As I write this, the strain of E. coli known as 0157:H7 is once again in the news. This time it"s not because of a disease outbreak associated with tainted hamburger, but one associated with contaminated spinach. As in similar outbreaks, hundreds of people have been sickened, and three have died. As a cattle veterinarian, when these outbreaks occur, I often get more questions about the disease. I also become more concerned about what role we should play in reducing the risk of this disease.
Understand the problem
E. coli 0157:H7 is a strain of bacteria first identified and associated with severe hemorrhagic diarrhea in humans in 1982. Outbreaks usually have been associated with contaminated hamburger, but other sources have included petting zoos, daycare centers, and, increasingly, fresh produce.
In humans, infection with this bacteria results in severe diarrhea because of our sensitivity to a toxin the bacteria produces causing dysentery. And, if the toxin and/or bacteria penetrate the intestinal lining, the kidney may be damaged, producing a hemolytic, uremia syndrome which is often fatal. Young children are more susceptible, as well as the elderly or those with compromised immune systems.
Compared to most foodborne illnesses, it takes very low numbers of the bacteria to produce disease — as few as 10. This is likely because E. coli 0157:H7 bacteria are resistant to acid, allowing more of the bacteria to survive in the acidic environment of our stomachs. That, in turn, allows more bacteria to colonize the intestine and produce disease.
Cattle, however, appear to be insensitive to the toxins produced by this strain of E. coli and can carry it in their digestive tract with no adverse effects. It colonizes in the anal-rectal junction, so it is often shed in large numbers in the feces of infected cattle.
Estimates vary, but it is likely that more than 20 percent of feedlot steers and 3 percent to 4 percent of slaughtered cows are infected at the time of slaughter. Estimates also suggest that more than 60 percent of herds and more than 80 percent of feedlots have at least one infected animal.
Meat usually becomes contaminated during the slaughter-and-dressing process. The source of the bacteria in fresh produce is uncertain, but the probability of it coming from water contaminated with cattle manure is considered to be high.
Greater attention to hazard analysis and critical control of likely problems in slaughter facilities has helped to reduce the incidence of this disease in the last few years. However, reducing its likelihood in cattle is also a critical-control point, and an area that all of us should be involved.
What you can do
What can be done to reduce E. coli 0157:H7 in cattle? The first step is cleanliness. After an infection, cattle routinely shed the bacteria for a limited time — probably several weeks. However, the bacteria can live in water reservoirs and tanks for four months or longer. Regular cleaning of drinking facilities can cut down on the problem.
In addition, the hides of cattle have been shown to be a likely source of contamination of beef in slaughter facilities. Preventing contamination of skin, by keeping animals dry and clean, can reduce the likelihood of infecting milk or meat.
A second method, although somewhat controversial, is to feed less grain. Reducing grain intake before slaughter has been shown to reduce concentrations of this acid-resistant organism in the manure of cattle. It is believed that high-grain diets favor selection of more acid-tolerant strains of bacteria, including E. coli 0157:H7. Feeding a diet higher in forage may help control the disease.
Foodborne illnesses affect all of agriculture. Understanding them and looking for ways to prevent them at all points in the food-production chain, including the farm, are important to everyone.
Brian J. Gerloff is a veterinarian and operates Seneca Bovine Services in Marengo, Ill.