KEY TAKEAWAYS
- Salmonella prevalence did not differ amongst tissue samples; however, Salmonella prevalence of edible liver samples was 18.75%.
- Salmonella prevalence was highest in warmer months (quarter three) which is consistent with existing literature.
- High intensity serotyping with CRISPR-SeroSeq aids in understanding serovar diversity within a given sample.
- Concordance amongst feedlot varied as three feedlots exhibited concordance for serovars, whereas three feedlots were diverse.
BACKGROUND
Salmonella contained within non-mesenteric, peripheral lymph nodes (PLN) serves as a potential source of ground beef contamination. Because these PLN are generally surrounded by a significant amount of adipose tissue, they are typically impervious to in-plant interventions designed to reduce pathogenic bacteria. While
Salmonella uptake in PLN has been the subject of considerable research in the past decade, relatively little is known regarding the dynamics of uptake and acquisition by the PLN. Conventional wisdom supports the idea that
Salmonella, a common resident of the bovine gastrointestinal tract (GIT), may escape the spiral colon, entering systemic and lymphatic circulation prior to sequestration by the PLN.
Salmonella has also been found in liver abscesses, further supporting its ability to escape the GIT. Even though other routes of transmission of
Salmonella to the PLN have been demonstrated, it is likely that the GIT may serve as the primary source.
The beef processing industry utilizes portions of the hindgut for export and human consumption. During this process, the small intestines are washed and braided, whereas spiral colons are trimmed of excess fat, segmented, and graded on the presence or absence of epithelial ridges, optimal surface area for absorption, and effective tight junctions (#1 or #2). Loss of epithelial ridges and compromised tight junction function of the spiral colon may release more Salmonella from the GIT to be ultimately captured by the PLN.
Factors contributing to epithelial ridge integrity are not well defined, but it is reasonable to assume that ridge presence and tight junction function are largely impacted by hindgut acidosis, which is a direct reflection of the complete ration and its influence on rumen pH. Low rumen pH has been demonstrated to compromise rumen wall integrity, thus allowing certain bacteria to escape the rumen and relocate to the liver, ultimately causing liver abscesses.
Whether or not Salmonella is the causative factor of the abscess or merely finds the abscess as a hospitable location remains to be determined. While the rumen likely harbors some Salmonella, due to the size and volume of contents, determining the exact contribution the rumen makes to overall Salmonella load within the bovine GIT is difficult and has not been fully elucidated. The hindgut however has been documented to contain significant populations of Salmonella. Hence it is possible that Salmonella found in liver abscesses is due to its escaping of the lower gut and entering systemic circulation.
Methodology
Six feedlots were enrolled in a one-year study, comprised of three feedlots each with a historic “high” and “low” prevalence of
Salmonella in beef lymph nodes from procured historic peripheral lymph node data. Management practices, geographic location, cattle type history and other demographics were collected for each feedlot through a questionnaire at the time of sample collection. Pertinent weather data (i.e. pen condition, temperature, humidity, precipitation and wind velocity) was recorded for a seven-day window, including the six days prior to sample collection and the sample collection day. Ration information and individual feed ingredient analysis was collected and conducted representing each feedlot. Pen surface samples were collected for each corresponding feedlot and evaluated for
Salmonella prevalence. Samples were collected from cattle from each feedlot on a quarterly basis for 1 year. During harvest, an individual animal ID was collected and transferred to the subsequent carcass via a unique carcass ID. Individual animal ID remained with the carcass and was transferred to any samples collected. Liver abscess prevalence and severity was collected by West Texas AMU personnel. Samples collected from each animal included: liver abscess, colon grade (#1 or #2), colon luminal content and mucosal tissue, and one subiliac PLN. Colons were aseptically excised and scored by plant personnel. Colonic mucosal tissue (n = 50; 25 each for #1 and #2 grade colons) were collected and cultured for
Salmonella prevalence and concentration. Colonic mucosal tissue samples were collected for histopathology and microscopy from a representative number of cattle from each feedlot representing equal representation of different colon grades. Luminal contents (n = 50; 25 each for #1 and #2 grade colons) were analyzed for pH and cultured for
Salmonella prevalence and concentration. Subiliac PLN (n = 50; 25 for each corresponding #1 and #2 grade colons) were collected and cultured for
Salmonella prevalence and concentration.
Salmonella isolates from all
Salmonella positive samples were further sequenced and relative frequency was captured utilizing the CRISPR-SeroSeq tool to capture the depth of multiple serotype populations within a sample.
In total, 176 colon, 192 liver, and 366 lymph node samples were collected.
Salmonella prevalence did not differ (P = 0.32) by feedlot however,
Salmonella concentrations differed (P = 0.02) by feedlot.
Salmonella prevalence and concentration increased during warmer months; the third quarter (July-September) was observed to have the highest (P = <0.01) total prevalence at 64.43% followed by the fourth quarter at 20.45%.
Salmonella prevalence did not differ (P = 0.14) for tissue type though prevalence was higher in LN (22%). Colon tissue had the highest (P = <0.01)
Salmonella concentrations (2.02 log CFU/g) when compared to liver and LN (0.1 and 0.44 log CFU/g), respectively. Prevalence of
Salmonella in the tissue from edible livers was 18.75%. Of the liver and colon samples, 19% of samples were positive for
Salmonella whereas 25% of the SLN samples were positive for
Salmonella. The greatest concentration of
Salmonella was detected in #2 colons (2.16 logCFU/g); the lowest prevalence of
Salmonella was detected in edible liver tissue (0.1 logCFU/g). Feedlot B had the highest average prevalence of
Salmonella (29.35%) whereas Feedlot A had the lowest average prevalence (17.19%) among all quarters and sample types.
Salmonella serovar identities were determined using CRISPR-SeroSeq. Overall, 6 different serogroups (13 different serovars) were detected. Slightly more than half (55%) of the samples contained two or more serovars, with 6 serovars found in one sample. Colon tissue samples exhibited slightly greater serovar diversity, with an average of three serovars compared to liver tissue and SLN which averaged two serovars.
Implications
Currently, very little is known regarding
Salmonella uptake by the PLN and no pre-harvest interventions have demonstrated marked benefit. This study could elucidate mechanisms that allow
Salmonella to escape the GIT and become sequestered by the PLN, thus enabling the design of effective intervention strategies.