Recent outbreaks in western Wisconsin and Georgia have added more lines to the rap sheet of E. coli O157:H7. The Wisconsin case was traced to contaminated cheese curds. In the Georgia outbreak, a child with diarrhea apparently tainted the kiddie pool at a water park, and children got sick after swallowing pool water.
O157:H7 is a particularly nasty serotype of the E. coli bacterium, sickening about 20,000 people a year in the United States, and killing about 5 percent of its victims. It causes cramping and bloody diarrhea; severe cases cause kidney failure.
The largest outbreaks have come from contaminated ground beef. For example, in 1992, undercooked, E. coli-tainted hamburgers from a fast-food chain left 500 people ill in the Pacific Northwest. A 1997 outbreak prompted a Hudson meat packing plant to recall 25 million pounds of ground beef. This is a serious issue in Wisconsin, which produces millions of pounds of ground beef each year, mostly from dairy cattle.
While the media began focusing on E. coli in the 1990s, food microbiologists at the University of Wisconsin-Madison have been concerned about it for nearly 20 years. Several years ago, a research team at the College of Agricultural and Life Sciences began a new project – hunting down E. coli O157:H7 on dairy farms.
“We want to find out how commonly it occurs on dairy farms, where it hides and how it gets from one animal to another,” says microbiologist Chuck Kaspar of the Food Research Institute. Kaspar believes this will help scientists discover where the bacterium comes from and identify practices that reduce or eliminate it from herds that harbor it.
Kaspar”s team includes FRI microbiologist John Luchansky and veterinary scientist Jack Shere of the USDA Animal and Plant Health Inspection Service. Shere is pursuing a doctorate in food microbiology at the FRI.
In an initial survey of 70 Wisconsin dairy farms, the researchers sampled calves, which are more likely than older cattle to shed pathogenic E. coli. The researchers found the bacterium in 10 of 560 calves on five of the 70 farms; rates similar to results from other states, according to Kaspar.
To better understand the nature of pathogenic E. coli and where it might be coming from, Kaspar and Shere conducted a year-long study of four dairy farms. They collected samples weekly from 15 calves, starting at birth until they were at least eight months old. If a calf started to shed E. coli through feces, the researchers quickly expanded the sampling to other cattle in the herd, domestic and wild animals on the farm, feed, and water.
Two of the four farms remained negative for E. coli O157:H7 and two farms had calves that shed the bacterium. During the study, the microbiologists did a barrage of sampling on farms that had been negative. “Our testing on farms without detectable E. coli was even more thorough than where animals were infected with O157,” Kaspar says. “We tested every animal we could catch.”
The complete study involved analyzing more than 3,000 samples from dairy cattle and other animals – everything from insects to raccoons.
To identify pathogenic strains and track their sources, the researchers determined the genomic fingerprint of each positive O157:H7 sample they collected. With genomic fingerprinting tools, scientists can identify more than 100 strains of the E. coli O157:H7 serotype based on differences in the strains” genetic makeup.
“Most farms with E. coli O157:H7 had a single dominant strain,” Kaspar says. “One farm had the same E. coli strain show up in testing over a two-year period. The E. coli O157:H7 we isolated from cattle, water and a pigeon from one farm all had genomic fingerprints that we could not tell apart. That strongly suggests a common source of E. coli O157:H7 on that farm.”
“Drinking water was an important source of the bacterium in a herd,” Kaspar says. “Once drinking water became positive, cattle that drank the water began shedding the organism and it quickly spread to other nearby cattle.” The contaminated drinking water contained relatively few cells of the pathogenic E. coli, according to Kaspar.
“We”ve seen that E. coli O157:H7 can hang around a farm for a long time, moving from animal to animal and through water back into the same animals again,” Kaspar says. “In our next project, we”d like to see if we can intervene on a farm when the water becomes contaminated and stop the transmission to other animals.”
In another CALS project, the research team of geneticist Fred Blattner has sequenced the genome (all the genes) of the E. coli bacterium. This information could help to further define and identify new virulence mechanisms associated with this type of pathogenic E. coli. The results will also be important in many other areas of biological research.