Please, please if you're buying pre-packed salads & giving them to your poultry as a treat make sure you wash them thoroughly. Have just found out that Kryten (our new gander) has died from listeria & after lots of tests & checks on the enclosure & our other feathered beasties the only thing he could have contracted it from was the salad we'd been giving them all as a treat. The chances are Smeg also had it (though it's too late to confirm it now) so I advise you to either wash any pre-packed chilled foods thoroughly (whether it's for human or animal consumption) or not give them at all. I hasten to add that the whole head of lettuce, loose "unpacked" salad stuff or the stuff grown in gardens/allotments is fine, it's just the pre-packed stuff that has higher levels of listeria.
Have also since been told that as the grass has been so slow-growing this spring animals have been grazing right down to the soil increasing the chances of picking listeria up from the soil. While the levels in our soil are down a normal, low "background" level the fact that the grass is so slow-growing at the mo means my feathered beasties possibly had higher than normal levels so the salad then pushed the levels up very high.