Bagged supermarket salads 'fuelling bugs that could make people ill'

Scientists found that the bugs include a new strain of Fusarium wilt which can trigger infections and eye conditions in humans.

The fungus is widespread in fruit and veg, and the latest type has turned up in the UK and Ireland.

Leaf spot, caused by bacteria, is also becoming more prevalent in bagged and pre-washed greens.

This may reduce the nutritional value of the ready to eat meals, a study found. More than £1.1 billion is spent on them each year in the UK alone.

In experiments under controlled conditions new diseases were identified in lettuce, rocket, chicory, endive, basil, spinach and Swiss chard.

Some should "be considered a threat to humans," according to the research team at the University of Toronto in Canada.

Lead author Professor Maria Gullino said: "Leafy vegetables can be infested, if hygienic conditions are not fully respected, by food-borne pathogens such as Escherichia coli, Salmonella enterica and Listeria monocytogenes."

Three years ago Leicester University researchers showed Salmonella and other food poisoning bugs thrive in the moist environment of packaged salads - and become more dangerous.

Prof Gullino said: "Ready-to-eat products go through intensive control programs, according to national and international regulations, which define the presence and microbiological limits for E. coli and other pathogens.

"Despite these rules, human pathogens on ready-to-eat products have been detected worldwide."

She added: "Some are not plant pathogens but rather use plants as vectors to humans."

Prof Gullino and colleagues say the dramatic growth of the pre-prepared salads industry is to blame for a number of new diseases.

They are a particular threat to farmers as they could wipe out crops - and leave them facing economic disaster.

Writing in Plant Disease, the the researchers say Fusarium wilts are among the most common diseases on leafy vegetables, especially lettuce.

Until very recently there were three different species, known as races - and now there is another.

Prof Gullino said: "In 2017, race 4 was detected in the Netherlands. This new race is rapidly spreading and has been reported to date in Belgium, the UK and Ireland, posing serious threats to growers and breeders.

"Until resistant varieties are developed, preventive management measures, such as the use of healthy seeds and seed treatments, are required to reduce the risk of its spread to new cultivation areas."

As salads are seasonal they are grown under high density in five to six cycles annually in the same specialised farms, explained the researchers.

This means there is a lack of adequate crop rotation, and a shortage of suitable fungicides.

What is more, international trade has moved the foods away from their original environments to foreign soils, where they encounter fresh infections.

In some cases, very low levels of seed contamination can lead to the rapid emergence of diseases in new geographic areas.

This results in severe losses, disrupting the environment's biological equilibrium and sometimes launching a devastating epidemic.

Climate change is also a threat as it interacts with globalisation to influence the development and spread of plant bugs. Rising temperatures also reduce resistance.

Prof Gullino called for disease control strategies to be re-evaluated to address the warmer temperatures. New pathogens tend to prefer hot weather.

The bugs are transferred onto salad leaves through water, compost or even animal manure used to grow vegetables.

Bagged salads have steadily gained popularity since their introduction in Europe in the early 1980s.

Health experts have previously warned against eating bagged salads, despite their convenience.

Those who do should consume them on the day of purchase as even a small number of bacteria can multiply rapidly, and make a person unwell.

Every year more than 500,000 cases of food poisoning are reported in the UK. Poultry is the most common cause, but some 48,000 are linked to non meat products.

Despite their wholesome nature, fresh green leaves and salad foods are often involved in food poisoning.

An outbreak that affected more than 2,000 people across Europe in 2011 was traced back to bean sprouts.

In 2016, officials in England traced an outbreak that killed two people back to bags of rocket leaves.

Prof Gullino added: "Among research priorities, there is a need for new varieties that are not only appealing to consumers but also resistant to the most frequent diseases, not only for lettuce and spinach but also in the case of minor leafy vegetables, such as rocket and lamb's lettuce.

"In addition, seed companies should invest more in the adoption of all measures needed to prevent the spread of diseases throughout infected seeds."

ENDS