Accuracy and Timeliness Of The Cryptosporidiosis Surveillance System Used By Utah Department Of Health

BACKGROUND:   Cryptosporidosis is a highly infectious illness caused by chlorine-tolerant protozoa of the genus Cryptosporidium. After experiencing the largest recreational water associated outbreak of cryptosporidiosis in United States history in 2007, Utah Department of Health (UDOH) began publishing a weekly surveillance report on its website. All confirmed cases of cryptosporidiosis in Utah are investigated to identify potential infection sources, with particular focus on recreational water exposure. UDOH is concerned with the ability of its surveillance system to accurately detect unexpected increases in cryptosporidiosis and to identify common recreational water exposures, enabling public health intervention in a timely manner. Therefore, we aimed to assess the accuracy and timeliness of the current system.

METHODS:   The UDOH surveillance report includes weekly cumulative year-to-date confirmed cryptosporidiosis cases and the percentage of case patients reporting recreational water exposure. The report is completed while case investigations are pending and case statuses may not be finalized. To assess surveillance report accuracy, using the same data source, we generated a second report, the PCSA (Post-Cryptosporidiosis-Season Analysis), after all investigations from 1/1/2012-11/3/2012 were closed and case statuses were irrevocable. Differences between weekly case counts from the UDOH report and the PCSA were tested using the Wilcoxon Signed Rank test. We also calculated the overall mean weekly difference between the two reports and the average duration between date reported to public health and date case confirmation was finalized.

RESULTS:   The total number of cases reported for the time period 1/1/2012-11/3/2012 was similar for the two reports (UDOH report: 168; PCSA: 165). However, the UDOH weekly surveillance report case counts were consistently lower than the PCSA case counts (p<0.01). The mean (standard deviation) weekly difference between the PCSA and the UDOH report was 20.5 (15.1) cases (median=17.5). The mean time from date reported to public health to case confirmation date was 27.9 (42.5) days (median=14 days).

CONCLUSIONS:   The PCSA was generated after case statuses were finalized and is therefore the most accurate account of confirmed cryptosporidiosis cases. However, it does not provide real-time data. The UDOH surveillance report is timelier, but consistently underestimates weekly cumulative case counts. The lower surveillance report case counts suggest that case confirmation timing influences reported confirmed cases. Updating case confirmations weekly, immediately preceding report generation, would improve data accuracy. Improved accuracy of the already timely surveillance reports will strengthen UDOH’s ability to rapidly detect unexpected increases in cryptosporidiosis and identify common recreational water source exposures.