BACKGROUND: Cryptosporidium is the most common cause of waterborne outbreaks and the second most common cause of waterborne disease in Indiana. The national surveillance case definition for cryptosporidiosis includes confirmed and probable case classifications and was revised by the Council of State and Territorial Epidemiologists in 2009, 2011 and 2012. While the 2009 definition allowed a variety of laboratory methods as diagnostic criteria for a confirmed case, the 2011 revision specified only two, and the 2012 revision added two more. We determined the impact of recent case definition revisions and incomplete laboratory reporting on case classification of cryptosporidiosis and identified areas for intervention.
METHODS: Frequency analyses were performed for confirmed and probable cryptosporidiosis cases reported in Indiana from 2009–2014. The completeness of laboratory data was ascertained for probable cases under the current case definition (2012–2014). A survey collecting information on Cryptosporidium diagnostic testing methods was administered to 71 Indiana microbiology laboratories, and selected out-of-state reference laboratories.
RESULTS: From 2009–2014, 1,318 cryptosporidiosis cases were reported in Indiana. Of these, 973 (74%) were confirmed and 345 (26%) were probable. The proportion of probable cases varied by case definition used from <1% (2009 case definition) to 70% (2011) and 32% (2012). From 2012–2014, 118 (75%) probable cases were classified as such because information on laboratory testing methods was not provided. Of these, 112 (95%) had been tested at Indiana laboratories. Thirty-nine of the 71 Indiana microbiology laboratories reported that they provide Cryptosporidium testing. Of these, only 13 (33%) conduct testing considered confirmatory based on the 2012 case definition.
CONCLUSIONS: In the 2011 case definition, the increased specificity of the cryptosporidiosis diagnostic testing criteria for a confirmed case, caused an increase in the proportion of probable reported cases. The proportion of probable cases decreased based on the 2012 revision, but a substantial number of cases were still classified as probable due to a lack of confirmatory testing and a failure to specify the diagnostic testing performed by Indiana laboratories. The Indiana State Department of Health will work with Indiana laboratories to improve diagnostic capacity and reporting to better understand the cryptosporidiosis disease burden in Indiana.