Escherichia coli: on-farm contamination of animals.

Escherichia coli is one of the main inhabitants of the intestinal tract of most mammalian species, including humans, and birds. Shiga toxin-producing E. coli (STEC), also called verotoxinogenic E. coli, usually do not cause disease in animals but may cause watery diarrhoea, haemorrhagic colitis, and/or haemolytic uraemic syndrome in humans. Zoonotic STEC include the O157:H7 strains and, with increasing frequency, certain non-O157 strains. The importance of non-O157 zoonotic strains is probably underestimated as they have been less well characterised and are more difficult to detect in samples than O157:H7. Another large subset of STEC strains has been isolated from animals but has not, at the present time, been associated with disease in animals or humans. Cattle and other ruminants are the most important reservoir of zoonotic STEC, which are transmitted to humans through the ingestion of foods or water contaminated with animal faeces, or through direct contact with the infected animals or their environment. The main sources of STEC infection of cattle on-farm are the drinking water, the feed, and the immediate environment of the animal. Risk factors that have been identified for infection of animals with O157 STEC include age, weaning, movement of the animals, season, feed composition, and the ability of the bacteria to persist in the environment. On-farm control of the zoonotic risk of human infection with STEC should primarily target the main source of contamination: the animal reservoir. Various strategies to reduce intestinal colonisation of cattle by zoonotic STEC have been tried with varying results, including vaccination, treatment with probiotics, such as direct-fed microbials or competitive exclusion, administration of bacteriophages, and modification of the diet.