Culled them all in the 90s with Salmonella,more nat tickets than i care to mention, but sometimes in a loft at breeding times the birds go right off and can be due to feeding differences and lack of condition rather than something sinister. A test should be carried out but many times this will be inconclusive! If a sinister disease was lurking such typhoid then signs should have been seen long before now especially during the moult? If the birds wer not checked in the moult then this must be something recent?