A very common dilemma for the cat breeder is 'my tom or queen has just produced a deformed kitten. What should I do about this? Should I breed from him or her again?'

This is a question which is rarely easy to answer. Each situation should be considered individually and many factors must be borne in mind. Your veterinary surgeon is therefore in the best position to advise on particular cases and you should consult him, providing as much background information as possible. An information sheet such as this cannot provide a set of rules which can be applied to every case. However, it will outline some guidelines which may be helpful to breeders planning the most appropriate course of action to adopt if a deformed kitten has been produced. This is an important subject which every responsible breeder should consider very carefully for the benefit of their own cats and the breed involved.

This information sheet is concerned with congenital deformities: these are abnormalities which are present at birth. Some of them are very obvious and will be noticed following the most casual observation - such as gross abdominal hernia. Others are much less obvious and may only be discovered following a thorough post-mortem examination - such as congenital malformations of the organs. For this reason, it is very important that a full post-mortem is performed on all young kittens which die for no obvious reason. There are also many very subtle abnormalities, such as errors of metabolism, which can only be diagnosed by sophisticated laboratory tests on blood samples from live kittens.

Although congenital abnormalities are present at birth, they may not necessarily be apparent at this time. For example, a congenital abnormality of the heart may not produce any signs in a young kitten, but in time, as the kitten grows and the demands on the heart increase, characteristic signs of heart disease may develop.

There are also some hereditary abnormalities which take time to develop. For example, retinal atrophy, although known to be hereditary in Abyssinians, Siamese, and possibly other breeds, is a slowly progressive disease in which no impairment of vision or abnormality of the retinal vessels may be recognised until the affected cats are quite old.

Not all congenital abnormalities are hereditary. Some certainly do result from gene mutations, but there is another important group of congenital deformities, the so-called non-genetic developmental abnormalities. These are caused by disturbance of the development of the kittens in the uterus by some non-hereditary factor.

Infections   

Some infections may lead to damage or death of the developing young if they occur during pregnancy. Most of these are viral infections since viruses tend to prefer rapidly dividing cells, and the cells of a developing embryo or foetus are dividing particularly rapidly, thus providing a prime target. For example, it is widely recognised that German measles can damage the developing baby if a mother is infected during pregnancy. There are similar examples recognised in cats, and panleucopenia infection may act in this way. It can selectively destroy cells in a certain area of the brain, the cerebellum. This part of the brain is important in controlling coordination and therefore affected kittens are ataxic (have difficulty in balancing).

Not only can field virus derived from natural infections cause this problem, but also virus used in some vaccines. Some vaccines contain live virus which is attenuated (it does not produce disease in kittens or adults), but it can, nevertheless, damage developing foetuses if a pregnant queen is vaccinated. For this reason, as a general rule, live vaccines are not used in pregnant animals.

Drugs  

Some drugs can damage developing foetuses and these are known as teratogens. Probably the best known example of a teratogenic drug is thalidomide which produced deformities in the children of mothers who had received this drug during pregnancy.

Very little is known about the teratogenic potential of most drugs used in cats. Drugs must be tested for teratogenicity before they are licensed for use in humans but their teratogenicity may vary considerably in different species, and it is clearly impractical to test drugs routinely for teratogenicity in cats. We certainly know that some drugs can be teratogenic, for example, griseofulvin, the antibiotic used for ringworm treatment, can cause congenital deformities in developing kittens if given to pregnant cats. But with most drugs used for cats, we simply do not know whether or not they are teratogenic. Therefore, as a general rule, it is best to avoid the use of drugs in pregnant queens unless absolutely necessary.

Substances other than drugs may also act as teratogens. For example, it is possible that some common household substances, or food preservatives could act in this way.

Environmental factors   

Certain environmental factors are known to be teratogenic. Exposure of pregnant animals to high temperatures can lead to developmental abnormalities in their unborn young. This can occur simply by maintaining the animals in a higher ambient temperature. We have encountered cases where there has been a strong suspicion of congenital deformities of kittens resulting from hyperthermia due to exposure to high environmental temperatures.

Other factors   

Some other factors may be responsible for developmental problems and these may be very difficult to identify. An example of this would be inadequate development of the placenta. This is not uncommon in humans and may lead to intra-uterine growth retardation. A similar situation may possibly occur in cats if the placenta fails to function correctly. It is the 'lifeline' for the foetuses, providing all their requirements for life, and if its function is impaired, the growth of the foetuses will suffer. Such a problem may be particularly common in a polytocous animal such as the cat which usually has many young in a litter. There may consequently be competition between the placentae for the available uterine space and some may fail to develop fully.

The question that the responsible cat breeder should ask on the appearance of a deformed kitten is: 'Is this an hereditary or non-genetic problem ?' Unfortunately, with many congenital deformities in cats, we simply do not know. With some of the more common conditions, such as cleft palate or umbilical hernia, it is quite clear that these may be of either hereditary or non-genetic, developmental nature.

There are a number of indicators which can be used. It must be stressed that these are not 'hard and fast' rules, and each case must be considered individually.

•  If there is a strong breed disposition (ie. the condition occurs particularly frequently in a specific breed) this suggests some hereditary involvement, although there are other possible causes of breed predispositions.

•  The presence of multiple abnormalities in the same kitten or in different kittens in the same litter, suggests a non-genetic, developmental problem, rather than an hereditary one. For example, a kitten may be born with cleft palate, a twisted spine and spina bifida - or in the same litter there may be one kitten with a cleft palate, one with an umbilical hernia, one with a malformed leg and one mummified. This would suggest that it is probably a non-genetic developmental problem and therefore not hereditary.

In considering an individual case, there is much information to examine (see table 1). In particular, if the mating has been used before, it is useful to know the outcome. It is also important to check whether the same deformity has been recorded in the progeny of either of the parents, and, if so, whether other partners in the mating were related to either the tom or queen.

Any change in management, especially of diet, and any history of illness during pregnancy may be relevant, as such factors may be responsible for non-genetic developmental abnormalities. In addition, difficulty during kittening may lead to damage to the young kitten.

Useful information to consider if a deformed kitten is produced   

•  How many kittens were produced in the litter and how many were abnormal ? Did they all show the same abnormality ?

•  Has this mating been used before and, if so, what was the outcome ?

•  Have either of the parents been mated to other cats before ? If so, were they related (to either of the original parents), and what was the outcome ?

•  Have you encountered this deformity previously, either in your own cats or in cats of the same breed ?

•  Did anything unusual occur during the pregnancy ? For example, was there

    1.) a change of diet or management ?

    2.) any illness during pregnancy ?

    3.) did the queen receive any drugs ?

•  Were there any problems with the kittening ?

If the available evidence strongly suggests that the problem is hereditary, and if the deformity is serious, the breeder may elect to have the queen spayed. However, it should be remembered that if the condition is hereditary, there will be far greater ramifications to consider. Most congenital deformities are inherited as recessive factors, in which case both parents must be 'carriers' of the condition and also 66% of the apparently normal littermates. Furthermore, the trait will be carried by at least 50% of the progeny of either parent mated to other cats. It is clear, therefore, that the implications for breeders who encounter severe deformities of an hereditary nature are extremely serious.

Unfortunately, in many instances, insufficient information is available to decide whether or not a deformity is hereditary, and this may only be determined by carrying out a series of test matings.

The ideal mating for this purpose is between affected offspring and their parents. If a recessive factor is responsible for the abnormality, 50% of the resulting kittens will be affected. If a simple recessive factor is suspected, it is necessary to produce at least five normal kittens to ensure a probability of over 95% that the condition is not hereditary, or seven kittens to increase the probability to over 99%. Just one affected kitten may be enough to indicate that the defect is inherited. However, if the abnormality is so severe that this is not practical, the original mating which produced the affected kittens should be repeated or, failing this, the parents should be mated with other cats known to have produced affected offspring and therefore suspected to be carriers. In this case only 25% of the progeny will be affected (although a further 66% of the apparently normal progeny will be carriers). In this case it is necessary to produce at least 11 and 17 unaffected kittens to ensure a probability of over 95% and 99% respectively that the condition is not inherited.

Such a mating programme may be rather an anathema to a breeder, arranging matings in an attempt to reproduce the condition, and breeders may be tempted to mate their cats to another individual from a different line. Indeed, all the young produced by this mating may well appear to be normal (unless the new partner is also a carrier), but if the abnormality is inherited as a recessive factor, 50% of these will also be carriers. This may not be discovered for several generations until a pairing is chosen between two cats with the original carrier in both their pedigrees. The trait may then be well disseminated throughout the breed and difficult to eradicate. The responsible breeder will therefore act immediately if deformed kittens are bred.

Occasionally it becomes apparent that a congenital abnormality is particularly common in a certain breed. This does not necessarily indicate that the abnormality is inherited, and it is possible that certain breeds may tend to express non-genetic developmental disturbances in a particular way. The importance of some hereditary conditions may be affected by the breed. An example of this would be hip dysplasia in the dog, a condition involving a deformity of the hip joints. In small breeds, the condition may have very little effect on the gait, but in large, and consequently heavier breeds, this can be a crippling deformity. Breed variation in the expression of congenital deformities is generally less significant in cats since there is not such a range in size and conformation between breeds.

Certain congenital deformities usually of a polygenic (ie. controlled by a number of genes rather than a single gene) may be associated with breed conformation. For example, lacrimal (tear) overflow may be seen in very 'typey' long-hair cats since a particularly short nose may lead to deformity of the naso-lacrimal ducts (which drain tears from the eye to the nose).

It is important that if a breed predisposition is suspected, investigations should be undertaken to determine whether the condition is inherited and, if so, its mode of inheritance. This information may be required if the trait is to be eliminated.

The first step is to determine the prevalence of the condition and to collect as much information as possible about individual cases (see table 2). In the case of some deformities, a thorough clinical investigation, possibly supported by laboratory tests or pathological studies, may be required to confirm the diagnosis and to differentiate from other conditions which cause similar clinical signs. Collation of pedigrees of affected individuals may also provide evidence of common ancestry and possibly an indication of the mode of inheritance. Unfortunately, the stigma that many breeders feel is associated with congenital deformities may discourage open discussion of such problems and free exchange of information. In this situation the breed club may elect to request the assistance of an impartial collator. Breeders may be more willing to provide information about cases to such an individual if they are assured of confidentiality. For this reason, and because of the availability of veterinary expertise, which may be required both to provide a definitive diagnosis and to investigate further the nature of the deformity, breed clubs may request the help of the Feline Advisory Bureau.

Investigation of deformities with suspected breed disposition   

•  Circulate a questionnaire to:

    1.) Determine the extent of the deformity

    2.) Obtain information about the nature of the problem

    3.) Collect information on the breeding and pedigrees of affected kittens

•  Collate the breeding results and pedigrees

•  Organise a test mating programme

The next step is to determine the precise mode of inheritance of the deformity. This will usually necessitate a test mating programme, but in some cases sufficient data may be available from random matings if detailed records and pedigrees are available. Considerable care, however, is required in collating the pedigrees of kittens from less common breeds. There may be a limited number of stud toms available for such breeds and the same studs will inevitably appear frequently in pedigrees, particularly if they have been very popular. Hereditary deformities are likely to appear more frequently in small breeds for this very reason, especially if it is a new breed which has increased in popularity rapidly and has been derived from a relatively small genetic pool. The genes responsible for congenital deformities may not be more common in such breeds but the chance of affected kittens may be higher due to a necessarily higher degree of inbreeding.

The success of an investigation of this type is dependent mainly on the participation of breeders. Unless breeders provide detailed information about cases, there will be insufficient data for meaningful analysis. It is also important for breeders to appreciate that investigations of this nature may take considerable time to complete. A test mating programme is a major undertaking and requires extensive resources, although it may be possible for a number of breeders to arrange a series of matings under the direction of a collator. However, a conflict may arise for the collator between the requirement to maintain confidentiality concerning information passed on by breeders and the need, inevitably, to disclose some information in suggesting suitable pairings for the test mating programme. Even if an extensive test mating programme of this type is performed, it may prove difficult to define the precise mode of inheritance for a number of reasons:-

•  The deformity may be polygenic rather than due to a single gene mutation. Its inheritance may therefore be highly complex.

•  It may be a deformity that can be either hereditary or of a non-genetic developmental nature.

•  The severity of the deformity may vary. This may be due to:

a.) A single gene with variable expressivity or incomplete penetrance

b.) Although the deformity may depend primarily on a single gene mutation, other factors, such as diet, may contribute to its severity in individual cases.

•  Threshold factors may be involved.

Once the hereditary nature of a defect has been confirmed and its mode of inheritance has been determined, the breed club can decide on an appropriate course of action. If the defect is mild, and is of purely aesthetic importance with no deleterious effect on the cat's health, it may not be considered necessary to take any action. If it is a serious defect, the course of action may depend on the prevalence of the defect. A rare deformity which is restricted to just one or two lines of cats may be eliminated relatively easily. However, if the deformity is widespread, it may prove impossible to eliminate. For example, if the incidence of a deformity is 1:100 cats, there may be 18 carriers in every 100 individuals.

It is therefore vital that every responsible cat breeder and breed club should make every effort to identify hereditary deformities as soon as possible to avoid perpetuation of the trait. Hereditary deformities do not disappear if ignored - they get worse. There is also considerable merit in the policy of collating information on all congenital deformities either through the breed club and the GCCF, or through the FAB register of such anomalies.

How to send a dead kitten for investigation   

(N.B. This applies to the UK only!)

If you intend sending a kitten for investigation at The Feline Centre, Langford, please observe the following:

•  You must consult your veterinary surgeon and obtain his agreement before you send the kitten.

•  Include as many of the details described in this information sheet as possible.

•  Send the kitten by first class post, wrapped in absorbent material (preferably cellulose wadding paper, but alternatively in tissue or cotton wool) and packed in a 'Jiffy' bag or an uncrushable package. DO NOT use a plastic bag, this is particularly important in summer as high temperatures will lead to 'sweating' and rapid deterioration.

•  If a kitten dies at the end of the week, or at the weekend, and a delay of more than two days is likely between despatch and receipt at Langford, the kitten should be stored temporarily. This can be done by placing the kitten in a fridge or deep freezer. Tissues will continue to deteriorate in a fridge (but not so rapidly) and subsequent histological interpretation may not be possible. If histology is likely to be important in a particular case, this problem can be overcome by fixing (pickling) small portions of the desired tissues in 10% formal saline. Freezing will not usually interfere with virus studies and will minimise deterioration of the tissues but does interfere with bacteriology (by selectively killing some bacteria), and histology (the ice crystals that form disrupt the tissues). It does have the advantage of enabling indefinite storage. Ideally, a kitten stored frozen should be received at Langford without thawing out.