Copyright 2001 - 2006, Sarah Hartwell

Minor mutations of fur type and body shape are often developed into new breeds. There are a growing number of curly-haired and hairless cat breeds; there are tailless, bobtailed and curly-tailed cats and cats with folded or back-swept ears. There are cats with shortened legs or extra toes, cats which resemble rabbits or squirrels, and cats with "wings" of loose skin. All of these can be found at Curious Cats (anomalous big cats can be found at Hybrid and Mutant Big Cats). There is also a whole host of medical curiosities which occur in cats, ranging from minor (extreme cases of extra toes) to lethal (two heads). This article looks at the uncommon medical anomalies which have occurred. My own interest is medical/genetic, though misguided people have sometimes attempted to breed the grosser deformities.

Note: Contrary to suggestions on some bulletin boards, the images here are not photoshop. With the exception of those labelled as artist's impressions these are photos of medical conditions. Offsite links to images on these pages is not supported - bandwidth costs money!


Deformities occur for two reasons - a genetic problem such as an inherited trait or mutation, or a non-hereditary birth defect due to something going wrong in the womb. If the defect - either genetic or a development defect - is only minor, then the kitten will grow into an adult and simply look a bit different from regular cats. It may live a perfectly normal life or it may need special care because it has some sort of disability. Nowadays, the term "freak" is unacceptable even for animals so I will use the term "curiosity" instead. The study of such curiosities helps scientists understand how genes work and how embryos develop - we can only tell what some genes or processes do when those genes or processes go wrong.

With a few exceptions, genes are inherited in pairs - one from each parent. If only one gene in a pair is needed to cause the defect then it is referred to as "dominant". If both genes must be faulty for the defect to show up, it is called "recessive" since if the cat only gets one bad copy of the gene it looks normal but can still pass the bad gene on to its kittens. Sometimes the cat has to inherit a whole bunch of different genes which only cause the defect if the right mixture is inherited from both parents - this is recombination of genes. Another possible cause is for a gene in the egg cell or sperm to mutate so the kitten inherits a deformity even if neither of the parent has it. These are all hereditary reasons because the bad genes can be passed on from one generation to another, even if they don't always show up.

Hybridisation can also cause defects because the chromosomes and genes can't match up. They try to build a foetus, but there are conflicting instructions and all manner of things can go wrong. Usually the foetus dies early on in the development. This occasionally happens when domestic cats are hybridised with wild cat species to create new breeds and the genes are incompatible. Usually the foetus is reabsorbed by the mother. Where the genes are completely incompatible, the egg cannot be fertilised at all or it stops dividing after a few days, for example "cabbits" are genetic impossibilities because cats and rabbits have different sets of genetic instructions which conflict with each other (real life "cabbits" are usually cats suffering from a form of spina bifida or a similar defect).

A birth defect happens when something goes wrong in the womb and the foetus does not develop properly. For example an egg starts to split, but doesn't split completely - this leads to conjoined twins (the old term is Siamese twins). If the umbilical cord gets wrapped around a leg or tail, the limb is starved of blood and withers away to leave a stump or an underdeveloped limb. Certain drugs interfere with the development of unborn kittens and cause deformities (just as Thalidomide causes birth defects if pregnant women take that drug). Certain drugs, environmental toxins and viral infections are "teratogenic" meaning they cause monsters (deformed individuals) to be born. An injury to the pregnant mother can also cause birth defects.

The old tale of fright causing birth defects is a myth. Extreme fear could possibly cause spontaneous abortion because there is too much stress on the mother's body. There are medical tales of humans with hare-lip who supposedly suffered the defect because their mother was "startled by a rabbit or hare" while pregnant. This is not possible.

Where something goes wrong early on in the development of the foetus, it is simply reabsorbed by the mother. If something goes wrong later on, the soft tissue is reabsorbed and all that remains is a "mummified kitten" or mass of calcified (turning to bone) tissue which is ejected when the other kittens are born. If something goes wrong immediately before birth, the kitten will be born dead or die within a few hours or days of birth because it is too badly made to survive outside the womb.

The popularity of this page and the number of images have led to slow-loading times; it has now been split into multiple pages. Each of the sections below will open in a new frame.


Although medical photographs are not be to everyone's taste, the point of this article is to discuss and illustrate some of the hereditary, developmental and acquired abnormalities seen in cats. Some are mild deviations from the norm. Others are gross abnormalities. Between these extremes is a huge range of abnormalities that can be treated or alleviated, giving affected cats good quality of life. For owners suddenly faced with an abnormal kitten, the big questions are "Why" and "What do I do now?"

This section only considers abnormalities evident at birth. Inherited abnormalities are largely due to recessive genes (often associated with particular breeds) and the parents are carriers of the faulty gene. The affected kitten's littermates are likely to be carriers of the gene. Other congenital abnormalities are not inherited from the parents. Some result from gene mutations in the embryo itself. Others are non-genetic developmental abnormalities i.e. a problem while the kitten developed in the womb. Where one kitten has multiple abnormalities, or where several kittens in a single litter have different abnormalities, the cause is likely to be developmental rather than genetic. Where several kittens have the same abnormality, albeit to different degrees, it is more likely to be genetic. These are probabilities, not certainties.

Infections, particularly viral infections, in the mother can damage her unborn kittens (much as Rubella is dangerous to pregnant women). Viruses generally target rapidly dividing cells; embryos and foetuses are divide rapidly during growth. Panleucopaenia can selectively destroy cells in the cerebellum; the part of the brain important in controlling co-ordination. Affected kittens are variously described as spastic or having cerebral palsy or cerebellar hypoplasia (CH). Pregnant females should not be vaccinated with live vaccines. How badly a kitten is affected by its mother's illness depends on its stage of development. A viral illness early in pregnancy can result in miscarriage. The same illness in late pregnancy may have less effect as the kittens are fully developed, or almost so.

Substances that damage developing foetuses are called teratogens ("monster forming"); a well-known example being thalidomide in humans. Griseofulvin (used to treat ringworm) can cause congenital deformities in developing kittens. Little is known about the teratogenicity of most drugs used for cats. Drugs are tested for teratogenicity before they are can be used in humans. Where veterinary medications are tested, what is teratogenic to a test species may act differently in cats. Testing for teratogenicity in pregnant cats is clearly distasteful to cat owners. As a rule, it is best to avoid giving medication to pregnant females. Other substances, such as household cleaners, environmental toxins and food preservatives might also cause birth defects in cats due to the relatively poor liver function of felines.

Hyperthermia (exposure to high environmental temperature) can also cause congenital defects.

An under-developed or poorly functioning placenta stunts foetal growth. Cats have litters of offspring and the placentas compete for space in the uterus. In larger litters, some placentas lose out, resulting in runty kittens and sometimes in deformities. In larger litters, one kitten may be squeezed for space and its limbs forced into an abnormal position resulting in the "twisted limbs" syndrome (usually correctable if treated early). If the umbilical cord gets wrapped round a limb, that limb is deprived of blood supply; it becomes stunted, shrivelled or is amputated by "umbilical strangulation". Finally, injuries and other problems (e.g. oxygen starvation) can occur during birth itself.

If your cat produces deformed kittens, you need to assess possible causes: illness or injury during pregnancy, inbreeding (mating closely related cats), medication or accidental poisoning. How many kittens were affected and did they have the same abnormality or different ones? Have the sire and dam been bred to each other, or to other partners, previously and did those mating result in abnormalities (if so, a genetic cause is likely)? Has the deformity been observed in other cats of the same household, bloodline or breed? In some cases, the only way to decide if a problem is genetic is through test-matings of the parent cats to each other and to other cats. The thought of deliberately producing abnormal kittens, especially if the kittens have severe abnormalities, is unacceptable to many breeders. If available data indicates a genetic cause, both parents and all offspring should be neutered. Breeding them to alternative cats simply spreads any hidden bad genes further around the breed and causes major headaches later on.

Some congenital abnormalities are particularly common in certain breeds. The most likely cause is genetic, but it is possible that certain breeds express non-genetic developmental problems more than other breeds. For example, hip dysplasia may be unnoticeable in small, light breeds of dog, but can be crippling in large heavy breeds. Other abnormalities are due to multiple genes interacting (polygenes). The extremely short faces of ultra-typed Persians can lead to deformed or absent tear-ducts. The problem is not due to a single tear-duct gene, but is due to the various genes that determine face shape. Some breed-specific abnormalities can be reduced or avoided through careful selective breeding e.g. spina bifida in Manx, skeletal thickening in Scottish Folds and gross abnormalities in Ojos Azules. Some breeds are bred to perpetuated what is essentially a deformity (a deformity being any deviation from the normal form) and great care must be taken to limit the degree of deformity.

If your cat produces a deformed kitten that subsequently dies or is euthanized, you (or your vet or the breed society) may wish to have the trait investigated. This generally means sending the body away for post mortem studies. Your veterinary surgeon can advise on how this can be done and may make arrangements for this (this is preferable). In general, you need his agreement (or at least need to advise him) before sending a kitten for investigation. As well as sending the body, you should provide as much information as possible about the circumstances of the birth and death and other factors mentioned in this section. If you are sending the kitten yourself, send it by the quickest method possible e.g. in your own car, by courier or by first class post/guaranteed next day delivery. Wrap it in absorbent material (wadding paper, tissue or cotton wool) and package it in an uncrushable package. If you use a courier or transport it yourself, you can surround the body in bags of crushed ice (especially in hot conditions, but make sure any melting ice cannot leak) in a cool-box. Do not use a plastic bag (especially in hot climates/conditions) as this causes sweating and speeds up deterioration/decomposition.

If immediate transportation isn't possible, the body can be stored temporarily in a fridge or, preferably, deep freeze. Tissue deteriorates more slowly in a fridge and this can make tissue studies (histology) harder. If tissue study is important, tissue samples can be pickled in 10% formal saline. Freezing minimises deterioration and generally doesn't interfere with virus studies. Freezing does interfere with bacteriology as it kills some bacteria. Freezing also affects tissue studies as ice crystals form and disrupt the tissues and burst cells.


If you are interested in medical curiosities, books worth reading are "Mutants: on the Form, Varieties and Errors of the Human Body" by Armand Marie Leroi and "Anomalies and Curiosities of Medicine Vols 1 and 2" by George M. Gould & Walter L. Pyle. The Gould & Pyle books were published in 1896 and are in the public domain. You can download text-only versions of Gould & Pyle from several websites so don't waste money on text-only versions of the book; but if you want the versions with photos, consider the Kessinger editions. The Leroi book explains why and how some deformities and anomalies happen - the mechanism is the same in cats as it is in humans.


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