Inflammation to the conjunctiva is probably the most common eye disorder you will see. It often occurs as a result of a direct conjunctival irritation but may also develop because of more serious problems elsewhere, in the eyeball itself or in other parts of the body. As well as causing local inflammation, conjunctivitis is associated with a variety of discharges that gives clues to the cause of the problem. Tear production and conjunctivitis are intimately associated.
Tear Production - What Tears Do
Tears not only keep the surface of the eye moist and glistening, they contain substances that fight infection. Tears are partly produced in lacrimal glands, one just under the bony ridge at the top of the eye socket, the other in the nicitating membrane. Excess tears drain from the eye through a large duct, the nasolacrimal duct, into the nasal cavity. Other components of tears are produced in the meibomian glands in the eyelids and by mucus-producing cells in the conjunctiva. Mucous holds water while the oil from the meibomian glands inhibits evaporation. Three tear problems may occur. There can be excess tear production, blocked tear drainage and most serious of all, reduced tear production in dogs. Reduced production dries the cornea and increases risk of corneal damage and conjunctival infection.
A clear, colourless discharge usually indicates physical or allergic irritation. Dust, wind and cold all trigger a watery discharge from both eyes. Allergens such as pollens do the same but allergy is often accompanied by face rubbing. Watery discharge from a single eye is usually caused by a foreign body.
Jelly-Like Mucous Discharge
Irritation or infection triggers increased mucous production from conjunctival cells.
A yellow-green discharge is pus. Infection is present and must be treated. The cause of infection, for example, reduced tear production also needs to be identified and treated.
Bacterial, Viral And Fungal Conjunctivitis
Like the skin, the conjunctiva acts as host to a population of normal bacteria and fungi. Injuries, foreign bodies in the eye, viral infections or, most important, a reduction in tear production allows some of these bacteria to grow exuberantly and cause infection. Any red, swollen, watery or mucousy inflammation of the eye membranes is ‘conjunctivitis’. The flat facial conformation of Pekes and Persians predisposes to tear overflow and conjunctivitis. So do droopy lower lids in some dogs. Allergy will also cause watery conjunctivitis but in cats the most common cause is rhinotracheitis or ‘Herpes’ virus infection. Many if not most cats are exposed to Herpesvirus as kittens and don’t develop conjunctivitis but Herpes can lay dormant for years and given a chance, when a cat’s immune system is under stress, it becomes active causing red, watery eyes. Opportunist bacteria infect the conjunctiva and the discharge changes to yellow-green and sticky. An affected cat squints either from pain or to avoid light. The preventative vaccines against this virus, calicivirus and Chlamydophila don’t actually prevent infection. They reduce the severity of the disease.
Allergy and foreign bodies account for most causes of sneezing in dogs but virus infection is an additional common cause in cats. In allergy and uncomplicated virus infection in cats the discharge is clear and colourless. Sometimes the only evidence of discharge in cats is the accumulation of small amounts of dry, dark debris around the nostrils, easy to remove with damp cotton wool, even with a slight rub with your finger. If bacterial opportunists complicate the nasal inflammation or ‘rhinitis’ the discharge becomes thicker with a yellow-green appearance. Sneezing and discharge from a single nostril usually suggests a foreign body in that side of the nose. In our experience, the most common foreign body in both dogs and cats is a blade of grass that, because of the texture of its surface, can only move in one direction, further in. Sneezing is the body’s way of trying to discharge the foreign body.
Prevention and treatment
Once more, inoculation reduces the intensity of infectious causes of rhinitis in cats while avoidance or antihistamines and anti-inflammatories may be needed to control allergic rhinitis in both dogs and cats. Foreign bodies such as blades of grass usually need to be helped out under general anaesthesia by us. Sometimes we can gently tease the grass back out through the nostril but on other occasions we need to use an endoscope to take it out through the ‘nasopharynx’ at the back of the mouth.
Tear Overflow - Watery Eye
The variety of causes of tear overflow include:
- Abnormal eyelashes
- Facial hair touching eye
- Growths on the eyelids
- Foreign bodies in eye
- Anterior uveitis
As well, tear overflow may occur because the normal drainage for tears, from the corners of the eyes by the nose, through the nasolacrimal duct into the front of the nasal chamber is blocked or anatomically poorly constructed. This is called EPIPHORA.
The common reasons for nasolacrimal duct problems are:
- The duct is swollen from infection
- Mucous produced in the conjunctiva plugs the opening of the duct or forms a blockage in the canal.
- The duct is narrow or absent from birth.
Diagnosis and treatment
A fluorescent dye is instilled on the eye. If the nasolacrimal duct is open (patent) dye is seen almost immediately in the nose. If it is not, under anaesthesia, a small cannula is inserted in the eye end of the nasolacrimal duct and dye is gently instilled. If the duct is patent, this dye is seen in the nose. Pets with chronic tear overflow develop mahogany staining to hair wet with tears.
Controlling Tear Stain
To control facial staining in pets with chronic tear overflow, clean away the overflow twice daily with saline eye wash.
Dry Eye In Dogs (Keratoconjunctivitis Sicca)
Any condition that reduces lacrimal tear production can cause "dry eye" or keratoconjuncivitis sicca, which means a dry cornea and conjunctiva. Predisposing conditions include:
- Immune-mediated disease - This is the commonest cause
- Viral infection - distemper
- Bacterial infection - Staphylococcus
- Underactive adrenal gland (hypoadrenocorticism)
- Sulfa drugs, including sulphonamides and trimethoprim-sulfa
- Lacrimal gland injury
- Lacrimal gland nerve injury following middle ear infection
The breed most prone to dry eye is the West Highland White Terrier.
It is not unusual for a dog to be treated for conjunctivitis and only when that treatment becomes chronic is it realised that there is an underlying dry eye. Dogs with dry eye initially produce more mucus than water in their tears. This becomes thick and tenacious. Secondary pus-producing bacterial infection is very common. Rather than bright and sparkling, the eyes look a little duller and dryer. Opaqueness is common The cornea becomes inflamed (keratitis) and ulcerates.
Diagnosis and treatment
We will measure tear production. Special sterile filter paper is placed in the corner of the eye for a minute. A normal tear test wets typical strips to 20 mm while in dry eye, moisture wicks up the strip to 10mm or less. While the underlying condition is eliminated and secondary bacterial infection controlled with appropriate topical and oral antibiotics, cyclosporin ointment and corticosteroids are used to reverse apparent immune-mediated destruction of the lacrimal glands.