It would be all too easy for this presentation to be merely a list of infectious diseases and their ophthalmic manifestations--indeed such is given in Figure 1, although this cannot hope to be a fully comprehensive treatment of the subject. Here, really, I want to ask the question to those of you interested in internal medicine and not primarily in ophthalmology, 'why look in the eye?'. Ophthalmoscopy isn't necessarily the easiest of techniques to master and one must ponder why someone who does not have the eye at the top of their priority list should bother examining it at each consultation, as I would argue should be the case.

Have you ever wanted to examine a vascular plexus in detail to evaluate hypertensive changes or vasculitis? Have you yearned to be able to open up a lymph node without taking a biopsy and see what is going on in a case of lymphadenopathy? Then look in the eye! The iris is the immunological heart of the eye and inflammatory changes, be they ever so mild, regularly accompany systemic immunological and inflammatory signs associated with infectious disease, be they something as widespread as ehrlichiosis or as apparently localised as pyometritis. The vascular plexus, which is the venules and arterioles of the fundus, provides an unsurpassed view of vascular changes in hypertension, anaemia and coagulopathies which can be seen with infectious and parasitic disease. For sure not every case of these conditions will have ocular signs, but for those that do, ocular examination is a real window not on the soul but on systemic disease in these animals.

Figure 1. Ocular manifestations of infectious disease.

Viral Disease

Systemic infectious disease can quite frequently result in ocular inflammatory signs, that is to say uveitis either involving the iris or the choroids. Feline uveitis is regularly associated with viral disease be it feline immunodeficiency virus, feline leukaemia virus, or feline infectious peritonitis, although the exact mechanism by which these viral infections cause the ocular lesions is unclear. Assessing whether there is specific intraocular immune response to the infectious agent is somewhat difficult, requiring antibody titres for both serum and aqueous humour. A high ratio of these two figures, the Goldmann-Witmer coefficient or C value, shows that antibody is being produced in the eye itself rather than merely leaking in from systemic production elsewhere. The difficulty of taking an aqueous sample in an already inflamed eye means that this test is rarely performed--finding an elevated serum antibody titre in an animal with a uveitic eye is usually taken as sufficient evidence for an association. The situation with canine adenoviral infection and blue eye related to corneal oedema is somewhat different: here it is antibody-antigen complexes which cause an immunological attack against corneal endothelial cells as part of a low-grade uveitis which leads to endothelial dysfunction and corneal oedema. While unvaccinated and exposed puppies are still seen with this condition, the change in vaccine from one based on CAV-1 to CAV-2 has prevented the condition seen commonly after vaccination in past decades. Ocular manifestations of distemper include conjunctivitis and keratoconjunctivitis and also multifocal non-granulomatous chorioretinitis, although the low prevalence of the disease in the UK now means that these signs are very rarely seen.

From a totally different perspective feline herpesvirus can give severe conjunctivitis and keratitis as part of the upper respiratory manifestations of the disease or an ulcerative keratoconjunctivitis as a sign of recurrent disease in later life. In this case the ocular signs are caused by the same sort of necrotic processes that result in severe nasal and oropharyngeal disease. Here famciclovir given orally can be a useful antiviral agent with ocular and systemic ameliorative effects.

Bacterial Disease

Bacterial disease either with a systemic condition such as Lyme disease caused by the spirochaete Borrelia burgdorferi or a more focal infection such as pyometritis, can both lead to uveitis, probably though breakdown of the blood-aqueous barrier but maybe in the former case through presence of the organism in ocular tissue; the pathogenetic mechanism is as yet unclear. Other bacterial causes of uveitis include brucellosis and leptospirosis as well as mycobacterial species, at least in the cat.

Parasitic and Fungal Disease

In a similar manner to infectious agents, several protozoal parasites and fungi can lead to intraocular inflammation. Toxoplasma gondii and Ehrlichia, as well as a host of fungi from Cryptococcus to Histoplasma, can give uveitis, most commonly involving the choroid and the serological diagnosis of the precise agent involved can be taxing and costly. Leishmaniosis particularly manifests as periocular alopecia with conjunctivitis, blepharitis, keratitis, secondary keratoconjunctivitis sicca and sometimes uveitis. Up to 80% of dogs with the disease can have ocular manifestations. Rickettsial diseases have ocular manifestations through coagulopathic effects in canine cyclic thrombocytopenia or the vasculitis seen in Rocky Mountain spotted fever. On occasion nematodes such as Angiostrongylus can show themselves in the anterior segment of the eye while visceral larva migrans from Toxocara canis may be noted as areas of post-inflammatory retinal degeneration.

We must not forget the rabbit, as a species of growing importance in small animal practice. Dacryocystitis is seen associated with dental disease. The protozoan Encephalitozoon cuniculi causes cataract and lens-induced uveitis which is best treated either with lens removal by phacoemulsification, or with oral fenbendazole.

Conclusion

It is always worthwhile examining the eye when confronted with a systemic disease. Many inflammatory, metabolic and neoplastic conditions have ocular signs which can aid in diagnosis, assessing severity of disease and monitoring response to treatment by direct observation of immunological and vascular structures.

References

1.  Cullen CL, Webb AA. Ocular manifestations of systemic diseases. In: Gelatt KN. ed. Veterinary Ophthalmology (fourth edition). Oxford: Blackwell Publishing 2007, 1470-1587.