The optimisation of Salmonella surveillance programmes for pullet and layer farms using local farm density as a risk factor

Samenvatting

Human salmonellosis cases are often caused by Salmonella serovars Enteritidis and Typhimurium and associated with the consumption of eggs and egg products. Many countries therefore implemented general surveillance programmes on pullet and layer farms. The identification of risk factors for Salmonella infection may be used to improve the performance of these surveillance programmes. The aims of this study were therefore to determine 1) whether local farm density is a risk factor for the infection of pullet and layer farms by Salmonella Enteritidis and Typhimurium and 2) whether the sampling effort of surveillance programmes can be reduced by accounting for this risk factor, while still providing sufficient control of these serovars. We assessed the importance of local farm density as a risk factor by fitting transmission kernels to Israeli surveillance data during the period from June 2017 to April 2019. The analysis shows that the risk of infection by serovars Enteritidis and Typhimurium significantly increased if infected farms were present within a radius of approximately 4 km and 0.3 km, respectively. We subsequently optimized a surveillance programme that subdivided layer farms into low and high risk groups based on the local farm density with and allowed the sampling frequency to vary between these groups. In this design, the pullet farms were always sampled one week prior to pullet distribution. Our analysis shows that the risk-based surveillance programme is able to keep the between-farm R₀ of serovars Enteritidis and Typhimurium below 1 for all pullet and layer farms, while reducing the sampling effort by 32% compared to the currently implemented surveillance programme in Israel. The results of our study therefore indicate that local farm density is an important risk factor for infection of pullet and layer farms by Salmonella Enteritidis and Typhimurium and can be used to improve the performance of surveillance programmes.

FDM, MB and AK were funded in the context of the EU Animal Health and Welfare (ANIHWA) ERA-Net initiative via the Epi-Risk Project (https://www.anihwa.eu/). The ANIHWA funds for AK were provided by the Dutch Ministry of Agriculture, Nature and Food Quality via project number BO-43-013.04-001 (https://www. government.nl/ministries/ministry-of-agriculturenature-and-food-quality). In addition, FDM was co-funded by Istituto Zooprofilattico Sperimentale dell’Abruzzo e del Molise, Teramo, Italy (https:// www.izs.it/IZS/). MB was co-funded by the Kimron Veterinary Institute, Beit Dagan, Israel (https:// www.gov.il/en/departments/Units/2vet). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.