High summer puts poison on our plates

With the increasing number of cases of food poisoning increasing every year a wide range of quick and easy tests is needed

Summertime ... and the living is easy for harmful pathogens.  Food that’s out of the fridge too long, undercooked barbecue meats and salad that’s inadequately washed all contribute to a rise in food poisoning cases during the summer months - which not only increases the workload of hard-working laboratories but also gives rise to a growing number of compensation claims.  Hotels, restaurants, travel companies, shops and all types of mass catering establishments are increasingly being targeted by lawyers in the wake of food poisoning outbreaks.  Is it a sign of the times that many firms of solicitors now specialise in suing those deemed responsible for endangering public health?

We now live in a so-called ‘compensation culture’, but it is also a culture in which we are all increasingly exposed to food poisoning risks.  As our standards of living rise, so do the chances of falling prey to lower standards of food hygiene, particularly where eating out and travel are involved.

Risks at home and abroad
Around four in ten British tourists are reported to suffer stomach bugs during holidays abroad (people taking all-inclusive foreign package holidays are thought to be more likely to fall ill than independent travellers).  The number contracting food poisoning while on holiday in the UK may also be as high as 10%.  Yet it is not only holiday-makers who are at risk.  The arrival of summer heralds an increase in outdoor eating as poor food preparation and storage practices combine with warmer weather to produce ideal conditions for the growth of pathogens.

Overall, salmonellosis (caused mainly by S. enteritidis and other Salmonella species) is by far the most frequently reported foodborne disease in Europe, although the available statistics are subject to differences in national regulations, reporting systems and varying levels of under-reporting.  However, campylobacteriosis (caused by Campylobacter) has become the most commonly reported gastrointestinal infection in many individual countries, including the Netherlands, England and Wales, Scotland, Finland, Denmark, Norway, Sweden, Iceland and Switzerland.  Other foodborne bacterial causative agents include Staphylococcus aureus, Clostridium perfringens and Clostridium botulinum (the latter being responsible for the potentially deadly condition of botulism), Bacillus cereus, Shigella, E. coli and Listeria.  Viruses (including Norwalk, Rotavirus and Hepatitis A), parasites (such as Trichinella, Giardia and Cryptosporidium) and chemical agents also play a significant role.

Eggs and egg-containing products, meat, poultry, shellfish, dairy products and confectionery are most commonly regarded as high-risk foods, but new and unexpected sources of food poisoning pathogens are emerging all the time.  For example, lettuce was implicated in an outbreak of salmonellosis in Lincolnshire last year, after contaminated leaves had been left lying around on display at high temperatures in a fast food outlet.  A toxin found in predatory tropical reef fish such as jack, grouper, barracuda and snapper has also recently attracted worldwide clinical attention as the cause of a severe food poisoning condition known as ciguatera.  19 people fell seriously ill in Turkey last year after eating local untreated honey which contained a toxin derived specifically from the leaves and flowers of rhododendrons and azaleas.

While untreated honey and tropical fish constitute risks which Britons are only likely to encounter on foreign holidays, the main sources of food poisoning will most often be found closer to home.  According to WHO statistics for Europe, most foodborne disease outbreaks (38%) occur in private houses, often following social events when large quantities of foods have been prepared in advance and not kept cool before serving.  Besides restaurants, hotels and bars (20%), different mass catering establishments such as schools and kindergartens (9%), canteens and medical care facilities (3%) have been found to be the next most significant locations where outbreaks occur.

Fast food outlets in particular are coming under increasing scrutiny.  In addition to the lettuce-related salmonellosis incident detailed above, the BBC has recently reported two other cases in England which highlight cause for concern.  In one of these a well-known deep-fried chicken chain was fined for selling improperly cooked food which was deemed unfit for human consumption, while the other focused on a kebab shop implicated in one of the largest outbreaks of salmonellosis in London in recent years, affecting over 160 people.

Thus, the idea that food poisoning is predominantly a threat to travellers abroad is clearly a misconception.  Popular wisdom dictates that tourists should avoid drinking tap water in foreign countries, yet - in Europe at least - contaminated water accounts for a mere 0.8% of recorded food-related illnesses3.  In order of importance, the main causative factors behind food poisoning outbreaks are temperature misuse, use of inadequate raw materials, inadequate handling and environmental factors - all of which can readily occur in the UK, particularly in summer.

Establishing blame
In the meantime, lawyers specialising in compensation suits are flexing their muscles.  “£3m demand from salmonella-hit tourists”; “Holiday makers seek compensation after fourth outbreak of Cryptosporidium and Salmonella”; “Further evidence shows holidaymakers not informed of illness outbreak at holiday hotel”; “Compensation call for holiday makers” ... such headlines have become increasingly frequent over the past six years.  One of the most high-profile cases to attract compensation claims in recent years is the ill-fated Mediterranean voyage of the P&O cruise ship Aurora in 2003, when some 1,800 passengers were affected by Norovirus.

Yet making a claim is one thing - proving it is quite another.  During the course of a single summer’s day, a person might cook their own breakfast, have a picnic lunch, consume a snack in a café and then eat dinner in a restaurant.  Unless it can be shown that other people have suffered the same illness after consuming the same food, substantiating a claim can be very difficult.

Laboratory tests clearly have a vital role to play, not only in terms of identifying the particular pathogen involved in an outbreak of food poisoning, but also in tracking down its source.  Whether the tests are requested by doctors treating those who have fallen ill or environmental health officers investigating an outbreak, time is very much of the essence. Rapid identification is essential if appropriate clinical and epidemiological decisions are to be made without delay.

The task is undoubtedly a challenging one. There are over 2,400 serotypes of Salmonella alone that may cause food poisoning, but a wide range of tests is now available to make life easier for laboratories.  For example, Oxoid’s Salmonella Rapid Test combines enrichment and detection in a self-contained disposable culture vessel, exploiting the motile characteristics of most Salmonella species.  This test will detect motile salmonellae in pre-enriched food and environmental samples after just 24 hours, indicating their presence through clear colour changes in two reaction tubes.  Positive cultures can quickly be confirmed using Oxoid’s Salmonella Latex which utilises latex particles coated with polyvalent antisera against the flagella antigens which are characteristic of the two most common illness-causing species:  S. enteritidis and S. typhimurium.  The particles agglutinate to form large visible clumps in the presence of these antigens, facilitating presumptive identification of Salmonella species from selective and enrichment cultures.

Oxoids Salmonella Rapid Test.

The Wellcolex Colour Salmonella test is formulated to identify Salmonella groups A, B, C, D, E/G or Vi antigen from Selenite Broth or solid media.  Two reagents are provided, each containing a mixture of three different coloured latex particles which are sensitised with antibodies specific to different groups.  A simple test procedure provides rapid results in terms of easily recognisable colour reactions.

It can often be difficult for laboratories to distinguish salmonellae from other organisms such as Citrobacter, Proteus and Morganella species which exhibit similar colonial appearance on standard diagnostic culture media.  The Oxoid Biochemical Identification System for Salmonella, OBIS Salmonella, incorporates two simple biochemical reactions on a single disposable test card, reliably identifying false positive colonies within five minutes.  This rapid screening test allows the laboratory to advise clinicians that Salmonella is not present in a clinical specimen up to a day earlier than by traditional methods, thus reducing the number of samples that need to be investigated further.

Oxoid also produce a chromogenic medium for improved selective identification of Salmonella species, which provides clear differentiation of Salmonella from other Enterobacteriaceae and detects more atypical strains than alternative media.

The number of different Salmonella testing products on the market reflects the extent of the illness caused by this particular pathogen.  The World Health Organisation reports the incidence of salmonellosis in Europe in terms of the number of cases per 100,000 inhabitants - and the figures for different countries vary dramatically3.  In Western Europe, Belgium reported the highest annual average incidence of 120 cases per 100,000 people between 1994 and 1998, compared with 55 in England and Wales and 57 in Scotland.  Portugal stands out amongst Southern European countries with a comparable incidence rate of only 5, contrasting sharply with the Czech Republic in Central Europe where the average rate was 458.

As stated earlier, these dramatic differences may reflect the varying quality and comparability of national reporting systems.  However, some clear temporal trends emerge, both seasonally and in the long term.  Countries that provide information about seasonal variations in Salmonella, such as the Czech Republic and Southern European countries consistently report a typical Salmonella incidence curve during the year with a peak in the warmer months.  Clearly, the lazy days of summer give us little cause to relax.

By Alison Smith, Marketing Manager, Oxoid Ltd