Conditions affecting guinea pig eyes are relatively common due to the prominence
and positioning of the eye. In addition guinea pigs have characteristics that predispose them to eye trauma. They blink less frequently than other animals, often sleep with their eyes partially open and have a tendency for burrowing and foraging through hay. Eye conditions can be very painful and can progress rapidly if left untreated. It is important you, as a guinea pig owner to regularly check your pets’ eyes and be familiar with what is normal.
Guinea pig eyes are located on either side of the head. The cross-over in vision from
the two eyes is relatively small. This means guinea pigs have a large field of vision
but limited binocular vision. This makes their three dimensional perception, as well as their judgement of distance, more limited than our own. It is thought that guinea pigs are able to see in colour. Like humans they have a circular pupil and a coloured iris. Guinea pigs are prey species so when they are handled they often release stress hormones, adrenaline and cortisol. This has the effect of dilating the pupils. The fact that guinea pigs are prey species is also the reason why they often have their eyes open. Guinea pigs have two eyelids, upper and lower, with most closure movement occurring from the upper lid. They do not have a third eye lid like dogs and cats.
A normal guinea pig eye looks bright and will glisten due to the protective tear film.
The two eyes will be open to an equal amount and have equally sized pupils. The
position and prominence of the eyes will be symmetrical. There shouldn’t be any
ocular discharge or crusting or hair loss to the face around the eye. The only exception to this is when the guinea pig is cleaning itself. It is normal for a guinea pig to produce a milky liquid from the corner of the eye (medial canthus) which it then clears with its front paws as part of the grooming process. This liquid contributes to lubricating and cleaning of the eye.
There are several eye conditions that guinea pigs suffer from quite regularly. As
described below:
Corneal Ulceration:
The cornea is the transparent covering to the front of the eye. It is made of many
layers, similar to the layers of an onion. Ulceration of the cornea occurs when some of these layers become damaged; this is usually due to trauma. Trauma most commonly occurs due to hay when the animal is foraging, hence the term hay poke. It can also occur due to hair rubbing on the eye (eg with entropian, see later), or due to an inability to blink following facial nerve paralysis from middle ear disease. Signs include a bluish haze to the front of the eye, increased tear production, inflammation and blinking or holding the eye partially closed. The ulceration is diagnosed with a yellow dye called flourescein. It is treated by removing the hay if present, followed by pain relief and topical medications. If ulcerations are left untreated they can progress into abscesses.
Corneal Ulceration; note the bluish haze to the front of the eye.
Entropian:
Entropian is a rolling inwards of the eyelid, usually the lower eyelid. This has the
effect of the hair of the eyelid constantly rubbing on the eye. This causes irritation and
often corneal ulceration. Treatment is frequently surgical.
1st image entropian; note a rolling in of the lower eyelid, and discharge in the fur. 2nd
image is the same guinea pig post surgery; note the eyelid margin is now visible.
Conjunctivitis:
Conjunctivitis is an inflammation of the conjunctiva (the mucous membrane that
covers the front of the eye and the inside of the eyelids). Signs include a reddening to
the inside if the eyelids as well as the white of the eye, increased tears and irritation. It
is often caused by a bacteria called chlamydophila caviae. Treatment is with antibiotic
eye drops.
Conjunctival Tissue Protrusion:
Conjunctival tissue protrusion is commonly referred to as fatty eye or pea eye. It is a
protrusion of the lower conjunctival sac caused by either enlargement of the glands in
this region or by increased fat in the conjunctiva. It is not thought to be painful for the
guinea or affect vision and therefore does not require treatment.
Conjunctival tissue protrusion.
“Cherry Eye”:
Guinea pigs can get inflammation or infection in the lacrimal (tear producing) gland
that sits at the corner of the eye. This results in this gland protruding as a red fleshy
swelling in the corner of the eye. It is treated with topical medications.
Heterotropic Calcification:
Heterotropic calcification is often seen as a white lesion at the junction between the
white of the eye and the coloured iris (it can, less commonly, also occur elsewhere in
the eye). This may be localised or form a complete circle. It is essentially bone
formation in an abnormal location. The lesions can be quite dramatic. However, it is
not thought to have clinical significance and therefore not require treatment.
Heterotropic calcification; note the uneven white lesions at the edge of the iris.
Cataracts:
A cataract is a hardening and opacity to the lens. This prevents light reaching the
retina and eventually causes blindness. It is seen as a progressive cloudiness within
the pupil. Some guinea pig family lines have congenital cataracts which develop
earlier in life. Cataracts can also be an age related change and can be seen associated
with diabetes.
Exophthalmus:
Exophthalmus is a protrusion of the eye. The most common cause of eye protrusion in
the guinea pig is due to a disease process in the retrobulbar space (area behind the
eye). This disease process can include tumours but is more often an abscess associated
with the roots of the maxillary cheek teeth. Treatment is likely to include antibiotic
therapy, correction of the underlying dental disease, and possible enucleation (surgical
removal of the eye).
Microphthalmus:
Microphthalmia is a relatively common congenital defect in certain guinea pig lines.
The guinea pig is born with either a very small, or non-existent, eye and is blind.
In conclusion there are many conditions that can affect the guinea pig eye. Many of
these conditions occur with greater frequency in guinea pigs than other species. It is
important that we check our guinea pigs eyes frequently for signs of disease as eye
conditions are often very painful and can progress very rapidly. A familiarity with what
is normal will allow a more rapid appreciation of abnormal and the need for veterinary attention.