Ophthalmological presentations in general practice can be some of the trickiest to diagnose and manage. The eye is an intricate organ, and considerable variation exists between different species. This is especially obvious when assessing the eyes of exotic pets, each of which has unique ocular anatomy and corresponding disease conditions.
This article aims to review the major anatomical differences between the eyes of exotic pets, as well as common disease states and their diagnosis and management.
Diagnostic methods for the ophthalmology of exotics
In general, there are several readily available diagnostic methods that can be used to assess the eye of any species in first-opinion practice.
The most common of these is direct ophthalmoscopy, which relies on the use of a hand-held ophthalmoscope to assess the globe, including the anterior and posterior chambers, and the adnexa in more detail. Indirect ophthalmoscopy offers a wider field of view, which is particularly useful in assessing peripheral lesions (Maggs, 2018). These can be used alone or in conjunction with fluorescein staining to assess for shallow defects in the surface of the cornea.
Measuring intraocular pressure with rebound or indentation tonometry can give an indication of ocular pressure, which is useful in diagnosing glaucoma and anterior uveitis.
Schirmer tear tests can be used to measure basal and reflex production of tear film (Maggs, 2018). However, in some exotic pets, due to their small size, the red phenol thread test is the preferred test to measure tear production (Holmberg, 2018).
Ultrasonography can also be used as a diagnostic modality in those species with eyes that are large enough to facilitate assessment of the posterior chamber (Donoldson and Hartley, 2022).
Avian ophthalmology
Anatomy of the avian eye
The avian eye has many similar components to the mammalian eye, but because birds rely so heavily on sight, there are specialised features you need to be aware of.
The avian eye has a smaller anterior chamber than mammals, and the globe is not round. Birds have several different globe shapes, which determine visual acuity. Most diurnal birds with narrow heads have a “flat” eyeball, which results in low visual acuity. Diurnal birds with wider heads, such as songbirds and birds of prey, have “globular” eyes, which improve visual acuity. Nocturnal birds of prey have an elongated, “tubular” eye, which gives superior visual acuity (King and McLelland, 1975). Most birds have eyes positioned laterally on the head, providing wide, monocular vision and allowing for up to 300-degree vision, whereas owls have more rostrally positioned eyes (Reese et al., 2016).
The muscles of the eye that control pupillary size are striated, so birds can consciously control their pupil size, making the pupillary light reflex of limited use
Birds have a cartilage-reinforced sclera, which contains 10 to 18 overlapping bones at the edge of the cornea; these are used for ciliary muscle attachment and help to strengthen the eyeball. The muscles of the eye that control pupillary size are striated, so birds can consciously control their pupil size, making the pupillary light reflex of limited use. The nictitating membrane is semi-transparent; it protects the eye and spreads tear film over the surface of the cornea.
Common pathologies of the avian eye
Chlamydia psittaci is a common infectious cause of ocular disease in parrots and other companion birds. It typically presents with unilateral or bilateral conjunctivitis (Figure 1), periocular swelling and mucopurulent discharge (Laroucau et al., 2019). Affected birds may also exhibit systemic signs such as lethargy and respiratory distress. The pathogenesis involves intracellular replication of the bacteria within conjunctival epithelial cells, eliciting an inflammatory response. Diagnosis is made via polymerase chain reaction (PCR), cytology or serology, and treatment includes systemic antibiotic therapy. All cases of conjunctivitis in birds should be considered as potentially carrying Chlamydia psittaci, which is a zoonotic infection.
Cataracts can be encountered in ageing parrots and are linked to genetics, trauma and nutritional deficiencies. Clinically, affected birds can exhibit vision impairment and may crash into objects or fail to locate food on the affected side. Direct or indirect ophthalmoscopy will reveal lens opacity (Figure 2). Treatment is possible through surgical phacoemulsification but requires general anaesthesia and specialist ophthalmological equipment. If owners do not wish to pursue this route, supportive care and environmental modification are recommended (Orcutt, 2016).
Ocular trauma is common due to the inquisitive nature of birds and can result from cage-related injuries or conspecific aggression (Figure 3). Corneal ulceration, hyphaema or uveal prolapse may be observed, and lower eyelid tearing is also a common traumatic injury of the avian eye. These cases should be treated as an emergency and should be assessed in-clinic as soon as possible. Initial assessment includes fluorescein staining and tonometry, followed by analgesia and antibiotic therapy. Consideration should be given to the provision of a topical local anaesthetic if indicated. Severe cases may require surgical repair or enucleation (Diehl and McKinnon, 2016).
Companion mammal ophthalmology
Anatomy of the eyes of small companion mammals
Small exotic mammals, including rabbits, guinea pigs and chinchillas, have distinct ocular features. Their eyes are positioned laterally, providing a broad field of vision which is advantageous for detecting predators.
In rabbits, the presence of a third eyelid or nictitating membrane offers additional protection to the globe, whereas other companion mammals only have a vestigial third eyelid. Guinea pigs are known to blink very rarely – recorded as only two to five times in a 20-minute period (Holmberg, 2018).
Common pathologies of the eyes of small companion mammals
Nasolacrimal duct blockage is a common ophthalmological presentation in rabbits. Primary blockage occurs due to debris within the duct or inflammation within the duct and/or the surrounding tissues. Secondary blockage occurs when dental disease is present and the roots of the teeth externally compress the duct (Reiter, 2008). Owners will often present rabbits with discharge from the medial canthus of the eye. In chronic or severe cases, this can result in secondary dermatitis at the medial canthus (Figure 4). Treatment involves anti-inflammatories, flushing the nasolacrimal duct (Figure 5) and treatment of the underlying cause.
Dental disease can also result in retrobulbar abscessation, wherein a tooth root infection of the caudal maxillary molar roots results in abscess formation behind the eye. This may present subtly, with conjunctivitis, exophthalmos and, potentially, exposure keratitis and corneal ulceration (Bozkan and Aybak, 2025). Diagnosis involves the use of computed tomography to assess for the presence of material behind the eye and to help determine if the cause is likely to be an abscess or a retrobulbar mass. In some cases, surgical removal of the affected tooth and long-term systemic antibiotics can result in favourable outcomes (Levy and Mans, 2023), but in others enucleation may be required.
Corneal ulceration can occur in rabbits secondary to trauma, entropion, lengthy general anaesthetics and facial nerve paralysis due to otitis media. Hay-poke injuries result from sharp hay particles causing corneal oedema and linear corneal abrasions, which are frequent in guinea pigs (Figure 6) and present similarly to corneal ulceration. It is recommended to instil a topical local anaesthetic and check under the nictitating membrane in the medial canthus as residual hay particles are often trapped in this area and will continue to cause corneal trauma until removed. Diagnosis is via visual examination of the eye and topical fluorescein staining (Figure 7). Management is typically conservative (unless infection develops) and includes multimodal analgesia, topical lubricants and topical antibiotics if indicated. If the ulceration is chronic, further diagnostics such as skull imaging is indicated. Debridement under local anaesthetic using a diamond burr or cotton tip can be useful to promote healing (Holmberg, 2018).
Encephalitozoon cuniculi infection in rabbits can lead to phacoclastic uveitis due to lens capsule rupture and the subsequent immune response. Clinical signs include a white mass within the anterior chamber, lens opacity (Figure 8) and possible secondary glaucoma. A diagnosis is confirmed by PCR of aqueous humour or lens aspirate. These lesions often are of minimal consequence, as the lens can only be affected if the infection is acquired in utero. Once the spores within the lens capsule cause it to rupture (Figure 9), protein escapes from the lens and a subsequent phacoclastic uveitis and pyogranulomatous inflammation occurs (Baney et al., 2021). Treatment is not always indicated; however, if there is discomfort, treatment can include lens removal and anti-inflammatory therapy.
Exophthalmos and proptosis are the most common ocular presentations in hamsters (Holmberg, 2018). Severe exophthalmos can be caused by scruffing or excessive pressure behind the head, as well as stressful events. With excessive exophthalmos the eyelids can become trapped behind the globe, resulting in proptosis of the eyeball. Ample lubrication and replacement of the globe under general anaesthesia is usually successful if performed quickly and if there is no trauma to the globe itself. In some cases, temporary tarsorrhaphy may be indicated, and in cases that are beyond management, enucleation is recommended (Figure 10).
Reptile ophthalmology
Anatomy of the reptilian eye
Most reptiles have well-developed eyelids, apart from some species of geckos and skinks, and all snakes, which possess a transparent scale (known as the spectacle) over the surface of the eye (Holmberg, 2018). Some reptiles have highly specialised eyes, for example chameleon species that can move each eye independently of the other.
Most reptiles have a ventromedial lacrimal gland and a dorsolateral Harderian gland (Holmberg, 2018). The nictating membrane is present in species with eyelids and is often well developed.
Common pathologies of the reptilian eye
Dysecdysis, or abnormal shedding, is a frequently encountered ocular issue. Retained spectacles are common in snakes and result from low humidity or poor health. Clinically, this appears as an opaque, retained cap over the eye (Williams, 2012). Treatment for retained spectacle includes increasing humidity and manual removal post-soaking. In lizards, blepharospasm is often an indication of a foreign body within the palpebral fissure; it may be necessary to flush the palpebral fissure to remove any stuck shed, dirt or debris. Topical anti-inflammatories and systemic analgesia should be provided to lizards with blepharospasm, and any underlying nutritional or systemic disease must be addressed.
Proptosis in reptiles, although rare, can occur following trauma (Figure 11). Lizards, especially iguanas, may suffer orbital fractures, leading to globe displacement. Immediate stabilisation, analgesia and enucleation are often necessary. Imaging helps assess the extent of trauma (Diehl and McKinnon, 2016).
Vitamin A deficiency can result in ophthalmic presentations and is more common in younger reptiles (Holmberg, 2018). Hypovitaminosis A causes squamous metaplasia of epithelium that lines the ducts of glands, which results in plugs of material that block the ducts. Blocking of the ducts results in oedema of the periocular tissues and conjunctivitis (Figure 12). While these patients often present for ophthalmic signs as these are external changes that the owner can see, it is important to remember that this occurs in all epithelium-lined ducts throughout the body, including the gastrointestinal and respiratory systems (Figure 13). Treatment should be swift and involve supplementation of vitamin A, as well as necessary dietary and husbandry adjustments to prevent deficiency from occurring again after treatment.
Conclusion
Many of the ophthalmic diagnostic tests used in companion animals can also be used, or used in a modified way, in order to investigate ophthalmic presentations in exotic pets
There is a vast array of variation between the ocular anatomy of exotic pets. Each group of animals has its own specific anatomy and corresponding pathology that general practitioners may be faced with. Many of the ophthalmic diagnostic tests used in companion animals can also be used, or used in a modified way, in order to investigate ophthalmic presentations in exotic pets.
