Sources of E. coli infection
E. coli O157:H7 bacteria and other pathogenic E. coli is believed to mostly live in the intestines of cattle (Elder, et al., 2000) but has also been found in the intestines of chickens, deer, sheep, and pigs. A 2003 study on the prevalence of E. coli O157:H7 in livestock at 29 county and three large state agricultural fairs in the United States found that E. coli O157:H7 could be isolated from 13.8% of beef cattle, 5.9% of dairy cattle, 3.6% of pigs, 5.2% of sheep, and 2.8% of goats. Over seven percent of pest fly pools also tested positive for E. coli O157:H7 (Keen et al., 2003). Stx-producing E. coli does not make the animals that carry it ill. The animals are merely the reservoir for the bacteria.
E. coli can be transmitted from several sources:
Foodborne Transmission of Stx-Producing E. coli
Throughout the 1990s and early into the 21st century, the majority of foodborne Stx-producing E. coli outbreaks occurred secondary to the consumption of contaminated ground beef; however, contaminated leafy vegetables have been increasingly identified as the source of outbreaks.
E. coli in Fresh Produce
Fresh fruits and vegetables can become contaminated pre- or post-harvest. Contaminated seeds, irrigation water, and flooding have contributed to E. coli outbreaks traced to sprouts, lettuce, spinach, parsley, and other fresh produce. Apples picked up from off the ground and used in the production of unpasteurized fruit juices were the source of a large E. coli O157:H7 outbreak in 1996.
June 2006 Lettuce E. coli Outbreak
In early August 2006, public health officials in a mid-sized city in Utah became aware that several people attending a teacher’s conference had contracted E. coli O121:H19 (a Shiga toxin-producing E. coli). The Weber-Morgan Health Department (HD) issued a news release indicating that three people had contracted E. coli O121:H19 from the same source, and that two had developed HUS.
Several days later, HD officials revised the number of outbreak victims to four, including three who had developed HUS (Weber-Morgan Health Department, 2006, August 7). One of the patients with confirmed HUS had not attended the teacher’s conference, but had eaten cheeseburgers with iceberg lettuce prepared at the same restaurant during the outbreak. The second confirmed HUS case was an attendee of the teachers’ conference, and a third was determined to be secondary transmission from a person infected at the conference.
Samples from three of the HUS patients with E. coli O121:H19 were laboratory-confirmed as genetic matches through DNA sub-typing using Pulsed Field Gel Electrophoresis (PFGE), confirming that their E. coli infections all came from the same source. Eventually, HD officials concluded that the source of the E. coli outbreak was iceberg lettuce prepared at the same fast-food facility. At least 69 people became ill.
August and September 2006 Spinach E. coli Outbreak
On Friday, September 8, 2006, officials at the Centers for Disease Control and Prevention (CDC) were alerted by Wisconsin Department of Health (WDOH) epidemiologists that a small cluster of E. coli O157:H7 infections with an unknown source had been identified. Separately, the State of Oregon Public Health Division (ODPH) also noted a small cluster of E. coli infections that day. Both WDOH and ODPH uploaded the PFGE patterns, or genetic fingerprints, of the E. coli O157:H7 strains that had been isolated from victims of their states to PulseNet. PulseNet is an early warning system for outbreaks of foodborne disease that is comprised of a national network of public health laboratories that performs DNA “fingerprinting” on bacteria that may be foodborne. PulseNet identifies and labels each “fingerprint” pattern and permits rapid comparison of these patterns through an electronic database at the CDC to identify related strains. Through PulseNet, CDC became aware that the Wisconsin and Oregon outbreaks had bee
n caused by an indistinguishable strain of E. coli, suggesting a common source.
On September 13, 2006, Wisconsin and Oregon health officials reported to CDC that interviews of ill individuals suggested that the consumption of fresh bagged spinach was common to victims in both clusters, and on September 14, 2006, the Food and Drug Administration (FDA) warned the public not to eat fresh bagged spinach. By September 15, CDC had received nearly 100 reports of E. coli infection among residents of several states, and the epidemiologic investigation indicated that the outbreak had been caused by consumption of bagged spinach produced in a single plant, on a single day, during a single shift.
Between August 1 and October 6, 2006, public health officials identified 199 individuals infected with the outbreak strain of E. coli O157:H7 in 26 states; 102 were hospitalized, 31 developed HUS, and 3 died.
November 2006 Lettuce E. coli Outbreak
On Jan 12, 2007, the Food and Drug Administration (FDA) announced that it had moved closer to identifying the source of an E. coli O157:H7 outbreak that had sickened approximately 81 individuals in November and December of 2006. Cases were reported in Minnesota (33), Iowa (47), and Wisconsin (1). Twenty-six people were hospitalized, and two suffered HUS.
The investigation into the outbreak revealed that all ill individuals had contracted E. coli after eating foods at chain Mexican food restaurants in Iowa and Minnesota. Epidemiologic studies by Minnesota and Iowa health officials identified shredded iceberg lettuce served in the restaurants as the likely vehicle of transmission.
Minnesota, Iowa, and Wisconsin health officials worked with public health agencies in California in a trace-back effort to determine where the E. coli-contaminated lettuce originated. During the trace-back investigation the strain of E. coli O157:H7 associated with the outbreak was found in two environmental samples gathered from dairy farms near a lettuce field in California’s Central Valley. The FDA was then able to locate the region where the lettuce was grown by reviewing records obtained from the lettuce processor.
E. coli in Ground Beef
At one time, prior to the widespread dissemination of E. coli throughout the food chain, hemolytic uremic syndrome secondary to E. coli O157:H7 infection was known as “Hamburger Disease”. The ground beef connection has not gone away. Numerous outbreaks and massive recalls of “tainted” ground beef continue to plague both the industry and the public.
Meat typically becomes contaminated with E. coli during the slaughtering process, when the contents of an animal’s intestines and feces are allowed to come into contact with the carcass. Unless the carcass is properly sanitized, the E. coli bacteria are mixed into the meat as it is ground. Because E. coli is mixed into the meat during the grinding process, and is not just on the surface, ground beef must be cooked through to a temperature of 160° F since only thorough cooking will kill E. coli bacteria (see prevention ).
In 2007, the United States Department of Agriculture’s Food Safety and Inspection Service (FSIS) announced the recall of nearly 30 million pounds of ground beef in 20 separate recalls for E. coli contamination. Many of the recalls were announced after illness had been traced to contaminated product.
One of Several September 2007 Ground Beef E. coli Outbreaks
On September 29, 2007, the United States Department of Agriculture Food Safety and Inspection Service (FSIS) announced that 21.7 million pounds of frozen ground beef patties were being recalled for possible E. coli O157:H7 contamination. The announcement came after health officials in several states who were investigating reports of E. coli O157 illnesses found that many ill persons had consumed the same brand of frozen ground beef patties.
Ground beef patties recovered from patients’ homes were tested by state public health departments and federal laboratories; tests conducted by the New York State Wadsworth Center Laboratory and by a FSIS laboratory on opened and unopened packages of the same brand of frozen ground beef patties yielded E. coli O157 isolates with several different “DNA fingerprint” patterns, as determined through PFGE.
An October 9, 2007 CDC news release stated that “several state health departments, CDC, and the United States Department of Agriculture’s Food Safety and Inspection Service (USDA-FSIS) are investigating a multi-state outbreak of Escherichia coli O157:H7 infections” (CDC, October 9, 2007).
Investigators compared the “DNA fingerprint” patterns of E. coli isolated from 35 ill individuals to E. coli strains isolated from the recalled ground beef patties and found that the strain isolated from the ill people matched at least one of the patterns of E. coli strains found in the frozen ground beef patties. Three cases had confirmed associations with recalled products because the E. coli strain isolated from their stool was also isolated from meat in their home. The ill persons, ages one to 77 years, resided in eight states [Connecticut (2), Florida (1), Indiana (1), Maine (1), New Jersey (8), New York (11), Ohio (1), and Pennsylvania (10)]. One patient developed HUS, and no deaths were reported.
Waterborne Transmission of Stx-producing E. coli
Water intended for recreation (e.g., pools, shallow lakes) and for human consumption can also become tainted. When lakes become contaminated, several weeks or months can be required for water quality conditions to improve or return to normal.
1998 E. coli outbreak at a water park
In 1998, an E. coli outbreak occurred among children who had visited a water theme park in the Southeast. Health officials traced the outbreak to an infected toddler who played in a pool while wearing diapers. Even though the pool was chlorinated, its concentration and contact time was presumably insufficient to kill the E. coli resulting from fecal contamination by the toddler, and other children who were in the pool ingested E. coli bacteria while playing in the pool.
1998 E. coli outbreak associated with a municipal water system
Also in 1998, the municipal water system in Alpine, Wyoming, became contaminated with E. coli, resulting in 157 illnesses, with four people developing HUS The outbreak investigation revealed that the town’s water supply, which was supplied by an underground spring and was unchlorinated, became contaminated with surface water prior to the outbreak. A large pool of water was found in the area over the water collection pipes, probably the result of a late snow melt combined with heavy rains and ground water outfalls. In addition, investigators found numerous deer and elk feces were present in the pool area, as animals came to the pool to drink (Olsen, et al., 2002).
1999 E. coli outbreak associated with exposure to recreational water
E. coli contamination at a lake in Connecticut led to an E. coli outbreak in 1999. Eleven people became ill with E. coli infections, and 3 children developed HUS; the attack rate was highest among those who were younger than 10 years who swam and/or swallowed water while swimming (McCarthy, et al., 2001; Tara, et al., 2001).
1999 E. coli outbreak associated with well water
Also in 1999, the New York State Department of Health investigated what is believed to be the largest outbreak of waterborne E. coli O157:H7 illness in United States history. The outbreak occurred at a fair in Washington County, New York, in August of 1999 (New York State Department of Health and Novello, 2000, March). A total of 781 persons were identified with suspected infections of E. coli O157:H7 and/or Campylobacter jejuni. Of these cases 127 persons were culture-confirmed to be ill with E. coli O157:H7, 71 individuals were hospitalized, 14 persons exhibited HUS, and 2 people died.
The environmental and site investigation indicated that unchlorinated water from a well serving the southwestern portion of the fairgrounds was contaminated with E. coli O157:H7 (DOH News, 1999, September 16). Samples of manure collected from a barn located 50 feet from the well and samples from the groundwater flow from the manure storage area located 80 feet from the well tested negative for E. coli O157:H7. However, samples from the septic system tested positive for E. coli O157:H7.
Consumption of only two food or beverage items, soda with ice or ice in any drink, was reported by a majority of the culture-confirmed case patients. MMWR Weekly (1999) reported that the pulsed-field gel electrophoresis testing by the New York state laboratory indicated that the DNA fingerprints of E. coli O157:H7 isolates from the well, the water distribution system, and most confirmed cases were similar.
The epidemiological investigation of this outbreak concluded that a significant relationship was associated with the incidence of the outbreak and the consumption of beverages purchased from vendors supplied with water from the unchlorinated well.
Animal-to-Person Transmission of E. coli
Animal-to-person spread also occurs, and has been identified in several outbreak-situations as well as in isolated settings, such as homes.
E. coli at Fairs and Petting Zoos
The mode of transmission at agricultural fairs, petting zoos, and farm visits was previously thought to be limited to hand-to-mouth transmission following contact with contaminated surfaces or animals. Conclusions reached by investigators in several recent fair and petting zoo-associated outbreaks of E. coli O157:H7, however, suggest that ingestion—and perhaps even inhalation—of contaminated dust particles, may be sufficient to contract E. coli and HUS.
2002 E. coli outbreak associated with a county fair
In August of 2002, 75 people, including 12 children, became ill with E. coli O157:H7 infections after attending a county fair. Although it was not confirmed, health officials hypothesized that possible exposures leading to the outbreak occurred at animal enclosures, including the cattle tent, horse barn, and exposition halls that housed goats, sheep, rabbits, chickens, ducks, and guinea pigs (Oregon, 2002). Scientists discovered a virulent strain of bacteria on pipes 15 feet above the goat pens in a fair exhibition hall (Capital Press, 2002, September). A state epidemiologist from the Oregon Department of Human Services suggested that the microorganisms must have been present in the dirt and dust, and henceforth accumulated on the tops of the pipes 15 feet in the air.
2004 E. coli outbreak associated with a petting zoo
In late October 2004, the North Carolina Department of Health and Human Services (NCDHHS) conducted an E. coli O157:H7 outbreak investigation among attendees at a fair; NCDHHS identified 180 potential E. coli cases, including those of 15 children who developed hemolytic uremic syndrome.
Cultures from 33 ill fair attendees exhibited indistinguishable PFGE patterns and environmental samples obtained from four fairground areas grew E. coli O157:H7. Nineteen of thirty specimens obtained from a particular petting zoo grew E. coli O157:H7 and were a PFGE match to ill patients (MMWR Weekly, 2005).
Direct contact with goats and sheep was strongly associated with illness. Ill children age three years or less were seven times more likely to have contact with manure than children who were not ill. Ill children were also five times more likely to fall or sit on the ground than children who were not ill.
2005 E. coli outbreak associated with 3 petting zoos
In 2005, an E. coli O157:H7 outbreak was first recognized after two separate HUS case reports were posted on the Florida Department of Health surveillance system. The two cases reported having visited a fair with a petting zoo run by the same operator a few days prior to becoming ill. The two children visited the same fair and did not have any other risk factors in common.
The Orange County, Florida, Health Department was contacted shortly thereafter by a local hospital administrator, who reported a cluster of pediatric HUS cases in her hospital. Interviews with the parents of the hospitalized children revealed that all had attended a petting zoo at a fair or festival held during the same timeframe as the first fair. E. coli O157:H7 isolates from the initial cases were sent to the Bureau of Laboratory in Jacksonville for PFGE typing.
E. coli O157:H7 with PFGE patterns matching those isolated from victims were recovered from the animal exhibit areas of the one fair and the festival. E. coli O157:H7 with matching PFGE patterns were also recovered from 6 samples collected from animals associated with the petting zoo (MMWR Weekly, 2005), which was present at both fairs and the festival, and was determined to be the source of the outbreak.
A total of 22 confirmed, 45 suspect and 6 secondary cases from 20 Florida counties were identified as victims of this outbreak. All but one culture-confirmed case had isolates with matching PFGE patterns. Twelve cases developed HUS. There were no fatalities.
E. coli case associated with exposure on a farm
One incident involved an infant whose father failed to remove his boots upon returning home from his dairy farm. After crawling on the living room carpet, the infant contracted E. coli O157:H7 and become ill with diarrhea that progressed to HUS.
E. coli case associated with exposure to a family pet
Another infant caught E. coli from the family dog that was in the habit of running with the cattle. The dog had developed diarrhea and soiled the living room carpet. Although the child’s mother did her best to clean the carpet, this infant also contracted an E. coli O157:H7 infection that progressed to HUS.
Person-to-Person transmission of E. coli
Outbreaks of E. coli O157:H7 can also be caused by person-to-person transmission, which has occurred in daycare centers, hospitals, nursing homes, and private residences. Because the infectious dose is so small it is very easy for the bacteria to be transmitted among people with close physical contact.
2000 E. coli outbreak associated with a daycare
In August of 2000, a daycare in California was traced as the source of an E. coli O157:H7 outbreak. Health department officials who investigated the outbreak determined that the probable “index case”—a child who unknowingly brought the bacteria into the facility--experienced “explosive diarrhea at the daycare on the afternoon of 8-3-00.”
Shortly thereafter, four other children became infected with E. coli O157:H7 on successive days, the 6th, 7th, 8th and 9th of August, 2000. All of the children were in the same day care group. In addition to the illnesses of the children, the mother of one child, and another child’s sibling became ill and tested positive for E. coli. Another toddler also became ill.
According to the Facility Evaluation Report by the Department of Social Services, “[t]he cause of the [E. coli O157:H7] outbreak was due to a sponge being used simultaneously for wiping down a changing table and wiping down a table used for serving meals.”
E. coli case associated with person-to-person contact
A toddler in Idaho who had mild non-bloody diarrhea routinely shared the family’s bath tub with a neighbor’s child. Several days after the two children bathed together, the neighbor child developed bloody diarrhea that progressed to severe HUS. A few days later, the first toddler was also admitted to the same children’s hospital with HUS. Tragically, the neighbor’s child died.
E. coli case associated with person-to-person contact
A father who worked on a dairy farm contracted mild, non-bloody E. coli diarrhea that was transmitted to his son who developed HUS. The same event reoccurred two years later. The son’s second episode was devastating. Although the son survived, he was left with blindness and severe brain damage.