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The long term goal of this longitudinal study is to determine the molecular epidemiology of multidrug resistant Salmonella in swine reared in the conventional and the antimicrobial free (ABF) productions systems. Our preliminary data clearly shows the presence of multidrug resistant (MDR) Salmonella in both the production systems. This proposal is relevant to the important priorities listed under the epidemiological approaches to food safety program and focuses on studies that seek to identify new risk factors that affect the prevalence, transmission and persistence of food borne organisms in the farm-to-fork continuum. Defining the role of these risk factors in the dissemination and persistence of antimicrobial resistant Salmonella and establish critical control points is important which will aid us in implementing intervention strategies to improve food safety.<P> With that information, we plan to achieve our long term goal of reducing the burden of pork associated salmonellosis by pursuing these objectives: <OL> <LI> Determine the sources, persistence and clonality of Salmonella in the conventional and ABF production systems at farm <LI>Compare and characterize the distribution of multi-drug resistant Salmonella between and within the different stages at farm in the two systems <LI>Identify farm and management level risk factors that are associated Salmonella dissemination in these two swine production systems </OL>This will be the first longitudinal study to provide valuable information to the scientific community and the stakeholders on the status of Salmonella and the risk factors involved in its dissemination in different swine production systems. The proposal is multidisciplinary and we have assembled an expert team from a wide range of fields including epidemiology, microbiology and biosciences to help us achieve our goal. The results will provide important information regarding dissemination of Salmonella in the ABF system which is currently not well defined in the United States. The data generated in this study will help to design and develop intervention methods to secure our food supply.
NON-TECHNICAL SUMMARY: Foodborne outbreaks due to antimicrobial resistant Salmonella strains have been reported due to consumption of pork products. The role of unidentified selective pressures that result in persistence of antimicrobial resistant strains in different swine production environments remains to be investigated. The available information on Salmonella dynamics in the conventional system does not apply to the antimicrobial free (ABF) system due to differences in them. The need to conduct a longitudinal based study to determine and compare the risk factors that are responsible for the persistence of Salmonella in these swine production environments is crucial. The long term goal of this longitudinal study is to determine and compare the molecular epidemiology and dynamics of Salmonella in the conventional and the ABF systems. Swine sampling will be done at three stages on farm (farrowing, nursery and finishing). Salmonella isolates will be characterized at the phenotypic and genotypic levels. Pertinent management data will be collected and appropriate epidemiological and statistical analysis will be performed to determine risk factors for Salmonella dissemination in pigs at farm and in these two distinct production systems. Information generated in this study will help us to determine the establishment of Salmonella infection and its dynamics within a herd and to better implement control measures. The proposal is relevant to priorities listed under the Epidemiological Approaches to Food Safety (32.0) with focus on studies to identify new risk factors that affect the prevalence, transmission and persistence of food borne organisms across the farm-to-fork continuum. <P>
APPROACH: Sampling will be done over a period of two years and every conventional farm sampled will be followed by sampling of an ABF farm. A total of 20 farms will be included in the study including 10 farms each from the conventional and the ABF farms in North Carolina. We will follow a cohort of pigs (n=30) all the way from the farrowing to finishing farms. At the farrowing unit, we will sample the sows (n=10) and their piglets (n=30; 3 piglets/sow). The same cohort of pigs will be sampled twice at the nursery farm, at 6 and 10 weeks of age. At the finishing level, pigs will be sampled twice at 16 and 26 weeks of age. At every farm visit, approximately 10 grams of fresh fecal samples will collected per pig with gloved hands directly from the rectum. Samples will be transported to the laboratory on ice and processed for Salmonella on the same day upon arrival. Confirmed Salmonella isolates will be stored at -80oC till further characterization. Salmonella isolates will be serotyped and antimicrobial susceptibility testing to determine the minimum inhibitory concentration (MIC) will be done by using the broth microdilution method. Genotyping will be done using Pulsed Field Gel Electrophoresis (PFGE) and analysis will be done using the Bionumerics software. We will use polymerase chain reaction (PCR) and sequencing for molecular characterizing the antimicrobial resistance determinants. Pertinent management, facility and herd information that could possibly play a role in emergence, dissemination and persistence of antimicrobial resistant Salmonella in pigs will be collected at farm. We have considered 2 outcome variables for the study that includes a) Presence of Salmonella in the fecal samples based on microbiological culture in the two production systems at farm. This will be recorded as a dichotomous variable (absent=0 and present=1) and, b) The occurrence of antimicrobial resistant Salmonella; exhibiting resistance to a single antimicrobial or multiple antimicrobials (resistance to greater than three or more antimicrobials) in the two production systems at farm. This will be also recorded as a dichotomous variable (absent=0 and present=1). All the data analyses will be run on SAS v.9.2 (SAS Institute, Inc, 2002). A combination of different modeling techniques and statistical models will be used including multivariate analysis techniques, logistic regression and cluster analysis will be used to provide a more thorough investigation of the associations described above. In addition to between production system comparisons, comparisons within a production system will be made to investigate potential risk factors that affect the occurrence of Salmonella. Trend analysis will be undertaken to ascertain the effect of time on the proportion of Salmonella within the cohorts. Logistic regression modeling will be performed using PROC LOGISTIC, repeated measures logistic analysis will be done using PROC NLMIXED and PROC GENMOD and GEE (generalized structural equations) will be used to account for the dependency structure of the observations registered along time.