Sources of E. coli
Where do E. coli O157:H7 and non-O157 Shiga toxin-producing E. coli (STEC) come from?
The primary reservoirs, or ultimate sources, of E. coli O157:H7 and non-O157 STEC in nature are domestic ruminant animals (cattle, goats, and sheep). Beef and dairy cattle are the most important reservoirs of STEC. STEC are present in virtually all cattle herds, and in many studies, more than half of individual animals in these herds have carried STEC.
The percentage of cattle that carry E. coli O157:H7 varies widely by age, season, production type, and testing method. Overall estimates of the percentage of cattle that carry E. coli O157:H7 range from up to 28% for beef cattle to up to 49% for dairy cattle. In feedlot cattle in the United States, multiple studies have shown E. coli O157:H7 carriage rates from 18% to more than 40% of individual animals. Non-O157 STEC are carried by cattle even more frequently, with about 45-75% of individual animals carrying the bacteria, depending on the study. However, a large proportion of non-O157 STEC strains recovered from cattle do not contain all of the virulence factors necessary to cause disease in humans. Still, virulent non-O157 are commonly carried by cattle.
Sheep and goats are important sources of STEC for humans as well, as indicated by foodborne outbreaks and outbreaks associated with animal contact venues such as petting zoos and agricultural fairs. 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 E. coli O157:H7 in 13.8% of beef cattle, 5.9% of dairy cattle, 5.2% of sheep, and 2.8% of goats. Wild ruminants, such as deer and elk, and farmed ruminants, such as bison, are also sources of the bacteria for humans. In all ruminant reservoir animals, STEC lives in the intestines without causing the host any harm. Therefore, it is not possible to tell whether an animal might be carrying STEC by looking at it, as perfectly healthy animals can harbor the bacteria. STEC can also be found in the intestines of other animals and birds, but they are of limited or no importance as a source for human infection.
STEC are passed in the feces of the ruminant animal reservoir, and ruminant feces will contaminate any environment in which the animals are or have been present. Subsequently, there are a multitude of ways that STEC bacteria can make their way to humans, be ingested, and cause infection. The bacteria can make their way from animal feces into foods such as raw milk or meat during collection or processing (often via the hide), or fresh produce (e.g., leafy greens, strawberries, hazelnuts) or other agricultural products (seeds for sprouting wheat flour) through contamination with animal waste or contaminated irrigation water in fields.
Infections associated with swimming in freshwater lakes or streams can occur when the water is susceptible to run-off from ruminant animal areas (e.g., pastures, feedlots). Similarly, drinking water can be contaminated when drinking water sources are impacted by animal sources.
Contact with ruminant animals, especially calves and goats, or their environments, in settings such as petting zoos, fairs, or private farms (your own or your relative’s) is a common source of infection. Moreover, the bacteria can survive in soil for months, and even dusty surfaces in these environments for weeks.
Once humans are infected via any of the methods described above, they can readily pass it on to others, because they too pass the bacteria in their feces, even well after the symptoms have resolved. Person-to-person transmission occurs frequently, particularly among diaper-aged children in child daycare centers. Transmission within households occurs frequently too, with caregivers and siblings affected. Infected people can be the source for others too when they share a bath, kiddie pool, freshwater swimming location, or even poorly chlorinated pools.
Successful transmission of E. coli O157:H7-from food, water, person-to-person contact, and animal contact happens so easily because of its very low infectious dose. As few as 20 organisms may be sufficient to infect a person. Therefore, infection occurs because of only slight undercooking, minor cross-contamination from meat to ready-to-eat food, ingestion of a small amount of water while swimming, and a single instance of hand-to-mouth contact after touching a contaminated surface in a petting zoo or in a daycare facility. Furthermore, compared with generic E. coli, the O157:H7 serotype is relatively heat-resistant (to a point), resists drying, and can survive exposure to acidic environments.