Allergic conjunctivitis in dogs

Overview

The conjunctiva and eye surface

The conjunctiva is a thin membrane lining eyelids and covering the front of the eyeball around the cornea. It also covers the third eyelid in the inner corner. It helps protect the surface from foreign material. It contributes to tear film formation and corneal healing.

Inflammation makes vessels more visible, producing redness. Swelling is called chemosis. Swollen tissue may appear around the corneal edge. Injury or insect stings, for example, can cause similar swelling. Allergy can affect only the eyes, though eye signs more commonly accompany atopic dermatitis.

Causes

Allergic conjunctival reactions

The surface can react to airborne allergens. Histamine mediates rapid allergic reactions. It can increase surface vessel permeability. In an experiment, redness and swelling began in one canine eye within a minute of histamine exposure. Inflammation altered tear protein concentrations.

Dust mite allergens can cause signs in sensitised atopic dogs. Pollen can aggravate atopy. Dogs may also become sensitised to animal or human dander. Sand can irritate the conjunctiva and cause inflammation too. Redness can therefore reflect irritation or infection rather than allergy.

Risk factors

Atopic dermatitis is an itchy disease associated with inherited susceptibility. Most affected dogs develop signs between six months and three years. Environment influences onset alongside genetics. Environmental allergen symptoms can vary seasonally.

Conjunctivitis has been identified in 21–30% of atopic dogs. One 60-dog study found allergic signs in or around the eyes in 60%.

Among 122 hospital-selected dogs with allergic conjunctivitis, 94% had concurrent atopic skin signs. Eye and skin severity were not associated. Ages ranged from 10 months to 13 years. Age was not associated with ocular severity.

Typical symptoms

Signs usually affect both eyes. Itching can be difficult to distinguish from other facial rubbing.

• Red conjunctiva with more visible vessels over the white of the eye.
• Swelling inside eyelids or around the eyeball.
• Scratching around the eye or rubbing the face.
• Tears overflowing the lid margin.
• Watery or mucoid discharge in the corner.
• Red surrounding skin.

The Portuguese 122-dog study found swelling in 84%. Redness affected 83%. Itching occurred in 79%. Excess tearing occurred in 69%. Itching, redness and swelling coexisted in 55%.

All dogs had bilateral signs. Severity differed between eyes in 27%. Watery, mucoid or mucopurulent discharge occurred in 62%.

Complications and consequences

Rubbing and surface inflammation

Repeated scratching can damage lids or the eye surface. Persistent or severe allergy can accompany corneal inflammation, called keratitis. Keratitis occurred in 15% of affected dogs. Severe corneal changes can impair vision.

Allergic eye inflammation can alter tear film composition. A weakened lipid layer accelerates evaporation. Conjunctival inflammation can alter the film despite normal Schirmer tear production. Chronic inflammation can enlarge lymphoid tissue into small follicles, visible on the third eyelid, for example. Other conjunctival inflammation can also cause follicles.

Diagnosis at the vet

Examining both eyes

Both are examined even if only one seemed affected at home. Pupil light responses can be assessed. Fundus examination checks retinal disease with impaired vision. Red-eye assessment excludes internal inflammation such as iritis. Foreign bodies can hide in conjunctival folds or behind the third eyelid. Extra rubbing eyelashes can cause similar redness and discharge. Inward-rolling lids can irritate the surface and cause conjunctivitis. Inadequate tears can inflame conjunctiva and cornea. Blocked tear ducts can irritate surrounding skin.

Slit-lamp examination magnifies surface changes. Schirmer testing measures tear production. A reference interval of 15–25 mm per minute was used. Fluorescein seeks corneal ulcers. Pressure measurement investigates glaucoma. All 122 Portuguese study dogs had normal pressure. Raised pressure with cloudiness can suggest glaucoma.

Investigating suspected allergy

Compatible recurring signs with other causes excluded suggest allergy. Conjunctival cytology can be collected when needed. A Croatian 24-dog atopy study found lymphocytes in 87.5% of samples. All 24 had inflammatory cells. One had no visible allergic conjunctivitis signs. Sampling helps confirm suspected bacterial infection.

Blood and skin tests identify allergens for immunotherapy after clinical atopy diagnosis. A positive allergy test alone does not diagnose atopic dermatitis.

Conjunctival provocation observes response to a specific allergen. Dust mite challenge involved two groups of 12 atopic dogs sensitised to different mite species. Controls included 12 healthy and 12 atopic dogs without mite sensitisation. Dermatophagoides farinae test sensitivity was 91.7%, with a 67.8–99.3% confidence interval. Dermatophagoides pteronyssinus sensitivity was 83.3%. Both tests were negative in 95.8% of controls.

Severity can be assessed using six signs. Among 122 graded cases, 64% were moderate. Mild cases comprised 24%. Severe cases comprised 12%.

Treatment

Protecting the eye surface

The vet chooses treatment according to the cause of inflammation. A cool compress can relieve itchy eyes. A lubricating product intended for the eye surface can support the tear film. Lubrication can reduce allergen attachment to the surface. A protective collar can prevent scratching when itching is severe. Sterile eye wash can remove discharge before applying prescribed medication.

Eye drops and atopy treatment

For mild cases, the vet may consider a topical allergy medicine such as an antihistamine. These can reduce histamine effects by blocking H1 receptors. Some products also stabilise mast cells involved in allergic reactions.

Severe inflammation may need prescribed short-term corticosteroid eye treatment. Ocular corticosteroids are unsuitable with corneal ulceration. Long-term steroid drops can increase corneal complication risk. The surface is stained to check for injury before prescribing them. Antibiotics are not routine treatment for an allergic reaction. Severe or persistent cases may prompt consideration of ocular ciclosporin or tacrolimus.

Systemic treatment can also help eye signs when other atopic symptoms are present. Allergen-specific immunotherapy may be considered once the responsible allergen is known. Atopic dermatitis management is adjusted to symptoms and response. Corneal involvement may prompt referral to a vet specialising in eye disease.

Prevention

Avoiding a known allergen can reduce eye signs. Exposure to widespread environmental allergens can be reduced, though complete avoidance is often difficult. Tobacco smoke can irritate the conjunctiva and worsen symptoms. Avoid strong cleaning chemicals close to your dog because they can irritate the eye surface.

Maintenance treatment of atopic dermatitis aims to reduce flare-ups. Reducing dust in your dog's living areas can ease surface irritation. If outdoor activity worsens signs, a vet-selected lubricating gel can protect the surface before outings.

Important for dog owners

Tell the vet when redness began and whether your dog has rubbed their eyes at home. Note whether one eye is more affected than the other. Mention whether eye signs worsen alongside skin symptoms.

Do not start human eye medication without veterinary advice. Leftover medication can also harm if the new problem has a different cause.

Wash your hands before applying prescribed medication to avoid contamination. Keep the bottle tip away from the surface and eyelids. Blinking spreads the drop over the surface. Contact the vet if redness increases after application. Recurrent signs may prompt repeat tear testing or sampling.

Frequently asked questions

Can allergy affect only one eye?

Both are usually affected, but unilateral signs are possible. A foreign body can also cause one-sided signs. Squinting or cloudiness needs prompt examination.

Does allergic conjunctivitis need antibiotics?

Anti-inflammatory eye medication can control the reaction when needed. Confirmed bacterial infection may require prescribed antibiotic drops. Sampling can investigate suspected bacteria. Allergy can also cause mucoid discharge, so discharge alone does not identify the cause.

Can I use steroid drops I already have?

First ask the vet whether the product is suitable. Corticosteroids are unsuitable with corneal ulcers. Fluorescein can identify a scratch or other surface injury. Leftover treatment may not suit a new condition.

Do itchy eyes mean food allergy?

Environmental allergen sensitisation can cause itchy eyes. Year-round skin itching can warrant a diet trial to investigate food involvement. If skin signs improve, supervised reintroduction of the previous food assesses its role.

Can signs return after treatment?

Re-exposure to an offending allergen can cause recurrence. Atopic dogs can develop eye signs during skin flare-ups. Repeated signs prompt assessment of additional causes such as inadequate tears. Persistent symptoms may require specialist examination.