Mareks or double-whammy?

I have a sick hen, a speckledy, age about 9 months. She stopped laying about 2 weeks ago which we thought was because of the cold snap. However, she is now limping badly, sometimes has gurgly breathing (like a chest cold), isn't feeding much and is withdrawn, sitting all day. Eyes are bright, no discharge from them or nose, no swollen cheeks, no gaping. Crop fairly empty, no obvious lumps or bumps anywhere, vent pink. Both legs look fine, no obvious lifted scales, cuts or thorns etc. No wincing when we prod the bad (left) leg. Sometimes she topples over and puts her wing out to steady herself. No further deterioration in the last two days.

I'm keeping her on pellets & ACV with plenty of fresh garlic as a tonic, hoping that it is a passing cold and coincidental limp. However, the breeder says there is an outside chance it may be Mareks Disease even though the hen was vaccinated against it - apparently it isn't 100% effective.

Advice please!? I only started keeping chickens in April and have already lost 1 of the 3 to Gapeworm. I'd hate to lose another and leave Chidgey all alone!

Despondently, Neil

It could be Marek's.  It could also be a chest infection combined with Marek's.

Only a vet would be able to tell you and only if they know how to treat poultry.