I spotted Gladys trotting about with a clear froth in the corner of her eye, and a runny nose; and thank heavens I did - this is a nasty, highly infectious respiratory disease which you only usually spot when the bird’s breathing becomes rattly. Once she’d wiped her head on her side the froth was gone so it’s easy to miss the early signs.
The vet came round and decided to vaccinate them all (I’m classed as a farm visit with my eight hens!) with Tylan 200 injectible. The dose is 0.25ml per bird, injected into the breast muscle, repeated in 48 hours if the symptoms are still there. Try not to inject into the same place as there is a risk of muscle damage. And try not to inject your own hand.