When your chicken suddenly starts limping, then the first item to check should be physical injury to the base of the foot. Chicken can also injure legs or break a bone. If none of these are present, but you notice swollen joints then this could be a case of Infectious synovitis.
What is Infectious Synovitis in chicken?
Infectious Synovitis, also called Mycoplasma Synoviae Infection, MS Infection, Enlarged Hock Disease, Synovitis is an acute to chronic, systemic disease of chickens caused by infection with Mycoplasma synoviae (MS).
It affects the synovial membrane of joints and tendons leaving the chicken unable to walk properly. Chickens with infectious synovitis develop swollen, red, and warm hock joints. They are in so much pain it is difficult for them to walk. The synovial membranes of tendon sheaths become thickened, edematous, with fibrinous exudates accumulating within and around the tendon sheaths.