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The quite remarkable feature of eyes across vertebrate species from the axolotl to the zebra is their similarity. The basic design of the eye, the cornea, the iris, the lens, the retina all enclosed in a tough collagenous sclera, is duplicated throughout vertebrate species, as are the similar functions across the animal kingdom with light refracted to form an image on the retina where the photoreceptors transform the incident photonâs energy into an electrical signal.
And yet these eyes have many differences in both their anatomy and their pathology: their appearance when normal and abnormal. Letâs face it, if this were not the case there would honestly be no need for this book! From the differences in conjunctival responses to infectious organisms to the variation in vascular anatomy of the orbit in the rabbit as compared to that of the dog and cat we are more used to enucleating, it is vital to understand the variation in anatomy and physiology, in pharmacology and pathology between the eyes of fish, amphibians, reptiles, birds and mammals.
Indeed this could be replicated across body systems but not perhaps quite so dramatically as in the eye. There is a dichotomy in exotic animal veterinary medicine. On the one hand quite a substantial proportion of what we understand about the diseases of cats and dogs, their aetiopathology and their management can be extrapolated to help us deal with disease in less familiar species, be they raptors or rabbits. But on the other hand there are differences between hounds, hamsters and horned toads that make extrapolation without due care and attention potentially ineffective or even dangerous.
Hopefully this volume will aid in identifying where extrapolation from canine and feline ophthalmology can be made and where new information is necessary. We start with areas where extrapolation is possible, the first of these being the straightforward techniques of ocular examination, which may, in many cases, be transferred from conventional companion animal species. Even here though, differences exist.
Before we continue, however, it may be that this is an appropriate point to make two confessions. First some might complain that there are several areas of duplication in the text. The book is designed with the assumption that many will not read it through from cover to cover, but rather use it as a reference dipping in to specific ocular diseases in particular species. Thus several areas are necessarily somewhat duplicated to ensure that the information needed is presented in a readily accessible form. Second it might be necessary to give an apology to some for the wording of the title of this book âOphthalmology of Exotic Petsâ. For some, it must be noted, rabbits and guinea pigs are hardly exotic species; recent evidence suggests that rabbits are the third most common species seen in small animal veterinary practice, certainly here in the United Kingdom and quite possibly elsewhere also. Nevertheless in many ways, from their teeth to their retinas, rabbits and guinea pigs are very different from cats and dogs and so they deserve inclusion in a volume detailing ophthalmic disease in what we might term non-dog-and-cat species. But that would hardly make an appropriate title for a book like this would it?! My first reason for producing a volume on this subject came when seeing how useful Sue Patersonâs volume âSkin Diseases of Exotic Petsâ was [1]. Sue cleverly gathered a group of other dermatologists with special interests in different exotic species to write her book with her, but somehow I failed to galvanise others in the field of veterinary ophthalmology to produce a similar volume. I hope that those with greater expertise and experience in the fields of reptile, avian and laboratory animal ophthalmology will forgive any resulting failings in this book. Perhaps a second edition can include their contributions to the subject.
Reference
1. Paterson S. Skin Diseases of Exotic Pets. Oxford: Blackwell Publishing, 2006.
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Chapter 2
A brief history of comparative ophthalmology
While we may think that a text on exotic animal ophthalmology is a new venture, and certainly within veterinary ophthalmology this is the first volume dedicated to the subject, ophthalmologists and visual scientists have for many years considered the delights of comparative ophthalmology worth pursuing. The great Sir Stewart Duke Elder started his momentous 16 volume System of Ophthalmology with a glorious first volume entitled âThe Eye in Evolutionâ, covering ocular anatomy and physiology together with visual ecology and behaviour from invertebrates through to higher mammals. But 50 years earlier the Canadian ophthalmologist Casey Albert Wood (Figure 2.1) was already publishing his remarkable overview of avian ophthalmology The Fundus Oculi of Birds as Viewed with the Ophthalmoscope linking his lifelong interest in ornithology with his acknowledged expertise in ophthalmology.
Wood, born in Ontario, Canada in 1856, first studied medicine in Montreal with further studies at Berlin, Vienna, Paris and London where he worked at Moorfields. He studied in Montreal under William (later Sir William) Osler and it was at this early stage that his passion for ophthalmology began. Before then however, even from childhood, his second passion, for nature study and particularly ornithology, took root. Together ophthalmology and ornithology would guide Wood through his life. Having no children, he, his wife Emma and their pet parrot John Paul toured the globe after his retirement in 1906, collecting material for his magnum opus on the appearance of the avian retina, published in 1917 (Figure 2.2). This was not just a work arising from a general interest in birds. Wood considered that the superior optics and visual capability of many birds when compared with the human eye may well lead to discoveries which would improve human vision.
But while The Fundus Oculi of Birds is the book for which Wood is remembered today, he was a key general ophthalmologist in North America at the end of the nineteenth century, one might even argue the key ophthalmologist. His 1896 publication implicating methyl alcohol in the etiology of toxic amblyopia was considered a classic work, and Wood was editor at various times of the Ophthalmic Record, the Annals of Ophthalmology, and the American Journal of Ophthalmology (of which he was a founder in 1884). He wrote of A System of Ophthalmic Therapeutics (1909), A System of Ophthalmic Operations (1911), and the seventeen-volume American Encyclopedia of Ophthalmology (1914â1920). But more than that, Wood was fascinated by the history of ophthalmology and, of course, by comparative ophthalmology. Wood translated numerous historical ophthalmic texts from all over the world into English. A masterpiece is his translation of the earliest printed book on ophthalmology, De Oculis Eorumque Egritudinibus et Curis, written by the twelfth-century physician, Benvenuto Grassi, and first published in Ferrara in 1474.
For most of his working life Casey Wood was an ophthalmologist in Chicago, joining the faculty in 1899 and becoming Head of Ophthalmology in 1913, when the College of Physicians and Surgeons of Chicago became the University of Illinois College of Medicine and holding that position until 1917. He was described as âone of the most colorful and outstanding figures in ophthalmology at the turn of the century, not only in Chicago but nationally and internationally as well.â In 1929 Wood organized the ornithological titles at the British Museum and prepared its first catalogue describing that collection. Travelling widely in pursuit of this interest, he collected specimens in British Guiana, the Caribbean, the South Pacific, India, Ceylon, Australia and New Zealand. Wood retired from the College in 1925, spending much of his last decade at the Vatican Library in the translation of medieval European and Arabic ophthalmic manuscripts.
It has to be said that from a political perspective Wood held some views with which we might not agree â he was an ardent fascist, supporting Mussolini in the 1930s while he worked in Europe towards the end of his life, but had progressive views on the importance of animal experimentation held in balance with animal welfare. These opinions stemmed from his interest both in human medicine and also in environmental conservation, developed in his early childhood in the fields around his home in Ontario.
Another important ophthalmologist of the turn of the twentieth century with an abiding passion for comparative ophthalmology was George Lindsay Johnson (Figure 2.3). Born in 1853 and dying in 1943, Johnson was almost exactly Woodâs contemporary. Interestingly, while Wood spent a formative period of his professional life in Germany, Johnsonâs early education occurred there although he had been born in Manchester. He was in Strasbourg in 1870 when the Prussians seized the city in a devastating episode in the Franco-Prussian war. Escaping from the beleaguered city, he spent a year on a relativeâs ranch in Australia before returning to his roots and studying at Owenâs College in Manchester. He then undertook his undergraduate degree at Caius College in Cambridge and St Bartholomewâs Hospital in London, becoming a Fellow of the Royal College of Surgeons in 1884 and taking his MD from Cambridge in 1890. His ophthalmic career began as a registrar at the Royal Westminster Hospital in London followed by a period at the Royal Eye Hospital in Southwark, London.
During this period in London, he spent much of his spare time at the Zoological Society of London performing ophthalmic observations on a wide variety of species; this resulted in papers in the Philosophical Transactions of the Royal Society of London first on the comparative ophthalmoscopy of the mammalian eye and secondly a sequel on the eyes of reptiles and amphibians (Figure 2.4).
In this he was ably assisted by Arthur Head, ophthalmic artist of no mean distinction, as may be observed looking at his illustrations of the fundus of species from tigers to rattlesnakes (Figure 2.5). Head was also responsible for the illustrations in Woodâs The Fundus Oculi of Birds, most of which were also undertaken in painstaking hours in the Zoological Collection at Regentâs Park.
Between these two ophthalmologists of a century ago and Duke Elder stand two further key figures in the subject of comparative ocular biology. These are the French ophthalmologist, Andre Jean Francois Rochon-Duvigneaud, and the American, Gordon Walls. Rochon-Duvigneaud (Figure 2.6) will be remembered for his comment that a raptor was âa wing guided by an eyeâ but had considerably wider comparative interests, documented in his book Les Yeux et la Vision des Vertebres of 1943 [1]. ...
