- 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).
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.
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.
- 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.
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).
There are several main causes of Malocclusion in rabbits.
· Hereditary/Genetic. Malocclusion is a hereditary disease that is passed on to the young from adult rabbits that also have the disorder. Malocclusion is due to an autosomal recessive trait with incomplete penetrance. Rabbits with malocclusion should not be bred and spaying or neutering is highly recommended for any rabbit that hasbeen diagnosed with malocclusion.
· Face or head trauma. Trauma to the mouth can also cause the teeth to become misaligned which will result in the teeth not grinding down correctly. This is often caused by bunnies pulling on the wire of their cages and so pulling their teeth out of alignment. Most breeders know if they have a ‘wire puller’ in the shed and take precautions to stop them doing it!
· Poor breeding or malnourishment of the doe or young babies. It can occur as a result of calcium deficiency which leads to osteomalacia of the jaw which allows the tooth to move in it’s socket.
· Bacterial infections. Bacterial infections can cause teeth to grow in odd directions.
· Unsuitablediet. Many rabbit owners think that feeding the well promoted commercial mixes from leading food manufacturers is enough. They may be‘nutritionally balanced’, but they do not take into account a number of very important factors including the eating pattern of rabbits in the wild, the problems of selective eating, and the lack of abrasive material found naturally in grass.
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