Drugs Used to Treat Conjunctivitis in Cats & Dogs

Renee Carter, DVM, DACVO, Louisiana State University

ArticleLast Updated August 20225 min read
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Conjunctivitis is inflammation of the conjunctiva (ie, mobile mucous membrane that lines the eyelids [palpebral] and surface of the globe [bulbar]) and can be primary (eg, allergic disease, immune-mediated inflammation, infectious disease process) or secondary (eg, to adnexal disease, tear film disorders, intraocular disease, systemic disease process).1 Clinical signs include conjunctival redness (eg, hyperemia; Figure 1), swelling, ocular discharge, and discomfort (eg, squinting or rubbing of the affected eye). Ulceration is less common but can occur following viral insult or injury to the conjunctiva.1

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FIGURE 1 Marked conjunctival hyperemia and mucopurulent ocular discharge in a dog with conjunctivitis. Image courtesy of Louisiana State University

Primary conjunctivitis should be differentiated from other ocular disorders with ocular redness (eg, episcleritis, keratitis, uveitis, glaucoma) that would require additional diagnostic investigation and different management strategies. Most causes of conjunctivitis in cats should be considered infectious until proven otherwise2; causes in dogs are commonly noninfectious.


Allergic Conjunctivitis

The most common cause of bilateral conjunctivitis in dogs is allergic disease (Figure 2). Periocular redness and alopecia are typical in patients with allergic conjunctivitis secondary to type 1 hypersensitivity reactions (eg, atopy).3 Lymphocytes and plasma cells can often be seen on conjunctival cytology.

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FIGURE 2 Follicular conjunctivitis with prominent lymphoid follicles involving the posterior aspect of the third eyelid in a dog with allergic conjunctivitis. Image courtesy of Louisiana State University

Acute, Bilateral Chemosis, & Blepharoedema

Acute, bilateral chemosis (Figure 3) and blepharoedema may be associated with an immediate-type hypersensitivity reaction mediated by histamine and immunoglobulin E.3 Reactions can be caused by contact allergy, vaccine reaction, or insect envenomation and should be treated with IM or IV short-acting corticosteroids and antihistamines, possibly in conjunction with topical corticosteroids.3 In patients with suspected ocular exposure to an irritating compound, the eye should be well irrigated with saline and stained with fluorescein; surface pH should also be monitored.

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FIGURE 3 Extensive chemosis with significant swelling of the conjunctiva and relative lack of inflammation in the right eye of a dog following insect envenomation. Image courtesy of Louisiana State University

Eosinophilic Conjunctivitis

Nodular Granulomatous Episcleritis

Eosinophilic conjunctivitis is most common in cats. Feline herpesvirus (FHV)-1 may be an etiology for this disorder, but this has not been verified.2 Both the conjunctiva and cornea may be involved. Eosinophils and/or mast cells are present on conjunctival cytologic samples.

Primary inflammation of episclera (ie, connective tissue between the conjunctiva and sclera) is often immune-mediated, resulting in multiple fleshy pink to tan-colored nodules (Figure 4). Collies, cocker spaniels, and Shetland sheepdogs are overrepresented.4

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FIGURE 4 Focal nodular mass in a dog with nodular granulomatous episcleritis. Image courtesy of Louisiana State University

Conjunctival Inflammation

The following medications control redness, discharge, pain, and inflammatory cell infiltrates that characterize conjunctivitis of any etiology. Topical treatment (ie, eye drops, ointment) is often sufficient for primary conjunctivitis because of disease distribution in the conjunctiva, ease of application, increased drug levels at the site of interest, and reduced risk for systemic adverse effects. 

Conjunctival inflammation is primarily managed with topical steroids or NSAIDs, depending on conjunctivitis etiology and severity. Secondary forms of treatment include topical antihistamines, mast cell stabilizers, and vasoconstricting agents. Topical management options are described here.

Tear Film Disorders

Immunomodulators that decrease inflammation in tear glands and lacrimomimetics that directly stimulate the glands are the primary classes of drugs that can improve tear production in patients with tear film disorders (Figure 5).

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Figure 5

Corneal vascularization, fibrosis, and pigmentation secondary to chronic KCS in a dog with conjunctivitis and keratitis. Image courtesy of Louisiana State University

Bacterial Conjunctivitis

Etiologic differences between dogs and cats are important when considering therapy for bacterial conjunctivitis.


Primary bacterial conjunctivitis is uncommon in dogs.1 Dogs have normal resident conjunctival flora, and bacteria can often be cultured from the conjunctiva.3 Secondary bacterial infections are more prevalent in dogs with tear film disorders or other disorders that compromise ocular surface health. Patients with a significant number of bacteria per high power field and neutrophils on conjunctival cytology may require short-term therapy while the underlying disorder (eg, atopy, KCS) is treated. Aerobic culture and susceptibility testing should be performed in patients with no improvement in clinical signs. 

Routine broad-spectrum antibiotics for gram-positive bacterial conjunctival infections in dogs include topical neomycin, polymyxin B, bacitracin, gramicidin, and erythromycin. Therapy for gram-negative conjunctival infections includes topical neomycin, polymyxin B, bacitracin, gentamicin, tobramycin, and fluoroquinolones.3


Primary bacterial conjunctivitis is common in cats. Important pathogens include Chlamydia felis (Figure 6), Mycoplasma spp, and Bordetella bronchiseptica. Treatment of the ocular surface alone is not curative due to presence of the organism in other systems (eg, respiratory, GI, and genitourinary tracts). Management should include topical and systemic therapy (eg, doxycycline, pradofloxacin, moxifloxacin) based on the pathogen responsible for disease, especially in patients presented with upper respiratory signs and positive via PCR.2,29

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Figure 6

Blepharoconjunctivitis and serous ocular discharge in a cat with C felis infection; clinical signs were bilateral. Image courtesy of Louisiana State University

Viral Conjunctivitis

FHV-1 and canine herpesvirus are the primary pathogens that cause viral conjunctivitis (Figure 7). There are no manufactured or approved antiviral drugs for treatment of veterinary ophthalmic viral infections.1,34 Herpetic viral infections are often associated with keratitis and conjunctivitis. FHV-1 is an alpha herpesvirus that causes upper respiratory tract infection, dermatitis, and ocular disease.34 Canine herpesvirus-1 is often self-limiting and infrequently requires antiviral therapy.1

Antiviral products are virostatic (ie, only act on a replicating virus) and herpetic viral infections can develop latency; antiviral drugs thus limit, but do not cure, disease.2,34 Safety and patient tolerance depend on how specifically the drug targets the virus without damaging host tissue. Therapy with an antiviral agent is generally not indicated in patients with mild disease and absence of corneal ulceration. Antivirals should be discontinued, not tapered, following resolution of clinical signs to reduce the risk for development of resistance.2

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Figure 7

Blepharoconjunctivitis in a cat with FHV-1; clinical signs were unilateral and recurrent. Image courtesy of Louisiana State University