What Genetic Disorders Are Produced In Breeding?
by Kitty Angell
Originally published in Cats Magazine
“Good and bad may not be dissevered. There is, as there should be, a commingling.”
– Euripedes, quoted in Plutarch’s “Contentment”
What are the usual elements that breeders consider when breeding for type?
When breeders endeavor to “set a look” with a particular breed, they concentrate on working with cats that possess the desirable traits they want in their line. What some do not realize, however, is that by intensifying the good traits of a cat (big eyes, heavy top-of-the-head doming, extremely fine boning. etc.) they are also doubling the bad things present in that cat’s genetic makeup. And once undesirable traits have been established in a line of cats, they can be more difficult to remove than the good ones.
It is wise to study a cat’s pedigree before breeding. When correctly used, a pedigree is like a road map. pointing out the notable features, both good and bad, of a cat’s background. Unfortunately, many breeders use pedigrees only as a shortcut to the winner’s circle.
A number of genetic disorders plague the cat fancy; some occur most often in particular breeds while others can affect cats of any breed.
What is Cryptorchidism?
Bilateral cryptorchidism is the failure of both testes to descend properly. Unilateral cryptorchidism, more commonly known as monorchidism, is the failure of only one testicle to descend. Cryptorchid males are usually infertile and therefore useless to breeding programs; monorchid males are not sterile, but they should not be used for stud service because in most cases they can pass along the genetic defect.
The defect occurs when the testes fail to pass through the inguinal canal into the scrotum, either because the inguinal canal is incorrectly formed or is too small. A developmental maladjustment can also prevent the testes from descending at the appropriate time.
The defect is an example of sex-limited inheritance: Only males can have undescended testes; females may carry- the defect but obviously cannot show it. In this respect the carrier female is as big a menace as the monorchid male. Although outwardly normal, she can potentially transmit the condition to her sons, which is one of the main reasons the defect persists.
Selective culling should be carried out immediately upon discovery of this genetic disorder. Not only should the male and his parents be removed from the breeding program, but brothers and sisters of affected cats should be carefully scrutinized as well.
Can you describe the types of tail abnormalities?
A variety of tail abnormalities occur in cats. Tails can be nodulated, kinked or bent; loss of the terminal portion of the tail is also possible, Many of these defects can be traced to injury, possibly at birth or some later stage. However, a certain defect may appear in cats of one particular strain or bloodline Then, of course, the possibility of genetic influence should be considered.
It’s difficult to determine whether kinks, nodules and fused vertebrae are all varying manifestations of the same genetic entity; difference in expression may be due to strain background. The advance of the deformity could be irregular, making it difficult to accurately determine the defect. It can also make simple culling of all affected animals a more difficult task. Breeders have noticed that tail defects often skip one generation, then turn up again to haunt the next. It is best not to use cats with tail abnormalities in a breeding program.
What is Hip Dysplasia and how does it occur?
Although hip dysplasia (HD) is seen most frequently in breeds known for their massive size (Maine Coons, for example), it can occur in cats of any breed, both short- and long-bodied, male and female. The pattern of inheritance suggests that it is polygenic, meaning that it is neither a simple dominant nor a simple recessive gene that can be easily bred out.
HD is a complex disorder in which the hip joint becomes damaged, inflamed and weakened. This causes the muscles to atrophy, which precipitates the onset of secondary osteoarthritis.
Dysplastic offspring may be born to parents with normal hips, but more commonly at least one parent has HD. Unfortunately. a breeder working toward certain desirable traits may also end up with a selection of cats that are susceptible to HD. Colleen Power, in her article “Hip Dysplasia in Cats” in the June 1992 issue of Persian News, says, “There is no single gene that is responsible for hip dysplasia. Instead there is a gradual piling up of genetic factors… The process is so dangerously gradual that, until the severe limp appears, the breeders’ will have no idea that they have been gradually altering the skeleton and musculature with each generation.”
The disease is progressive and often crippling. Intense activity can irritate the condition, but also makes the symptoms more evident to veterinarians. HD may be diagnosed differently in cats than in dogs, due to the dissimilarity of the hip structure and movement of dogs and cats.
How is Hip Dysplasia diagnosed and what are the indications of severity?
HD is best diagnosed through radiography of the pelvic region, a tedious procedure that requires the cat to be anesthetized. Working with veterinarians skilled in reading the subtleties of X-ray images, the Hip Dysplasia Registry of the Orthopedic Foundation for Animals at the University of Missouri in Columbia, Missouri, assigns nine variations of congruity and fit between the femoral head (top of the thigh bone) and the acetabulum (hollow area in the pelvic bone into which the thigh bone fits):
1. Excellent conformation.
2. Normal conformation, for age and breed
3. Less than ideal. but within the normal limits
4. Near normal, minor hip joint abnormalities
5. Borderline minimal dvsplastic change
6. Grade 1 dysplasia, subluxated (partially dislocated) 25 percent
7. Grade 2 dysplasia, subluxated 50 percent
8. Grade 3 dysplasia, subluxated 75 percent
9. Grade 4 dysplasia, femoral head luxated (dislocated) out of acetabulum
Only cats with hips rated I through 3 should be used for breeding.
I have vaguely heard of Patellar Luxation, but what is this condition?
Patellar luxation is a dislocation of the kneecap that manifests itself in one of two ways, both of which affect a cat’s gait. Medial luxations usually cause a bowlegged appearance while lateral luxations result in a knock-kneed stance. Luxation may be intermittent and the effect on gait may not be apparent with every stride. The condition can become particularly severe by the age of 1 to 2 years.
The occurrence of patellar luxation in certain breeds suggests a genetic influence, but the mode of inheritance is unknown; there is a possibility the defect is polygenic. The Devon Rex was one of the first breeds to be diagnosed with this problem. Abyssinians and Chartreux have also had increasing incidence of dislocated kneecaps.
Clinical signs are the best indicators for diagnosing patellar luxation. The kneecap can also be palpitated or X-rayed. Radiography and arthroscopy help evaluate osteochondritic lesions and deformities of the trochlear (pulley-like) ridges.
Treatment for cats with serious congenital deformities of the trochlear ridges can be difficult, especially if the patella does not stay within the trochlear groove for a reasonable length of time following manual replacement Cats diagnosed with patellar luxation should not be used in a breeding program.
What can you tell me about Progressive Retinal Atrophy?
Progressive retinal atrophy (PRA) is often quite advanced before outward signs are apparent. Affected cats display cautious behavior and stumble into things as a result of diminished vision.
Clincal signs of PRA include bilateral dilation of the pupils, jerking movement of the eye (nystagmus). hyperreflectivity of the capetum lucidum (reflective tissue layer) and progressive diminishing of the retinal blood vessels The photoreceptor layer of the retina (containing the rods and cones) also undergoes steady degeneration.
It has been noted that some Siamese display a familial tendency for this disorder.
At present there are three forms of PRA: rt, Rdy and rdg. They are differentiated from each other according to age of onset and mode of inheritance.
Monogenic recessive (rt) PRA is clearly inherited in some Persian cats. The condition becomes apparent at around 12 to 15 weeks of age. Abnormal pupil dilation (mydriasis) is a clinical sign. Examination reveals only remnants of rods and cones and thinning of the retina’s outer layers.
PRA expressed through the dominant gene designated Rdy has been diagnosed in Abyssinians at 4 to 5 weeks. By 12 weeks the degeneration is quite advanced. In one study, a stock of affected Abyssinian cats was found to have descended from a single male. The disorder resembles the effects of taurine deficiency, although no problems with taurine homeostasis have been noted.
Some Abyssinians were also found to have a third type of retinal degeneration, expressed in the recessive gene rdg. This third form begins later in life and progresses slower than the other two forms of PRA. Most cats with rdg PRA are diagnosed between 18 and 24 months of age. The advanced stages of the disorder are not reached until affected cats are about 4 years old.
What is Osteodystrophy and how does it occur?
Osteodystrophy results in a skeleton that is seriously and cripplingly deficient in calcium. Radiography reveals distinctly rarefied (less dense) bones. Osteodystrophic cats usually resent being handled, especially when gently manipulated in the lumbar region. Affected cats become lame and, in some instances, are unable to even stand. In severe cases the hindlimbs may become paralyzed
While the disease may be congenital, there has been some discussion as to whether or not other genetic factors are involved; some researchers believe the condition may be linked to recessive genes
In 1974, Dr. Oliphant F Jackson of the Royal Free Hospital School of Medicine in London, England, conducted a study of congenital bone lesions in cats with folded ears. He warned that heritable osteodystrophy in cats was associated with the fold-ear breed and suggested their breeding be limited to only fold-car/straight-ear matings.
Patrida Turner, an English breeder, was one of the first to study heritable osteodystrophy in Scottish Folds. She felt the condition might be inherited independently, but given expression by the presence of the fold gene. Unfortunately, no further studies have been done to support her theory.
This disease is apparently the result of a diet low in calcium. Diets consisting almost exclusively of meat are particularly conducive to the onset of osteodystrophy. Familial incidence could be simulated if the same deficient diet is fed to successive litters or generations. Whether or not this is a factor is best determined by the response of the skeletal dystrophy to corrective feeding.
Genetic influence cannot be totally ruled out because certain strains or breeds have particularly high calcium requirements. This may explain the regular recurrence of the disease in the Burmese and Siamese breeds.
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.