Nausea on a plate

Salmonella can survive a spin in the microwave. It's enough to make you ill, says Bernard Dixon

Public health workers in Cardiff have called attention to the danger that food-poisoning bacteria can survive inadequate microwave cooking. The warning by Meirion Evans and his colleagues appears in their report in Epidemiology and Infection of a food-poisoning outbreak caused by Salmonella enteriditis in a savoury rice dish that had not been properly microwaved. Several people became ill after eating a buffet meal prepared at one of their homes.

S. enteritidis infections began to increase worldwide in the late Seventies. During the late Eighties, scientists realised that part of this rise was attributable to the contamination of hens' eggs, and it was as a result of publicly exaggerating the size of the problem, just before Christmas 1988, that the Health Minister Edwina Currie was forced to resign.

The warning from Cardiff is timely, since sales of prepared meals for microwaving have gone up dramatically over recent years. In future, any danger of infection from such foods may be eliminated entirely by so-called "smart" microwave ovens, now coming on to the market, which guarantee thorough cooking. But for the moment a risk remains whenever users fail to follow the precise instructions that accompany ovens and particular food products and recipes.

In the Cardiff incident, the host and at least five of her six guests developed diarrhoea, abdominal pains, fever and headache the day after eating a dinner consisting of cheese and onion quiche, a savoury rice dish, potato salad, green salad, vegetarian sausage, strawberry mousse and chocolate cake. Three of the victims also had bouts of vomiting.

Mr Evans, of the South Glamorgan Health Authority, investigated the outbreak together with colleagues from Cardiff Public Health Laboratory and Cardiff Environmental Services. To determine exactly what the diners had eaten, they used questionnaires, which all but one of the seven people completed. The investigators also obtained samples of faeces from four of the individuals.

Tests on the only two food items still available showed that the rice dish, eaten by all six of those who became ill, contained S. enteritidis, a well-known cause of food poisoning. The quiche contained S. enteritidis, too, but only at a very low level which was probably a result of contamination from the rice dish. The four samples of faeces also carried S. enteriditis and not only the same microbe but exactly the same strain of that microbe.

These findings heavily implicated the rice dish as the cause of the outbreak. But Mr Evans and his colleagues were unable to discover how it, in turn, had become infected. One possibility was that the S. enteritidis had come from the host, who had prepared the dinner. This seemed extremely unlikely, however, as she did not become ill until the day after the meal, in common with her guests. Of the three most common carriers of S. enteritidis, no poultry or meat were served at the meal. The third potential source, eggs, did fall under suspicion because two raw eggs had been used to make the rice dish. However, tests on the remainder of the pack of six were negative.

But why had the heat of the microwaving not destroyed the bacteria, whatever their origin? The host had cooked the rice dish, composed of rice, raw eggs, grated carrot, cheese and curry powder, in a 500Watt oven for five minutes on a rotating turntable. The investigators believe this may not have been sufficient to sterilise it. "This incident was almost certainly a consequence of inadequate microwave cooking, either due to uneven heating of the rice dish or to insufficient duration of cooking," they conclude.

Microwave ovens can undoubtedly produce temperatures similar to those achieved by traditional cooking. However, there is a possibility that disease-causing microbes will survive in "cold spots" inside the food, unless users follow the manufacturer's instructions rigorously. One previous study of chicken contaminated with S. typhimurium showed that five out of nine samples cooked in a microwave still contained the living bacteria. But none of nine samples cooked in the conventional way remained infected. Similar research has demonstrated that localised concentrations of another food poisoning bacterium, Listeria monocytogenes, can survive in broth and on chicken skin even when a relatively high temperature is reached overall.

The food poisoning outbreak in Cardiff underscores the lessons of another incident, reported last year, in which microwave cooking was also implicated. Shortly after a community picnic in Juneau, Alaska, several people developed gastrointestinal symptoms as a result of S. typhimurium infection which originated in roast pork. Of 60 of those attending the picnic who took leftover food home later, 30 ate some of the roast pork after reheating it. Ten of these people used a microwave oven and all became ill. Yet not one of the 20 of those who warmed up the meat in a conventional oven or saucepan suffered any symptoms.

The "smart" microwave ovens that are now appearing in the shops should obviate these problems entirely because they monitor the steam generated when food is cooking and ensure the correct temperature and time accordingly. For those without such sophisticated technology, Mr Evans and his co-investigators have a straightfoward message: "All microwave oven users need to be aware of the importance of carefully following the standard recipe or food product instructions when heating food. If no instructions are available, food should be heated until piping hot and, to ensure it has attained an even temperature, stirred carefully during heating (where possible) and allowed standing time after cooking."