Cataract in rabbits
Esther van Praag, Ph.D.
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The word cataract comes from the Latin “cataracta”, or from the Greek katarraktēs, meaning waterfall. It reminds the progressive increase in opacity of the eye lens, which is sometimes referred to as “looking though a waterfall”. Consequently, a reduced amount of light passes through the lens. The ability to focus and eyesight sharpness will decrease with time. This is accompanied by a loss of sensitivity to contrast.
The ability to see objects in bright light is reduced and an affected rabbit will start to hop furniture or any object that is on its path. The changes in the lens are due to the oxygen metabolism and the recycling of the gluthatione protective molecule. Since the lens has no direct contact with the blood circulation, its level of oxygen is the second lowest in the body, after the nervous system and the adrenal cortex. Oxidative respiration, which is carried out by the mitochondria organelles within the cells of the lens, is sufficient to produce the ATP (form of stored energy in organisms) necessary for the proper functioning of cells. This is accompanied by the formation of free radicals and other oxidative molecules. These molecules are neutralized by a smaller protein, gluthatione. While gluthatione is oxidized, the free radicals are reduced and neutralized. The oxidized gluthatione will move to the surface of the lens, where it will be reduced by the enzyme gluthatione reductase, with the help of a co-enzyme derived from vitamin B3. The cycle enables to regenerate the glutatione, so it can be used again. Vitamin C also plays a protective role and its concentration within the lens is about 40 times higher than in the blood. Once the vitamin C has entered the lens cells, it will also start to reduce free radicals and other oxidizing molecules.
Cataracts observed in mammals and human beings can be classified in three categories:
· Nuclear cataract: it is characterized by a degeneration of the proteins in the center of the lens (nucleus) due to age. It is typically related to increased levels of oxidized (dangerous) gluthatione in the lens. Possibly the movement of the later is slowed down due to age or to a disbalance between proteins and lipid oxidation. The lens becomes white and later brown.
§ Cortical cataract: it is related to the disruption of the lens at the periphery and spreads towards the center. It is typically related to a decrease in the level of gluthatione, accompanied by the excessive destruction of proteins, damage of the fiber plasma membrane or disruption of the calcium homeostasis (maintained relatively constant state within the body).
§ Posterior subcapsular cataract: it is characterized by the development of clusters of swollen cells in the back of the lens. Several clusters can develop, independently from each other. This type of cataract is rare and typically stress induced (e.g. UV) or due to medication (corticosteroids).
In rabbits, there is a fourth cause for cataract and lens rupture, related to protozoal parasite Encephalitozoon cuniculi.
The appearance of cataract is generally related to age. Further contributing factors are heredity, nutrition, medication, exposure to sun light, presence of the protozoal parasite Encephalitozoon cuniculi, head-trauma, or a diet poor in caretonoids. The incidence and the causes of cataract in rabbit is not well known. On the contrary to other animals, cataract development is not related to diabetes, a metabolic disorder that is very rare seen in rabbit.
Increased oxidative stress, due to the presence of free radicals, a breakdown of the protective mechanism, or a decreased gluthatione cycle, lead to an accumulation of hydrogen peroxide in the aqueous humor of the eye. Although gluthatione will reduce the peroxide, the energy-producing metabolism will be destroyed on the long term, enabling the diffusion of sodium into the lens. Osmolality (natural tendency to maintain water balance) will lead to edema (accumulation of water) in the lens. The proteins inside the lens oxidize, become opaque and insoluble (similar process as heat induced denaturation of ovalbumin and egg white proteins).
The free radicals attack the lipids present in the membrane, which leads to a shrinkage or swelling of the lens capsule. These changes of pressure inside the lens break the lens fiber membranes and the space will be filled with water and waste.
A comprehensive diagnosis enables to diagnose a cataract and monitor its development with time. Rarely, lens rupture is observed.
The treatment of choice for cataract is surgical removal, using the phacofragmentation of the lens techniques, without replacement of the lens. Indeed, regeneration of the lens has been observed in numerous rabbits. If this will not be the case, the rabbit will nevertheless be able to discern light and differentiate shapes.
If the cause of cataract relates to the parasite E. cuniculi, living in the nervous system of rabbits, the treatment includes the administration of fenbendazole (20 mg/kg, q 24 h., during a month) during 28 days. Longer treatments must be avoided as onset of secondary effects such as bone marrow depression has been observed in rabbits. The use of albendazole should be avoided in rabbits. It has lead to sudden death in healthy and/or young rabbits, immediately after administration of the drug (private communication, Prof. P. Deplazes, DVM, Faculty of Veterinary Medicine, University of Zurich, Switzerland)
If uveitis is present and the lens cannot be removed surgically, the use of a topical NSAID or non-NSAID medication (e.g. prednisolone acetate 1%) is necessary.
Cataract may cause pain. If this is observed, the use of analgesics is recommended. If treatment does not bring relief or improvement, eye surgery or enucleation may help the rabbit.
My gratitude goes to Amy Carpenter (USA), to Susan L. (USA), to Lisa Hutcheon (USA), to Christine Goodhand, to Melanie Kuenzel and Heather Bechtel (USA), to Sandy Minshull (Canada) and to Akira Yamanouchi (Veterinary Exotic Information Network), for the permission to use their pictures and/or their help. Many thanks also to the rabbits that helped illustrate this page.
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