text: Are you partial to a prawn mayo sandwich, drawn by the lure of a shrimp cocktail on a menu? Once only enjoyed by the affluent, prawns are now a healthy, cheap option filling supermarkets, sandwich bars and restaurants across the world. But have you ever stopped to think about where the ones you're eating have come from? Forget sun-kissed seas and a lone fisherman casting his nets into the Indian Ocean to haul in his catch. For a new report shows that idyllic picture could not be further from the truth. A new study by experts at Consumer Reports in the US has found 60 per cent of packaged prawns tested were found to contain traces of harmful bacteria . Experts in the US found the majority of the shrimp sold on supermarket shelves arrive there after starting life on vast prawn farms in south east Asia. Many prawns sold in the UK also come from places such as Thailand and Indonesia. They tested 342 packages of frozen shrimp - 284 raw and 58 cooked samples - from large, chain supermarkets and natural food stores in 27 cities across the US. And their investigation revealed 60 per cent of the prawns were found to have traces of  harmful bacteria, including E.coli, which can cause violent food poisoning. Furthermore, in 11 samples researchers detected antibiotics, and seven samples tested positive for MRSA - an infection that is resistant to a number of antibiotics. Other traces of bacteria included salmonella and vibrio, both of which can cause serious food poisoning. Dr Urvashi Rangan, executive director of Consumer Reports Food Safety and Sustainability Centre, warned: 'Bacteria and algae can begin to grow and disease can set in, prompting farmers to use drugs and other chemicals that can remain on the shrimp and seep into the surrounding environment. 'Vibrio is the most common cause of food poisoning from eating raw oysters. 'And even though most bacteria on shrimp would be killed during the cooking process, our test results raise real questions about how shrimp is raised, processed, and regulated.' Scientists found traces of E.coli, MRSA, salmonella and vibrio - a major cause of food poisoning . Responding to the discovery of MRSA, Dr Michael Crupain, director of the Consumer Reports Food Safety and Sustainability Centre, said: 'It's spread through contact, so if MRSA gets on your skin while you're preparing raw shrimp, it can potentially cause an infection, especially if you have an abrasion or cut.' A medium-sized shrimp farm in South-East Asia can produced close to a million pounds of prawns every year, a 'powerful incentive for farmers to maximse production', the report notes. The vast majority of prawns eaten on these shores come from such farms in India, Indonesia and Thailand. There, prawns are 'farmed' in huge industrial tanks, or shallow, man-made ponds that can stretch for acres. In some cases, 150 shrimp can occupy a single square metre. They are typically fed commercial pellets, some of which contain antibiotics to ward off disease. If the ponds are not carefully managed, 'a sludge of faecal matter, chemicals and excess food can build up and decay'. And wastewater can be 'periodically discharged' into nearby waterways. The experts advise wild shrimp, a number of samples of which were also tested, appear to be worth the higher prices. The report reveals: 'Of all the shrimp we tested, they were among the least likely to harbour any kind of bacteria or contain chemicals.' But, it is worth considering the effects on the environment, marine scientists warn. Amanda Keledjian, from the not-for-profit conservation group, Oceana, said: 'Nets dragged along the ocean floor can severely damage the sea bottom and anything that lives there.' The report goes on to state, that while estimates vary, between one and three pounds of other species, including endangered sea turtles, can be killed for every pound of shrimp caught in the wild. A large number of the prawns available on supermarket started life in vast prawn farms in Thailand, India and Indonesia. They are farmed in huge industrial tanks, or in shallow ponds, like those pictured in Krabi, above . If the ponds are not carefully managed, 'a sludge of faecal matter, chemicals and excess food can build up'. Experts said on the face of their findings it is better to opt for wild prawns over their farmed cousins . So faced with the facts, how can consumers make the right choices when it comes to which shrimp to put in their cocktail? Marianne Cufone, an environmental solicitor and executive director at Recirculating Farms Coalition, explains how you can spot a farmed prawn from their wild cousins. 'Wild shrimp often vary in size, shape, and colour because they don't all have identical genetics,' she said. 'Batches of farmed shrimp often all hatch at the same time, eat the same food and live in the same environment, so they're more likely to look the same.' And her second tip, is to look for evidence of prawn faeces. 'Look for poop, or what is politely called a 'vein',' she added. 'Frequently, shrimp farmers stop feeding shrimp before harvesting them so that the vein empties. 'If you see a dark line there's a better chance it's a real wild shrimp.'
target: Study found 60% of prawns tested had traces of harmful bacteria .
  Included E.coli, antibiotic resistant MRSA, salmonella and vibrio .
  Warned farmed prawns are more likely to trigger violent food poisoning .
  Majority of supermarket prawns from Indian, Thai and Indonesian farms .