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Polymelia is a genetic birth defect and type of congenital malformation of the limbs that can sometimes occur in chickens, and is defined as the growth of additional limb(s) attached to various body regions. The additional limb or limbs are often shrunken in size and/or deformed. The exact cause of this congenital anomaly is unknown, however it is thought to be associated with genetic factors involving transgenes, chromosomes, and/or possible exposure to environmental toxins.
Cases of chickens with polymelia
In 2005, a four-legged chicken named Henrietta was hatched at a small farm in Somerset, Pennsylvania and raised by the farm owner's 13-year-old daughter. The bird readily adapted to its condition, and was found to peacefully coexist with other chickens on the farm.
In 2011, a 9-week old broiler was brought to the University of Abuja's Veterinary Teaching Hospital in Nigeria, where it was diagnosed with polymelia, confirmed macroscopically and radiographically. The chicken had one undeveloped extra-limb that was attached to the caudal region of the pelvic bone, which was surrounded by a mass of muscular tissue. Radiographic imagery revealed that the right half of the chicken's pelvis had an extra femur, which was attached to the acetabulum in addition to the normal femur.