Transmission of and Infection with E. coli
How does food become contaminated with E. coli?
While many dairy cattle-associated foodborne disease outbreaks are linked to raw milk and other raw dairy products (e.g., cheeses, butter, ice cream), dairy cattle still represent a source of contamination of beef products. Male calves of dairy cattle usually enter the beef production cycle, and dairy cows themselves are sent to slaughter for hamburger production at the end of their useful milk production life (termed “cull” or “spent” dairy cows, flies, or other animal vectors. Similarly, other agricultural products (e.g., seeds that are to be sprouted, wheat flour) can be contaminated during production, as plants are commonly grown in areas with domestic (and sometimes wild) ruminants. Sprouts in particular are a frequent cause of outbreaks because seeds cannot really be effectively decontaminated, or they won’t sprout; furthermore, conditions for sprouting (heat, moisture) are the same conditions conducive to bacterial growth.
How do beef products become contaminated with Shiga toxin-producing E. coli (STEC)?
Up to 80% of hides of cattle tested at processing plants are contaminated with E. coli O157:H7, depending on the study. Contamination rates of hides with non-O157 STEC are comparable to those of O157. Hides routinely have higher STEC contamination rates than do fecal samples, and hides tested in processing plants often have significantly higher contamination rates than hides of cattle tested in feedlots.
Contamination rates of E. coli O157:H7 in beef products have declined substantially since the mid-1990s, primarily because of immense and persistent efforts by the United States Department of Agriculture (USDA) Food Safety and Inspection Service, academic institution partners, and the beef production industry itself. By the mid-2000s, ground beef positivity rates were generally <0.2%. In 2008, a USDA study indicated that 0.7% of boneless beef manufacturing trimmings designated for use in raw beef were positive for E. coli O157:H7. In 2019, testing by USDA in beef processing plants indicated that different classes of beef components that end up in ground beef had very low contamination rates with E. coli O157:H7: 1/146 (0.7%) “raw ground beef components,” 6/4131 (0.1%) “manufacturing trim,” and 1/471 (0.2%) “bench trim” samples. These types of beef components are frequently obtained from cuts of beef like steaks and so give a general idea of risk from those types of products. USDA testing of finished raw ground beef in processing plants in 2019 revealed that only 5 of 10,932 (0.05%) samples were positive for E. coli O157:H7. These are very small percentages, but caution is still warranted, as ground beef is still a common source of outbreaks because it is so frequently consumed.
Up-to-date data on contamination rates of ground beef with non-O157 STEC are not available, but USDA testing of manufacturing trim in beef processing plants in 2019 revealed that 36 of 4035 (0.9%) samples were positive. Therefore, ground beef remains as much, or more, of a risk for non-O157 STEC infections as it is for E. coli O157:H7.
Mechanically tenderized (a.k.a. blade-tenderized) steaks have emerged as a vehicle of multiple E. coli O157:H7 outbreaks in the last 20 years. Ordinarily, if steaks and other intact cuts of beef are contaminated with STEC, that contamination should only be on the surface and simply cooking the outside of the beef would be enough to eliminate that contamination. However, mechanical tenderization involves stabbing the steak with hundreds of tiny needles or blades to tenderize the product. This, in turn, transfers any STEC bacteria that are on the surface of the meat to the inside of the meat. Consequently, if steaks are only cooked to a medium rare or even medium level, the STEC can survive and cause illness. If steaks are mechanically tenderized, they have to be labeled as such.
Environmental Sources of Shiga toxin-producing E. coli (STEC)
Prolonged environmental persistence of E. coli O157:H7 was documented in an Ohio outbreak of E. coli O157:H7 infections in which 23 persons became ill at a fair after handling sawdust, attending a dance, or eating and drinking in a building where animals were exhibited as part of a fair during the previous week. Fourteen weeks after the fair ended, E. coli O157:H7 was isolated from multiple environmental sources within the building, including sawdust on the floor and dust on the rafters. Forty-two weeks after the fair ended, E. coli O157:H7 was still present in the sawdust.
In a 2009 E. coli O157:H7 outbreak associated with rodeo attendance, the outbreak strain was isolated from the rodeo grounds 90 days after the end of the event. In a study prompted by a single E. coli O157:H7 case, the bacteria survived for >2 months in garden soil fertilized with cattle manure from a relative’s farm.
STEC also survives well in water, remaining viable for weeks. Feces from cattle or other ruminant animals can be washed into water sources, resulting in contaminated recreational water, drinking water, or water that is used to irrigate produce or other crops.