Evaluation of Cryptosporidiosis Case Status Using a Revised National Case Definition

BACKGROUND: Cryptosporidiosis is a diarrheal disease caused by Cryptosporidium, a microscopic parasite. It is the leading cause of waterborne disease outbreaks in the United States. Historically, a diagnosis of cryptosporidiosis was made by examination of stool by microscopy. Laboratory testing methods have moved away from this gold standard to antigen-based detection methods. Of the many available methods, the rapid cartridge assay (RCA) is preferred, given its ease of use. In 2011, the Council of State and Territorial Epidemiologists (CSTE) revised the national cryptosporidiosis case definition to classify  disease diagnosed using a RCA or those with an unknown testing method as probable, rather than confirmed, cases. Recent data in the literature have raised concerns regarding the positive predictive value (PPV) of RCA tests and their ability to diagnose cryptosporidiosis as a stand-alone test.  Foodborne Diseases Active Surveillance Network (FoodNet) conducts population-based active surveillance for laboratory-confirmed Cryptosporidium infections in 10 U.S. states and classifies all diagnosed cryptosporidiosis cases as confirmed, regardless of testing methodology. We applied the revised national case definition to FoodNet surveillance data to assess the impact that the definition change would have on the number of cryptosporidiosis cases in FoodNet.

METHODS: FoodNet cryptosporidiosis surveillance data from 2012 and 2013 were analyzed using SAS 9.2. The type of test performed by the clinical laboratories was documented as the test type for each case. Cases were then classified according to the national cryptosporidiosis case definition (11-ID-14).

RESULTS: FoodNet sites reported 2475 laboratory-confirmed cases of cryptosporidiosis in 2012-2013. Eighty percent (n=1997) of cases had documented the clinical laboratory test kit used for identification; 33.1% used non-RCA test kits and 66.8% used an RCA test, of which 7.4% were confirmed by microscopy, direct fluorescent antibody (DFA) or direct immunofluorescent antibody (IFA). Twenty percent of cases were missing information on laboratory method.

CONCLUSIONS: Application of the revised national case definition to FoodNet surveillance data would result in only 26.8% of cryptosporidiosis cases counted as confirmed cases. Given the difficulty in obtaining test type information on a national level, a change in definition has the potential to dramatically impact national surveillance rates. When RCA tests are used, confirmatory testing could be considered. Further investigation of factors contributing to decreased PPV of RCA tests is needed