EFFECT OF CLEANING AND SUBTHERAPEUTIC CHLORTETRACYCLINE ON SALMONELLA

The objective of this study was to evaluate the effect of stringent cleaning and subtherapeutic chlortetracycline (CTC) on Salmonella enterica (SE) prevalence in market age swine. Twenty-four finisher barns from one farm were enrolled in a 2X2 factorial study design. Treatments included 1) assignment to a “stringent” or standard cleaning protocol and 2) inclusion of 50g of CTC/ton of feed or no feed antimicrobials. Barn swab samples were obtained preand post-cleaning for SE detection. Post-cleaning aerobic plate counts (APC) were collected. Feces were collected to determine SE prevalence pre-market. The stringent cleaning protocol resulted in decreased mean APC post-cleaning, but there was no difference in isolation of SE from the barn environment between cleaning protocols. Pigs housed in barns that received CTC had decreased odds of being SE positive, but there was no difference in the odds of a pig being SE positive between cleaning protocols. Introduction Contamination of the barn environment with Salmonella enterica (SE) has been suggested to be an important risk factor for SE infection in swine. All-in, all-out production practices with cleaning and disinfection between groups is often utilized to aid in the control of production impairing illnesses in swine. However, the effectiveness of these practices on the prevalence of SE is not clear. Relatively high SE prevalence has been reported on US farms that utilize all-in, allout pig flow with cleaning and disinfection between groups.(Davies et al., 1998; Davies et al., 1997; Funk et al., 2001; Funk and Gebreyes, 2004) Subtherapeutic antimicrobial use, in particular for the purpose of growth promotion, is under increasing scrutiny regarding its contribution to antimicrobial resistance in human pathogens. It is currently unclear what the effect of subtherapeutic antimicrobial use has on the prevalence of SE. Few on-farm clinical trials evaluating the effect of subtherapeutic antimicrobials on SE prevalence have been conducted. Previous research regarding SE shedding and antimicrobial resistance subsequent to antimicrobial therapy have been conducted in laboratory facilities involving experimental infection with S. enterica (Abou-Youssef et al., 1979; Baggesen et al., 1999; Dealy and Moeller, 1976; Delsol et al., 2003; Ebner and Mathew, 2000; Evangelisti et al., 1975; Girard et al., 1976; Gutzmann et al., 1976; Jacks et al., 1988; Jones et al., 1983; Mathew et al., 2002; Wilcock and Olander, 1978; Williams et al., 1978). The goal of this study is to determine the effect of stringent cleaning and the use of subtherapeutic CTC on the prevalence of SE in market ready swine. Materials and Methods One production company with 24 finisher barns at six different production sites (4 barns per site) was selected. Entry criteria was willingness to participate and proximity to the laboratory as substantial investment in travel and labor were required. The study was a 2X2 factorial design. Treatments included 1) assignment to a “stringent” or standard cleaning protocol and 2) inclusion of 50g of CTC/ton of feed or no feed antimicrobials. Prior to the cleaning of any barns 30 environmental swabs were taken to get a baseline of SE contamination in the environment. These samples were taken using a 4x4 swab that was moistened in 30ml of buffered peptone water (BPW). The samples originated from surfaces throughout the barn including; gates, feeders, waters, overhead pipes and air vents, fans, the floor and the entry door. Samples were transported to the lab on ice and cultured for SE using standard methods. Briefly, 100ml of BPW was added to each sample and incubated at 37°C for 18-24 hours. Following incubation 100μl was transferred to 9.9ml of Rappaport-Vassiliadis Broth (RV) and incubated at 42°C for 18-24 hours. Finally samples were struck onto XLT-4 agar and compared to control strains. The suspected positive samples were confirmed using Triple Sugar Iron agar slants and urea broth incubated at 37°C for 18-24 hours. Following the environmental sampling barns were then cleaned according to their randomly assigned protocol. The standard protocol, which was conducted by the farm personnel, included a cold water soak, followed by cold water pressure wash (2500 psi), and ended with the application of Virkon-S applied using the pressure washer. The farm personnel estimated that the standard protocol required a single person 12-13 hours to complete. The stringent protocol, conducted by laboratory personnel, started with the same cold water soak, which was then followed with a hot water (180-190°F) high pressure wash (4000 psi). Barn equipment such as gates, feeders, and waters were hand scrubbed during this same time period