Differentiation of Acute and Latent Salmonella Infection in Cattle Using the Glyoxylate Pathway Gene aceA, Flagellar Gene fimA, and an Attempt at Validation by Bacterial Culture

Aims: To test the hypothesis that it is possible to detect latently Salmonella infected cattle using a glyoxylate pathway gene as a qPCR target.

Study Design: Convenience sample of tissue specimens.

Place and Duration of Study: Department of Biomedical Sciences and Pathobiology, Virginia Maryland College of Veterinary Medicine, between April 2015 and July 2017.

Methodology: Supra-mammary lymph nodes from 40 dairy cattle and mesenteric lymph nodes from 100 cattle from a slaughter house were collected, sampled, and bisected. One half of each sample was cultured on Salmonella chromogenic agar, while the other half was tested using qPCR for both an acute infection associated gene (fimA) and a chronic/latent infection associated gene (aceA). The Salmonella culture isolation results were compared with the qPCR results for the above two genes.

Results: All 40 dairy cattle lymph nodes were qPCR negative for both fimA and aceA, and none of the lymph nodes were culture positive. From the 100 cattle mesenteric lymph nodes, 11 were qPCR positive for aceA, none were qPCR positive for fimA, and 7 were culture positive for Salmonella isolation. Of those 11 aceA qPCR positive, only 5 of them were culture positive and six were culture negative.

Conclusion: The qPCR results for mesenteric lymph nodes showed high specificity and predictive value negative. The results between qPCR and bacterial culture in supra-mammary lymph nodes may suggest that the number of Salmonella in the sample cultured may be below detection limit for both the assays. But the results demonstrate the advantage of using gene(s) primers to identify latent Salmonella infections in clinically normal cattle from slaughter house samples. In addition, the assay may be able to differentiate between latent vs active Salmonella infection. The sample size might be the reason for the lack of results in the case of the dairy cattle samples.