Abstract
Dear Editor:
Fullerton et al. (2012) authoritatively reviewed methods for case-control studies of sporadic disease to identify risk factors at a population level. However, the cost, complexity, and duration of these studies limit their intended usefulness to inform public health action. Foodborne diseases result from dynamic interactions between agents, hosts, and environment. These may vary across the geographic catchment area and time frame over which the study is conducted. For example, 85% of cases enrolled in the 1996 FoodNet case-control study of Escherichia coli O157:H7 were from two sites (Kassenborg et al., 2004). When the prevalence of risk factors varies across the population, attributable risk estimates may not be generalized to the whole population. Similar concerns exist for studies conducted over several years. Large, complex studies are inherently limited in their ability to evaluate geographic and temporal shifts in risk factor distributions that have important public health implications.
These changes are much more readily captured by smaller, more focused outbreak investigations. For example, eating melons at a commercial establishment was identified as a significant risk factor for listeriosis in the 2000–2003 FoodNet case control study (Varma et al., 2007). The finding suggested a role for retail establishments in contamination or amplification of Listeria monocytogenes. However, the findings did not alert cantaloupe producers or consumers about the risks from contamination and distribution of whole cantaloupes that led to the 2011 outbreak involving 146 cases across 28 states (CDC, 2012).
Outbreaks result from amplifying events that may select a subset of sporadic case risk factors. However, sporadic cases may also represent outbreaks that go undetected. By increasing the sensitivity of outbreak detection and investigation, a higher percentage of apparently sporadic cases may be linked to smaller outbreaks (Rounds et al., 2010). Understanding the sources of these outbreaks may go a long way towards reducing uncertainty regarding the source of sporadic infections.