Introduction
Grey mouse lemurs are found throughout the dry deciduous forests and spiny deserts of the south and west cost of Madagascar. This is a primarily insectivorous species, which also consumes fruits, flowers, leaves and small vertebrates. Grey mouse lemurs are nocturnal, solitary foragers who congregate at daytime sleeping sites. Their lifetime expectancy is thought to be around 6 years in the wild, 10 years in captivity.
Because of the general concern about the space of habitat destruction in Madagascar, all species of Microcebus are listed as endangered by USDI and are on CITES appendix I. The mouse lemurs are still relatively common and may be the only Madagascan lemurs not undergoing a general decline.
Recently, scientists’ interest for mouse lemurs grew because Microcebus have cerebral lesions with ageing which are similar to lesions found in Alzheimer patients in humans. First mouse lemurs arrived in Paris Zoo in 1990. Our collection is reaching now 71 individuals; most of them are off public. Some of the older specimens are part of a behavioral study to evaluate their cerebral ageing. Among the tests they undergo is a visual discrimination test.1 In 1998, more and more specimens could not be tested because of visual deficiency, and an increasing number of cases of cataracts was observed even on relatively young animals. Thus, we decided to perform an ocular examination on all the animals in order to determine the incidence of cataract in the colony, describe the type of cataract, its progression and attempt to identify its possible cause.
Methods
Over a 1-year period, 75 mouse lemurs (45 males, 30 females) between 7 months and more than 10 years of age, underwent an ocular examination. Seven and 36 of them, respectively, were re-examined 5 months and 13 months after the initial examination. Ocular examinations were performed under subdued illumination. Pupillary light reflexes (direct and indirect) were assessed with a Finoff transilluminator. Slit-lamp biomicroscopy (Kowa SL-2) examination was performed before and after pupil dilation using one drop of 1% tropicamide (Mydriaticum 1%, MSD laboratory). Fundoscopic examination was performed with an indirect ophthalmoscope (Heine EN20) and a 28 D lens.
Observations for each animal were recorded on a separate form. Results of ocular histopathologic findings were available for five animals that died during that period.
Results
Ocular lesions were observed in 45 lemurs. These lesions included cataracts (36/45), keratitis (4/45), corneal dystrophy (2/45), corectopia (1/45), chorioretinitis scars (1/45) and phthisis bulbi (1/45). Blindness due to cataract or to total opacification of the cornea was seen in 14 cases.
Cataracts were predominantly bilateral (34/36) and all stages of progression were observed (21/36 incipient, 7/36 immature, 4/36 mature and 4/36 hypermature). A majority of incipient cataracts (19/21) were posterior subcapsular opacities. Cataract-associated lesions were observed in 10 animals and included hyphema (one case), posterior synechiae (five cases), pupil seclusion (three cases), keratitis or corneal edema (three cases). Cataracts affected both males (n=18) and females (n=18) and were observed in animals as young as 3 years and 10 months old. Cataract incidence increased with time and more than 50% of lemurs over 6 years of age were diagnosed with cataract.
Discussion
Cataracts in this colony of mouse lemurs appeared to be acquired and slowly progressive. In 11 cases, no cataracts were observed at initial examination but were diagnosed subsequently. Initial opacities were predominantly localized, both clinically and histologically, to the posterior subcapsular area. Later stages included anterior and posterior subcapsular opacities progressing to nucleo-cortical cataracts.
No cases of uveitis in the absence of cataract were diagnosed, suggesting that, when present, the uveitis was a lens-induced inflammation.
Early age of onset observed in some lemurs, as well as lens morphology, tends to exclude that these may be senile cataracts. Possible causes of these cataracts include nutritional, hereditary and infectious disorders.
Conclusion
We report an important number of cases of acquired slowly progressive bilateral cataracts in a colony of mouse lemurs (Microcebus murinus). Further studies need to be addressed to determine the aetiology of this blindness causing disease.
Acknowledgments
We thank Seth A. Koch, Diplomate of the American college of veterinary ophthalmology for his precious help. Also our thanks to Monique Wyers, Olivier Albaric and the staff from the laboratoire d’histologie animale de l’école nationale vétérinaire de Nantes who did histopathologic exams. We thank Bernard Clerc from the service d’ophtalologie de l’école nationale vétérinaire d’Alfort for providing ophthalmologic equipment and keepers from lemur house for their patience and technical support.
Literature Cited
1. Dhenain, M, Michot, JL, Volk, A, Picq, JL, Boller, F.1997. T2-weighted MRI studies of mouse lemurs: a primate model of brain aging. Neurobiol Aging. 18(5):517–521.