What Genetic Disorders Are Produced In Breeding? Part II

by Kitty Angell - Originally published in Cats Magazine

It is extremely important that breeders be aware of the congenital malformations that can be produced by breeding. Heavy linebreeding (breeding cats that are in the same family but not closely related) or inbreeding (the mating of closely-related cats) can create a heritable trait that affects the future health of an entire breed. Without extensive breeding studies, it is generally impossible to isolate the genetic basis for most congenital malformations

When planning a breeding program, breeders must realize that doubling on the good traits in a cat also results in doubling the defects. There is no way to genetically separate "the wheat from the shaft" other than by systematically removing from a breeding program any cats suspected of carrying defects

What is hydrocephalus?

Hydrocephalus, sometimes called “water on the brain,” is the result of an accumulation of cerebrospinal fluid combined with an enlargement of the cerebral ventricular system. The defect inhibits the transportation of cerebrospinal fluid from the ventricles (small cavities) to the meningeal spaces (thin covering of the brain and spinal cord) and bloodstream. This abnormal accumulation of fluid causes enlargement of the skull and compression of the brain, destroying much of the neural tissue.

The skull of a hydrocephalic kitten is larger than normal and the soft spots where the bones of the skull come together are more open. The kitten's skull may have a bulge in the calvarium (dome-like upper portion of the cranium) In most cases, the condition is fatal within a few weeks of birth. Mildly affected kittens that survive are mentally retarded.

Affected kittens may be overly excitable or very depressed. They may walk in circles or be unable to stand or walk. They may appear to be blind. Other symptoms include head pressing, disjointed gait and seizures. Diagnosis is confirmed by radiography of the skull.

The deformity can manifest itself in almost any breed, but is more likely to appear in breeds in which breeders are genetically lengthening or shortening the skull. In the Siamese cat, there is evidence that hydrocephalus may be the result of a hereditary autosomal-recessive trait. Persians' head type can make them more susceptible to this anomaly.

What can you tell me about spina bifida?

Spina bifida, or “divided spine," is a defect of the neural tube (the precursor of the central nerve system in an embryo) that occurs when the vertebral column fails to close properly during prenatal development. In milder cases, the affected cat displays hindlimb weakness, fecal and urinary incontinence, numbness around the anus and a weak anal sphincter. In more extreme cases, the spine is completely open and there is severe neurological disability The spinal cord and its membranes may actually protrude within a fragile sac, and the intestines may be visible on the outside of the body. Diagnosis is confirmed by radiography and myelography (radiography of the spinal cord after injection of a contrast medium). The prognosis for cats with spina bifida is poor.

This disorder is most often seen in the Manx breed; however, it can occur in any breed that is highly inbred. Careful breeding of the Manx helps to prevent this problem

What about protruding sternums?

Protrusion of the cranial sternum is seen mostly in breeds of oriental body type: Siamese, Oriental Shorthairs and Longhairs, Colorpoint Shorthairs, Balinese and Javanese. It has been observed less frequently in other breeds such as Abyssinians and American Shorthairs. As long as the protrusion is outward there do not seem to be any health problems. If a kitten’s sternum protrudes inward, however, it can be damaging to the lungs and other parts of the chest cavity. Protruding sternum is a heritable condition but the mode of inheritance is unknown.

Foreleg micromelia? What is that?

Cats with foreleg micromelia have unusually short long-bones in the forelegs and are referred to as "kangaroo cats.” Although seemingly healthy, the front legs of an affected cat measure only about 6 3/4 inches from shoulder to ground, whereas the rear legs measure 9 1/4 inches from the highest part of the rump to ground. Studies show that micromelia is inherited but no mode of inheritance has been established.

And polycystic kidneys or PKD?

Polycystic kidneys can be congenital, developmental or acquired. Congenital polycystic kidney disease (PKD) was documented in a litter of longhair kittens that had large, distended abdomens and died by 7 weeks of age. Examination of their kidneys found them to be enlarged with a spongy appearance and many small cysts.

In adult cats this disease resembles autosomal-recessive PKD in human beings. Its clinical signs include lack or loss of appetite. weight loss, depression, vomiting, excessive urina¬tion and excessive thirst. Affected cats show signs of kidney failure; the kidneys become enlarged and cystic structures can be seen with ultrasound.

Heritable PKD has been described in both mixed-breed and pedigreed cats. In the Persian cat, PKD is familial and inherited as an autosomal-dominant trait.

I’ve heard of renal amyloidosis. What do we know about this?

Renal amyloidosis (RA) is a kidney disorder characterized by deposits of insoluble proteins in the kidneys The word amyloid means “having characteristics similar to starch. " Amyloid deposits are believed to interfere with the flow of blood through the vessels that supply the kidneys, causing cell death within the kidneys as a result of enzymatic degradation.

Clinical signs in cats include chronic weight loss, dehydration. gingivitis and a rough coat. Excessive urination and drinking is not uncommon. Lethargy and anorexia also affect cats with RA. Laboratory evaluation of the kidneys finds them firm, irregular on palpation, small and of paler color than normal.

This disorder most often strikes young adult cats before the age of 7. The male/female ratio of affected cats is approximately 1.4 to 1, meaning males are nearly 50 percent more likely to have it than females. Amyloid deposits first appear in the kidneys of affected cats between 9 months and 2 years of age. Some cases of RA are mild and the affected cats live fairly long lives, while others develop kidney failure within one year of diagnosis.

Although RA is quite prevalent in Abyssinians, it is rarely seen in other cat breeds. The disease is known to be inherited, but despite extensive study, researchers have been unable to determine the mode of inheritance because of the variability in severity and progression of the disorder.

What is the story on congenital heart disease?

Anomalies of the heart can occur in any breed of cat. If a cat with a known heart defect appears in a pedigree it is wise to breed as far away from that cat and its ancestors as possible. Congenital cardiovascular defects include atrioventricular septal defects (ASDs), mitral and tricuspid value dysplasia, endocardial cushion defects, patent ductus arteriosus, tetralogy of Fallot and stenosis of the ventricular outflow tracts.

Symptoms of a poorly functioning heart include an unusually fast rate of breathing, respiratory distress, weakness, anorexia, stunted growth, sudden loss of consciousness and cyanosis (blueness of the skin due to lack of oxygen).

The veterinarian must search for acquired signs of heart disease by carefully studying a medical history of the cat and its ancestors before pursuing the diagnosis of a congenital heart problem. Palpating the cat’s heart and listening to it with a stethoscope are the first diagnostic steps. Examining the cat and determining its temperature, pulse rate and respiratory rate are also necessary. Cars with a chronic or recurrent fever could have bacterial endocarditis (bacterial inflammation of the endocardium), although it is somewhat uncommon in cats.

Siamese seem to have a higher incidence of patent ductus arteriosus (abnormal opening between the aorta and the pulmonary artery) that suggests a genetic basis. Endocardial fibroelastosis (the accumulation of collagen and elastic fibers in the endocardium along with left ventricular hypertrophic dila¬tion) has been observed in both Siamese and Burmese cats. The condition appears to be heritable in Burmese, but has not been proven to be so in Siamese. Clinical signs of this disorder are difficulty breathing and cyanosis, followed by heart failure. Fluid may accumulate in the chest cavity as the heart begins to fail. Marked heart enlargement is seen in X-rays and during autopsy.

Before 1987, feline dilated cardiomyopathy (congestive primary myocardial disease) was one of the most frequently diagnosed heart diseases in cats. Then scientists discovered an association between taurine depletion and dilated cardiomyopathy. Once commercial cat food companies started adding taurine to their food, feline dilated cardiomyopathy in cats decreased dramatically. Taurine deficiency can cause the same type of cardiomyopathy as congenital and viral infections of the heart. Therefore. it is very important to find out the exact cause of heart failure before diagnosing it as a heritable form of the disease.

What can you tell me about glycogen-storage disease?

Certain lines of Norwegian Forest cats have been found to have an inherited deficiency of the branching enzyme, which causes glycogen-storage disease. Glycogen is a polysaccharide that is the main form of carbohydrate storage in animals and is found primarily in the liver and muscle tissue. It is quickly converted to glucose as needed by the body to satisfy its energy needs. In this disorder, various metabolic defects are found in the enzymatic processing of stored glycogen to glucose.

Clinical signs include generalized muscle tremors, weakness and fever beginning at 5 months of age. By the age of 8 months, the cats experience paralysis of the extremities along with severe muscle atrophy and limb contractures. Examination of the tissues verifies the presence of PAS-positive material in skeletal and cardiac muscles and nervous tissues. The glycogen-glucose content of the gray matter of the brain and spinal cord of affected cats is about six times greater than normal. The overall clinical signs closely resemble a human disorder called Pompe's disease, in which glycogen accumulates in muscle, nerve and heart tissue. The prognosis for cats with this disease is poor.

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