Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners

Netherland Dwarf

Netherland Dwarf

Dental Malocclusion

Related terms: Congenital malocclusion, mandibular prognathism, maxillary brachygnathism

Outline: Netherland dwarf rabbits quite commonly have a genetic disease in which the upper and lower incisor teeth – which grow throughout life - do not meet together when the animal bites and thus do not wear. The result is that the teeth over-grow. This causes injury and infection in the mouth and, unless treated, prevents normal feeding (leading to starvation) and prevents normal grooming. The problem tends to begin early in life, is life-long, and can cause severe and long-term discomfort and pain.


Summary of Information

(for more information click on the links below)

1. Brief description

Dental malocclusion is the condition in which upper and lower teeth fail to meet in the correct way when the mouth is closed. This problem can be acquired during life (for example as a result of poor diet) but here we are focusing on a form of the disease that is present from birth (congenital). Incisor malocclusion involves the incisor teeth at the front of the rabbit’s mouth. The rabbit has four incisors in the maxilla (top jaw) and two in the mandible (lower jaw). The incisors are chisel-like teeth used to gnaw or slice through vegetation as well as playing a role in grooming and fighting (Harcourt Brown 2009a).

Rabbit’s teeth grow throughout life. These teeth have evolved to cope with the rabbit’s high fibre diet which is abrasive and which wears away the teeth. The rate of growth normally equals the rate of wear so that the teeth neither become too long or too short. However, if the teeth are not positioned correctly in the mouth, they do not wear against each other normally and become severely overgrown. Incisor malocclusion in Netherland dwarfs is diagnosed when the upper and lower incisors do not meet correctly; the teeth are not worn down as necessary and have lost their shape and functionality.

In dwarf rabbit breeds, including the Netherland dwarf, along with much of the rest of the skeleton overall skull length is reduced. However, the mandible does not always show a similar proportional reduction in size. This condition is termed maxillary brachygnathism (Harcourt Brown 2002, Verstraete and Osofsky 2005), and it can lead to incisor malocclusion. (As the lower jaw appears relatively too long, some authors use the term mandibular prognathism rather than maxillary brachygnathism).

In incisor malocclusion, as the teeth grow, the lower teeth start protruding from the mouth, and the upper teeth overgrow and curl round inside the mouth. As these upper teeth continue to grow they may cut the surface of the soft tissues of the mouth causing pain and sometimes infection. Affected rabbits find it difficult and sometimes painful to feed normally, thus become anorexic (not eating adequately) and ultimately can starve. Also, their coats become unkempt as they are unable to groom adequately and their perineums (the area around their anuses) may become soiled.

2. Intensity of welfare impact

This is a severe welfare issue in affected rabbits. Without treatment the continually lengthening teeth lead to a complete inability to feed adequately and ultimately death by starvation. The elongated upper incisors cause pain and infection when cutting into the hard palate or other tissues in the mouth. The protruding teeth may get caught and damaged. Other health problems connected to the rabbit’s inability to groom may occur, such as fly strike and mite infestations both of which have severe welfare consequences (Harcourt Brown 2009b). Even when treated properly, the necessary and regular veterinary treatment can cause significant stress in rabbits.

3. Duration of welfare impact

Veterinary surgeons see animals with signs of overgrowth from as young as 8 to 10 weeks of age (Redrobe 2000). From the point at which the problems begin, this is a life-long problem in rabbits, unless the incisors are regularly burred or surgically removed.

4. Number of animals affected

Lindsey and Fox (1994) considered congenital incisor malocclusion to be a common inherited problem in rabbits. Congenital malocclusion is seen more commonly in dwarf breeds under 1.5kg and especially the Netherland dwarf (Harcourt Brown 2002, Hobson 2006), though the percentage of affected individuals is not recorded.

5. Diagnosis

Congenital incisor malocclusion is diagnosed by examining the mouth of a young rabbit.

6. Genetics

Mandibular prognathism, having a longer lower than upper jaw, which is the cause of congenital incisor malocclusion in rabbits, has been shown to have a simple autosomal recessive inheritance with incomplete penetrance of 81% (Huang et al 1981). Thus, for a rabbit to have this condition it needs to have a pair of recessive genes, one inherited from the dam and one from the sire. The condition is said to have incomplete penetrance as only 81% of individuals who have a pair of the affected genes will show the disease.

7. How do you know if an animal is a carrier or likely to become affected?

Affected individuals can be detected from as young as 3 weeks of age on careful inspection by a knowledgeable examiner. Animals with any abnormalities at time of purchase should be rejected. Rabbits with congenital incisor malocclusion should not be bred from (Harcourt Brown 2002).

Currently the particular gene or genes responsible for incisor malocclusions are unidentified. The only way of judging if an animal is a carrier or likely to be a carrier is from knowledge of whether any of its relatives, particularly siblings and offspring, are affected.

8. Methods and prospects for elimination of the problem

Currently there is no genetic test for this condition and no breeding programme in place to try and eliminate this condition. Possible courses of future action are discussed below.


For further details about this condition, please click on the following:
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1. Clinical and pathological effects

A normal rabbit has 28 teeth. It has two pairs of maxillary (upper) incisors which are the sharp, chisel-like teeth at the front of the mouth. One pair of these maxillary incisors sits behind the other pair. They are smaller “peg” teeth. There is just a single pair of incisors in the lower jaw (the mandibular incisors). Rabbits have no canine teeth; instead they have a large gap, called the diastema, between the incisors and their other (cheek) teeth. The cheek teeth consist of three premolar and three molar teeth in each side of the upper jaw (maxilla) and two premolars and three molars in each side of the lower jaw (mandible).

Dental Malocclusion figure 1

Figure 1.  The location of teeth in the rabbit.

Dental malocclusion is the condition in which the upper and lower teeth fail to meet in the correct way when the mouth is closed. Acquired dental malocclusion in pet rabbits is not present at birth but develops later. It is an important and common problem which can affect both incisors and cheek teeth. It is connected with inappropriate feeding and husbandry protocols. However, the focus here  is the congenital (present from birth) form of the disease.

Rabbits’ teeth grow throughout life (teeth which do this are known as aradicular hypodont teeth (Redrobe 2000)). These teeth have evolved to cope with the rabbit’s high fibre diet, which is abrasive, and because the teeth constantly grow they have to be positioned correctly in the mouth to ensure upper and lower teeth meet, rub against and wear each other down. This also ensures that they stay sharp.

The incisors are chisel-like teeth evolved for gnawing or slicing through vegetation as well as playing a role in grooming and fighting (Harcourt Brown 2009a). The outer surface of each tooth has a coating of hard enamel that provides protection and this enamel forms a sharp cutting edge. In incisors the enamel edge is designed to be longer and sharper on the labial aspect (the surface towards the outside of the mouth).

To maintain the specific shape and edge to the incisor teeth, upper and lower tooth-to-tooth contact must be very precise. In the mouth of a normal rabbit at rest, the tips of the lower incisors rest against the tiny “peg” incisors of the upper jaw. This helps to maintain upper incisor shape (Harcourt Brown 2009a). Normal rabbits can then protrude (jut out) their lower jaw and, with movements of the mouth, bring the labial side of the lower incisor in contact with the sharp edge of the upper incisor. This also helps maintain the lower incisor shape (Harcourt Brown 2009a).

Incisor tooth growth rates are reported to be 8 to 12 cm per year (Redrobe 2000) or up to 2 to 2.4 mm per week in normal rabbits (Harcourt Brown 2009a). When incisor malocclusion is present and the teeth are not worn down as necessary, they lose their shape as they grow and their functionality and in this situation it seems that their growth rates can be even higher (Harcourt Brown 2009a).

The defect is caused by by the upper and lower jaws being of different lengths. Some authors consider that this congenital deformity is due to the lower jaw being relatively long compared withthe upper jaw, and describe it as mandibular prognathism (Harcourt Brown 1997, Redrobe 2000). However, others suggest the underlying problem is due to the overall skull length (including the upper jaw) being reduced (most obviously in dwarf breeds), whilst mandibular length has remained normal and thus consider that the term maxillary brachygnathism is more appropriate (Lindsey and Fox 1994, Harcourt Brown 2002, Verstraete and Osofsky 2005).

Dental Malocclusion figure 2

Figure 2.  Dental malocclusion can affect the function of both the incisors and the cheek teeth due to incorrect alignment. (Image property of Esther van Praag at www.medirabbit.com, to whom we are grateful for the permission to reproduce it here).

Signs of incisor malocclusion include obviously overgrown incisors, both upper and lower. When over-long, the lower incisors tend to protrude from the mouth. Over-long upper incisors may, as they grow, curl round within the mouth and cut into the hard palate (the roof of the mouth). This can lead to drooling and wound infections. The rabbit may be unable to close its mouth properly. The overgrowth also leads to problems of anorexia (not eating enough), and lack of grooming and coprophagy (Redrobe 2000).

Dental Malocclusion figure 3 

Figure 3.  Overgrowth of the incisors due to dental malocclusion can lead to a number of other severe health and welfare problems. (Image from Wikipedia: http://en.wikipedia.org/wiki/Domestic_rabbit )

As a result of their adaptation to their high fibre diet, which requires them to pass partially digested food through their digestive system a second time to ensure they extract all the necessary nutrients from it, rabbits normally produce two types of faeces. One type, produced after the second passage through the gut, is the standard waste material produced by mammals. Faeces produced on the initial passage are cecotrophes; small, soft pellets about the size of a pea. These contain high levels of important nutrients which rabbits need to regularly consume and re-digest. If dental disease prevents this then the animal may become thin. Affected animals may look unkempt as they are unable to clean and groom themselves. The skin and fur around the anus (the perineum) may become soiled and this can lead to significant damage to the skin (Lindsey and Fox 1994). Certain species of flies lay eggs on the skin and rabbits with dental malocclusion are unable to groom them off. In this situation, the emerging maggots eat the surrounding flesh. This condition of fly strike is common in rabbits with skin damage, particularly of the perineum. This causes a major welfare problem and may require euthanasia.

As all teeth grow continually throughout their lives, managing rabbits with congenital incisor malocclusion is demanding and difficult. Regular veterinary attention is required involving trimming of the incisors (often under sedation or anaesthesia). An alternative is the major surgical procedure of removing all six incisors but this also has deleterious welfare consequences.

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2. Intensity of welfare impact

This is a severe welfare issue in affected rabbits. Unless treated, the continually lengthening teeth lead to a complete inability to feed adequately and ultimately death by starvation. The upper elongated incisors may cut into the hard palate or other tissues of the mouth causing pain and infection. Treatment involves regular trimming or surgical removal of the teeth, procedures which themselves can cause significant stress and pain. Rabbits can manage to eat without incisors if hard food is cut up into small pieces. However, rabbits without incisors cannot groom themselves adequately (Harcourt Brown 2002), so a programme of regular grooming needs to be undertaken to ensure that the rabbit does not become affected by fly strike, which can cause severe welfare problems if not swiftly addressed.

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3. Duration of welfare impact

Veterinary surgeons see animals with signs of overgrowth from as young as 8 to 10 weeks of age, though some animals are not presented with problems until 12 to 18 months of age (Redrobe 2000). It can be identified in kits as young as 3 weeks old if they are carefully examined (Lindsey and Fox 1994). From when the problems start, this is a lifelong condition. Treatment involves regular trimming of the teeth throughout the animal’s life. This usually has to be done by a vet. An alternative treatment is surgical removal of the incisors. However, this also has adverse welfare consequences that are, likewise, lifelong.

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4. Number of animals affected

Lindsey and Fox (1994) considered congenital incisor malocclusion to be a common inherited problem in rabbits. It is seen more commonly in dwarf breeds – those of less than 1.5kg average body weight, and especially in Netherland dwarf rabbits (Harcourt Brown 2002, Hobson 2006), though the proportion of animals affected has not been recorded.

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5. Diagnosis

Congenital incisor malocclusion is diagnosed by examining the mouth of a young rabbit. In older animals it has to be differentiated from the acquired disease that has developed due to inappropriate husbandry and nutrition. Acquired disease is very common.

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6. Genetics

Mandibular prognathism, or the relatively longer lower jaw which is the cause of congenital incisor malocclusion, has been shown to have a simple autosomal recessive inheritance with an incomplete penetrance of 81% (Huang et al 1981). Thus, for a rabbit to have this condition it needs to have inherited a pair of recessive genes, one from the dam and one from the sire. As the gene has incomplete penetrance only 81% of individuals with a pair of the affected genes will show the disease. The reason for the incomplete penetrance of this genetic disease is unknown but it may be due to the actions of other genes or environmental influences.

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7. How do you know if an animal is a carrier or likely to become affected?

Affected individuals can be detected from as young as 3 weeks of age on careful inspection by a knowledgeable examiner. In order to avoid perpetuating the problem animals with any signs of dental malocclusion should not be purchased. Rabbits with congenital incisor malocclusion should not be bred from (Harcourt Brown 2002).

There is no genetic test for dental malocclusion therefore it is currently impossible to identify carrier animals i.e. those animals that are physically normal but carry one abnormal gene which might be passed on to offspring, The particular gene responsible is unidentified. However, any rabbit that has produced an affected offspring must be a carrier (although just because it has not does not rule out carrier status). Any rabbit with an affected sibling has a high chance of being a carrier as both of its parents must have either been affected or have been carriers.

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8. Methods and prospects for elimination of the problem

There currently is no genetic test for congenital malocclusion, thus it is difficult to identify carrier animals. If animals produce any affected offspring then clearly these are carriers and their future use for breeding should be considered very carefully. The obvious choice would be to not breed from any known carrier animals. However, as the prevalence of this condition is not fully known and it is possible that it is relatively common, then it is possible that excluding all carriers would significantly reduce the gene pool of the breed and lead to high levels of inbreeding and the increased occurrence of other unwanted genetic problems. However, these could be avoided by out-crossing with healthy animals of other breeds.

When trying to eradicate an autosomal recessive condition for which no genetic test is available, Bell (2010) suggested that an open health database should be kept for the breed and that a relative risk analysis programme should be run. This database would consist of records of all breeding animals and their relatives and including data on congenital malocclusion and other genetic conditions so that the ancestry of potential breeding animals can be checked and the risk of their being a carrier can be assessed. This can be compared with the average risk of an animal of a particular breed being a carrier. Matings with a combined risk lower than the breed average are then chosen and, by breeding in this way, gradually animals which are less likely to carry the harmful gene replace higher-risk animals as breeding stock and the incidence of the condition within the breed falls (Bell 2010).

Consideration should also be given as to why dwarf rabbit breeds are predisposed to this condition. It may be that selecting for small breeds with shortened heads, tends to lead to animals with maxillary brachygnathism - mismatched upper and lower jaw length. Maxillary brachygnathism is observed also in other species selected for shortened head length (brachycephaly), such as the English bulldog and Persian cat. These species have very different dentition to rabbits and so have far fewer problems arising from malocclusion.

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9. Acknowledgements

UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section, to Nina Taylor for her contribution to it and to Stephanie Kaufman for assistance in illustrating it.

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10. References

Bell J (2010) Genetic Testing and Genetic Counselling in Pet and Breeding Dogs. World Small Animal Veterinary Association. World Congress Proceedings, June 2-5th, 2010, Geneva

Harcourt Brown F (2002) Textbook of rabbit medicine. Reed Educational and Professional Publishing Ltd: Oxford

Harcourt Brown F (2009a) Dental disease in pet rabbits. 1. Normal dentition, pathogenesis and aetiology. In Practice 31: 370-379.

Harcourt Brown F (2009b) Dental disease in pet rabbits. 2. Diagnosis and Treatment. In Practice 31: 432-445

Hobson P (2006) Dentistry in Meredith A and Flecknell P (Eds) Manual of Rabbit Medicine and Surgery. 2nd Ed. Gloucester, England: BSAVA. p184-196

Huang C, Mi M and Vogt D (1981) Mandibular prognathism in the rabbit: discrimination between single-locus and multifactoral models of inheritance. Journal of Heredity 72(4): 296-298

Lindsey JR and Fox RR (1994) Inherited Diseases and Variations in Manning PJ, Ringler DH and Newcomer CE (Eds) The Biology of the Laboratory Rabbit. 2nd Ed. Academic Press Limited, London, p 293-320

Redrobe S (2000) Surgical procedures and dental disorders. In Flecknell P (Ed) Manual of rabbit medicine and surgery. Gloucester, England: BSAVA

Verstraete F and Osofsky A (2005) Dentistry in pet rabbits. Compendium of Continuing Education for the Practising Veterinarian 27: 671-684

© UFAW 2011


Credit for main photo above:

ttp://commons.wikimedia.org/wiki/File%3ANetherland_Dwarf_On_Brick.jpg by Erebus555 at en.wikipedia [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons