By Tim Lundeen
The latest research says about 2% of all cattle, including those in feedlots and those on pasture, may be supershedders," a term scientists have coined to describe cattle which turn out high levels of pathogenic organisms.
One of those problem organisms, of course, is Escherichia coli O157:H7.
This supershedding of pathogens in manure by the few is believed by some to be a primary source of contamination for all cattle entering slaughter facilities.
Findings from studies by researcher Terrance M. Arthur and his colleagues at the USDA Agricultural Research Service Roman L. Hruska US Meat Animal Research Center in Clay Center, Neb., hope to provide a scientifically sound basis for new and effective strategies to curb shedding of this bacterium.
Arthur and his co-workers have designed and conducted studies of 6,000 head of feedlot cattle and more than 13,000 manure, hide and carcass samples. To discover more about supershedding, Arthur and his colleagues gathered data representative of the entire U.S. cattle population to estimate the incidence of supershedding. Their analysis determined an average incidence of 2%.
ARS says this team was the first to show that in supershedders, E. coli O157 colonization may occur not just in the lower digestive tract but throughout the entire digestive system.
Scientists generally agree a supershedder is any animal that sheds 10,000 pathogenic organisms per gram of manure.
"It isn't the amount of manure that's shed; it's the amount of the pathogen in the manure," Arthur says.
Short but long
Also, supershedding is a transitory condition that researchers currently think lasts less than a month. Regardless of duration, ARS says the basic problem with supershedding is the same: The copious amounts of E. coli O157 in the manure don't necessarily stay where the manure was deposited. Instead, shedding may lead to spreading.
An animal which takes a dust bath, for instance, may pick up some E. coli-contaminated manure on the feedlot floor and end up with bacterial cells stuck to its hide. Later, some of that manure-borne E. coli may spread to pen mates during the usual milling about.
E. coli O157 which is swallowed might then colonize a previously uninfected animal's gastrointestinal tract, and that animal's manure could later become a new source of infection in other cattle, Arthur says.
Understandably, high levels of E. coli O157 on cattle hides could stress packinghouse sanitation systems designed to prevent the spread of the pathogen.
Finding the origins
In other work, the researchers monitored E. coli O157 contamination on hides of cattle in 10 feedlot pens and determined that supershedders were responsible for the majority of contamination, ARS says.
The study Arthur et al. found that in supershedders, E. coli colonization may occur throughout the digestive system.
"If you are operating a packinghouse sanitation system with the expectation that O157 occurs primarily in the lower digestive tract, it's important to know that a supershedder is apparently an exception to that generalization," Arthur says.
What's more, the researchers discovered that supershedding was not restricted to any particular E. coli O157 strain.
"Our work rules out the idea that a strategy should target a specific strain or strains to reduce supershedding," Arthur says. "The O157 in the manure samples collected for our research was mostly a mixture of strains in which no single E. coli predominated."
The research has also yielded criteria to gauge the success of candidate strategies for reducing or eliminating supershedding. Such interventions might include treating cattle with an O157 vaccine or adding an ingredient to their feed that helps suppress the pathogen, ARS says.
For an intervention to be deemed successful, the scientists say, two criteria must be met:
• First, none of the cattle in the pasture or pen would be supershedders.
• Second, the rate of fecal contamination (the number of cattle in a pen that are shedding O157 in their manure) would be kept below 20%.
Though preliminary, these criteria are apparently the first statistically sound targets for developing and testing a feedlot intervention.
"When you have a supershedder or more than 20% of the animals shedding O157 in their manure, you have a dramatic increase in the number of hides contaminated with manure-borne E. coli O157," Arthur says. "Hide contamination is typically 80% or higher in those pens.
"It may seem surprising that having 20% of the herd shedding O157 at low levels or one animal supershedding could lead to having 80% of the hides contaminated with O157, but cattle tend to congregate, and that promotes contamination," he adds.
Looking for reasons
Related work at Clay Center conducted by ARS researchers Bono and Jim Wells, along with Andy Benson of the University of Nebraska-Lincoln and others, may fill in other pieces of the supershedder puzzle. In particular, that research may help answer the question of why some cattle are supershedders while others are not.
In one set of studies, these scientists are inventorying and comparing the microbes that live in the gastrointestinal tract of supershedders with those which dwell in non-supershedders, ARS says.
This work may provide clues about whether some microbial species and strains help O157 flourish or, conversely, whether some "beneficial" species outcompete and suppress it. Such data may be useful in developing approaches to help the beneficial strains proliferate in cattle.
Lundeen is a staff writer for Feedstuffs.