PSVI-29 Do lungs serve as a portal of entry and a source for dissemination of Salmonella enterica into the liver and lymph nodes?

Abstract Salmonella enterica, an enteric pathogen that is shed in the feces, can get aerosolized in the beef cattle production environment and inhaled by cattle. We hypothesized that lungs may serve as a portal of entry, harbor Salmonella, and be a source for dissemination into the liver and lymph nodes. Our objective was to determine Salmonella prevalence in lungs of feedlot cattle collected at slaughter, and to test for associations with prevalence in livers and internal lymph nodes (LN). A total of 275 cattle with healthy (n=156) or abscessed (n=119) livers were sampled at slaughter to collect tissues of lungs, healthy livers, abscessed livers, mesenteric, subiliac, and mediastinal lymph nodes (LN), and colonic contents. Homogenized samples, before and after enrichment, were subjected to culture and qPCR methods targeting S. enterica to determine prevalence and concentration. Species-confirmed isolates were serotyped. Data were analyzed using generalized linear model with a Tukey adjustment for pairwise comparisons with detection method as a fixed effect. For analysis of prevalence when lung was positive for Salmonella, a two-way interaction between Salmonella presence and sample type was used with a generalized linear model. Pairwise comparisons were made using Tukey adjustment. Of the 275 cattle, 3 (1.1%) were positive and 11 (41.3%) were negative for Salmonella in all six samples, and 160 (59.7%) were positive in at least one sample. The prevalence of Salmonella in samples were not different between the two detection methods. The total prevalence, by culture or qPCR method, was highest in the colonic contents (39.7%), followed by lungs (21.7%), abscessed livers (21.7%), subiliac LN (21.5%) and healthy livers (21.0%). Among the LN, the subiliac and mesenteric LN had higher prevalence (P < 0.01) than mediastinal LN. Samples from 32 cattle (11.6%) had quantifiable concentrations (> log 3 CFU/g), and 13 were in colonic contents, 10 in mesenteric LN, 6 in subiliac LN, 2 in mediastinal LN and 1 in lungs, with mean concentrations between log 5 to 7 CFU per g. In cattle that harbored Salmonella in lungs (n=60; 21.7%), prevalence in LN and abscessed livers, but not in healthy livers, were higher (P < 0.01) than cattle that were Salmonella-negative (n=215; 78.3%). A total of 307 isolates were serotyped and 18 different serotypes were identified. The 4 dominant serotypes were Montevideo (25.4%), Cerro (16.3%), Anatum (14.7%) and Muenchen (13.4). Lubbock, reported to be the dominant serotype in liver abscesses, was detected only in the lung, liver and subiliac LN. Our study provides evidence that lungs harbor Salmonella and prevalence was next highest to the gut prevalence, and the positive associations between prevalence in lungs and liver and LN suggest that lungs could be a source for dissemination into the liver and lymph nodes.