Assessment of residual Salmonella contamination in German broiler farms and their surroundings after cleaning and disinfection

Abstract The presence of Salmonella on broiler farms after cleaning and disinfection (C&D) poses a significant risk to public health as Salmonella remains a leading cause of human foodborne infections. In this study, retrospective investigations were conducted to evaluate the persistence of non-typhoidal Salmonella after C&D. Samples were collected from 21 German broiler farms which had a recent history of Salmonella contamination. Samples were collected after the C&D process over a period of seven years. A total of 1,200 swab samples from different locations inside and outside the poultry houses were tested for the presence of Salmonella using real-time polymerase chain reaction, followed by culture confirmation and biochemical serovar identification of positive samples. Sampling locations were classified into four Risk Groups based on their spatial proximity to the broilers and the associated likelihood of direct or indirect Salmonella transmission. Salmonella was detected in 27.6% of swab samples ( n = 331), with S. Infantis ( n = 162; 48.9%) and S. Java ( n = 52; 15.7%) being the predominant serovars. A significant difference was observed between positive internal samples (25.1%) and positive external samples (34.5%). The highest percentage of positive samples was found in Risk Group 4, which involved areas outside of farm buildings with minimal human contact. Additionally, Salmonella detection was significantly associated with season, particularly autumn. The findings highlight that both internal and external environments of broiler houses may act as infection sources for newly placed flocks. Strict implementation of C&D procedures is essential to prevent transmission, especially for persistent serovars such as S . Infantis or S . Java. Although categorizing sample locations into Risk Groups can be a useful tool to identify critical control points and weaknesses, it needs to be critically evaluated. To reduce the risk of Salmonell a on broiler farms, further research on transmission pathways, evaluation of disinfection protocols, and continuous staff training are recommended.