“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.”e. coli intervention in beef industry quote

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.