The U.S. beef industry has done a tremendous job of dealing with carcass contamination involving E. coli. In fact, E. coli O157 positives in ground beef sampling have dropped by more than 90% in the last decade.
The same drop hasn’t been seen in incidence of Salmonella in ground beef, however, says Guy Loneragan, Texas Tech University professor of food safety and public health.
“We haven’t seen a decrease in human incidence of Salmonella. Admittedly there are many sources of Salmonella – in poultry, eggs, etc. – but the beef industry is asking itself why our intervention can so effectively manage E. coli O157, and not Salmonella.”
Focusing on lymph nodes
Salmonella is an occasional disease problem in cattle. However, in the past few years, researchers have been investigating the discovery of what is probably an old issue – Salmonella in healthy animals’ lymph nodes at the time of slaughter.
One theory is that cattle contract a low-grade Salmonella infection, which their immune system combats by engulfing the bacteria and transporting it to the lymph nodes for destruction. The Salmonella bacteria, however, are able to remain alive and are retained in the lymph nodes. The concern with this translocation is that some of the lymph nodes are later incorporated into ground beef at processing, which presents a potential food safety issue.
Dayna Harhay, a microbiologist and molecular biologist with the Meat Safety and Quality Research Unit at the U.S. Meat Animal Research Center (MARC) in Clay Center, NE, is among specialists working on the Salmonella/lymph node link. She’s done extensive work with Loneragan, as well as with Steve Carlson, a former MARC colleague now at Iowa State University.
“Over the course of a year, we collected lymph nodes from feedlot cattle and cull cattle (beef and dairy) at slaughter facilities in three regions – West Coast, South and Midwest,” she says. “We found that the presence of Salmonella in cattle lymph nodes generally is low (about 1%). But, in feedlot cattle, we found higher levels (11% on average) than in cull cattle.”
The researchers matched cull and fed cattle within each region, and found that the highest levels of Salmonella in the lymph nodes of fed cattle occurred in animals from the southern U.S. Interestingly, however, the levels of Salmonella in the lymph nodes of cull cattle from that region were low – about 1%, she reports.
“We’re trying to understand the epidemiology. Since mostly peripheral lymph nodes are contaminated, we theorize that the bacteria entry may be transdermal, from a cut or scrape on the hide, biting flies, or foot rot,” rather than their typical route via the digestive tract, she says.
“The animal’s body does what it’s supposed to – it localizes and contains the invader in the lymph nodes. The tricky part is that Salmonella appears to have evolved the ability to survive within the lymph node that is designed to kill it.”
Loneragan has been engaged in Salmonella research for the past 15 years. He says the industry needs to rethink its understanding of Salmonella and how it behaves within and between cattle. “In many situations, it doesn’t sicken the animal. Obviously, some strains are very pathogenic and we want to control those, but there are also many strains that don’t cause harm,” he says.
Seasonal, regional differences
“This is a regional and seasonal phenomenon, involving a warmer climate and warmer time of year,” Harhay says. “We see seasonality with pathogen prevalence on hides and in feces. We see more Salmonella in warmer climates and warmer times of year, but we were surprised to find that about 11% of lymph nodes isolated from fed cattle had Salmonella. In summer at some locations, it was higher.”
Loneragan says the prevalence of Salmonella in lymph nodes spikes at midsummer through late fall. “In the South during peak season, we generally recover Salmonella from 30%-50% of the lymph nodes we’ve checked in the flanks of these animals,” he says.
Meanwhile, the Salmonella strains inhabiting the intestines of cattle seem to be seasonal. “Prevalence goes up in summer, and then down during transition from fall to winter,” Loneragan says. “When we look at cattle operations here in Texas, we routinely recover Salmonella from 30%-60% of fecal samples from healthy animals during summer and fall.”
However, the overall prevalence in Colorado is usually less than 1%. “We don’t find the diversity or level of Salmonella that we find farther south,” he adds. “We’re trying to understand the seasonal and regional factors that affect the levels of Salmonella.”
Food safety issues
Loneragan says the peripheral lymph nodes of concern are located between the muscles of the legs, shoulder or flank. “When we grind this tissue into hamburger, it may include lymph nodes. It’s impossible to sort them all out, as many are embedded in lean muscle tissue, and deep in the fat trimmings.”
In most situations, the levels of Salmonella in lymph nodes are low. “If the fat trim containing those nodes ended up in ground beef, it would be significantly diluted,” Harhay says. “However, we also found some positive nodes with a higher load. These could be more of a problem, and a likely source for Salmonella getting into ground beef.”
She says the serotypes she’s found in these lymph nodes that have higher concentration happen to match what surveys indicate are the dominant serotypes of Salmonella found in ground beef – S. montevideo and S. anatum. The focus of her research, however, is on the serotypes that are likely to cause sickness if undercooked ground beef containing Salmonella is consumed – S. newport and S. typhimirium. “Even though we did find both of these in lymph nodes, we found them only rarely, which is good news,” Harhay says.
The understanding that not all serotypes of Salmonella are equal is crucial, she says. “This is important to the beef industry in the same way that we don’t target all E. coli. To impact human health, we need to know which Salmonella serotypes we must be more concerned with.”
She says researchers are looking at virulence, as well as genetic determinants the industry can use to screen for the strains of concern. “Right now, if we find one lymph node positive in a carcass, we don’t know what the likelihood might be of finding any other lymph nodes positive. Also, there are so many lymph nodes in a carcass that it’s impossible to effectively remove them. We’ll have to find upstream methods to deal with this,” she says.
Packing plants have tried to reduce carcass contamination based on the assumption that Salmonella (like E. coli) comes from manure-contaminated hides coming in contact with the surface of the carcass, but this hasn’t worked to the same extent. “So we’re exploring whether lymph nodes may be contributing to the problem,” Loneragan says.
Is management the answer?
As animals are going to slaughter with Salmonella already inside the carcass, specialists are considering what preharvest practices could be employed at feedlots to reduce the risk. This may involve use of a probiotic, or management to eliminate or reduce biting flies, Harhay says.
“Monitoring might show whether there are higher levels of Salmonella in the environment. After all, there are differences from one location to another. If certain practices can be identified that might help keep Salmonella levels down, then broad implementation of those practices might address this problem,” she says.
Loneragan says work is underway on vaccine technology as well, such as Pfizer’s Salmonella SRP vaccine. “We’re trying to identify where vaccine works and where it might not work. We’ve also been looking at a probiotic called Bovamine Defend, made by Nutrition Physiology Co. This also looks promising. Perhaps we can control this issue preharvest, or reduce it enough to have a meaningful impact,” he says.
Genetics is another area of focus, since some non-black cattle have been found to have two copies of a gene that makes them very resistant to Salmonella. This finding is being pursued by a company called PSR Genetics, in Scott City, KS.
Harhay adds the differences in Salmonella lymph node contamination between feedlots, and between dairy and feedlot cattle in the same region, would seem to indicate that interventions may be possible at the farm or feedlot level.
Not all Salmonella are dangerous
Salmonella can cause disease in cattle and humans, but there are many serotypes; not all of them are dangerous. Many that exist in cattle only rarely cause disease in cattle.
“Some are more pathogenic than others, in both animals and people,” says Guy Loneragan, Texas Tech University professor of food safety and public health. “S. newport and S. typhimurium are quite pathogenic and can cause substantial disease in cattle and people, yet the farther south we go, the more diverse collections of serotypes we find in cattle, and more total numbers,” he says.
“We often find a carcass that has at least one lymph node in which we can recover Salmonella. These animals are healthy, with no history of clinical disease. These diverse strains of Salmonella seem to get in from the environment or the intestines, and enter the peripheral lymph nodes. Our hypothesis is that much of the Salmonella we observe in these locations are getting into the animal through the skin,” he says.
Heather Smith Thomas is a rancher and freelance writer based in Salmon, ID.
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