'I understand your concern about watering your vegetables using water from a butt in which a mouse drowned and which may have been contaminated with bird poo,' says John Negus, a gardening expert of Homes & Gardens.
'Normally, when rain barrel water is used  for irrigating crops, the contaminants we are talking about break down into basic elements – nitrogen, phosphorous and potassium – which are used by plants as food. I do not think you need be alarmed about this situation, though if you remain concerned you can buy rain barrel purification tablets from garden centers and online.'