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‘Supershedding’ cows could be the biggest cause of E. coli

Looks so innocent but is she a supershedder? b3d_, CC BY

Poisoning by E. coli is too familiar. As far as food-poisonings go, it’s one of the most serious and is frequently fatal in children and the elderly. Infection can also lead to lifelong kidney damage and acute kidney failure. One of the worst outbreaks saw 21 people die in Wishaw, Scotland in 1996.

Infection is caused by the Escherichia coli bacteria, and certain strains produce a dangerous toxin that can cause illness in humans. The worst of these is E. coli 0157, a pathogen that is transmitted from animal to human. Cattle are the main reservoir for the pathogen as cows harbour the bacteria in their gastrointestinal tract without becoming ill or showing any symptoms. Although it can spread between people, the usual routes of infection are either via the consumption of contaminated food and water, or by contact with livestock faeces.

If we are to control the spread of such diseases, we have to become better at identifying the major sources of risk in their spread. However, our understanding of how disease spreads across species boundaries is typically poor, which makes control difficult.

One idea is to vaccinate cattle. Vaccines have already been developed but their adoption is being hampered by licensing delays. Medical and veterinary agencies also have conflicting responsibilities. Those responsible for licensing vaccines in animals must typically show that a new control is not just safe, but that it improves the health of animals receiving it; this poses a problem for zoonotic pathogens that are benign in their reservoir hosts. It is also not easy to test a control measure used in animals against the outcome of a reduction in human illnesses, and this lack of data hampers effective decision-making when it comes to policy.

It is also difficult to mathematically model transmission of infection in animals to humans. Different factors such as the infectiousness of cattle, pathogen strains and infection routes, mean it’s not as simple as just measuring how much E. coli there is in cattle and using this to predict the risk for humans.

‘Supershedding cows’

But so-called “supershedding” might be one way of tackling the problem. Supershedding is a rare but epidemiologically important process where some cows are responsible for much more disease transmission than others. In the case of E. coli, some cattle shed the pathogen in faeces at unusually high concentrations. This is important because, despite being relatively rare, supershedders appear to be the major source of E. coli in the environment, from where other animals and humans can become infected.

A recent study in PNAS we published looked at this link between supershedding in cattle and transmission risk to humans. We found that although relatively rare, it was these cows that were significantly contributing to human risk. We used mathematical modelling to predict the impact of vaccination on the number of human cases. This was based on observations of more human cases associated with strains shed at high densities and fewer cases from strains shed at low densities. This information allowed us to predict the reduction in human cases from using vaccines that reduce high shedding.

This new understanding of animal-to-human transmission means a potential new way to target vaccines better. Vaccines could be used to reduce the frequency of bacterial shedding by particular cattle and also the number of bacteria shed when this happens. By doing this, the benefit to people from cattle vaccination should be substantially greater than previously anticipated by blanket vaccination. Our study showed that vaccines that cut shedding frequency in half could reduce human cases by nearly 85%, and could be an especially effective public health control against a serious disease.

In addition to the challenge of finding the best way to deal with the problem, it’s clear that veterinary and public health agencies aren’t helping by acting in apparent conflict. What the E. coli finding shows is the need for an understanding of animal and human health to be fundamentally integrated, rather than treated as discrete issues to be dealt with by separate organisations. It is only in this way that we can link together animal health findings to preventing infection in people.

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