Case-Control Studies of Sporadic Enteric Infections: A Review and Discussion of Studies Conducted Internationally from 1990 to 2009

Abstract

Epidemiologists have used case-control studies to investigate enteric disease outbreaks for many decades. Increasingly, case-control studies are also used to investigate risk factors for sporadic (not outbreak-associated) disease. While the same basic approach is used, there are important differences between outbreak and sporadic disease settings that need to be considered in the design and implementation of the case-control study for sporadic disease. Through the International Collaboration on Enteric Disease "Burden of Illness" Studies (the International Collaboration), we reviewed 79 case-control studies of sporadic enteric infections caused by nine pathogens that were conducted in 22 countries and published from 1990 through to 2009. We highlight important methodological and study design issues (including case definition, control selection, and exposure assessment) and discuss how approaches to the study of sporadic enteric disease have changed over the last 20 years (e.g., making use of more sensitive case definitions, databases of controls, and computer-assisted interviewing). As our understanding of sporadic enteric infections grows, methods and topics for case-control studies are expected to continue to evolve; for example, advances in understanding of the role of immunity can be used to improve

Attribution of human infections with Shiga toxin‐producing Escherichia coli (STEC) to livestock sources and identification of source‐specific risk factors, The Netherlands (2010–2014)

“…Many case–control studies use similar sources of controls, often relying on random or sequential digit dialling from telephone directories in addition to general population registries. However, it is well recognized that acquiring cases and controls from different sources may lead to some selection bias, as even when controls are selected from the same catchment area of the surveillance system, there is no guarantee that, had the controls become ill, they would have sought medical attention and been reported as were the cases (Fullerton et al., ). Our study was based on countrywide laboratory surveillance data for STEC, and controls came from the same population and geographical area (i.e., catchment area of the surveillance system) as the cases, so we can assume that they had the same chance to be recognized as cases if they had developed the disease.…”

Risk factors for sporadic listeriosis in the Netherlands, 2008 to 2013

“…This could be due to similar food advice given to both cases and controls; however, we had not asked about received food advice. Overall, identification of high-risk foods in a case-control study with sporadic cases can be difficult due to the ubiquity of the microorganism in the environment and fluctuating rates of contamination of food products, but also because some risky food products are frequently consumed in the control population, and (13) 347 (20) the varying incubation period [7,10,13,14]. Besides host and environmental components, virulence of the pathogen is another important risk factor [7].…”

“…However, other often mentioned explanations include bias through differential recall of exposure between cases and controls or association of the food product with other unmeasured factors, such as the way and how long a product is stored at home. Furthermore, controls could possibly be less susceptible because underlying diseases are less frequent or less severe, or acquired (partial) immunity following frequent exposure to the high-risk food products [10]. Although it is assumed that immunity to L. monocytogenes in humans is long-lived, it has only been shown in mice [17][18][19].…”

“…Case-control studies in sporadic cases are commonly complex and resource-intensive [10]. In the present study, almost all cases in the case-control study were derived from the notification system as listeriosis has been a mandatory disease since December 2008.…”

“…In the Netherlands, no outbreak of listeriosis has been identified yet. Case-control studies of sporadic cases are useful for identifying risk factors, although it is more difficult to detect sources, as the cases do not necessarily share a single common source or food product [10].…”