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The beef industry’s stellar record in reducing E. coli contamination hasn’t extended to Salmonella. But the industry is working on it.
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.