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According to The Health Protection Agency (HPA), there has been an investigation into why there has been an 80 per cent increase in listeria cases over the first 21 weeks of this year, compared with the same period last year.

The number of hospital cases reported to the end of last week was 79, of which nine were pregnant women. Majority of cases were from the North East, Wales and London, but there have been cases reported in every region of England and Wales.

A high percentage (about 55 people) with the illness were aged over 60 and likely to have been suffering from other chronic conditions. The figures suggest the highest incidence of listeria since surveillance for listeriosis was introduced in 1990.

Pregnant women are one of the groups of people most vulnerable to the infection. While symptoms may be mild, the infection can trigger miscarriage, stillbirth, premature birth and serious illness in a newborn baby.

Other groups particularly at risk are the elderly, very young and people receiving medical treatments that compromise the immune system. For most healthy adults the risk is low, but severe cases can lead to fatal complications.

Most people become infected from eating certain foods. Soft cheeses are especially linked to the disease, but caution is advised over feta if it is made from unpasteurised milk. Pâtés, smoked fish and unwashed salad can also be contaminated. There is also concern over some bought, cooked and chilled meats.

Health chiefs are anxious to find out if there is a common food source because many more people may be incubating the illness. Symptoms can take as long as 90 days to develop, although the average is 30 days

They have ruled out any link with a Food Standards Agency poisoning alert in March that concerned 250,000 packs of sandwiches sold in the South East only, under the Anchor or Pomegranate label, which may have been contaminated with listeria bacteria.

But disease experts are now studying the eating patterns of surviving patients to see if there is any connection with food. Microbiological testing of food samples is also under way to see if a link can be identified

The HPA has reported a steady rise in incidence of the disease since 2001, but the scale of this year’s increase is huge.

Iain Gillespie, the agency’s head of bacterial diseases in the environmental and enteric disease department, said: “We really are unable to say what has caused the rise in cases. There are no particular clusters of cases and the cases seem to be spread sporadically though the highest numbers were in the North East, Wales and London. The most important thing to remember with listeria is that, if you are fit and healthy, you are unlikely to get listeria symptoms. It should not be a cause for alarm as it is still extremely rare.”

One theory is that as treatments for people with chronic conditions are improving and people are living, the increased survival rate from chronic conditions may be a factor linked to the increase in listeria cases.

Mr Gillespie denied that listeria was an infection acquired in hospitals and he said that most people were only admitted after reporting symptoms of the disease.

The Food Standards Agency has been informed of the sharp rise in new cases this year but is waiting for the results of further work by health chiefs.

Danger signs

Listeria is a rare but potentially life-threatening disease

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