Cryptosporidiosis Outbreaks — United States, 2009–2017

Radhika Gharpure, DVM; Ariana Perez, MPH; Allison D. Miller, MPH; Mary E. Wikswo, MPH; Rachel Silver, MPH; Michele C. Hlavsa, MPH


Morbidity and Mortality Weekly Report. 2019;68(25):568-572. 

In This Article


The 444 outbreaks characterized in this report highlight the public health importance of Cryptosporidium, which is the leading etiology of waterborne outbreaks[1] and the third leading etiology of enteric infections attributable to animal contact[2] in the United States. In part, this is because Cryptosporidium oocysts are immediately infectious upon excretion, are excreted in numbers multiple orders of magnitude higher than the human infectious dose (≤10 oocysts), and are extremely tolerant to chlorine. These factors should be considered in the development of effective cryptosporidiosis prevention measures.

The number of treated recreational water–associated outbreaks caused by Cryptosporidium drives the summer seasonal peak in both waterborne cryptosporidiosis outbreaks and cryptosporidiosis outbreaks overall. A treated recreational water–associated cryptosporidiosis outbreak can result in hundreds or thousands of cases, because 1) an infected swimmer can excrete 107–108 oocysts in one diarrheal incident in the water;[3] 2) Cryptosporidium oocysts can survive >7 days at CDC-recommended concentrations of >1 ppm free available chlorine;[4] and 3) swimmers might use multiple recreational water venues.

The summer seasonal peak of cryptosporidiosis outbreaks associated with child care is similar to that of treated recreational water–associated outbreaks. Contributing factors include 1) cryptosporidiosis disproportionately affects children aged 1–4 years;[5] 2) young children, who have no or limited toileting skills and who ingest recreational water, often use one or more kiddie/wading pools, water playgrounds, and other treated recreational water venues; and 3) chlorine (or bleach) is the primary barrier to pathogen transmission in child care facilities. Consequently, community-wide cryptosporidiosis outbreaks, in which an outbreak associated with a single treated recreational water venue evolves into one associated with multiple venues and settings (e.g., child care facilities), have been documented.[6] Thus, primary prevention of Cryptosporidium contamination is important. CDC recommends not swimming or attending child care if ill with diarrhea and not swimming for an additional 2 weeks after diarrhea has resolved.†† If a cryptosporidiosis outbreak occurs, substantial decontamination measures are needed, including hyperchlorinating§§ public treated recreational water venues (e.g., at a hotel, apartment complex, or waterpark) and using hydrogen peroxide¶¶ to disinfect surfaces in child care settings to inactivate Cryptosporidiumoocysts.

Cryptosporidium contamination can be unavoidable and widespread in environments where ruminants such as cattle, goats, and sheep live. Cryptosporidiumtransmission from preweaned calves to humans has been well documented, and the spring seasonal peak in outbreaks associated with contact with cattle coincides with the spring calving season.[7] Bovine calves can shed >1010 oocysts daily.[8] To minimize further contamination and risk for infection, CDC recommends hand washing*** after coming in direct or indirect contact with ruminants or their living environments. Additional preventive measures include, but are not limited to, removing clothing and shoes worn in the animals' living environment before entering other environments (e.g., a home) to reduce risk for cross-contamination.

Cryptosporidium caused 13 outbreaks associated with unpasteurized milk or apple cider during 2009–2017. Outbreak sources might include contaminated udders, apples, or processing equipment. CDC recommends consumption of pasteurized milk and apple cider because of the risk for infection from unpasteurized products in general and the risk for severe illness in young children, pregnant women, and immunocompromised persons.†††

The findings in this report are subject to at least five limitations. First, the outbreaks described in this report likely underestimate the actual number of cryptosporidiosis outbreaks, and the reported number of cases likely underestimate the actual magnitude of individual outbreaks. Second, the advent of multipathogen molecular testing panels, which include Cryptosporidium, could have contributed to the increase in reported outbreaks in recent years. Third, requirements and capacity to detect, investigate, and report outbreaks vary across jurisdictions. Thus, it is unclear if approximately half of the outbreaks actually occurred in the Great Lakes states; further investigation is warranted. Fourth, only two outbreaks were determined to be the result of transmission by environmental contamination; this might be because of difficulties inherent to implicating fomites as an outbreak source. Finally, only 67 NORS outbreak reports included molecular characterization data, precluding analysis of mode of transmission by Cryptosporidium species and genotypes.

Reversing the increasing trends in annual numbers of reported cryptosporidiosis outbreaks overall and those associated with treated recreational water, contact with cattle, or contact with infected persons in child care settings will require implementing effective prevention measures.§§§ Approximately 40 Cryptosporidium species have been identified to date, of which 17 species and four additional genotypes have been reported to infect humans.[9] Most Cryptosporidium species and genotypes cannot be distinguished by traditional diagnostic tests (microscopy or immunoassays). Therefore, advancing molecular characterization methods, such as those used by CryptoNet, the first U.S. molecularly based surveillance system for a parasitic disease, might help optimize efforts to prevent cryptosporidiosis. Given that individual species, genotypes, and subtypes can have unique host ranges, molecular characterization can provide insight into outbreak exposures and sources. CryptoNet has already demonstrated its ability to elucidate Cryptosporidium transmission chains when used in investigations of treated recreational water–associated outbreaks[10] and has the potential to do the same for investigations of cryptosporidiosis outbreaks not associated with treated recreational water.

§§ At water pH ≤7.5 and temperature ≥77°F (25°C), 2- to 3-log10 (99%–99.9%) Cryptosporidium inactivation can be achieved by raising the free available chlorine concentration for a prolonged period ( CDC does not recommend hyperchlorinating residential venues (e.g., those in a backyard).
¶¶ Do not mix hydrogen peroxide and bleach solutions; the two can react violently. If the health department instructs a child care facility to disinfect surfaces with both a bleach solution and hydrogen peroxide, facility personnel should first clean the surface and then disinfect it with the bleach solution, thoroughly rinse it with water, soak it with hydrogen peroxide for 20 minutes, and thoroughly rinse it with water.
*** Alcohol-based hand sanitizers do not effectively inactivate Cryptosporidium. Furthermore, hand sanitizers might not effectively inactivate pathogens, in general, when hands are visibly dirty or greasy.