Congenital and Inherited Disorders Affecting Multiple Body Systems of Dogs
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. Defective development may also cause embryonic loss, fetal death, mummification, abortion, stillbirth, or a newborn not capable of living.
Susceptibility to environmental agents or genetic abnormalities varies with the stage of development and species, and decreases with fetal age. The fertilized egg is resistant to agents or factors that cause or increase the chances of a congenital defect (teratogens), but it is susceptible to genetic mutations and changes in the chromosomes. The embryo is highly susceptible to teratogens, but this susceptibility decreases with age as the critical developmental periods of various organs or organ systems are passed. The fetus becomes increasingly resistant to teratogens except for structures that develop late such as the cerebellum, palate, urinary system, and genitals.
The frequency of individual defects varies with the species, breed, geographic location, season, and other environmental factors. It is estimated to occur at a rate of 0.2 to 3.5% of all canine births. Commonly reported congenital and inherited defects in dogs include neurologic defects, eye defects, heart defects, skeletal muscle defects, failure of one or both testicles to descend into the scrotum (known as cryptorchidism), and hip and elbow abnormalities. Most congenital defects have no clearly established cause; others are caused by genetic or environmental factors or interaction between these 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 associated with the X chromosome and not the Y chromosome).
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, increased embryonic mortality, or reduced litter size. 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.
Factors tending to produce abnormalities of formation include toxic plants, viral infections that occur during pregnancy, drugs, trace elements, nutritional deficiencies, and physical agents such as radiation, abnormally high body temperature, uterine positioning, and pressure during rectal examination. These factors may be difficult to identify, 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.