, stomach cramps, fever, and vomiting, and while it can affect anyone, it poses the greatest risk to individuals with weakened immune systems.
How does *Cryptosporidium* spread through water systems?
Cryptosporidium spreads primarily through the fecal-oral route, often via contaminated water sources. The parasite forms a hard shell called an oocyst, which allows it to survive for long periods in water. When water sources become contaminated with feces from infected humans or animals, the oocysts can enter drinking or recreational water supplies, including lakes, rivers, swimming pools, and water parks.
Because oocysts are too small to be seen in a glass of water and chlorine does not kill them effectively, they can go undetected in water systems. Outbreaks often occur from ingestion of contaminated drinking water, but also are frequently associated with recreational settings, such as swimming pools and water parks, where transmission can occur when one person contaminates the water and others swallow it. Proper filtration and monitoring of public water systems are essential tools in preventing the spread of this parasite.
Who is most at risk for developing severe cryptosporidiosis?
While cryptosporidiosis can affect individuals of any age or health status, those with weakened immune systems face the greatest risk for developing severe, chronic illness. This includes people living with HIV/AIDS, individuals undergoing chemotherapy, transplant recipients taking immunosuppressive drugs, and those with inherited immune system disorders. In these populations, infection can lead to prolonged diarrhea, significant weight loss, malnutrition, and even life-threatening complications.
Children, especially those under the age of five, are also more likely to develop severe symptoms or experience complications due to dehydration. However, in healthy adults and older children, symptoms may be less severe and typically resolve on their own in about one to two weeks. Despite this, even otherwise healthy individuals can experience significant discomfort and impaired quality of life during the course of the illness, making prevention and water safety important for everyone.
What are the symptoms of a *Cryptosporidium* infection?
The primary symptom of cryptosporidiosis is watery diarrhea, which is often accompanied by stomach cramps, nausea, and vomiting. Additional symptoms may include fever, fatigue, loss of appetite, and abdominal pain. Symptoms generally begin 2 to 10 days after infection, with a median onset of 7 days. They can last for up to two weeks in otherwise healthy individuals, and in some cases, may last even longer or recur even after initial improvement.
The duration and severity of the symptoms depend largely on the strength of the infected person’s immune system. Immunosuppressed individuals may experience a prolonged illness that can last for several months. In addition to the physical discomfort, the illness can lead to electrolyte imbalance, dehydration, and in serious cases, hospitalization may be required. People who suspect they have cryptosporidiosis should consult a healthcare provider for diagnosis and monitoring, particularly if symptoms persist.
How is *Cryptosporidium* infection diagnosed and treated?
Cryptosporidiosis is typically diagnosed through laboratory testing of stool samples. Because the symptoms are similar to those of several other gastrointestinal illnesses, specific tests that detect Cryptosporidium antigens or DNA are required for accurate diagnosis. These include enzyme immunoassay (EIA), polymerase chain reaction (PCR), and microscopy using special staining techniques such as acid-fast staining, which helps visualize the oocysts under a microscope.
Treatment for cryptosporidiosis in healthy individuals focuses on managing symptoms and preventing dehydration by ensuring adequate fluid intake. There is no universally effective cure, although nitazoxanide is approved for treating the condition in otherwise healthy people. For immunosuppressed patients, restoring immune function is often critical for recovery, and drugs such as atovaquone or combination therapies may be used. Research into more effective treatments continues, especially for those with weakened immune systems.
What steps can individuals take to prevent *Cryptosporidium* infection through water?
Prevention of Cryptosporidium infection requires attention to water quality and hygiene practices. Since the parasite is resistant to chlorine, it’s important that public water suppliers use filtration or ultraviolet (UV) light treatments to effectively remove oocysts from drinking water. At the individual level, people can use home water filters labeled as effective against parasites or boil water (especially during boil-water advisories or when traveling to areas with questionable water safety).
Additional preventive measures include avoiding the ingestion of untreated recreational water (e.g., from lakes, pools, or water parks), practicing good hand hygiene, especially after using the restroom or changing diapers, and ensuring that children with diarrhea are kept out of swimming pools and other shared water environments. These steps are especially important during community outbreaks, and can help reduce the spread and impact of cryptosporidiosis in the general population.
What role does public health play in preventing waterborne diseases like cryptosporidiosis?
Public health systems play a crucial role in preventing waterborne diseases such as cryptosporidiosis by monitoring water quality and enforcing regulations designed to keep drinking and recreational water sources safe. Agencies like the Environmental Protection Agency (EPA) set standards for water treatment, while the Centers for Disease Control and Prevention (CDC) track disease outbreaks and help coordinate responses to waterborne illness threats.
These efforts include routine testing of water sources for contaminants, ensuring compliance with national water standards, investigating outbreaks to identify sources and implement control measures, and educating the public about risks and prevention. The CDC also collaborates with the EPA and local health departments to improve treatment methods and implement public advisories when contamination is detected. Through these coordinated strategies, public health officials work to limit the occurrence and spread of infection in communities.