Horse Genetics: Contracted Foal Syndrome

The skeletal anatomy of a horse's front and hind limbs is comparable to the anatomy of the human hands and feet. The horse's cannon bone, or metacarpal, is the same as a bone in the palm of a hand. The human phalanges, or finger bones, are comparable to the bones making up a horse's hoof and pastern. Fortunately, most humans and horses are born with normal limbs. However, both children and foals can be afflicted with contracted limb abnormalities.

When a child is born with tightly clenched fists and club feet, it may have one of several muscle contracture syndromes collectively known as Distal Arthrogryposis (DA). The child may or may not have other congenital abnormalities, such as spinal curvature (scoliosis), facial muscle contractures, or a small mouth.

DA is caused by mutations in one or more genes that control skeletal muscle contraction. These gene mutations cause abnormal muscle protein production that disrupts normal muscle function. The skeletal muscles contract but are unable to relax, causing limb contractures during fetal development. Most of the mutations are inherited, but the mode of inheritance can vary. Symptoms can vary markedly within and between families. Mutations can arise spontaneously as new (de novo) mutations. Some children with DA respond to surgery and physical therapy; others do not.

When a foal is born with limb contractures preventing it from standing or walking normally, it is said to have contractures, or Contracted Foal Syndrome (CFS). CFS is the most common congenital anomaly in horses diagnosed at the University of Kentucky's Livestock Disease Diagnostic Center (LDDC). The signs of CFS are similar to those seen in human DA. The severity of CFS varies in horses, just as DA varies in humans.

Foals may have a mild form of CFS or exhibit only one affected limb. They may recover with surgery, splinting, or other therapy. Alternatively, foals may present with severe CFS involving contractures of all four limbs and may exhibit other anomalies, including neck flexion (torticollis), facial bone deviation (wry nose), and spinal curvature (scoliosis). Muscle, tendon, and ligament tissues appear normal when examined microscopically by pathologists. CFS has been found primarily in Thoroughbreds at the LDDC, which may be a reflection of the Thoroughbred-dominant equine population in Central Kentucky-other breeds may dominate in other parts of the country. The condition appears to be distributed equally between males and females. If severely affected foals survive delivery, they are usually euthanized due to the inability to stand and nurse.

Mares carrying foals with CFS may experience dystocia at delivery, thus endangering the life of the mare. Although CFS is not widely regarded as an inherited disease, breeding records indicate that a hereditary risk factor may exist. Some mares have produced up to four CFS foals, each one sired by a different stallion. These mares may have been housed on different farms during each pregnancy, therefore ruling out a management component. Findings in individual cases suggest CFS may be inherited in a dominant fashion, with its development depending on other genetic factors. That would explain why some foals have a mild form of CFS and others have a severe form.

The availability of the horse genome sequences is facilitating our study of CFS. The DNA sequence information for any horse gene of interest can be downloaded from Internet databases. The necessary tools can be prepared for sequencing candidate genes from affected and non-affected individuals. Based on the gene mutations causing DA in humans, we have selected and are currently sequencing candidate genes in an effort to identify mutations that cause CFS.

Even though this research has begun, success will depend on continued support from the horse industry, especially through providing research samples from foals and information on sires and dams that have produced one or more foals with CFS. All information is kept confidential, including the identity of horses and farms. We expect one day to develop a diagnostic test that will provide information to allow farm managers to avoid matings that will produce CFS foals and to determine which foals will respond to treatment. The participation of breeding farms, veterinarians, and horse owners is imperative if the cause of CFS is to be identified.