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Development Disease in Horses


As spring approaches, so does the anticipation of next year’s foal crop. Wrapped up within all this excitement are concerns about possible developmental issues that can arise as the foal grows into the athletic horse we desire. Now is a good time to revisit some of the hurdles that may be encountered during the first days and beyond.

Developmental issues can arise in utero (in the uterus) or post birth. Many in utero developmental defects are recognized as fetal failures, usually resulting in abortions or loss of pregnancy. When issues are recognized in the newborn foal, they are termed congenital, meaning present at birth. Congenital defects may involve one or several body systems and can range from mild to life threatening. Congenital defects can be further categorized as atrophic/atretic or hypoplastic. An atrophic/atretic defect is one where there is a completely undeveloped organ or limb. A hypoplastic defect means that development is incomplete, resulting in an organ or limb that is not the expected size.

Major Defects

Atrophic or hypoplastic defects that involve major body/organ systems are life threatening. One example is major defects of the cardiopulmonary system, resulting in an inability to transport oxygenated blood to the rest of the body. In general, there is not much that can be done surgically to treat atrophic or hypoplastic anatomy because there is insufficient tissue available to reconstruct the needed part. The most favorable cases involve atretic segments of bowel, where there is normally more redundant tissue to harvest for creating a functional gastro-intestinal tract. However, most severe cases require humane euthanasia due to a poor or nonfunctional quality of life. Thankfully, these cases are extremely rare.

Minor Defects

Minor defects are more common and can often be corrected or managed. Umbilical hernias are minor defects in the body wall. Umbilical hernias appear as soft, non-painful protrusions of intestines that are held in by a covering of skin. While they may sound severe, hernias are easily correctable by medical or surgical management. Umbilical hernias under 5cm may spontaneously close or correct through manual reduction. Small umbilical hernias can even remain as harmless defects.

Some other defects, especially those of the musculoskeletal system, manifest as inappropriate function (either laxity or contracture) or as extra tissue, such as in polydactyly, in which extra toes are present. Mild defects such as these can be managed, and most can result in a productive horse. Surgery for tendon contracture or polydactyly, and corrective shoeing for tendon laxity can often put these foals on track with their peers by weaning time.

Origin of Congenital Defects

There is always a question as to the origin of congenital and developmental defects. Most horse owners are concerned about the next foal after a defective one has been born, so there is a question as to the heritability of a defect – in other words, is it genetic? Unfortunately, the cause of most neonatal problems is unknown.

Some congenital abnormalities are linked to a specific gene. Among these are lavender foal syndrome, lethal white syndrome, and severe combined immunodeficiency (SCID). However, just because a defect is congenital doesn’t mean that the defect is heritable. Some, such as angular limb deformities, may result from in utero stress (i.e. placentitis, twins, premature birth, etc.), or from post-foaling nutrition or trauma.

The best advice for avoiding congenital defects has not changed over the years. Make sure broodmares have adequate veterinary care during the pregnancy (vaccinations, deworming, dental care) and adequate, balanced nutrition for the developing fetus.

Identification of Congenital Defects

Post-foaling examinations should be done as early as possible to identify congenital abnormalities and begin treatment in a timely fashion. Neonatal examinations go from the tip of the nose to the tip of the tail. Particular attention is paid to the heart to identify ventricular defects; to the lungs; to the eyes for cataracts or entropion; nasal passage for choanal atresia; oral cavity for cleft palate; umbilicus; gastro-intestinal tract, and anus for atresia. Every major joint, bone, ligament, and tendon in every limb, as well as the patella location and external genitalia is examined.

Developmental Defects

Some developmental defects are not congenital, but may appear later, during the post-weaning period. These conditions, which may or may not be heritable, generally involve the musculoskeletal system. Classically, they are categorized in six basic groups:

  • flexural limb deformities (contracted tendons),
  • angular limb deformities (crooked legs),
  • physitis (epiphysitis),
  • OCD (osteochondritis dessicans or osterochondrosis),
  • subchondral bone cysts, and
  • cervical vertebral malformation (wobblers syndrome).

Many of these deformities are caused by or associated with factors such as genetics, pain, trauma, and nutrition (either an excess or imbalance of trace minerals or energy).


Regardless of the causes, many congenital and developmental problems can be addressed medically with successful outcomes. Surgery can often be avoided by early treatment. When surgery is required, success depends on manipulating growth potential to correct the deformities. Once growth ceases, surgically correcting deformities is very difficult.
For example, angular limb deformities are among the most common issues seen in foals. These conformational faults can have serious consequences as the horse matures. But if treated immediately, they can be easily fixed. Corrective trimming involves shaping the hoof to change the forces on the affected joint. In lateral deviations, the outside of the hoof wall is shortened, and extension shoes are applied medially, and vice versa for medial deviations. Rest and splinting may also help correct minor defects. For severe or resistant cases, there are a variety of surgical techniques for straightening the limb. Periosteal transection, where a small flap of periosteum is lifted from the bone, can be used to accelerate growth on the concave side of the leg. Conversely, screws and/or plates placed across the physis can be used to slow growth on the convex side of the deformation, straightening the joint.

While there are many congenital and developmental disorders that can affect a foal, there are also a multitude of medical options for treating these issues. Foaling season is an exciting and often stressful time. Being prepared and knowledgeable about neonatal health can help smooth these creatures’ transition into the world. The most important part of raising a healthy foal is close observation and early action, because many abnormalities have a much better prognosis if treated within the first few weeks of life. Early neonatal exams and medical care will put your foal on the right track for a bright and healthy future.

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Pilchuck Veterinary Hospital


11308 92nd St SE Snohomish, WA 98290


24 HOUR EMERGENCY SERVICES Small Animal Reception Monday - Thursday: 8 AM to 7 PM Friday - Saturday: 8 AM to 5 PM Sunday: Emergency Only Equine/Large Animal Reception Monday - Friday: 8 AM to 5 PM Saturday: 8 AM to 12 PM Equine Reproduction Center Monday - Friday: 8 AM to 5 PM Saturday: 8 AM to 12 PM Integrative Medicine Tuesday - Friday: 9 AM to 6 PM Saturday: 9 AM to 4 PM