The potential problem was confirmed late last month, after testing of catering and wholesale mushrooms sampled by environmental health officers in Lancashire. They discovered the presence of Salmonella kedougou, which has in the past been associated with turkey meat and tripe. The extent of the problem is not yet known but it has been found in mushrooms from some - but not all - growers in Northern Ireland. Investigations are continuing to identify any other affected growers and suppliers of compost and casing throughout the UK.
The Food Standards Agency is co-ordinating further investigations with the Mushroom Growers' industry - and suppliers of the medium used to grow the mushrooms - to pinpoint the source and extent of the problem. Preliminary findings indicate that the most likely source may be the casing, a layer of organic material added on top of the compost to promote mushroom fruiting.
Mushroom growers and suppliers of compost and casings that are potentially implicated in this incident are being advised of test results on their products and requested not to sell their products until the problem has been resolved.
There have been only three reported cases in the UK this year associated with this bacteria. None of the reported cases was associated with eating mushrooms, nor was the type of Salmonella kedougou the same as the one found in the mushrooms.
Although most mushrooms are cooked, they are also widely eaten raw in salads or dips. Like other salad vegetables, they may become contaminated by bacteria from the soil or other medium in which they are grown.
The Food Standards Agency advises that vegetables and salads should be washed carefully to remove any soil and dirt which can carry bacteria and other organisms, making sure that any soil is also washed from hands, kitchen surfaces and equipment. Consumers are advised to peel their washed mushrooms, in order to minimise any risks from bacteria. Mushrooms for use in salads may be cooked, then allowed to cool before serving.
There are about 2,400 different types of Salmonella. The Public Health Laboratory Service reported14,844 isolates (provisional figures) of Salmonella from humans in England and Wales last year, of which only 20 or so were Salmonella kedougou. This particular strain was first isolated in Africa in 1970. Total numbers of human infections in the last four years are as follows:
The Food Standards Agency Northern Ireland has so far taken between 110 and 120 samples of mushrooms, complete with their compost and casing. Of the 22 positive results so far obtained 14 relate to the casing, four to the compost, and only four to the mushrooms themselves.
According to the Mushroom Growers' Association, approximately 300 million pounds in weight of mushrooms are consumed each year in the UK.