Inherited disorders are conditions that arise due to abnormal genes that are passed down from one generation to another. Genetically determined disorders can be obvious at birth, but some may not develop or may not be obvious until later in life.
Cats suffer from inherited disorders like other animals, but they tend to be more common among pedigree cats because the selective breeding and in-breeding (breeding together very closely related cats) used to develop particular characteristics of the breed may also increase the risk of inherited disorders. Indeed in some cases, the breed itself is based on an inherited disorder which, potentially can be harmful to health.
Genetic testing of cats
Recent advances in genetic investigation and testing have meant that it has been possible to identify the gene defects associated with a number of inherited conditions, confirm their genetic basis, and also in many cases develop diagnostic tests to identify affected cats (and occasionally ‘carrier’ cats).
Although some diseases have a simply genetic basis with the disorder being determined by a single pair of genes, in other disorders that may have an hereditary component, the inheritance may be much more complicated. Multiple genes can be involved in some disorders (polygenic) and there may be a combination of genetic and environmental effects (multifactorial) in others. Single gene disorders are much easier to characterise, investigate and develop diagnostic tests for.
With different populations of cats in different parts of the world, some genetic disorders may be seen in more commonly, or even exclusively, in some geographical locations. However, with international travel of both owners and cats, many disorders are seen worldwide (although the frequency of disease may vary in different regions).
Many different veterinary diagnostic laboratories offer genetic (DNA) tests for different animal diseases. Many laboratories offer DNA testing for cat coat colours and cat parentage, as well as tests for inherited disorders.
Are some breeds genetically ‘healthier’ than others?
Inbreeding, such as is used to fix traits within pedigree breeds, will inevitably increase the risk on inherited defects coming to light. Although inherited diseases can occur in both non-pedigree and pedigree cats, they are generally more likely to occur in pedigree cats than in outbred domestic cats (domestic shorthairs or domestic longhairs).
Some pedigree breeds are much more inbred than others and so, in theory, would be at greater risk of having inherited diseases present. However it is difficult or impossible to say that some breeds are genetically ‘healthier’ than others. In some breeds a large number of different inherited diseases have been identified, but this usually reflects more widespread surveillance and testing within these breeds rather than necessarily a higher frequency of inherited diseases.
However, an exception to this is where an inherited disorder is specifically bred for within a breed, but is also detrimental to the health of the cat. Clear examples of this include the breeding of Manx cats, Scottish folds, and extreme-type (very flat-faced) Persians. In these cases the characteristic of the breed itself is based on gene mutations or selecting genotypes that express a phenotype (trait or morphology) that is harmful to the health of the cat.
Can inherited diseases be controlled?
Where there is a relatively simple mode of inheritance, and where there is a DNA test widely available, controlling an inherited disease may be relatively straightforward. In other cases it may be more difficult.
A good example of controlling an inherited disease is polycystic kidney disease (PKD) in Persian cats and related breeds. In the past, up to 50% or more of Persian cats may have been affected by this disease, which will frequently result in chronic kidney disease and premature death of affected cats.
It was recognised that PKD was inherited as a simple autosomal dominant trait. This means the disease was determined by a single gene. Within each cell in the body, genes are present as pairs one on each of two strands of DNA (chromosomes). With an autosomal dominant disease, if one of the pair of genes is abnormal, this is ‘dominant’ over the normal gene, and results in disease development. With this type of disease, there are no unaffected ‘carrier’ cats – all cats with an abnormal gene will be clinically affected.
Cats with PKD were initially identified by ultrasound scanning of their kidneys, but now a readily accessible and accurate DNA test is available (performed on a blood sample or cheek swab). Affected cats can easily be identified in this way and breeding from them prevented. This is highly successful, and responsible breeders have their cats tested before embarking on a breeding programme. While there is still more work to be done, and not all breeders accept or undertake responsible breeding, in many countries the frequency of PKD has now been dramatically reduced.
What precautions should be taken when doing DNA testing for a breeding programme?
When undertaking DNA testing of cats (to determine whether they are suitable for a breeding programme) a veterinarian should always be present to supervise, and a reputable and reliable testing labratory should be used.
International Cat Care believe that whenever genetic tests are run on cats for the selection of breeding stock, the gene test result should be linked to a method of permanently identifying the cat that has been tested (eg, a standard, internationally recognised microchip number), and that a vet should collect the sample (blood sample or cheek swab) so that the identification (microchip number) can be verified and recorded on the submission form and result.
For certain diseases, International Cat Care has set up a register, where the results of DNA tests for individual cats can be viewed (when samples are collected and reported with the above precautions), to assist in the selection of breeding stock.
Common and/or important inherited disorders in cats
Some of the more common and important inherited disorders of cats include:
- Blood group incompatibility or neonatal isoerythrolysis
- Burmese Head Defect
- Devon Rex Myopathy
- Glycogen storage disease type IV
- Hypertrophic cardiomyopathy
- Hypertrophic muscular dystrophy
- Hypokalaemic polymyopathy
- Manx syndrome (spina bifida)
- Niemann-Pick Disease (sphyngomyelinosis)
- Osteochrondrodysplasia or Scottish Fold disease
- Polycystic kidney disease
- Polydactyl cats
- Progressive retinal atrophy
- Pyruvate kinase deficiency
- Scottish Fold disease - osteochondrodysplasia
- Spinal muscular atrophy in Maine coons
Some pedigree breeds have been deliberately bred and selectively developed for some extreme traits or characteristics. These are all heritable traits, and where these compromise the health or welfare of the cat, International Cat Care believe such traits or mutations should not be perpetuated by continued breeding.
- Severe brachycephalia in Persians and similar breeds
- Hairless cats
- Short-legged cats
There are many diseases where a very marked breed predisposition has been demonstrated (ie, the disease occurs much more commonly in certain breeds or in certain lines within breeds). This gives a very strong indication that the disease is likely to be inherited, or that there is likely to be an underlying inherited component to the disease, but in many cases the gene(s) involved have not yet been discovered.
Examples of this include:
- Feline orofacial pain syndrome
- Diabetes mellitus in Burmese cats
- Asthma in Siamese and related cats
- Flat chested kittens in Burmese cats
- Mediastinal lymphoma in Siamese cats
- Patellar luxation
- Hip dysplasia
- Pyloric stenosis or dysfunction in Siamese cats
- Small intestinal adenocarcinoma in Siamese cats
- Strabismus (squint, cross-eyed) in Siamese cats