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Group occupational risks for Salmonella prevalence associated with swine production facilities

Leslie Hoobler*1, H. Morgan Scott1, R.B. Harvey2


1Department of Veterinary Integrative Biociences, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, Texas, USA; Food and Feed Safety Research Unit, USDA-ARS, College Station, Texas, USA; 2Food and Feed Safety Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Southern Plains Agricultural Research Center, College Station, Texas, USA

Structured Abstract

Objective - Pathogen and disease surveillance programs are important tools for public health.  Ongoing wastewater surveillance is one method which has been used to monitor aggregated at-risk populations.  In this study, we assessed whether the prevalence of Salmonella detected in human wastewater samples over a one-year period increased with occupational exposure to swine.  Since swine are known to be reservoirs of Salmonella, we hypothesized that there would be an increase in Salmonella isolates derived from wastewater draining multiple human populations known to have direct contact with swine, as compared to those with no contact.

Animals or Sample Population - From 2004-2007, monthly human wastewater sampling and swine fecal sampling were conducted in a systematic manner in 13 geographically distinct locations of a multi-site population in Texas.  Two different groups existed in the human population: 1) swine-worker group cohorts that had direct contact with the swine population, 2) the other group cohorts that had no direct occupational contact with swine.

Procedure - Monthly swine samples collected between the dates of February 2006 to January 2007 were cultured for detection and isolation of Salmonella spp..  A subset of samples from human populations located in: 1) units with a high prevalence of Salmonella among swine fecal samples in their farrowing units, and 2) units exhibiting a low prevalence of Salmonella were then selected, isolated for Salmonella, and then compared.  Thus, the farm-level swine Salmonella prevalence served as the potential risk factor for the human cohorts.  Grower and finisher units and associated human populations were likewise classified and selected in this manner.  Human wastewater samples collected from February 2006 to January 2007 within the chosen populations were cultured for detection and isolation of Salmonella.  Odds ratios (both adjusted and unadjusted) were calculated to determine if there was a significant (P < 0.05) association between elevated Salmonella infection status of the swine herds and the swine-workers, relative to non-workers at the same location, and swine workers at other locations with low swine prevalence of Salmonella.  Serogrouping was performed on all human isolates, but only on Salmonella isolates from those swine isolates that were concurrent with positive human samples, in order to determine if both populations were infected with similar serogroups. 

Results - Salmonella spp. were isolated from 24 human wastewater samples, indicating an overall 5.3% prevalence of positive samples across both occupational cohorts.  Prevalence did not differ by occupational exposure (P = 0.775), however there were differences by month and unit.  Salmonella were isolated from 18.9% of the selected swine samples (n = 185 fecal samples).  There were great differences in prevalence across swine production groups and locations.  However, the latter (location) had no effect on the risk of Salmonella in associated human populations.  Serogrouping indicated very few matches of human isolates with concurrent swine isolates.

Conclusions and Clinical Relevance - There appeared to be no association between Salmonella infection status of a swine herd and the risk of Salmonella shedding in associated human occupational group cohorts.  These results suggest that foodborne disease remains the greatest risk to human populations, rather than direct occupational exposure to swine.  Serogroup results showed little evidence for swine involvement with those outbreaks that did occur in the human population during our study.

Impact for Human Medicine - The results of this study will contribute to a better understanding of the potential utility of ongoing wastewater monitoring as a public health tool and point to little evidence for group risk of Salmonella infection among those working directly with swine.