Along with most of the country, I was introduced to E. coli O157:H7 suddenly in 1993, when hundreds of people became ill and four children died after eating Jack in the Box hamburgers.
However, the then little-known strain of bacteria was first recognized as a new food-related threat more than a decade earlier, in 1982, after an outbreak of hemorrhagic colitis (bloody diarrhea) in Michigan. The victims had all eaten at a McDonald’s restaurant (Riley, et al., 1983).
By the following year, microbiologists had determined that Escherichia coli – a common bacterium that exists harmlessly by the billions in the intestines of animals and people – had picked up the ability to integrate a dangerous Shiga toxin-secreting gene. Along with botulism and tetanus, Shigella is one of the most deadly toxins on earth.
This variant, toxin-armed E. coli was designated O157:H7 for the protein compounds on the strain’s surface. An E. coli infection often begins with diarrhea but can progress to kidney failure and sometimes cause death.
Over the next 10 years, approximately 30 E. coli O157:H7 epidemics were recorded in the United States (Griffin & Tauxe, 1991). The actual number was probably much higher because few hospitals tested for E. coli, and those that did weren’t required to report cases to public health authorities until 1987 (Keene et al., 1991 p. 60, 73). It is also important to note that only about 10 percent of E. coli infections occur in outbreak clusters, the rest are among individuals and thus sporadic. As a result, only the most geographically concentrated illnesses drew enough attention to prompt further investigation (Keene et al., 1991 p. 583.)
The Centers for Disease Control and Prevention estimate that E. coli O157:H7 now sickens as many as 73,000 Americans a year, hospitalizes 2,200 and kills 61. And O157:H7 isn’t the only Shiga-producing E. coli. The CDC says non-O157:H7 Shiga toxins are annually responsible for an additional 36,700 illnesses, 1,100 hospitalizations and 30 deaths.
In analyzing 334 outbreaks and 7,864 illnesses caused by E. coli O157:H7 from 1998 through 2007, the CDC found that 69 percent of the infections were caused by food and 18 percent by water contaminated by animal waste. But 8 percent of the illnesses were the result of contact with animals – petting zoos have been a common source of infections — while 6 percent were so-called “secondary infections” from exposure to an infected person.
Of the non-O157:H7 E. coli infections during the same period, 83 percent were food borne, 9 percent water borne, 5 percent were from direct contact with animals and 4 percent involved person-to-person contact.
In 1982, during the investigation into the McDonald’s outbreak, the U.S. Department of Agriculture failed to find E. coli O157:H7 in any animals. By 2003, however, a study on the prevalence of the pathogen in livestock at 29 county and three large state agricultural fairs found that 13.8 percent of beef cattle, 5.9 percent of dairy cattle, 3.6 percent of pigs, 5.2 percent of sheep and 2.8 percent of goats carried the organism. Even 7 percent of pest fly pools tested positive for E. coli O157:H7.
While people can get sick from consuming food or beverages that have been contaminated by animal feces, especially cattle manure, cattle and other animals are merely carriers of Shiga toxin (Stx) producing E. coli – they suffer no ill effects from it. That’s because animals lack receptors – the tiny protein structures on the surface of cells — that are specific for the Shiga toxin. In host cattle, for example, receptors merely provide a “docking station” for the toxin while in humans; the toxin can bind to receptors and then penetrate and destroy cells.
Hamburger has been such a major source of E coli illness that hemolytic uremic syndrome (HUS), the serious complication that strikes five to 10 percent of people infected with E. coli O157:H7, was once called “Hamburger Disease.”
But any meat can become contaminated with E. coli if fecal matter on a carcass is not washed off when an animal is slaughtered and butchered. Hamburger presents heightened risks because a contaminated carcass ground up with others can poison an entire batch. And while cooking can kill the disease-causing bacteria, E. coli is mingled throughout ground beef, not just on its surface. Hamburger must be cooked to an internal temperature of 165° F. to make it safe.
Pathogenic E. coli is not limited to meat; it has become widely spread throughout the food chain. The bacteria has surfaced in drinking water as well as in venison; sausages; dried (uncooked) salami; unpasteurized milk and cheese; unpasteurized apple juice and cider, alfalfa and radish sprouts, parsley, lettuce, cabbage, spinach, fruit, nuts, berries, and, somewhat mysteriously, cookie dough.
In 1996, apples that had dropped to the ground were contaminated by cow manure, and then harvested and used by Odwalla Co. to make unpasteurized apple juice. On Nov. 1, 1996, after more than a dozen children who drank the juice developed HUS, and one died, Odwalla recalled and later stopped selling unpasteurized juice.
Another national outbreak in 2006 that sickened 205 people in 26 states, including 102 who required hospitalization and five who died, was traced to bagged, fresh spinach. The outbreak strain was isolated from cattle pastures near the spinach crops, as well as in a wild boar killed in one of the fields.
Also in 2006, iceberg lettuce grown near dairy farms in California’s Central Valley caused an outbreak of E. coli that sickened at least 81 people, hundreds of miles away in Minnesota, Iowa and Wisconsin.
In its Sept. 14, 2009, report, “Update on the Epidemiology of Shiga-toxin producing E. coli (STEC) in the United States,” the CDC warns of changes in the kinds of food that may be contaminated with E. coli. Prior to 1995, the CDC said, no leafy green vegetables were implicated in any outbreaks, but from 1995 to 2005, 27 outbreaks were traced back to leafy greens.
The majority (21) of those illness clusters involved lettuce and lettuce salads, with cabbage responsible for three outbreaks, parsley for two and spinach for one. Fresh vegetables can become contaminated either before or after harvest and contaminated seeds; irrigation water, floodwater and wash water have contributed to E. coli outbreaks involving produce.
The trend toward contaminated produce may be on the upswing. From 1998 through 2002, the CDC reported 11 outbreaks of E. coli linked to leafy vegetables. That number jumped to 41 from 2003 through 2007.
Beef’s connection with E. coli has not gone away — numerous outbreaks and massive recalls of contaminated hamburger continue to plague both the industry and the public. The fall of 2007, for instance, was a particularly dreadful season in which the Food Safety and Inspection Service (FSIS) of the U.S. Department of Agriculture (USDA) recalled nearly 30 million pounds of ground beef in 20 separate recalls.
From 1998 through 2002, there were 33 beef-related E. coli O157:H7 outbreaks in the United States, compared with 42 outbreaks from 2003 through 2007. During the same four-year periods, the number of dairy-linked outbreaks remained steady at 13.
Advances in microbiology, such as pulse field gel electrophoresis (PFGE), have transformed the way food-borne illness is investigated. By running electrical pulses through isolates obtained from patients’ stool samples, or suspected sources, the process establishes a DNA fingerprint that can be used to identify related E. coli strains.
On the same day in September, 2006, public health departments in Wisconsin and Oregon separately uploaded PFGE patterns of E. coli strains isolated from a cluster of victims in each of their states onto PulseNet, a national network of laboratories. PulseNet permits rapid comparison of patterns through an electronic database, and quickly established that the Wisconsin and Oregon outbreaks were caused by an indistinguishable strain of E. coli, suggesting a common source.
Five days later, health officials from both states told the CDC that all the ailing individuals had eaten fresh-bagged spinach, which led to the public warning about the potential hazard from the Food and Drug Administration and the national recall. Further epidemiologic investigation indicated the source of the E. coli outbreak was spinach packed in California’s Salinas Valley in a single plant, on a single day, during a single shift.
On June 18, 2009, the Colorado Department of Public Health and Environment (CDPHE) issued a news release announcing an investigation, in cooperation with the CDC and other state health departments, of an outbreak of E. coli O157:H7 infections in people who had eaten pre-packaged, raw chocolate chip cookie dough. The outbreak sickened 80 people in 30 states; 10 of the victims became seriously ill after developing HUS. So far, how the cookie dough became contaminated with E. coli remains a mystery.
My work continues.