Super-Shedders: How Big of an Impact Can one Animal Have on Food Safety?

By: Suzanne Osborne, Ph.D

Ruminants, such as cattle, are a major reservoir for Escherichia coli O157:H7 which causes bloody diarrhoea and potentially fatal kidney failure in humans. Although the pathogens are harmless in cattle, these animals release (shed) the pathogens into the environment through their feces. The resulting contamination of manure, soil and ground water leads to transmission to other animals and humans.

The first attempts to examine transmission of E. coli between cattle perplexed researchers. Using data on the number of bacteria shed in the faeces across multiple farms, the mathematical models developed only seemed to work when it was assumed that a few select cattle had much higher transmission rates than others. Subsequent studies revealed that of cattle testing positive for E. coli, 75% of them shed less than 100 bacteria per gram of faeces. In sharp contrast, certain individual animals were found to shed up to 10 million. To add insult to injury, these individual animals, now know as super-shedders, shed bacteria for an extended period of time. Although super-shedders make up less than 10% of the cattle population, they are responsible for 80-96% of bacteria shed into the environment.

Super-shedders pose a huge risk to food safety and human health. Super-shedders are associated with increased spreading of bacteria to other cattle on the farm, during transport, slaughter and processing. In addition, the elevated release of bacteria into the environment poses a higher risk of direct human contact. It is a strongly held belief that the ability to identify and remove super-shedders from a population may significantly reduce transmission and contamination with E. coli O157:H7.

Extremely little is known about the causes of super-shedding. Theories range from differences in the genetics of the animal or their diet, to variability in the bacteria. The location of the bacteria within the intestinal tract of the animal has, however, been noted as a distinguishing feature of super-shedders. In contrast to the more diffuse bacterial location in a typical animal, super-shedders seem to have bacteria specifically attached to and replicating at the terminal rectum. The reason for this specific site of colonization is unknown. As a result, methods to identify super-shedders focus on the detection of E. coli O157:H7 from swabs of the recto-anal junction. However, the need for culture enrichment and molecular techniques to identify the pathogen using this strategy limits its utility for routine screening. Normal dips and spikes in bacterial shedding levels further complicates identification of super-shedders and dictates that multiple samples over a longer period of time would be required.

If one thing is clear, there is a need to better understand the mechanisms that underlie the cause of super-shedding and to develop diagnostics amiable to routine screening at the farm level. The identification and removal of super-shedders from the population is expected to significantly improve food safety and human health.

    • Chase-Topping, M., et al, 2008. Nature Reviews Microbiology, 6:904-912.


    • Matthews, L. et al 2006. Epidemiology and Infection, 134(1): 131-142.

About the Author

Dr. Suzanne Osborne’s expertise is in the field of host-pathogen interactions and foodborne bacteria. She obtained her doctoral degree at McMaster University and worked as a Research Fellow at the Hospital for Sick Children (Toronto). She has received numerous awards for her research. Suzanne currently does freelance science writing and grant writing.

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