- Feeding vitamin E to feedlot cattle did not reduce the prevalence of Salmonella in lymph nodes.
- In comparison to the control group, which was fed a significantly lower IU/head/day vitamin E, treated cattle returned higher rates of Salmonella-positive LNs.
- Serovar findings and antibiotic results support the importance of monitoring Salmonella in bovine lymph nodes due to its food safety risk in ground beef products.
has been found in the peripheral lymph nodes (LNs) of beef cattle. Therefore, Salmonella
in LNs can pose a threat to consumers by potentially contaminating ground beef. Annually, it is estimated that 9.4 million cases of foodborne illness occur in the United States, with 11% of cases believed to be attributed to Salmonella
. It is imperative that new strategies are developed for both pre- and post-harvest sectors of beef products to mitigate Salmonella
prevalence in bovine LNs. Protected within the carcass, the LNs avoid many of the antimicrobial interventions applied to carcass surfaces during harvest. As a result, LNs can contaminate ground beef with Salmonella
. This study was designed to offer a potential pre-harvest solution for the beef industry.
From each crop, feeder calves were assigned to one of four feedlot pens. Two pens were designated as control and fed a typical commercial ration, with an average of 240 IU vitamin E per head per day. The other two pens were fed the same typical base ration with an average of 1,348 IU of vitamin E per head per day throughout the duration of the finishing program. Crop 1 was harvested during August to October and Crop 2 was harvested during April to May. Live weights were recorded upon arrival at the commercial feeding location and after 100 days on feed. The first crop entered the commercial feeding location in February 202 and was harvested in September and October 2021 for a total of about 210 to 240 days on feed. The second crop entered the commercial feeding location in August 2021 and was harvested in April and May 2022 for a total of 230 and 260 days on feed, respectively. Upon reaching market weight, cattle were transported to a commercial beef harvest and processing facility in Texas. From the first crop, one control pen and one treatment pen were processed in September 2021, and the other two pens (treatment and control) were processed in October 2021. From the second crop, one control pen and one treatment pen were processed in April 2022, and the remaining two pens (treatment and control) were processed in May 2022. Following harvest, carcass weights were recorded, and liver and lung scores collected.
Following harvest, left and right superficial cervical and subiliac LNs were removed from each carcass and pooled by type. LNs were placed in sterile bags and transported to 25 the Meat Science Laboratory at Texas A&M University in insulated containers with refrigerant materials. Upon arrival, all LNs were flame-sterilized and then trimmed of fat. LNs, once denuded, were again flame-sterilized. Each sterilized LN sample was weighed and placed in a sterile, labeled filter bag. A rubber mallet was used to pulverize the LNs samples. Pulverized LN samples were transported to the Texas A&M University Food Microbiology Laboratory for microbiological analyses. Pre-enrichment and isolation of Salmonella followed the USDA-FSIS Microbiology Laboratory Guidebook (MLG) 4.11. Samples were pre-screened using the Hygenia BAX Q7 with Real-Time Salmonella PCR Assay. A positive result was indicated by a plus-sign (+) on the results file. Negative results were indicated by a minus-sign (-) in the results file. All presumptive positives as indicated by the BAX were subjected to traditional cultural microbiological methods.
From incubated plates exhibiting growth, at least three viable colonies were identified, and one was chosen and onto Lysine Iron Agar (LIA) and Triple Sugar Iron Agar slants. Salmonella-typical colonies were selected. Once an ideal colony was selected, half of the colony was removed via sterile disposable needle. Subsequently, the needle was stabbed into the butt of the TSI slant and upon removal, used to streak the angled agar surface. Using the same needle, this step was repeated for the paired LIA slant. The remaining half colony was picked using a disposable sterile loop (VWR) and was streaked on a Tryptic Soy Agar (TSA) plate. Slants and plates then were incubated at 35°C for 18 to 24 h and 35°C for 16 to 24 h, respectively. Following incubation, slants were pulled and screened for gas exchange representative of Salmonella spp. presence and presumptive-positive slants were identified. One representative presumptive-positive colony from the corresponding TSA plate then was selected and ½ of the colony was streaked onto a TSA nutrient slant. TSA slants were incubated at 35°C for 22 h before being prepped to ship. The other half of the colony was swabbed onto cryovials for storage and placed in the -80°C freezer located in the microbiology lab for reference. Incubated TSA slants were transported to USDA-APHIS National Veterinary Service Laboratory (NVSL) for confirmation via MALDI-TOF. Confirmed positive isolates were serotyped. Additionally, the minimum inhibitory concentration for 14 antibiotics was determined for each confirmed positive Salmonella isolate. NVSL utilized the Sensititre system with the National Antimicrobial Resistance Monitoring System (NARMS) Gram-negative CMV4AGNF plates.
Live and carcass weights did not differ (P > 0.05) between control and treated groups for either harvest season. Average daily gain (ADG) differences were calculated for control (CNTRL) and treatment (TRT) groups for both harvest seasons. No differences (P < 0.05) in ADG were seen between CNTRL (1.59 ± 0.04) and TRT (1.46 ± 0.04) for warm season, but no differences between treatment groups in ADG for cattle harvest during the cooler season. Liver and lung scores did not differ (P > 0.05) between treatment groups for either the warm or cool season harvests. Of the 382 LN samples collected, 186 were confirmed positive for Salmonella
. No difference (P > 0.05) in Salmonella
-positive LNs was seen between the CNTRL (58.16%; 57/98 LNs) and TRT (61.22%; 60/98 LNs) groups harvested in the warm season (August 2021 and September 2021). The CNTRL group harvested in the cool season (April 2022 and May 2022) returned a lower (P < 0.05) rate (23.33%; 21/90 LNs) of Salmonella
-positive LNs when compared to warm season TRT cattle (50%; 48/96 LNs). A greater number (P < 0.05) of Salmonella
-positive LNs was recovered from CNTRL cattle harvested in the warm season (58.16%; 57/98 LNs) when compared to CNTRL cattle harvest in cooler months (23.33%; 21/90 LNs). No difference (P > 0.05) in Salmonella
-positive results was reported across harvest seasons for TRT cattle. All one hundred and eighty-six confirmed positive samples were serotyped. Fourteen serovars were identified, the majority being: Salmonella
Anatum (42/186), Salmonella
Kentucky (35/186), Salmonella
Muenchen (33/186), Salmonella
Montevideo (31/186), and Salmonella
Findings support the importance of continued research focusing on Salmonella
being present in bovine lymph nodes. Identifying methods that can help reduce Salmonella
prevalence in beef would lessen the risk to public health resulting in fewer recall-driven economic consequences. A pre-harvest approach is beneficial in reducing Salmonella
in lymph nodes of naturally-exposed feeder cattle by identifying serovars and their prevalence, along with aiding in recent findings of antibiotic resistance.