Case-Control Study of MRSA Carriage and Infection and Proximity to Swine Farms in Eastern North Carolina
Open AccessBackground:Antibiotic-resistant Staphylococcus aureus, and MRSA in particular, is a rising public health threat. Antibiotic use in animal feed and the growth of CAFOs, or confined animal feeding operations, have been examined as potential contributors to the rise of antibiotic-resistant bacteria. Methods:Three case-control studies and one retrospective cohort study were conducted to examine the association between residence in a swine-dense area and MRSA nasal carriage and MRSA-positive clinical culture (as a proxy for infection) in Eastern North Carolina. Study participants were patients admitted to Vidant Health System (VHS) hospitals, including Vidant Medical Center (VMC). More specifically, case-control study 1 examined MRSA carriage on admission among patients at VMC; case-control study 2 evaluated MRSA carriage on admission among “high-risk” patients at all other VHS hospitals (excluding VMC); and case-control study 3 examined MRSA infection on admission among patients at all of the VHS hospitals. The cohort study followed patients with and without MRSA nasal carriage for the development of infection during hospitalization and also examined the role of residence in a swine-dense area. Propensity score matching was utilized where possible to reduce the potential for confounding.Residential proximity to swine farms was defined using 3 separate exposure metrics: (1) 2 county-based classifications of swine exposure (residence in a county with <250 000 or ≥250 000 swine and residence in a county with <500 000 or ≥500 000 swine); (2) a distance-based classification of swine exposure (residence within 1500 m of a swine farm); and 3) an inverse distance-squared swine exposure metric that takes into account the number of swine farms within 1 mile of the participants’ residence, the distance(s) between the home and the swine farm(s), and swine count on the farm(s). Agreement between exposure metrics was examined in each study using Cohen’s Kappa statistic. Results: In case-control study 1, MRSA nasal carriage was found to be associated with living within 1500 m of a swine farm (aOR 3.10, 95% CI 1.28-7.51) but no association was found with regard to the other exposure metrics. In case-control study 2, a significant negative association was reported between MRSA nasal carriage and residence in a county with 250 000 or more swine among high-risk patients (aOR 0.39, 95% CI 0.15-0.98). No significant associations were found between MRSA carriage and residence within 1500 m of a swine farm or the inverse distance-squared exposure metric. In case-control study 3, no significant associations were found between any of the exposure metrics and MRSA infection on admission. In the retrospective cohort study, MRSA carriage was significantly associated with the development of MRSA infection (HR 9.94, 95% CI 5.00-19.73). However, the hazard ratio decreased dramatically and became non-significant after adjusting for Mupirocin, Linezolid, and contact isolation (aHR 2.36, 95% CI 0.41-13.58). When residence within 1500 m of a swine farm was added to the model, the aHR for MRSA carriage increased (aHR 4.48, 95% CI 1.13-17.84); the aHR corresponding to residence within 1500 m of a swine farm was also significant (aHR 5.10, 95% CI 1.66-15.65). Agreement between exposure metrics was low, and agreement between specific metrics varied greatly among studies. Conclusion: An association between residence in swine-dense areas and MRSA carriage or infection was not evident in this study. The inverse distance-squared swine exposure metric was determined to be the most accurate measure of community swine exposure as it incorporated swine count on farms and distance between the patient’s home and the farms and was only calculated for patients residing within 1 mile of a swine farm. Because agreement was so low, evaluation of community swine exposure in relation to health effects should be assessed using multiple metrics in future studies.
- All rights reserved
Notice to Authors
If you are the author of this work and you have any questions about the information on this page, please use the Contact form to get in touch with us.