Hi Anna_MN, sorry to hear about your problem.
How old are the birds? What are you feeding them?
If they were my birds I suppose I'd try to rule out the most common problems first. Coccidiosis is still the easiest way to lose chicks, but usually you see hunching, drooping wings, ruffled feathers, head pulled in on the neck, and sometimes blood in the droppings. You wouldn't see much in the way of limping really. Treatment would be corid or sulmet.
Nutritional deficiencies can also cause some leg problems; if they're on a fully formulated diet this is unlikely (though still possible). If they definitely don't have coccidiosis then perhaps it would be a good idea to buy a vitamin tonic and give it to see if it helps.
There are some diseases to think off too. Older chicks may develop Marek's but I would imagine if they came from a meat bird hatchery they'd be vaccinated against that. I guess it's possible yours weren't vaccinated properly, or the virus has mutated. I'm sorry, I don't know much about lymphoid leucosis which is another limping disease. It might be worth googling both diseases to see if either sounds similar. Unfortunately as I understand it meat birds are quite prone to Marek's, and some Marek's strains cause symptoms far earlier than others.
I can't think what else could be going on, sorry. There are other causes of leg troubles such as mycoplasma synoviae but I can't think of many that frequently cause deaths. Is it possible they become unable to walk and then starve or get trampled?
So sorry I can't help more,
Erica
