All About E. coli O157:H7
Where does E. coli O157:H7 come from?
Ruminant livestock such as cattle, deer, goats and sheep naturally carry E. coli O157:H7 in their systems.
Cattle, however, are considered to be one of the primary sources of E. coli O157:H7 worldwide. Numerous studies have shown that E. coli O157:H7 prevalence is widespread in dairy and beef animals, and can be found in, on and around cattle in most parts of the world.
E. coli O157:H7 does not cause disease in cattle, so most farmers do not know whether their animals are carriers or not. The pathogen lives in an animal’s intestinal tract and is spread to the outside environment through its manure. This is why cattle are considered to be the primary reservoir and source of infection, even though many different foods—and water sources—are often implicated in human e.coli outbreaks.
Recent reports estimate that as many as half of all cattle shed enterohemorrhagic E. coli O157:H7 in their manure at some stage of their life (Gansheroff and O’Brien 2000; Elder et al, 2000). The level of e.coli prevalence seems to be high and variable, ranging from 10% in some countries to almost 100% in others (NASIM, 2002). Data out of the US suggests that almost 30% of animals going to slaughter could be infected with E. coli O157:H7, and an average of 43% of carcasses coming out of the slaughter line at processing plants are contaminated (NASIM, 2002).
In beef feedlots, the number of animals that shed E. coli O157:H7 varies greatly over time on both a pen and feedlot level. Peak shedding of E. coli O157:H7 in cattle manure happens in the summer and early fall, with variation from 0% to 61% on some farms (Laegreid et al, 1999; Heuvelink et al, 1996; Jackson et al, 1998). Coincidentally, summer is also when contamination rates in ground beef and the number of human infections are higher.
According to Dr. David Smith, a dairy and beef veterinarian at the University of Nebraska-Lincoln, it’s the survival instinct of the bacteria that causes this phenomenon.
"It’s their success at getting back into another host animal that assures their survival,” says Smith. “When it’s wet or warm, there are more opportunities for the organism to be re-ingested and, under those conditions, we see lots of cattle shedding the organism.”
This means that in colder fall, winter and spring seasons, the bacteria are more likely to stay inside the host animal instead of exposing themselves to the harsh elements.