ON average, about 2% of the cattle grazing in a pasture or eating high-energy rations in a feedlot pen may be "supershedders" that shed high levels of pathogenic organisms such as Escherichia coli O157:H7 in their manure, according to research led by U.S. Department of Agriculture scientist Terrance M. Arthur.
Supershedding is of concern because it could increase the amount of E. coli O157:H7 that makes its way from the pasture or feedlot pen into packinghouses.
Findings from studies by Arthur and his colleagues at the USDA Agricultural Research Service (ARS) Roman L. Hruska U.S. Meat Animal Research Center in Clay Center, Neb. — which recently marked its 50th anniversary — may 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.
The team was the first to show that, in supershedders, E. coli O157 colonization may occur not just in the lower digestive tract but also throughout the supershedders' entire digestive system, ARS said.
Scientists generally agree that 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 said.
Supershedding is a transitory condition that researchers currently think lasts less than a month. Regardless of duration, ARS said 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 that takes a dust bath, for instance, may inadvertently roll over 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 that's 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 said.
Understandably, high levels of E. coli O157 on cattle hides could stress packinghouse sanitation systems that are designed to prevent the spread of the pathogen.
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%.
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 noted.
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 said.
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 said. "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 said.
For an intervention to be deemed successful, the scientists said, 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 said. "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 added.
Arthur and his co-investigators at Clay Center, including ARS scientists Joseph M. Bosilevac, James L. Bono, Dayna M. Brichta-Harhay, Norasak Kalchayanand, John W. Schmidt, Steven D. Shackelford and Tommy L. Wheeler, have documented these and related findings in peer-reviewed scientific articles published in 2014, 2013 and 2009 in Applied & Environmental Microbiology.
ARS and the beef checkoff program funded the research.
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, their 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 dwelling in non-supershedders, ARS said.
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.
Beef cattle lameness
Beef cattle are naturally adept at hiding signs of weakness, and as a result, lameness prevalence in beef cattle is often underestimated.
"We see people that come out and say, 'We don't have issues with lameness,' but generally speaking, if you say that, you're not looking," said Dan Thomson, director of the Beef Cattle Institute at Kansas State University. "The experiences from veterinary school and veterinary practice prove that you miss more by not looking than by not knowing."
Thomson said it takes getting out with the crews and spending time watching the animals to see what's really going on.
Dr. Dee Griffin with the University of Nebraska's Great Plains Veterinary Educational Center explained that time spent with the animals is necessary, because lameness may not be observed on a day-to-day basis due to the instinctive nature of the animals.
"A steer will stand there with a clearly painful injury, and it will look at you and act like nothing is wrong. Cattle are prey animals, and they can hide the worst symptoms in the world," Griffin said.
Griffin explained that spending time with the cattle helps develop trust. After trust has been established, one can get a true picture of how the animals walk, which can provide clues about lameness and other ailments.
Once cattle can be observed walking at a steady pace or gait, the Step-Up Management Program helps provide a systematic approach to identify and manage beef cattle lameness. Developed by Zinpro Corp. in conjunction with the Beef Cattle Institute and Kansas State University, the program focuses on locomotion scoring to assess the prevalence and severity of lameness.
"I think the critical aspect about this scoring system is that it's a simple and effective means to identify cattle that need to be treated for lameness," said Connie Larson, ruminant research and nutritional services manager at Zinpro. "We can easily train individuals on how to identify and assign cattle to different locomotion categories."
Locomotion scoring is based on the observation of cattle walking (gait), with an emphasis on head bob and stride length. The system uses a simple scale of zero to three to assess the severity of lameness in beef cattle.
"We're looking at mild to moderate lameness, and that allows an opportunity to have an intervention," she said. "Can we treat these animals? Can we manage these animals to prevent them from becoming a category 3 (severe lameness)?"
Also, when lameness is recognized early, the success rate of treatment is higher, and it is easier to mitigate the pain the animal is experiencing.
Michigan State University AgBioResearch animal scientist Jason Rowntree has been awarded nearly $500,000 by USDA to improve the profitability of producers of grass-finished beef in the upper Great Lakes region. The grant is one of four recently awarded to Michigan State to help small-scale agricultural producers build secure food systems.
"Michigan has a climate that is conducive to grass-finished beef production, but producers face two challenges: obtaining efficient (cattle) weight gains in the last 60 days of finishing and supplying product for a majority of the year," Rowntree said.
His project will address these challenges by identifying economically feasible production strategies that are well suited for producers in the Upper Midwest. Rowntree said he believes that forging relationships between processors and distributors is the key to meeting long-term retail supply needs, and he will use this project to refine a working model for building these partnerships.
For some producers, the absence of in-state processing facilities forces them to ship cattle hundreds — and, in some cases, thousands — of miles out of state.
"The longer producers keep beef cattle in their state, the more value it gives to their local economy, regardless of if they are grass fed or grain fed," he said. "Local packing and processing is the lifeblood of our communities, and if we don't have them, we're completely dependent on someone else to feed us."
Rowntree aims to develop a pasture-based local food system that he refers to as a "pasture-to-plate" model. He will also assess consumer beliefs, attitudes and acceptance of beef derived from a local food system.
"Improving the efficiency of grass-finished beef production and collaborating with local retail and culinary partners will enable small and midsized farms to improve their profitability and sustainability in addition to increasing the overall food security of the Upper Midwest," Rowntree said.