Foodborne zoonoses

Food poisoning can be serious—as well as diarrhoea and vomiting, infection can have long term implications. Sarah J O'Brien looks at the key roles that doctors and vets have in tackling the hundreds of millions of cases a year worldwide

Foodborne disease (food poisoning) tends to be regarded as a comedy illness—not pleasant to have, or talk about, but little more than an inconvenience. Yet trivialising foodborne disease ignores the size of the illness burden: estimates vary from 76 million cases of foodborne disease annually in the United States^w1 to 5.4 million in Australia^w2 and 1.3 million in England and Wales.^w3 Three of the major pathogens—Campylobacter spp, Salmonella spp, and Shiga toxin producing Escherichia coli O157 (STEC O157)—are zoonoses (that is, transmitted from vertebrate animals to humans). As well as causing acute symptoms including diarrhoea and vomiting, infection can have long term implications.

Campylobacter is the principal bacterial cause of gastroenteritis in the developed world. The World Health Organization estimates that about 1% of the population of Europe will be infected with Campylobacter spp each year. In England and Wales about 45 000 acute cases are diagnosed annually. Infection is associated with development of Guillain-Barré syndrome, the commonest cause of acute flaccid paralysis in polio-free regions in the world.^w4 The risk of developing it after campylobacter infection is about 1 in 1000 (the risk rising to around 1 in 160 for infections with certain serotypes).^w5 Both campylobacter and salmonella infections can result in reactive arthritis, although the precise mechanisms are still unclear, and infection with STEC O157 is a leading cause of haemolytic uraemic syndrome, the most common preventable trigger for acute renal failure in children.^w6

Clinical management of patients with acute symptoms of foodborne disease in primary care is generally the same regardless of cause (rest and rehydration), and treatment with antimicrobials is rarely indicated for uncomplicated diarrhoea. For example, there is no evidence that antibiotic treatment of uncomplicated salmonella diarrhoea in otherwise healthy children and adults is beneficial, and it may even prolong salmonella carriage.^w7 Antimicrobial treatment in cases of STEC O157 infection might do positive harm, potentially precipitating the onset of haemolytic uraemic syndrome.^w8 Limiting inappropriate antimicrobial use also has broader public health benefits.

Although knowing the causative organism might not affect clinical management, it is vital for public health protection, and primary care is in the front line as regards outbreak detection. Differentiating infection with foodborne zoonoses from other causes of acute diarrhoea is not always easy, but clinical acumen, appropriate laboratory tests, and prompt alerts to local health protection teams allow public health professionals to assess whether an apparently sporadic case is simply that or whether it is necessary to initiate the detailed detective work required to track an outbreak to its source. Clinical, epidemiological, microbiological, and environmental studies require the combined effort of doctors and vets, alongside laboratory and environmental health colleagues.

Epidemiological studies of sporadic infection also yield important clues about transmission paths and potential sources. The now familiar association between Salmonella enteritidis phage type 4 infection in humans and consumption of poultry eggs^w9 was revealed through such endeavour. More than a decade after those observations were published in the BMJ, the epidemic of infection in humans is seemingly coming under control. However, a continued public health challenge is to ensure that good work undertaken by UK industry is not jeopardised by buying contaminated products from overseas.^w10

Primary prevention of foodborne zoonoses is mainly a veterinary responsibility. However, organisms causing human illness do not necessarily have animal health implications—campylobacter and STEC O157 are cases in point. Surveillance data amassed from clinical investigations can help policy makers target resources. For example, the Food Standards Agency's campaign “Cleaner farms, better flocks” reflects the fact that reducing the impact of UK foodborne disease chiefly depends on controlling campylobacter contamination of chickens.^w11 Safeguarding the flow of surveillance data remains a priority in the wake of developments in data protection, and in changes to the delivery of primary care, including the introduction of telephone triage systems.

Like all communicable diseases, foodborne zoonoses do not respect administrative or professional boundaries. Acknowledging the importance of an integrated approach to investigation and control of zoonoses, including foodborne zoonoses, several European countries have formed unified administrative structures. Although this has not formally happened in the UK, a combined approach at government level has led to several initiatives, including the production of UK zoonoses reports (available at www.defra.gov.uk/animalh/diseases/zoonoses/reports.htm).

It is, perhaps, worth reflecting that vets and doctors form either end of a chain (from primary prevention at one end to dealing with its failures at the other) and that decisions made by many others along the food chain, including in the food industry (primary production, retail, and catering) affect our exposure to foodborne pathogens. The recent outbreak of STEC O157 in Wales^w12 serves as a timely reminder of the importance of foodborne zoonoses and of the collaborative efforts needed to control them.

DAVID GREGORY AND DEBBIE MARSHALL/WELLCOME PHOTO LIBRARY
 False colour scanning electron micrograph of Escherichia coli, showing pili

Competing interests: SJO'B is a member of the Advisory Committee on the Microbiological Safety of Food.

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