Escherichia coli (E. coli) O157 is a particularly important bacterium that can cause very serious disease in people, including diarrhea and a severe kidney disorder called hemolytic uremic syndrome (HUS). Infection in people typically results from ingesting the bacterium from contaminated food products. The main reservoir of E. coli O157 is in the intestinal tract of cattle. There have been a few reports of suspected transmission of E. coli O157 from pets, but overall pets are thought to play a very minor role in the spread of this disease.
However, minor role does not mean no role. A recent report in the journal Veterinary Record described suspected transmission of E. coli O157 from dogs to people. Three children and two adults in a household developed diarrhea, and E. coli O157 was isolated from their stool, as well as from the stool of a healthy sibling in the house. Molecular testing showed that all the people were infected with the same strain of E. coli. An "outbreak response" was initiated by the UK’s Health Protection Agency to determine the source of infection. One of the affected children, the first to get sick, visited a farm five days before developing diarrhea, so the investigation focused on that farm.
The same E. coli strain was found in 7/29 samples collected from the farm: three samples from calves, two from dogs, one from a manure pile and one from a calf pen gate. Finding E. coli in samples from the calves was not surprising, as it is commonly found in healthy cattle. However, the visiting child did not have direct contact with the calves. Finding the bacterium in the dogs was somewhat surprising. Since the child had contact with both dogs, this was thought to be the most likely source of infection. Cattle were probably the source that infected the dogs, the dogs were then probably able to transmit the infection to the child, and the child then infected other people in the house. Infection from contact with E. coli in the environment is also possible, but considering there was confirmed contact with dogs who were carrying the same strain, the conclusion that the dogs were likely the source is reasonable.
Like many of the other bacteria we worry about, E. coli is transmitted by the fecal-oral route – infection is spread by swallowing feces/stool/manure (even in minute quantities) that contains E. coli. This can occur more easily than people think, as low-level fecal contamination of hands and other surfaces is common. Identifying animals that carry this bacterium is not practical. The most important protective measure is close attention to handwashing after contact with animals, especially farm animals or pets exposed to farm animals.
This study provides more information about dogs as potential sources of E. coli O157 and the need to include testing of pets during outbreak investigations. However, dogs are probably still a minor source of this important pathogen, and it’s most likely only of concern in dogs with close contact with cattle.