A Review of E. coli O157 Outbreaks From 2003-2012

Escherichia coli was first recognized as a possible foodborne pathogen in 1975 in a California hospital with a patient experiencing symptoms of gastroenteritis including bloody diarrhea. Then in 1982, E. coli serotype O157:H7 was associated with two outbreaks of hemorrhagic colitis in Oregon and Michigan linked to contaminated frozen hamburger patties from the same fast food chain restaurant. Shortly after these outbreaks, E. coli O157:H7 isolated from the stools of children experiencing hemolytic uremic syndrome (HUS) was found to produce a substance that was toxic toward mammalian cells in tissue culture (Vero cells), now known as shiga toxin (and classifying E. coli O157:H7 as a Shiga-toxigenic E. coli [STEC]). In 2006, an E. coli O157:H7 outbreak that was associated with bagged spinach that affected 26 states, caused 238 illnesses and 5 deaths spread awareness to consumers and producers about E. coli O157:H7 as a foodborne pathogen.

E. coli is a Gram-negative, facultative anaerobic rod-shaped bacterium that is found in the lower intestine of humans and animals. Most E. coli strains are part of the normal microflora of the gut and are nonpathogenic. They have a symbiotic relationship with the host by having a place to thrive in the gut while protecting the host from other pathogenic bacteria that may enter the intestine. E. coli is a part of the Enterobacteriaceae family which includes other foodborne pathogens such as Salmonella, Shigella, and Yersinia.

While most strains of E. coli are nonpathogenic, one of the leading pathogens responsible for causing severe foodborne illness, E. coli O157:H7, is within this same genus and species of bacteria. Birds, pigs, and humans are among susceptible hosts to symptomatic colonization whereas cattle can be nonsymptomatic carriers. The gastrointestinal tract, and kidneys are some of the target organs that are affected by strains of pathogenic E. coli. There are many types of pathogenic E. coli, which differ in their abilities to cause disease.

The types of E. coli that can cause gastroenteritis can be transmitted in a number of ways, including through contaminated food, drinking water, recreational water, or person-to-person contact with an infected individual. The median incubation period for an EHEC infection is typically three to four days and symptoms of infections include severe stomach cramps, diarrhea, and vomiting. More serious symptoms may include septicemia, hemolytic-uremic syndrome, and thrombocytopenia. Those most at risk, especially for more severe symptoms are young children, the elderly, and people who are immunocompromised.

E. coli O157 outbreak data from 2003 – 2012 was reviewed by a team of researchers at the CDC and recently published in the journal Emerging Infectious Diseases. They found that outbreaks were mostly associated with beef with epidemic curves peaking in the summer seasons. This is not surprising since cattle serves as a reservoir for the bacteria. However; multiple outbreaks of E. coli O157 in a single summer season has not been reported since 2012. They also found that there was an unusual outbreak pattern in the time period studied with 6 foods other than beef being associated with E. coli O157 outbreaks. Some of these foods included poultry, dairy, leafy vegetables, fruits, sprouts and nuts. The broad range of complex foods (foods containing more than one ingredient) that were implicated in outbreaks also included guacamole, pico de gallo, salsa, potato salad, cookie dough, baked beans, meatballs, lime and bean dip, macaroni, Mexican wheat snacks, sandwiches, vegetable-based salads and many others. During the decade studied, 390 E. coli O157 outbreaks occurred including 4.928 illnesses, 1,272 hospitalizations, 299 cases of hemolytic uremic syndrome (HUS) and 33 deaths. Of those outbreaks, the majority (65%) were attributed to food sources. Authors hypothesized that outbreaks associated with other transmission sources may be more severe than those related to beef, specifically those related to leafy vegetables, dairy products, fruits and meats (other than beef) according to the data. Possible factors influencing severity of an outbreak included virulence of the bacterial strain as well as age and sex of affected individuals.

Sources: New England Journal of Medicine, Lancet, AEPP, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention, Bacterial Analytical Manual, Human colonic bacteria: role in nutrition, Physiology and Pathology, Foodborne Disease Handbook, Escherichia coli: virulence mechanisms of a versatile pathogen, Food Safety News.