CategoryPost-Harvest Pork Safety
Date Full Report Received
Date Abstract Report Received
Since the mid-1990’s, Salmonella Typhimurium has been the predominant serotype of Salmonella enterica identified in diagnostic samples as a significant cause of clinical disease in swine. However, over the past 5 years, identification of Salmonella Typhimurium in samples from diseased swine has decreased substantially while identification of the Salmonella I 4,,12:i:- serotype has concurrently increased. The overall goal of this study was to enable the pork industry to better understand the significance of increasing identification of this emerging serotype as well as provide veterinarians and producers with improved diagnostic tests to allow for more rapid identification of I 4,,12:i:- in diagnostic samples. This study addressed this overall goal in four ways: 1) direct comparison of the pathogenicity of I 4,,12:i:- in swine when compared to the known pathogenic Typhimurium and lesser pathogenic Derby serotypes; 2) assessment of the fecal bacterial population over time in Salmonella-infected versus non-infected swine; 3) evaluation of the ability of I 4,,12:i:- to outcompete Typhimurium in swine when co-infected; and 4) development of a rapid PCR test for identification of suspected pathogenic (i.e.- I 4,,12:i:- and Typhimurium) versus lesser pathogenic serotypes of Salmonella in diagnostic samples.
To complete the first two objectives, 72 pigs were inoculated with either Salmonella Typhimurium, Salmonella I 4,,12:i:-, Salmonella Derby or sham-inoculated and followed for up to 28 days following inoculation. During this study, fecal scores and samples, rectal temperatures, and gross and histopathologic necropsy data were collected to compare pathogenicity and shedding of Salmonella between serotypes. Sequencing of the bacterial population present in the feces of these pigs using 16S metagenomic analysis was also completed. The results of this study clearly demonstrate that Salmonella serotype I 4,,12:i:- possesses similar ability as Salmonella Typhimurium to cause significant disease in growing swine and can be carried in the tonsils and lymph nodes and shed in the feces of infected animals for weeks following exposure and illness. While Salmonella Derby can also be carried in the tonsils and lymph nodes for extended periods of time, fecal shedding is limited as is clinical disease. Despite the significant clinical disease caused by infection with Salmonella, large scale changes in the bacterial populations present in the feces of infected swine identifiable via 16S metagenomics analysis are minimal. Infection with pathogenic Salmonella serotypes such as Typhimurium and I 4,,12:i:- do, however, lead to a decrease in the overall diversity of the bacterial populations in the feces which could lead to increased susceptibility to other gastrointestinal disease.
A second animal study utilizing 18 pigs, 12 co-inoculated with both Salmonella Typhimurium and Salmonella I 4,,12:i:-, and 3 each singly inoculated with either Typhimurium or I 4,,12:i:-, was completed to evaluate the comparative competitive fitness of I 4,,12:i:- in the porcine host. When co-inoculated at the same levels, I 4,,12:i:- was consistently detected in the feces of a higher percentage of pigs and at higher levels than Typhimurium. This indicates that serotype I 4,,12:i:- may possess the ability to outcompete serotype Typhimurium when inoculated simultaneously into naïve pigs, which may partly explain why I 4,,12:i:- has been increasingly identified in swine diagnostic samples over the past several years.
Finally, validation of a rapid PCR test for identification of pathogenic Salmonella serotypes was completed utilizing samples submitted to the Iowa State University Veterinary Diagnostic Lab. Results of this validation shows that PCR can be used equivalently to culture for Salmonella detection within diagnostic samples. For accurate identification of Salmonella serotype, enrichment prior to PCR is recommended to ensure that adequate levels of Salmonella are present in the sample for reliable identification. The use of this PCR in place of standard culture and serotyping, particularly when combined with enrichment, can reduce the turn-around time for diagnosis of pathogenic serotypes Typhimurium and I 4,,12:i:- in swine diagnostic samples from upwards of 6 weeks to a maximum of 2 days.
The combined effect of completion of these objectives will assist producers and practitioners in better understanding the role of serotypes Typhimurium and I 4,,12:i:- in clinical disease in swine. With the use of the newly developed PCR, veterinarians can more rapidly identify pathogenic Salmonella as the cause of disease within herds to initiate appropriate interventions to decrease further production losses.