in my opinion its often better [and cheaper] to have their droppings tested,,,, and a swab taken,,,then if treating,s a must, so be it,, but as they say,,,if it aint broken you cant fix it ,, many treatments such as cocci, canker, worms etc are cures,,, not preventatives,, good loft management should keep these levels to a minimum, i,e overcrowded loft, un-ventilated lofts,, and dirty, damp lofts ., avoid these at all costs,,,good luck