Note to self.
Do not assume 9 weeks is sufficient quarantine.
Or to put this another way: always stress one's chickens before assuming they are free from disease and moving them to proximity with existing birds.
Unfortunately the change to the new pen has brought out sneezes in the little ones. They have had no contact with sick birds, though their tractor pen was stationed under trees, so of course bird droppings may have fallen onto the ground inside the cage. But having raised many chicks in this way, I don't believe such disease transmission is common. What I'm thinking, of course, is that the birds may harbour mycoplasma gallisepticum (MG, the germ associated with CRD).
MG is transmissible via eggs and thus can appear in hatchlings even if they've had no contact with other birds. However it usually remains sub-clinical in a well flock, and tends to show up as an outbreak only when the birds are stressed. Often this isn't until point of lay for female birds, but changing to a new pen is probably the ideal way to make birds show the disease.
Now I don't intend to jump any guns here. I've separated 2 of the chicks and have put them onto commercial (medicated) chick starter in case the slight droopiness is coccidiosis instead of something respiratory (remember, they've been off meds, so the coccidiostats should be quite effective at stopping cocci escalating and giving the chicks time to get over the load they have). The 2 chicks are also on wire and off the ground in the short term to give them the best chance to get over cocci if that's their only serious problem (and if the sneezes are just coincidental). Given that they haven't started to do bloody droppings and that they're still eating, I feel I've caught it early if cocci is the case.
The question of moving to active anticoccidials (Sulphaquin or similar) is harder to decide. In a show or pet situation it would seem unquestionable to use such medications to help these 2 sick birds. However these birds are going to be meat (the spare ones) or breeders (the best male). Furthermore, Sulphaquin may have a slight antibiotic effect on respiratory illnesses like MG, masking underlying disease. In this case, I'd rather know exactly what I'm dealing with in regard to the sneezing. Thus for the next few days I'll watch the chicks closely and try to rely on coccidiostats alone. If they develop bloody droppings and other direct cocci signs then it will be clear they need Sulphaquin, and I won't hold back. However if the respiratory symptoms worsen then that's another story.
One thing is sure: MG is *not* something I want in my flock. It produces a cycle of revolving illnesses when birds are stressed and cannot be eradicated once breeders have acquired it — that is, you can produce a 'clean' flock by using antibiotics on the breeders, dipping eggs before incubation, and practicing impeccable hygiene, but you also have to cull all the carriers. I've bought MG carriers before, and always regretted it. As soon as a grey goshawk swoops or I have to change pens, the birds turn rattly and unwell, and then it's often a battle to get them back into shape.
According to most agricultural perspectives, in the case of egg-borne organisms like MG, it's best practice to cull the entire suspect flock and start again with known non-carriers. I know that seems harsh, but because my interest is backyard production, I tend to follow agricultural rather than pet or show advice. My preference is to keep the disease out rather than put up with cycles of illness and poor production, so culling may be necessary.
Unfortunately, having already put the chicks in proximity to my adult birds, the next couple of weeks is going to be a time of watching and waiting. I'd rather know there's a revolving illness than hide it behind a veneer of temporary (and artificial) wellness. But that doesn't mean I enjoy seeing sick birds — in fact it's distressing and depressing, since my entire approach has always been about being humane.
But I count humaneness in the breeding (for vigour and hardiness) as much as in the treatment of birds. And unfortunately in this case I need to know what's going on.
UPDATE: I've just taken out a third sneezer, so that makes 3 in the sick cage. But the third bird doesn't look unwell apart from the sneezing, so we'll see.