Cryptosporidium is a parasite that causes cryptosporidiosis, a profuse; watery diarrhea that can last up to 3 weeks in immunocompetent patients and can lead to life-threatening malnutrition and wasting in immunocompromised patients.* Fecal-oral transmission can occur by ingestion of contaminated recreational water; drinking water, or food, or through contact with infected persons or animals. Prevention and control measures can be optimize by improving understanding of Cryptosporidium transmission; through regular analysis of systematically collected epidemiologic and molecular characterization data.

A cryptosporidiosis outbreak was define as two or more cases epidemiologically link; to a common source by location and time of exposure. Public health officials in the 50 states, the District of Columbia, U.S. territories,§ and Freely Associated States voluntarily report outbreaks to CDC via the National Outbreak Reporting System (NORS).

NORS outbreak reports include data on etiology; counts of primary cases, hospitalizations; and deaths; transmission mode; exposures and settings; molecular characterization; and earliest illness onset date. Negative binomial regression analyses were conducted to assess trends in annual outbreak counts using SAS (version 9.4; SAS Institute).

Public health importance of Cryptosporidiosis

However, 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. But 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. But these factors should be considered in the development of effective cryptosporidiosis prevention measures.

However, 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.

Outbreak sources might include contaminated udders

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.