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Scientists Studying Salmonella in Cattle Lymph Nodes

If you eat undercooked beef, it’s not just E. coli you have to worry about. Salmonella may be predominately associated with poultry, but beef can also be contaminated with the pathogen.

According to the Centers for Disease Control and Prevention, 23 percent of the outbreaks attributed to beef between 2009 and 2013 were caused by Salmonella. Recent multi-state outbreaks linked to ground beef include Salmonella Enteritidis, which infected 46 people in nine states in 2012, and Salmonella Typhimurium, which infected 22 in six states in late 2012 and early 2013.

A few years ago, scientists began to study lymph nodes as a significant source of Salmonella in cattle, and some think the organs could lead to the contaminated ground beef.

“What we have found is that, in certain times of the year and in certain regions of the country, it’s not uncommon to recover Salmonella from some of the peripheral lymph nodes” in cattle, says Guy Loneragan, a veterinary epidemiologist and professor of food safety and public health at Texas Tech University.

Loneragan is part of a broad research group working on the issue, which includes USDA’s Agricultural Research Service scientists Dayna Harhay, Tom Edrington and others.

At the North American Meat Association’s (NAMA) recent Beef Safety Conference, Loneragan presented some of the newest research on the issue.

In one study, the team found a diversity of Salmonella among the lymph nodes within carcasses, which Loneragan tells Food Safety News means that the Salmonella probably got there from multiple different routes.

The original understanding of Salmonella contamination is that the pathogen is transferred from the animal’s hide or intestines to the carcass during slaughter and processing. When slaughtering cattle, lymph nodes are not typically removed for the grinding process, which is why finding Salmonella in them is troubling. It may open up a different avenue for Salmonella to contaminate ground beef.

“We don’t think that traditional paradigm is sufficient to explain what we actually observe with Salmonella in these peripheral lymph nodes,” Loneragan says.

Flies may have a part to play in some of the transmission. Researchers have hypothesized that biting flies or other parasites transfer Salmonella into a cow’s skin, causing infection.

In another study, they looked at smaller lymph nodes dispersed throughout the carcass and found that, as opposed to the larger ones, it’s rare to recover Salmonella from the very small ones. This is a good thing because it means interventions don’t need to be as targeted on the small nodes.

Loneragan’s third presentation to NAMA was about the development of a model for producing Salmonella-positive lymph nodes and incorporating them into a beef-grinding process. The model helps scientists understand the distribution of Salmonella in ground beef when it comes from lymph nodes rather than the animal’s hide. They also hope to use it to evaluate interventions for reducing contamination.

Understanding distribution is important for risk assessment and for developing interventions important for process control, Loneragan says.

Animal interventions could include vaccines or certain feeding approaches, and in-plant interventions might include treatment of trim before grinding.

FSIS has been testing for Salmonella in meat since 1996, and, historically, 1.5 to 2.5 percent of ground beef samples contain Salmonella. In 2013, 1.6 percent of samples tested positive for Salmonella, compared to 5.5 percent for whole chickens. 

One element of USDA’s Salmonella Action Plan released in December 2013 is to explore how animal lymph nodes contribute to Salmonella contamination.

FSIS planned to survey research that’s been done on the topic, decide whether data from FSIS and the Agricultural Marketing Service would provide information about the effects of lymph nodes, work with industry to determine the extent of the problem and identify possible solutions, and possibly begin collecting samples of beef and pork tissue containing certain lymph nodes.

FSIS emphasizes that correlation is not the same as causation. In other words, just because Salmonella is found in cattle lymph nodes does not prove that it causes contamination in ground beef.

What needs to be established is the extent to which the Salmonella remains in the lymph nodes. Because the purpose of the lymphatic system is to fight infection and then dispose of attackers, “You might find Salmonella in a lymph node today, but if you tested that lymph node a week from now, that Salmonella might have cleared the lymphatic system,” Dr. David Goldman, acting USDA chief medical officer, tells Food Safety News.

According to the industry blog Meatingplace, Rachel Edelstein, deputy assistant administrator for FSIS, told the NAMA conference during a presentation from Loneragan that the agency may issue guidance on controlling Salmonella in cattle lymph nodes.

But it’s still relatively early days for the research. In discussing the nature of scientific inquiry, Goldman says, “You think you have a hypothesis based on an analysis, and then you need to put together other information and data to make that determination, and then use that to consider the policy options.”

The research Loneragan presented to NAMA was funded by the National Cattleman’s Beef Association beef check-off program and primarily commissioned by the association.

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