Transmission pathways and risk factors for sporadic salmonellosis and campylobacteriosis: A source attribution meta-analysis of European case-control studies

Case-control studies can provide attribution estimates of the likely sources of zoonotic pathogens. We applied a meta-analytical model within a Bayesian estimation framework to pool population attributable fractions (PAFs) from European case-control studies of sporadic campylobacteriosis and salmonellosis. The input data were obtained from two existing systematic reviews, supplemented with additional literature searches, covering the period 2000-2021. In total, 12 studies on Campylobacter providing data for 180 PAFs referring to 5983 cases and 13213 controls, and five studies on Salmonella providing data for 75 PAFs referring to 2908 cases and 5913 controls, were included. All these studies were conducted in Western or Northern European countries. Both pathogens were estimated as being predominantly linked to food- and waterborne transmission, which explained nearly half of the cases, with Campylobacter being mainly attributable to poultry (meat), and Salmonella to poultry (eggs and meat) and pig (meat), as specific foodborne exposures. When also considering contact with animals, around 60% of cases could be explained by the larger group of zoonotic transmission pathways. While environmental transmission was also sizeable (around 10%), about a quarter of cases could be explained by factors such as travel, underlying diseases/medicine use, person-to-person transmission and occupational exposure.