BACKGROUND: Traditionally, population-based case-control studies have been used to identify risk factors for cryptosporidiosis and giardiasis. However, these studies are expensive and include controls with characteristics that likely differ from reported cases since they are selected from the general population while only a relatively small proportion of cases of cryptosporidiosis and giardiasis are reported to public health authorities. In this analysis, we use a variation of the case-control analysis wherein reported cases of a different disease are used as a comparison group to identify potential risk factors for cryptosporidiosis and giardiasis.
METHODS: Surveillance data from the National Notifiable Disease Surveillance System (NNDSS) were accessed through the NNDSS Data Availability Project (NDAP). Exposures reported for cryptosporidiosis and giardiasis case-patients were compared to exposures of salmonellosis case-patients reported by 16 states (2005 – 2015). Odds ratios adjusted for age and reporting state (aOR) and 95% confidence intervals (95% CI) for exposures were calculated using SAS 9.3.
RESULTS: A total of 14,996 cryptosporidiosis, 22,595 giardiasis, and 123,180 salmonellosis cases were reported. When compared to cases of salmonellosis, exposure to treated recreational water (aOR: 5.3; 95% CI: 5.0-5.6) and livestock (aOR: 2.8; 95% CI: 2.5-3.1) were risk factors for cryptosporidiosis, and exposure to untreated drinking (aOR: 5.8; 95% CI: 5.3-6.4) or recreational water (aOR: 3.4; 95% CI: 3.2-3.7) and previous travel (aOR: 2.5; 95% CI: 2.4-2.7) were risk factors for giardiasis.
CONCLUSIONS: Risk factors for cryptosporidiosis and giardiasis identified through national surveillance data were comparable to those identified in outbreak investigations or population based case-control studies. Case-case analyses can be used to better understand disease epidemiology and more efficiently identify risk factors to guide the development of prevention and control measures.