Escherichia Coli O157:H7

Also known as

Food poisoning (in humans)

Description

Escherichia coli are a large and diverse group of bacteria (family Enterobacteriaceae) commonly found in the lower intestine of warm-blooded animals like birds and mammals (including cattle and humans).These bacteria are Gram-negative, rod-shaped, anaerobes (organisms that thrive in a habitat without oxygen) and there are hundreds of serotypes.

A serotype is a subgroup within a species. Each subgroup that has unique characteristics that distinguish it from other strains of E. coli. Even though these differences may only be detectable at the molecular level, they may result in changes in function or life cycle of the bacteria. New serotypes of E. coli evolve through natural biological processes of mutation and horizontal gene transfer (the ability to transfer DNA through an existing bacterial population).

Most E. coli strains are harmless, and many are a normal part of the gut flora; they benefit their host animals and humans by aiding food digestion, producing vitamin K and B-complex vitamins, and filling the microbial “space”--preventing establishment of harmful bacteria within the intestine. Humans, for instance, need certain types of E.coli and other bacteria within the intestinal tract to remain healthy.

E. coli is not always confined to the intestine. Its ability to survive for brief periods outside the body makes it a convenient organism to test for fecal contamination in water, soil, or food. Most serotypes of E. coli are host-specific, making it possible to determine whether the source of fecal contamination originated from birds, humans or other mammals.

Although most strains are harmless, others can cause illnesses like diarrhea, urinary tract infections, pneumonia and other clinical disease in humans and some can cause disease in animals.

E. coli 0157:H7 is a resilient bacterium that is very common in the environment. It can survive and replicate in a wide range of conditions, including with or without presence of oxygen, and survives in spite of changes in pH and temperature. This particular strain rarely causes disease in cattle but often causes disease in humans.

Signs

Cattle rarely show any clinical signs

Cause

E. coli is best known for its ability to cause intestinal disease in humans. There are five classes of this bacterium that cause diarrheal diseases: enterotoxigenic, enteroinvasive, enterohemorrhagic, enteropathogenic and enteroaggregative. Each class falls within a serological subgroup and has distinct features in how it causes sickness.

E. coli O157:H7 is one particular serotype in the enterohemorrhagic category that has become a major concern for cattle producers and their customers because it sometimes causes human illness when it gets into the human intestine (even though cattle that carry it do not get sick).

Microbiologists differentiate E. coli strains based on antigens associated with a particular strain, using an “O” and “H” naming system. The “O” describes the particular antigen associated with the cell wall of the microbe, and the “H” refers to the particular flagella antigen of the cell. This O:H combination is called the serotype.

The feature that makes enterohemorrhagic bacteria like E. coli O157 so problematic is the fact that they produce a toxin that can damage the lining of the human intestine and other tissues. The E. coli bacteria that create these types of toxins are called “Shiga toxin producing E. coli,” or STEC. The STEC that cause hemorrhagic colitis and hemolytic uremic syndrome are called enterohemorrhagic E. coli.

E. coli O157:H7 is the most commonly identified STEC in North America. Whenever E. coli infections or outbreaks are in the news, E. coli O157 is usually the strain implicated. However, many other kinds of STEC can cause disease and are sometimes referred to as “non-O157 STEC” bacteria.

E. coli O157:H7 is nonpathogenic for cattle, but can be very pathogenic for humans, so ingestion of anything contaminated with these bacteria can pose a potential threat to human health. Unfortunately, cattle and many other animals are common hosts. Cattle are often blamed, but food contamination can come from many different sources.

E. coli O157:H7 is shed in the feces of many warm-blooded animals including cattle.
Producers, processors, retailers and consumers play important roles in reducing or eliminating incidence of E. coli O157:H7 illness in humans.

Prevention

Cattle are not at risk for disease from this strain of E.coli so prevention measures are aimed at reducing contamination of the environment or food with fecal matter. Cattle producers should assume that E. coli O157:H7 is present in their herd, since these bacteria are common in healthy cattle.

Making sure pens and transport trailers are clean and have adequate, clean bedding, and that water from cattle pens does not flow into or near surface water or wells, and that animals shipped to processors have little or no dried manure on them may minimize carcass contamination.

A vaccine for cattle was developed in Canada to aid in reduction of E. coli O157:H7 shedding, but is not currently available. Designing a vaccine that consistently controls bacterial shedding in live cattle is difficult, because E. coli O157:H7 does not cause disease in cattle.

Methods used by meat processors to improve sanitation have proven very effective in reducing the pathogen load in meat, and contamination today is rare.

Humans are at risk of consuming E. coli O157 when they eat, drink or touch their hands to something that has been contaminated with any type of animal fecal matter.

The frequency that E. coli O157:H7 has been reported in cattle ranges from zero percent to 41.5% of animals tested. A three-decade review of published reports summarized the prevalence of STEC in beef cattle manure and persisting on hides. Prevalence rates of E. coli O157 generally ranged from 0.3% to 19.7% in feedlot cattle, 0.7% to 27.3% in cattle on irrigated pasture, 0.9% to 6.9% in cattle grazing rangeland, and 0.2% to 27.8% at slaughter.

The concentration at which STEC are shed in manure varies from animal to animal, as demonstrated by a U.S. study in which a range from 100 to 100,000 colony-forming units of E. coli O157:H7 per gram of wet feces was reported.

Shedding of E. coli O157:H7 and other serotypes of STEC seems to be related to age of the cattle. The lowest rates occur in calves before weaning, and the highest rates in calves during the immediate post-weaning period, while rates are intermediate in adult cattle.

Beef cattle also shed more STEC in warmer months.

Environmental studies have shown that these organisms can persist in manure, water troughs and other places on cattle farms. Water runoff from feedlots or pastures is a way that these bacteria can be spread. Sanitation and good manure management are important.

Treatment

There is no treatment needed for cattle since they do not become sick.