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Anatomy & physiology

 

Ophthalmologic examination of the normal eye of the koala  

Hirst, LW, Brown, AS, Kempster, R & Winney, N 1992, Journal of Wildlife Diseases, vol. 28, no. 3, pp. 419 -423.  

  Examination of the eyes of 28 koalas (Phascolarctos cinereus) was performed to document the typical in vivo formation. 22 of the 28 investigated koalas had eyes that were normal. Upon examination using slit-lamps, the cornea was found to have atypical lamellae with average keratometric measurements at 59.3 ±  3 diopters. The measurement of the endothelial cell area of the cornea was 381.4 ± m2  ± 46.8 ± m2. No identifiable features that would explain the ocular infection susceptibility of this species by Chlamydia psittaci infection were noted.  

  In this study of 12 months only koalas, of varying captivity times, which had no indications of infections or ocular diseases were included. Transportation of the koalas was organised by using hessian bags with alphadolone acetate, and alphaxalone injected intravenously for anaesthesia. The animals were aged based on teeth-wear and estimated birth date, and sexed from genitalia observations. Ocular, urogenital and nasal samples were taken to isolate Chlamydia psittaci. Extracted blood was used to perform Chlamydial serology. The curvature of the central cornea of both eyes was measured, and a pneumotonometer used to blow a puff of air at the cornea to quantify intraocular pressure. Researchers took macro lens photographs, and anterior segments of each eye analyzed with a Zeiss photo slit-lamp. The koalas were between twelve months and ten years old with 32% males and 68% females. Six animals had unusual corneal pathology when examined with slit-lamps and were excluded from results. The koalas were found to have to a narrow interpalpebral fissure that exposed the cornea and no visible sclera between eyelids. The interpalpebral fissure refers to the space between the upper and lower eyelid. There was poor definition between the mucosal membrane and skin. Slit-lamp observations showed low surface structure, highly vascularized conjunctiva and defined smooth nictitating membrane, or third eyelid. No peripheral vasculature and limbal differences were seen surrounding the circular cornea. The majority of the epithelial tissues consisted of smooth and clear stroma. Specular microscopy of the cornea displayed a consistent hexagonal cell array that presented some distinctly different forms. Most of the features were deemed ‘unremarkable’.  

  Previous extensive documentation of normal koala eyes was not conducted, and thus the chlamydial response in the eye is unknown without base-line characteristics, which this study aimed to provide by examining koala populations not infected by Chlamydia psittaciSix koalas, initially described as normal, presented minor pathological states with a more rigorous ophthalmological examination. These findings suggest  superficial examination without using a slit-lamp may miss evidence of pathology. There is increased curvature in the cornea/air interface when compared to other species, but its significance is unknown. Overall, no defining features could be identified to predispose koalas to chlamydial infection.  

  These findings contribute to the accumulation of baseline data for normal koala eye morphology. These detailed findings can better describe corneal diseases that may have previously been mistaken for chlamydial keratitis.. Future studies should further investigate the in vivo morphology of the koala eye to better understand the changes that occur in a chlamydial eye infection. 

 

Summarised by Alexander Murdoch

 

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