Thankyou Chris. And I put the chicks in the garden (on their own) because the junior vet was sure the blood test was 100% reliable. I very much hope as they were recently treated with antibiotics, there will be a low risk of infection. I've now spoken again to the main vet, who tells me the chicks may not yet have developed antibodies and the blood test is only reliable after 6 weeks old. The DNA swab is reliable but is more expensive and it takes a week to get the results. He tells me one way to reduce the risk of mycoplasma is to treat suspected birds with Tylan every month. There is apparently no egg withdrawal on Tylan. If new birds are isolated for two weeks and show no signs of illness, they are generally safe to mix with a flock, but this is only proven if the flock then remains healthy. If they go down with eye bubbles and streaming colds after 4 days, it is likely to be mycoplasma.
Re mycoplasma, the danger is that it is a virulent, highly infectious bug. Mycoplasma on its own is innocuous, like a common cold. On its own it is not a killer. However in combination with e-coli or infectious bronchitis, it becomes a major issue and kills. Infectious bronchitis is extremely common. When mycoplasma is in a breeding flock, offspring are not healthy and fertility is poor. It is carried within the egg, so he advises only buying from one source that you know well, and know the history of their birds, or from reliable breeders who blood test for mycoplasma. It is dangerous buying eggs from multiple sources as each parent flock builds a resistance to different bugs. He says its like us going to Spain for a holiday and not being resistant to new bugs there. It is always best to keep a closed flock which is happy with its own bugs and passes on a resistance to these.
It seems most backyard flocks don't have mycoplasma. He is not sure what percentage do - maybe around 25%. If a flock has had it they will eventually show few symptoms but will remain positive and carriers. But anyone who is breeding intensively and expanding their birds quickly, who has poor hygiene, or who gives their birds less than 1 square meter per bird (which he says a lot of backyard breeders do), is more or less certain to have it. Mycoplasma is extremely common in commercial birds because of the number of eggs they produce, lack of space, stress and conditions, so many commercial farms are treating with tylan and other drugs on a regular basis to keep their birds free of disease signs. Even with the three vaccines they give commercial birds for mycoplasma including the reliable live eye drop, if they are groups of thousands they can still get it. Poor abused things.
Re my own chicks, sadly I've arranged to cull them this afternoon. I am away next week so its difficult to wait for another test and I just don't want the risk of having chicks from a mycoplasma postive breeder - so all back to the original advice you gave me. But I have perhaps found a knowledgeable vet in the process (Stuart Young in Cullompton) who tells me he imports a huge range of specialist medication for poultry. And I'm very grateful to Marigold for the recommendation of Chalk Hill Poultry.
