Feline conjunctivitis is a common and often frustrating clinical problem. It is most frequently caused by feline herpesvirus-1 (FHV-1) or Chlamydophila felis, primary feline pathogens capable of infecting the healthy ocular surface. Non-infectious feline conjunctival disorders include eosinophilic conjunctivitis, keratoconjunctivitis sicca and lipogranulomatous conjunctivitis.

Owing to the frequently infectious nature of feline conjunctivitis, symptomatic therapy with topical and/or systemic corticosteroids is contraindicated until an infectious aetiology is disproved. Clinical inflammation may initially decrease with steroid therapy, but the immunosuppressive effects increase the risk of herpes-related corneal ulceration, lengthen the period of ocular virus shedding and encourage deeper corneal infection.

Infectious Conjunctivitis

FHV-1 and Chlamydophila felis infections tend to occur more frequently in catteries, shelters and multicat households. Natural (pregnancy, lactation, oestrus and systemic illness) and exogenous (relocation, new pets, exhibitions and corticosteroids) stress factors have a proven role in reactivating latent ocular infections.

FHV-1

The typical patient with viral conjunctivitis is a neonatal or adolescent cat with an acute conjunctival-respiratory infection. The conjunctivitis is bilateral and characterised by pronounced hyperaemia and serous discharge that becomes mucopurulent as the disease follows its 10-14-day course. Conjunctival adhesions known as symblepharon frequently occur in very young kittens, resulting in epiphora due to occlusion of the nasolacrimal puncta, reduced eyelid mobility and permanent corneal opacity.

An estimated 80% of cats become latently infected; 45% of these will either shed the virus asymptomatically or develop recurrent clinical disease. Cats may have intermittent ocular disease, after which they appear clinically normal, or may have chronic clinical signs that persist or fail to respond to therapy. Recurrent herpetic conjunctivitis is often unilateral, characterised by intermittent blepharospasm, mild conjunctival hyperaemia and serous discharge, without signs of respiratory infection.

Decreased tear production (Schirmer tear test <5 mm/min) can occur, attributed to occlusion of lacrimal ductules or lacrimal adenitis secondary to FHV-1. Response to ciclosporin is limited. Because Schirmer tear test values can decrease substantially in anxious cats, a single low reading in the absence of other clinical signs may be insignificant.

Chlamydophila felis

In contrast to herpes-infected cats, patients with acute Chlamydophila rarely demonstrate signs of upper respiratory disease beyond a mild rhinitis. Conjunctivitis is initially unilateral, with marked conjunctival oedema (chemosis) and serous to mucopurulent discharge. Second eye involvement occurs 5-7 days later. Conjunctival follicles are described but can occur in any chronic conjunctivitis. Chronic chlamydial conjunctivitis may be unilateral or bilateral, with mild blepharospasm, serous discharge and mild hyperaemia.

Other Infectious Agents

Although Mycoplasma spp. have been isolated from the cul-de-sacs of cats with chronic conjunctivitis, the organisms are also prevalent in the conjunctival sacs of normal animals. This ubiquitous nature, coupled with the difficulty of experimentally establishing mycoplasmal conjunctivitis in the absence of other organisms, suggests that Mycoplasma is significant only as a secondary pathogen. Calicivirus and reovirus are insignificant ocular pathogens. Histoplasma capsulatum has been identified in conjunctival nodules on rare occasion. Recent reports have implicated Bartonella spp. in feline conjunctivitis.

Diagnostics

Since the initial herpetic conjunctivitis has a short clinical course and acute Chlamydophila conjunctivitis responds readily to antibacterial therapy, diagnostic tests are not often performed in the acute presentation. Instead diagnosis is based on clinical signs and response to therapy. Basophilic intracytoplasmic inclusions are only seen in conjunctival scrapings taken during the first few days of an acute chlamydial infection. Herpesvirus intranuclear inclusions are not identifiable in routinely prepared conjunctival scrapings.

Determination of the aetiological agent in chronic or recurrent conjunctivitis in the adult cat is especially difficult. Multiple studies have concluded that an aetiological diagnosis cannot be made in the majority of chronically infected cats using aerobic bacterial and chlamydial cultures, cytology, virus isolation, serum neutralisation, immunofluorescent antibody (IFA), or enzyme-linked immunosorbent assay (ELISA). Although polymerase chain reaction (PCR) technology is vastly superior at detecting virus compared to other methods, the test's sensitivity actually complicates interpretation. A positive PCR result may reflect viral DNA of vaccine origin rather than wild type virus, as well as low levels of latent or persistent FHV-1 DNA that are not contributing to disease.

A clinical feature that clearly distinguishes herpesvirus from Chlamydophila is the presence of corneal ulceration. Linear, branching epithelial erosions termed dendrites are pathognomonic for herpetic keratitis, but these early indicators of viral replication are usually subtle and may be overlooked without use of Rose Bengal stain. Patients should be routinely screened for feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV) which occur in a higher percentage of cats with chronic conjunctivitis compared to the general population.

Therapy: Acute Conjunctivitis

The usual approach to acute feline conjunctivitis of suspected infectious aetiology is to 'treat the treatable'. Symptomatic ocular therapy is directed against Chlamydophila, coupled with supportive systemic care. Since the primary viral infection is self-limiting in 10-14 days, antiviral therapy is rarely used during the initial infection except in patients with corneal ulceration. There is no known way to prevent viral latency, but dietary supplementation with L-lysine (in kittens, 250 mg twice daily in food) may limit frequency and severity of recurrences. Chlamydophila is resistant to many common topical antibiotics including bacitracin, neomycin and gentamicin. The treatment of choice is topical tetracycline applied four times daily for 3 weeks, but because the ointment is often quite irritating, suitable alternatives include erythromycin, chloramphenicol and the fluoroquinolones such as ofloxacin. Systemic therapy may also be considered. Although tetracycline would be an excellent choice with respect to sensitivity of the organism, the risk of dental staining precludes its use in young animals. Doxycycline is not as likely to discolour enamel in the short term, but the possibility still exists in the 3-week regimen recommended to discourage the carrier state (5 mg/kg q12h for 21 days). Azithromycin has the advantage of a longer half-life and less frequent administration but is more expensive. The usual dosage is 5 mg/kg for 5 days, then 5 mg/kg every 72 hours for five to seven additional doses. The recommended dosage of azithromycin for Bartonella-associated conjunctivitis is 10 mg/kg daily for 6 weeks.

Therapy: Chronic Conjunctivitis

Therapy for the adult cat with recurring bouts of conjunctivitis is first directed against Chlamydophila, as described for the acute episode. Failure to respond to a 3-week course of therapy implicates herpesvirus as the inciting cause and subsequent treatment is based on the severity, duration and frequency of the outbreaks.

In mild cases of presumed herpetic conjunctivitis, oral L-lysine supplementation limits viral replication by suppressing arginine incorporation into viral proteins. The recommended lifelong dose in adult patients is 500 mg twice daily. The product is available as a powder, paste or over-the-counter capsule that can be opened and mixed with moist food. Topical interferon alpha-2b (3000 units/ml) applied three times daily may limit spread of the virus to adjacent cells and potentially shorten the course of the disease. A topical antibiotic ointment such as erythromycin or a viscous artificial tear supplement may provide subjective relief of discomfort. Topically applied 0.5-1% povidone- iodine solution has been used three to four times daily for its antiviral effect, but its extracellular site of action may limit efficacy.

Antivirals should be considered when clinical signs are severe, persistent or recurrent, or when there is notable corneal involvement. The in vitro potency of specific antiviral drugs against FHV-1 is: trifluridine > ganciclovir = idoxuridine = cidofovir = penciclovir = vidarabine > acyclovir >> foscarnet. Idoxuridine is a less irritating, more economical antiviral than trifluridine and can be compounded as a 0.1% solution or 0.5% ointment. Topical antivirals are virostatic, requiring frequent application to be effective (every 2 hours the first day, then five to six times daily.) The exception is 0.5% cidofovir applied twice daily. Topical antivirals should not be applied for more than 3 weeks at a time due to epithelial cell toxicity. Rather than topical antivirals, the author prefers to use oral famciclovir, dosed at 32 mg (1/4of a 125 mg tablet) q12h for 10-14 days.

Eosinophilic Conjunctivitis

Eosinophilic conjunctivitis is a presumably immune-mediated disorder characterised by unilateral or bilateral conjunctival hyperaemia, thickening and discharge. A nodular variation occurs in the third eyelid. Both herpesvirus-1 and Chlamydophila have been implicated as a cause or secondary opportunist. Diagnosis is based on conjunctival scrapings containing eosinophils, with occasional mast cells, lymphocytes and plasma cells. Treatment consists of a topical corticosteroid such as 1% prednisolone acetate two to four times daily, tapering the frequency of therapy as clinical signs resolve. Signs may recur if treatment is discontinued.

Keratoconjunctivitis Sicca

Keratoconjunctivitis sicca (KCS) is rare in cats. Decreased tear production occurs occasionally in patients with chronic infectious conjunctivitis or keratitis. Schirmer tear test values are consistently less than 5 mm/min. FHV-1 is usually implicated in the pathogenesis of the problem owing to secondary ductal occlusion or lacrimal adenitis. Because the Schirmer tear test can decrease substantially in anxious cats, a single low reading may be insignificant. Topical ciclosporin is rarely effective. Symptomatic therapy with topical artificial tear supplements is recommended.

Lipogranulomatous Conjunctivitis

Older cats (mean age 11 years) demonstrate an inflammatory reaction to lipids, presumably of meibomian gland origin, causing multifocal palpebral conjunctival nodules adjacent to the eyelid margins. Biopsy specimens demonstrate accumulations of lipid surrounded by macrophages, giant cells, lymphocytes and plasma cells. Surgical excision of the abnormal tissue is reportedly curative.