Can you post a picture? How old is the chick? Sometimes a vitamin defiency can present itself in this way as well. Was your chick vaccinated for Mareks at hatch? Does it seem she is paralyzed? Are her toes curled? Are the legs sticking out at odd angles? If the chick is very young its more. Likely the vitamin defiency. Likely riboflavin.it can effect the legs if the bird is lacking. Please post a pic so we can better determine what might be the cause. If it IS Mareks, sadly there is no cure and its HIGHLY contagious. It can even travel miles in the wind. It does NOT mean your entire flock will have symptoms if its Mareks. But it DOES mean they will ALL likely be carriers and will infect new birds. If you bring new birds on your property they should get the vaccine and be given at least 3to4 weeks for immunities to kick in before exposure. Exposure would mean taken outside or brought around your other birds. If it is Mareks the disease can stay on the land for quite sometime unfortunately. But remember not all mareks birds will have symptoms. They can carry the virus and never become symptomatic, however like i mentioned these same birds can and will spread the virus even without symptoms.many people have Mareks in their flock and some breed for resistance while others vaccinate any new comers and some keep a closed flock. it does not mean the end of your flock. With some adjustments and precautions your flock can still continue. If your bird DOES have Mareks, read as much as you can about it!! Remember, knowledge is power.then you can decide which course of action is best for you. Hope this helps. Best wishes
It's not possible to know precisely what's responsible for the condition of your chick unless you get a necropsy done if/when the chick dies. I highly recommend it. If you got the chick from a private breeder, it is possible their flock has a virus or even Marek's that you've imported into your flock.
Besides the possibility of it being Marek's, there are other viruses that cause the same symptoms that are passed through the egg from hen to chick, resulting in these early onset symptoms and early death, if not death in the egg before the chick even hatches. These viruses are all extremely contagious, spread on wind currents as well as direct contact.
Right now, you can try to treat the symptoms with vitamins and maybe antibiotics and see if there's improvement. The fact your chick has labored breathing leads me to believe vitamins aren't going to fix it.
But should the chick die, it's very important you get a necropsy so you know what you're dealing with. It could affect your entire flock.