Salmonellosis in laying flocks is one of the major health concerns worldwide and the size of the flock has been introduced as an important risk-factor associated with increased risk of Salmonella. In this study a total of 113 holdings was selected using simple random sampling, stratified by active layer holdings in each province of Iran. Two pooled fecal samples were obtained from each flock of holding and information of sampled holdings was acquired from Geographic Information System (GIS). The prevalence of Salmonella contamination in laying hen holdings was 3.5% and the risk of Salmonella contamination was associated with the size of the poultry-holding (OR=5.6; CI95%=1.35, 23.57; P=0.018). None of the positive farms were in high density surface at farm level. Two of the positive farms were at the density surface of more than 13 flocks per square kilometer and all the positive farms were in the regions with poultry density of more than 319,000 per square kilometer. According statistical and spatial analysis keeping large number of poultries in a certain area is an important risk factor for Salmonella contamination.

Key words: Kernel density, Laying farms, Salmonella infection, Spatial analysis