Beware of lemon meringue pie

Microbe of the month: Bernard Dixon on the return of salmonella

With summer around the corner, a warning to campers. Life under canvas can provide the perfect conditions for bacteria to proliferate in foodstuffs, according to a new report of a gastroenteritis outbreak caused by Salmonella enteritidis during a camping holiday in north Wales. Thirty-three people were admitted to hospital with diarrhoea, vomiting and abdominal pain.

Describing the incident in Epidemiology and Infection, Ruairi Brugha of the Central Public Health Laboratory in Colindale, with colleagues in Cardiff and Bangor, points out that infected eggs pose a particular danger to campers. The victims in the Wales case - the first reported outbreak caused by a new type of S. enteritidis discovered in 1993 - were on a trip organised by a Boys and Girls Brigade group. The 12 adults and 37 children (aged nine to 17) brought tents, ovens, cooked and uncooked food from their home town 300 miles away. But they had no refrigeration facilities. Three days after they arrived, 46 of the 49 members of the party went down with gastroenteritis. The condition of 33 of them soon deteriorated, and ambulances ferried the victims to hospital. Thirty-one of them had to be intravenously rehydrated.

The bacteriologists who investigated the incident found the campers had discarded the remains of the meal consumed the evening before the first symptoms appeared. Though they could not test the foods, the investigators screened samples of faeces from all the campers for disease-causing bacteria.

A comparison of these results with the food items eaten by individual campers quickly highlighted lemon meringue pie as the most likely - indeed the only - source of infection. Every one of the 42 individuals who had eaten the pie had become ill. And every one of them had S. enteritidis in their faeces, except one who had eaten only three or four spoonfuls. None of the faecal samples contained any other hazardous microbes.

The pie had been made from eggs, pastry, sugar and lemon powder. The eggs came from a batch laid 13 days before theoutbreak, transported from the campers' home town and stored at the camp in warm summer temperatures for two days before the pie was made. Two cooks produced the pie mixture, containing 20 egg yolks, boiled it for one or two minutes and then left it at ambient temperature for four hours. They added a meringue topping, made with egg white that had also been standing for four hours, and put portions of pie into three ovens at gas mark 6 for five to 10 minutes to brown. The pie then stood at ambient temperature for another 2-3 hours before being eaten.

Tests on the remaining six eggs failed to show S. enteritidis, so the investigators could not pinpoint the source of the outbreak with 100 per cent certainty. However, the dangers posed by this bacterium in eggs and egg products have become clear over the past decade (though Edwina Currie exaggerated the risk in 1988). Together with other evidence, this makes the origin of the Welsh incident virtually certain.

Given that the bacterium can occur in eggs, the ways of preventing it from causing illness are to reduce its chances of growing (by refrigeration) and to kill it (by thorough cooking). On this occasion, S. enteritidis had ample opportunity to multiply while the eggs, pie mixture and pie were left at ambient temperature and the gas ovens were probably too small for the amount of food being cooked, so they did not reach a temperature high enough to kill the bacteria.

Three years ago, the Government's Advisory Committee on the Microbiological Safety of Food produced a report on the dangers posed by bacteria such as S. enteritidis in eggs. It included clear guidelines on the handling and storage of eggs. However, these need to be supplemented by advice specifically directed at campers. Meanwhile, people holidaying outdoors should use cold boxes to carry eggs, and should not prepare lightly cooked egg products under field conditions. Safer still, give up eggs completely for a few days.