Conjunctivitis in cats

©Barnett, KC & Crispin, SM Feline Ophthalmology (2002) Saunders

This is a very common eye disease in cats with many causes, ranging from allergies, scratches from other cats and objects, to herpes virus infection and other parasite infections such as toxoplasmosis. Probably the most common cause of conjunctivitis is herpes virus (cat flu) and Chlamydia infection. The cat normally has one eye infected and squints and rubs the eye constantly.

Figs. 1-3. Conjunctivitis secondary to herpes virus, trauma and Chlamydia.

	Eosinophilic keratitis
Superficial dendritic branching secondary to FHV

Viral conjunctivitis

The feline herpes virus most commonly infects kittens and causes sneezing, ocular and nasal discharge, and a reluctance to eat and play. With good nursing care, the vast majority of kittens return to normal within 3 weeks. Vaccinated kittens may still develop disease, but the illness is less severe. Approximately 80% of FHV infected cats become latent carriers with a 45% chance of viral re-activation. Adult cats with eye disease due to FHV are more likely to be suffering from viral re-activation than from a primary FHV infection. 

Feline eosinophilic keratitis is a rare infiltrative and progressive keratopathy of the cat. The lesions are typically tan and have a granular or cobblestone appearance. They often originate from the limbus. Vascularization is commonly present and the unaffected cornea is frequently edematous ( 1). At the time of presentation, 24% of affected cats have a corneal ulcer in the affected eye. Conjunctival involvement is also commonly present and is indicated by thick hyperemic edematous conjunctiva ( 2). The disease seems to have no age, breed, or sex predilection and appears unilaterally 66% of the time ( 1). It has been suggested that as the disease progresses, it can become bilateral. Other presenting signs include ocular discharge, chemosis, blepharospasm, depigmentation of external eye lids, and thickening of the third eyelid ( 2). The differential diagnoses for feline eosinophilic keratitis include herpetic keratitis, keratomycosis, acid fast granuloma, corneal neoplasia, and foreign body granuloma ( 1, 2).

Currently, feline eosinophilic keratitis is considered idiopathic; however, many possible etiologies have been speculated. Most of the debate has been over the relationship between feline eosinophilic keratitis and Feline herpesvirus. One study detected Feline herpesvirus in 76% of cases with eosinophilic keratitis ( 3), while in another study it was only 33% ( 1). The study showing a 76% correlation used more sensitive diagnostic tests. There has also been a suggestion that the cornea may be a depot for latent Herpesvirus ( 3).

Many authors have suggested that feline eosinophilic keratitis is linked to the feline eosinophilic granuloma complex, but this association has yet to be confirmed ( 3, 4). Prasse ( 5) has suggested that the eosinophilic response could be explained by a type I or type IV hypersensitivity, but this theory has not yet been researched. It is currently believed that eosinophilic keratitis is related to feline eosinophilic conjunctivitis, another idiopathic syndrome observed in cats ( 5).

Feline eosinophilic keratitis is usually suspected when there is a history of no response to antibiotic treatment, along with the characteristic appearance of the lesions. Definitive diagnosis is made on cytologic examination of corneal scrapings, or light microscopy of corneal biopsies. The presence of eosinophils, eosinophilic granules, and mast cells in significant numbers is considered diagnostic for feline eosinophilic keratitis ( 1, 2, 6). Histologically, there is an associated mast cell and eosinophilic exocytosis. The superficial stroma of the cornea may be infiltrated by a variety of leukocyte types, and there is corneal epithelial hypertrophy, hyperplasia, and thinning ( 6).

The recommended initial treatment for feline eosinophilic keratitis is topical corticosteroids ( 1, 2, 6). Other suggested treatments include systemic corticosteroids and megestrol acetate (Ovaban; Schering-Plough Animal Health, Pointe Claire, Quebec). Topical corticosteroids is the first choice for treatment due to its efficacy, high local concentrations, and minimal side effects ( 7). Oral corticosteroids should not be used as a first line drug due to the high dosage needed for immunosuppression and the possible adverse effects. It has been suggested that megestrol acetate is useful in cats with a history of recurrent eosinophilic keratits ( 2). Megestrol acetate has been recommended, but it should be used cautiously, as it also causes adrenocortical suppression, is not licenced for use in cats, and has an 8-day half life ( 1, 8). Recurrence rates for feline eosinophilic keratitis are high. Morgan et al ( 2) reported a recurrence rate of 64% in a study with 65 cats. Recurrence is reported with all treatment modalities.

Bacterial conjunctivitis

Chlamydia psittaci (an obligate intracellular bacterium) is the most important of the feline conjunctival pathogens and the clinical signs may be observed in cats from 4 weeks old onward. Clinical signs are of a unilateral conjunctivitis initially, which becomes bilateral several days later. Initially there is a serous discharge with obvious chemosis and conjunctival hyperaemia, later the discharge can become mucopurulent and other organisms may be isolated. There is no corneal involvement and no primary respiratory disease, although mild rhinitis may be present. In a proportion of cases, both respiratory tract viruses and Chlamydia psittaci will be isolated. Lymphoid follicle formation is common in chronic cases.

Diagnosis is confirmed by chlamydial isolation from swabs taken in VCTM and from conjunctival scrapings to demonstrate intracytoplasmic inclusion bodies in Giemsa and Giema-stained material. Intracytoplasmic inclusion bodies can be difficult to differentiate from intracytoplasmic pigment granules. Serology is of limited value in unvaccinated cats and of no value in vaccinated animals.

Treatment consists of topical tetracycline or systemic doxycycline for some 3-4 weeks. Oral treatment with doxycycline (25mg/kg in divided doses) is well tolerated and effective. A proportion of previously affected cats become chronic carriers and may be a possible source of infection for other cats (the organism can be isolated from the urogenital and gastrointestinal tract). This may pose problems in catteries, especially for breeding colonies. In this type of environment all the cats will require systemic tetracycline, erythromycin or doxycycline for at least 4 weeks. Systemic doxycycline is probably the drug of choice and is suitable for younger cats.

Mycoplasma felis is recorded as an occasional cause of conjunctivitis, although the pathogenicity of the organism is equivocal, particularly as the condition is self-limiting and usually resolves within 30 days, although the cat may remain infectious for up to 60 days. Mycoplasma felis can be isolated from the conjunctiva of both normal cats and those with conjunctivitis, so it is important to ensure that no other potential pathogens are present in suspect cases. The clinical appearance if often spectacular as chemosis, hyperaemia and conjunctival thickening are marked. Slit lamp examination may reveal papillary hypertrophy in the initial stages. In an untreated case, the hyperaemia becomes less obvious after 14 days and the conjunctiva becomes pale with a friable white diphtheritic membrane (pseudomembrane) as an obvious feature.

Other bacteria identified (such as Pasteurella spp., Staphylococcal spp., Streptococcus spp., Salmonella spp., Moraxella spp.) are of uncertain pathogenicity. Any underlying primary problem should be identified and eliminated and appropriate antibiotic therapy initiated for the bacterial conjunctivitis.