KEY TAKEAWAYS
- Salmonella levels in finishing pen environmental samples appear related to Salmonella levels in peripheral lymph nodes.
- Peripheral lymph node Salmonella levels and pathogenic subtypes were highly predictive of each other.
- Additional research should be performed to broadly assess the abilities of a single peripheral lymph node sample and finishing pen environmental samples as sentinels for Salmonella in peripheral lymph nodes.
BACKGROUND
Salmonella is responsible for an estimated 1.35 million infections, 26,500 hospitalizations, and 420 deaths in the United States annually. An estimated 6.4% of U.S. foodborne
Salmonella illnesses are attributed to beef. Finishing beef cattle gastrointestinal tracts and feedyard pen environments are known to harbor
Salmonella. Cattle peripheral lymph nodes have been demonstrated to harbor
Salmonella. Current processing interventions targeted at pathogens (including
Salmonella) on carcass surfaces are not effective against
Salmonella present in peripheral lymph nodes since they are located below the carcass surface.
Salmonella present in peripheral lymph nodes when cattle are harvested may contaminate final products, since portions of peripheral lymph nodes may be present in trim and ground beef. It is hypothesized that pre-harvest factors contribute to
Salmonella lymph node and fecal contamination in cattle and, subsequently,
Salmonella contamination in ground beef. Major peripheral lymph nodes can be removed from carcasses during processing to reduce the food safety risk posed by
Salmonella. However, removal of major peripheral lymph nodes reduces value since high value products are damaged and additional labor is required. Identification of sentinel samples for
Salmonella contamination of peripheral lymph nodes would enable processing interventions targeted to high-risk groups and evaluation of pre-harvest interventions. The first goal of this research was to define the relationship between
Salmonella populations in cattle pen surface samples from finishing yards, cecal samples, mesenteric lymph nodes, and peripheral lymph nodes. The second goal of this research was to determine if finishing pen surface samples, mesenteric lymph nodes sampled at harvest, and cecal content sampled at harvest are suitable sentinels for peripheral lymph nodes contaminated by high-risk
Salmonella.
Methodology
Cattle intestines contain a variable number of smaller mesenteric lymph nodes (MLNs) with diverse locations and morphologies we have named sporadic small MLNs (ssMLNs) as well as the larger ileocecal lymph node. 50 cattle intestines were sampled with 5 methods: ileocecal MLNs, proximal ssMLNs, distal ssMLNs, cecal content, and cecal swab. For each intestine, 4 to 10 ssMLNs were assigned to two separate samples, proximal ssMLNs and distal ssMLNs, based on their relative proximity to the cecum. An incision was made in the cecum and 10 grams of contents were removed to obtain the cecal content sample. To obtain the cecal swab sample a sponge-stick was inserted into the interior of the cecum, the sponge was placed against the interior surface and swabbed.
Salmonella were cultured for detection and enumeration.
Four 10-gram pen surface material samples were obtained from each of 40 and 100 finishing cattle pens distributed equally across 10 commercial feedyards. Salmonella were cultured for detection and enumeration. From the third objective samples, eight pens with high Salmonella levels and eight pens with no Salmonella were identified and followed to processing for the fourth objective. For the fourth objective, five sample types (cecal content sponge, ileocecal lymph node, superficial cervical lymph nodes, popliteal lymph nodes, and subiliac lymph nodes) were obtained from between 20 and 25 randomly sampled carcasses. Salmonella were enumerated and detected using BAX System SalQuant and BAX Real-time PCR Salmonella assays, respectively. For the first objective, culture methods were used to isolate over 2,000 Salmonella. The Salmonella isolates were serotyped using molecular and slide agglutination methods. The isolates were screened by PCR for the presence of 7 virulence genes.
The ileocecal lymph node was reliably identified and quickly recovered from intestines. Recovery of ssMLNs was more difficult and time consuming.
Salmonella detection by ileocecal MLN and distal ssMLNs sample methods did not differ significantly. The ileocecal lymph node should be sampled to represent the MLNs because
Salmonella was detected more frequently, and it was logistically easier to obtain.
Salmonella detection did not differ significantly between cecal sponge and cecal content sample methods. A sponge should be used to sample cecal contents since it detected
Salmonella at least as often as cecal contents and is logistically easier to collect. High
Salmonella levels were present in peripheral lymph nodes from the pens with the highest
Salmonella levels in pen surface samples. Low
Salmonella levels were present in peripheral lymph nodes from the pens with no
Salmonella in pen surface samples.
Salmonella levels in cecal sponges and ileocecal lymph nodes were not predictive of
Salmonella levels in peripheral lymph nodes. However, peripheral lymph nodes were highly predictive of each other.
Implications
Cecal samples obtained at harvest and mesenteric lymph nodes cannot be used as sentinels for
Salmonella peripheral lymph node contamination. Samples of the cattle finishing pen surfaces hold promise as reliable sentinels for
Salmonella peripheral lymph node contamination at harvest, however, additional research is needed. Also, a single peripheral lymph node may serve as a sentinel for other peripheral lymph nodes. Again, this finding must be confirmed with additional research.