Purpose: The objective of this study is to determine the transmission of Salmonella due to swine manure application in the environment.
Methods: At the different time points of application: day 0, 7, 14, and 21, the soil and lagoon samples were collected representing swine farms in North Carolina (n = 3) and Iowa (n = 7). A total of 1,000 soil samples (NC = 300; IA = 700) and 100 lagoon samples (NC = 30; IA = 70) were included in this study. Antimicrobial susceptibility (AST) was characterized using Sensititre® with a panel of 15 antimicrobial drugs. PCR was performed to identify the resistant determining genes. Genotypic characterization was done using pulse field gel electrophoresis (PFGE).
Results: Overall Salmonella prevalence was 11.73% (129/1100). The prevalence in soil and lagoon were 9.8% (98/1000) and 31% (31/100), respectively. Decrease in prevalence of Salmonella in the area from Day 0 to Day 21 was observed over time. We identified 8 serotypes including Anatum (17.14%) and Litchffield (5.71%) in IA, while Altona (20%), Johannesburg (1.43%) Muenster (22.86%), Worthington (14.29%), Mbandaka (7.14%), and Uganda (14.29%) in NC. Sixty-eight percent of Salmonella isolates were MDR with the most frequent antibiotic resistance against Streptomycin (66.7%), sulfisoxazole (56.5%), and tetracycline (52.2%). Streptomycin resistance was encoded in 39.3% of the isolates by the addA2 gene, while sul1 gene (22.6%) was detected in sulfisoxazole resistance. TetA (17.24%) and TetB (27.6%) were also observed. According to PFGE pattern, we detected clonal relatedness among Salmonella recovered from lagoon and soil at multiple time points with relatively close geographic proximity.
Significance: Our study highlights the potential role of manure application on Salmonella persistence and transmission in the environment.