Breed-related disorders of cats
Article first published online: 12 MAR 2008
© 2008 British Small Animal Veterinary Association
Journal of Small Animal Practice
Volume 49, Issue 4, pages 167–168, April 2008
How to Cite
Gunn-Moore, D., Bessant, C. and Malik, R. (2008), Breed-related disorders of cats. Journal of Small Animal Practice, 49: 167–168. doi: 10.1111/j.1748-5827.2008.00572.x
- Issue published online: 12 MAR 2008
- Article first published online: 12 MAR 2008
Human beings have been keeping cats as pets for over 100,000 years (Driscoll and others 2007). It has only been since the 19th century, however, that we have been selectively breeding them for show and novelty - the first cat show was held as recently as 1871 in Crystal Palace, England. The General Council for the Cat Fancy (GCCF: the cat version of the UK Kennel Club) was established in 1910. Since then there has been active selection for numerous apparently “desirable traits”, particularly those associated with coat (colour, length or nature), facial conformation, eye colour and shape, body size and somatotype. This has resulted in an increasing number of recognised cat breeds and, unfortunately, in an increasing number of breed-related anomalies, disorders and outright diseases.
In Europe there are now believed to be over 60 million pet cats, with the percentage of pedigree cats varying from country to country. In the UK, about 12 per cent of our nearly nine million pet cats are purebreds (PFMA 2004, Euromonitor 2004, Ross Tiffin, personal communication, 2006, www.onswitch.co.uk). This means that UK vets currently care for about one million pedigree cats. Unfortunately, when we selectively breed for certain genetic traits we cannot help but inadvertently select for other genes that are closely linked to them. Furthermore, when most breeds were originally established, a number of undesirable traits were “fixed” by inbreeding (i.e. breeding to closely related cats such as siblings or the dam) at the same time as allegedly “desirable” traits were fixed.
To see what a significant impact this can have it is useful to look at the prevalences of dystocia and kitten mortality and to see how much higher they are in pedigree cats. A recent survey revealed pedigree kitten mortality to be 16 per cent from birth to eight weeks-of-age, with a staggering 25 per cent mortality in Persian cats (Sparkes and others 2006). The risk of dystocia, which is a significant risk factor contributing to neonatal mortality, can be dramatically altered by selective breeding. For example, only 0.4 per cent of births were problematic when domestic short haired cats were selectively bred for trouble-free kittening, compared to an average of 6 per cent for pedigree cats in the UK in 1990 (Gunn-Moore and Thrusfield 1995). Disturbingly, this percentage has increased further over the last 10 years to a current level of approximately 15 per cent (Sparkes and others 2006). While these higher levels of dystocia and neonatal mortality do not result from any single gene defect, they do reflect the deleterious effects that inbreeding can incur.
The long-term effects of selective breeding can be difficult to predict. The selective breeding of Burmese cats with dome-shaped heads in the USA resulted in craniofacial defects (meningoencephalocoele) due to an autosomal recessive gene (Sponenberg and Graf-Webster 1986). As a result, the GCCF refused to allow UK Burmese cats to be bred with those from the USA. While this prevented that defective gene from entering the UK, selective breeding (and often “in breeding”) has continued for almost 20 years. Thus, amongst a number of different disorders, Burmese cats from the UK, Australia and New Zealand are now almost four times more likely to develop type II diabetes mellitus than other cats (Baral and others 2003, Rand and others 1997), with one in 50 of the breed in the UK being affected (McCann and others 2007). In contrast, diabetes mellitus is not commonly seen in Burmese cats residing in North America (Panciera and others 1990).
Unfortunately, not all defects have been propagated inadvertently. Many actually define particular breeds. Some of these defects are esoteric genetic mutations that generally have little impact on overall health, such as the hair follicle defect that gives Devon Rex cats their short fragile curly coat, and the acromelanosis that causes Siamese and Tonkinese cats to have “points” (temperature dependent melanin formation). However, other breeds are defined by genetic mutations that actually have substantial adverse impact on overall health and longevity. For example, Manx cats have a lethal autosomal dominant gene for the Manx mutation that results in a short or non-existent tail (sacrocaudal dysgenesis) and a variety of secondary abnormalities including constipation, megacolon, rectal prolapse, congenital urinary tract defects, incontinence and spina bifida (Robinson 1993, Howell and Siegel 1963). Other examples include Sphynx cats with their baldness which puts them at risk from sun burn and chilling (depending on the weather), Munchkin cats with their chondrodystrophic shortened limbs and associated osteoarthritis, Persian cats with brachycephalic upper airway issues, and Scottish Fold cats as described in the paper by Takanosu and others in this issue of JSAP (see page xx). This paper confirms the deleterious nature of this autosomal incompletely dominant gene that results in the forward folding of the ears. Unfortunately the gene does not limit its influence to the ear cartilages, so even heterozygotic cats can develop osteochondrodysplasia and suffer degenerative osteoarthritis in their limbs and even their tail (Malik and others 1999). The paper by Takanosu and others (2008), appropriately, recommends that these cats should not be bred or kept as pets. However, even though the GCCF and the Fédération International Féline refuse to accept the registration of this breed because of its cartilage issues, these cats continue to be bred and they are still very popular pets. Why is this the case? People who own them may be “charmed” by their round faces and open expression (and they may not realise that the reason the cats do not move around too much is because they are variably crippled with arthritis).
It is only by recognising the risks of selective breeding – and speaking out against it - that veterinarians can play a role in curbing people’s desire for ever more extraordinary and unusual cat breeds (with an increasing number of deleterious genes). The Feline Advisory Bureau (FAB), a charity dedicated to the promotion of the health and welfare of cats, has been concerned about breed-related disorders for many years. To help veterinary surgeons and breeders recognise potential problems, the FAB has just launched an online list of many breed-related defects in cats (http://www.fabcats.org/breeders/inherited_disorders).
The FAB list provides details of over 100 conditions that can occur in cats and which appear to have an inherited basis. The conditions are coded as those:
- i. where the genetics have been confirmed and/or a genetic test is available,
- ii. where a breed predisposition is recognised and the condition is strongly suspected to be inherited and
- iii. where a potential breed predisposition is recognised but it is not currently known whether or not the condition is inherited, or where there are only single case reports, or evidence is anecdotal.
Detail is also given on the frequency of occurrence within the particular breed, and whether or not national differences are known to exist, including pertinent references from the scientific literature. While this is an extremely valuable resource, it is important to recognise that any such list is limited. Deficiencies result from limited access to information from other countries, particularly those that do not publish in English, and from the difficulty in keeping the list updated. Perhaps the most critical limitation, however, results from the need to understand that the amount of information available for a particular breed is not necessarily a reflection of the overall health of that breed. This is because the length of any breed-related list will depend to a great degree upon the level of surveillance within the breed, and with the breed’s long term popularity or special factors such as usage in medical research. For example, it would be wrong to contend that the Burmese breed is significantly less healthy than the Tonkinese (which has fewer disorders listed): it is simply that Burmese are more numerous and have an active and vigilant breed club, while the Tonkinese is rarer and less well studied. In fact, much of our data would suggest that in general terms, the less popular a breed, the smaller the available gene pool and the more likely it is that there will be inherited disease issues per se.
Only by recognising the occurrence and severity of particular conditions and trying to determine their prevalence within individual breeds can we educate breeders, and cat owners, about the actual significance of particular problems. The development and promotion of genetic disorder screening schemes plays an important role in this. For example, the FAB established the polycystic kidney (PKD) screening scheme for Persian and related breeds in the UK over five years ago. This confirmed that 30-40 per cent of Persian cats in the UK, as in most other countries, were affected by this autosomal dominant disease (Eaton and others 1997, Cannon and others, 2001, Barrs and others 2001, Barthez and others 2003), and the scheme has facilitated the breeding of non-affected cats. Pleasingly, the prevalence of the disorder now appears to be declining in the UK (FAB data on file). Additionally, in collaboration with the Veterinary Cardiovascular Society, the FAB established the hypertrophic cardiomyopathy (HCM) screening scheme for Maine Coon cats. Such schemes are greatly enhanced by the availability of accurate molecular tests, reliable screening tests (e.g. accredited diagnostic ultrasound schemes) and by accurate identification of individual animals (e.g. by use of microchip transponders). Part of the FAB/European Society for Feline Medicine pre-BSAVA Congress day this year will focus on the breed-related disorders list and testing for the most common and important of these disorders.
Veterinary surgeons should not abdicate the responsibility for breed-related disorders on the grounds that they are not responsible for actually producing the animals. In our duty of care to our patients, we should be prepared to counsel breeders and potential owners as to the possible negative effects that selective breeding can incur. Further, we should be vigilant in looking for common breed-related disorders, and should speak out against the current desire to have ever more extreme pets: including tea-cup sized cats, huge cats, cats with shortened legs, bald cats, and even cats that have been crossed with wild animals.
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Danièlle Gunn-Moore is Professor of Feline Medicine at the University of Edinburgh. She set up the Feline Clinic 10 years ago where she sees referral cases and teaches the veterinary students. She undertakes clinical research and publishes widely in many areas of feline medicine, delivers continuing education lectures worldwide, and is a Trustee/Director of the Feline Advisory Bureau.
Claire Bessant is Chief Executive of the Feline Advisory Bureau and the European Society of Feline Medicine, which is a division of FAB. The charity generates and disseminates feline information to a range of interested groups including vets, breeders, catteries, cat rescue and cat owners.
Richard Malik is an Adjunct Professor of Veterinary Medicine. He divides his time between organising the delivery of veterinary continuing education for the Post Graduate Foundation in Veterinary Science at the University of Sydney, doing infectious diseases research and seeing cases in companion animal practices and the RSPCA.