A variety of structural and functional defects have been described in animals. These birth defects are usually classified by the body system primarily affected, and many are discussed in this book under the appropriate body system section. Defective newborns have survived a disruptive event during embryonic or fetal development.

The frequency of individual defects varies with the species, breed, geographic location, season, and other environmental factors. It occurs at an estimated rate of 0.2 to 3.5% of all foal births.

Commonly reported congenital and inherited defects in horses include the following: contracted tendons, hydrocephalus (“water on the brain”), failure of one or both testicles to descend into the scrotum (known as cryptorchidism), displacement of the kneecap, cataracts, and failure of the umbilical opening to close (“leaky navel”). Other congenital defects are known, but they are reported less often. Most congenital defects have no clearly established cause; others are caused by genetic or environmental factors or interaction between these factors.

Genetic Factors

Inherited defects resulting from mutant genes or chromosome abnormalities tend to occur in patterns of inheritance. Such patterns include dominant (in which the defect will occur if either parent supplies an abnormal gene to its offspring), recessive (in which both parents must supply an abnormal gene) or others, such as sex-linked (in which the gene is nearly always associated with the X chromosome and not the Y chromosome).

Some Congenital and Inherited Defects in Horses
Contracted flexor tendons (see Bone, Joint, and Muscle Disorders in Horses: Contracted Flexor Tendons) Cryptorchidism (apparent absence of one or both testicles) (see Reproductive Disorders of Horses: Congenital and Inherited Disorders of the Reproductive System in Horses) Angular limb deformities (see Bone, Joint, and Muscle Disorders in Horses: Angular Limb Deformities) Cataracts (see Eye Disorders of Horses: Cataracts)

Some common diseases or disorders caused by genetic defects include deficiencies of particular enzymes that lead to the body's inability to perform normal metabolic functions, and chromosome abnormalities that can result in sterility, abnormal growth or development, or fetal death. Viruses, certain drugs, and radiation are common causes of chromosomal damage.

The complex interaction between genetic and environmental factors is being studied and is slowly becoming better understood.

Environmental Factors

Factors that tend to produce abnormalities in body formation include toxic plants (such as poison hemlock, which may contribute to contracted tendons in foals), viral infections that occur during pregnancy, drugs, trace elements, nutritional deficiencies, and physical agents such as radiation, abnormally high body temperature, uterine positioning, and twisting of the umbilical cord. These factors may be difficult to identify. They often follow seasonal patterns and stress and may be linked to maternal disease. They do not follow the pattern of family inheritance that is shown by genetic changes.

Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Delores E. Hill, PhD; Barton W. Rohrbach, VMD, MPH, DACVPM; Charles J. Issel, DVM, PhD; Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Daniela Bedenice, DrVetMed, DACVIM, DACVECC; Farouk M. Hamdy, DVM, MSc, PhD, MPA (Deceased); Kenneth R. Harkin, DVM, DACVIM; Johnny D. Hoskins, DVM, PhD; Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; John E. Madigan, DVM, MS; Dale A. Moore, MS, DVM, MPVM, PhD; J. Glenn Songer, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM; Brian J. McCluskey, DVM, MS, PhD, DACVPM; Bert E. Stromberg, PhD; Peter J. Timoney, MVB, MS, PhD, FRCVS