PREVALENCE OF YERSINIA ENTEROCOLITICA ACROSS PHASES OF SWINE PRODUCTION

The objective of this study was to describe the prevalence of Yersinia enterocolitica (YE) in different swine production phases. In this cross-sectional study, individual pigs on eight farrowto-finish farms were sampled for YE by collection of both feces and oral-pharyngeal swabs. Samples were cultured with a three-week cold enrichment followed by culture onto selective media. Presumptive YE isolates were confirmed as YE and assayed for the presence of the ail gene using a multiplex PCR. A pig was considered positive if either the fecal sample or oral-pharyngeal swab tested positive. Of the 2321 pigs sampled, 141(6.1%) were YE positive and of those,109 were ail positive (77.3 % of YE isolates). There was a consistent trend of increasing prevalence with maturity. These results represent the first on-farm description of YE in US herds providing first step for future studies to understand the epidemiology of YE in US market swine. Introduction Yersinia enterocolitica is a food borne pathogen which causes an estimated 96,000 human illnesses in the United States each year (Mead et al., 1999). While the bacterium has been found in a variety of food and environmental samples, swine are the only food animals that regularly harbor pathogenic Y. enterocolitica (Kapperud, 1991). Thus swine and pork products have been implicated as the primary reservoir of pathogenic Y. enterocolitica. Recent outbreaks in the US have been related to the preparation and consumption of chitterlings (pig intestines). (Anonymous, 1990; Anonymous, 2003) Of particular food safety concern is the ability for Y. enterocolitica to grow at refrigeration temperatures and survive repeated freezing and thawing.(Toora et al., 1992) Previous studies of Y. enterocolitica have investigated swine at harvest (Hanna et al., 1980; Harmon et al., 1984) but little research has focused on the epidemiology of Y. enterocolitica at the farm level. Understanding the on-farm epidemiology of Y. enterocolitica is the first step towards identifying risk factors and potential interventions in swine production that may decrease the risk of product contamination during harvest and processing. The objective of this study was to describe the prevalence of Y. enterocolitica in different production phases on swine farms. Methods A cross-sectional study to survey individual pigs for the presence of Y. enterocolitica was undertaken on eight farrow-to-finish swine operations in Ohio. On each farm, during a onetime visit, gestating sows (G), farrowing sows (S), their suckling piglets (P), nursery pigs (N), and finishing pigs (F) were cultured for the presence of Y. enterocolitica. When possible, the youngest (1) and oldest animals (2) within each phase were sampled. The number of pigs sampled in each production phase was calculated in order to estimate prevalence with 95% confidence and a 610% confidence interval based on previous estimates in the literature of 25% prevalence in swine at harvest. This sampling scheme resulted in the sampling of 2321 pigs from May to August 2003. Fecal samples (10g) and oral-pharyngeal swabs were collected and tested for each animal with the exceptions of 1) sows in the farrowing room where oral-pharyngeal swabs were not collected and 2) in the cases where 10g of feces could not be collected (i.e. piglets under 10 weeks of age) rectal swabs were used in place of a fecal sample. Animals were considered positive if either the fecal or oral-pharyngeal sample tested positive for Y. enterocolitica. Samples were cultured according to the gold standard method of three-week cold enrichment in phosphate buffered saline (PBS) (Aleksic et al., 1999). Briefly, swabs were placed into 10mL of PBS and fecal samples were diluted at a 1:10 ratio with PBS (EMD Chemicals Inc.). Swab and fecal samples were incubated for 3 weeks at 4°C. Following enrichment, 10μL of the inoculated PBS was streaked onto Yersinia Selective Agar (CIN) plates (Becton Dickinson and Company). Plates were incubated at 25°C for 48 hours before being examined for colonies resembling Y. enterocolitica. Colonies having morphologies typical of Y. enterocolitica were tested on Kligler’s Iron Agar (KIA) (Becton Dickinson and Company) slants and urease broth (Becton Dickinson and Company). Colonies that produced an alkaline slant with an acid butt on the KIA within 24 hours and an alkaline urease broth reaction within 48 hours were classified as presumptive Y. enterocolitica. Presumptive Y.