Feline conjunctivitis. A cat is not a small dog!

Conjunctivitis is a common presenting complaint in companion animals. Yet although the clinical signs of the disease are generally similar in dogs and cats, there are significant differences in its etiology between the 2 species, as implied by the title of this review. In general, primary (and particularly primary bacterial) infections of the canine conjunctiva are quite rare, and most cases of conjunctivitis in dogs are due to some other disease or disorder which triggers inflammation and may facilitate secondary, opportunistic bacterial infection. In cats, on the other hand, nearly all cases of conjunctivitis are due to infection by a primary pathogen, most notably feline herpesvirus (FHV-1). Consequently, the clinical approach, diagnostic workup, and treatment of canine and feline conjunctivitis differ significantly.

Clinical signs of conjunctivitis

Though conjunctivitis is not painful, it may cause a patient discomfort. Clinical signs of conjunctivitis are mostly nonspecific and are similar in cats and dogs regardless of the cause. They include the following:

• Hyperemia of conjunctival vessels—“red eye” (Figure 1)
• Conjunctival edema or chemosis, as well as swelling and thickening (Figure 2)
• Ocular discharge, which may be serous, mucoid, or purulent (Figures 1, 2)
• Hypertrophy of lymphatic follicles, particularly on the inner aspect of the third eyelid; probably more common in dogs than in cats (Figure 3)
• Conjunctival ulceration and subconjunctival hemorrhage, which is sometimes seen
• Concurrent involvement of the cornea, which is not unusual; many patients may present with keratoconjunctivitis rather than conjunctivitis (Figure 4)

Figure 1.

Figure 2.

Figure 3.

Figure 4A.

Figure 4B.

Causes of feline conjunctivitis

FHV-1 is undoubtedly the most common cause of feline conjunctivitis. Findings from studies suggest that 95% of cats worldwide have been exposed to the virus, and at least 80% of cats are latent carriers of the virus. Chlamydophila felis is another common cause of feline conjunctivitis, especially in young kittens (and clinicians should remember its zoonotic potential). Other infectious agents, including calicivirus, Mycoplasma species and Bordetella bronchiseptica, are occasionally mentioned as potential causes, but their clinical significance is questionable, or they cause a transient and self-limiting disease.

Primary herpetic infection commonly occurs in kittens. It is caused by exposure to wild type FHV-1, which is transmitted between cats by microdroplets (frequently from the dam) or fomites (possibly by the owner). The disease is characterized by conjunctivitis, respiratory tract signs (fever, sneezing, purulent nasal discharge), and occasionally corneal ulcers. Following primary, and usually self-limiting, disease, FHV-1 establishes itself permanently in the trigeminal ganglia. The virus is cleared only in a limited number of cats. Stress, or treatment with steroids, will induce subsequent reactivation and shedding of the virus, resulting in recurring, recrudescent disease. The recurrent disease is usually milder than the primary disease, and can affect the cornea, conjunctiva, and respiratory system.

Diagnosis of feline conjunctivitis

Various diagnostic tests are available, including the immunofluorescent antibody test, serology, and viral isolation for the diagnosis of FHV-1; culture and sensitivity for C felis; or polymerase chain reaction (PCR) testing for either agent. However, due to subclinical shedding of FHV-1 by normal animals and reduced shedding in recrudescent stages of the disease, as well as a high prevalence of antibodies due to vaccination and exposure, these tests suffer from high percentages of false negatives and false positives. Likewise, conjunctival scrapes to demonstrate intracytoplasmic elementary bodies of C felis are rarely diagnostic. Because it is difficult to reach a definitive diagnosis using the aforementioned tests, most clinicians base their diagnosis on patient signalment, history of exposure, and clinical signs.

FHV-1 is characterized by greater conjunctival hyperemia and amounts of ocular discharge than C felis (Figure 1). Also, the signs of respiratory disease are usually more severe in FHV-1, especially during the primary disease. Recurrent disease, keratitis, and corneal and conjunctival ulceration are definitive hallmarks of FHV-1 infection (Figure 4). Consequently, the presence of symblepharon, or adhesions between the cornea and conjunctiva, indicates previous FHV-1 disease (Figure 5). On the other hand, the chemosis is much more severe in C felis cases and is the most notable clinical sign (Figure 2). Although the C felis is rather mild and may resolve, chlamydial conjunctivitis can be very persistent, especially if untreated.

Figure 5.

Alternatively, clinicians will base their diagnosis on response to treatment, assuming that a certain degree of FHV-1 involvement must always be suspected in every case of feline conjunctivitis and adding treatment for C felis when it is suspected. The rationale for this approach is that it is better to invest the owner’s resources in treatment (as antiviral drugs are costly) rather than in inconclusive laboratory testing.

Considerations in treatment of feline conjunctivitis

Client education is critical in patient management. The owners must understand that none of the antiviral drugs are virucidal. Few cats will clear the virus and in most patients it will remain latent. Therefore, even after clinical signs have resolved, recurrence of disease is quite possible, especially in stressful situations (eg, in multicat households, due to travel, etc). Indeed, it is impossible to overstate the importance of reducing stress in the treatment of FHV-1. This may be an important consideration for clinicians when deciding on therapy, as some of the topical antiviral drugs have to be administered very frequently. In mild cases, it is possible that no treatment is preferable to medical therapy if the latter involves the stress of frequent treatment. Likewise, whenever possible, feline conjunctivitis cases should not be hospitalized but rather sent home where owners should provide a nurturing, and less stressful, environment.

For similar reasons, clinicians are urged to carefully weigh any use of topical glucocorticoids in cats, as the drug may also induce viral shedding. In cases where such treatment is unavoidable (eg, eosinophilic keratitis patients), concurrent antiviral treatment should be provided and the patient closely monitored for recrudescent disease. Likewise, because FHV-1 may be reactivated due to immunosuppression, the prognosis is poor in immunosuppressed patients (ie, Feline Leukemia Virus or Feline Immunodeficiency Virus-infected) because the recrudescence rate can be high.

Finally, beware that many of the commercially available antiviral drugs are effective against human herpes simplex virus but are ineffective against FHV-1, or may even be toxic in cats. Don’t be tempted to use an antiviral drug simply because it is commercially available.

Medical treatment of feline conjunctivitis

Cases of C felis conjunctivitis are treated with topical 1% tetracycline ointment, 3 to 4 times daily for 1 to 2 weeks. Findings from recent studies suggest that this treatment be supplemented, or even replaced, with oral doxycycline (10 mg/kg once daily for 3 weeks). Systemic treatment is definitely indicated in cases of systemic, respiratory disease. A rapid resolution of signs would suggest that the patient had indeed suffered from C felis. The treatment is also useful in preventing secondary bacterial infection in FHV-1 patients.

Antiviral therapy

Older texts mention oral Lysine and topical interferon as potential antiviral therapies. However, although lysine may reduce shedding in carriers and improve clinical signs in primary infections, clinical studies of the efficacy of both of these preparations are lacking.

More conventional topical antiviral solutions include trifluridine 1%, idoxuridine 0.1%, and vidarabine 3%, which are variably effective against FHV-1. Trifluridine has the highest efficacy as well as transcorneal penetration but may be more irritating to cats. Idoxuridine and vidarabine are less irritating but are difficult to obtain because they are not widely available commercially, although they may be ordered from compounding pharmacies. However, all these drugs have to be administered 5 to 6 times a day, with treatment continuing for 10 to 14 days after resolution of signs, and thus may exacerbate the disease due to stress associated with frequent treatment.

These drugs are therefore being replaced by very efficacious drugs that are administered only twice daily. Cidofovir 0.5% solution, which is not commercially available as an ophthalmic preparation, has strong in vitro and in vivo efficacy against FHV-1 infection, with treatment reducing the severity of clinical signs and viral shedding while being less toxic than other antivirals. Famciclovir, a prodrug of penciclovir, is a very safe and effective oral drug for the treatment of FHV-1. The recommended dose is 90 mg/kg twice daily.

As FHV-1 reduces the number of conjunctival goblet cells consequently causing qualitative tear film disorders, all patients greatly benefit from the frequent application of high-quality artificial tear (hyaluronate) preparations. Finally, as all treatment is associated with potential stress, you should always ask yourself whether the severity of the disease justifies treatment. Indeed, in very mild cases of herpetic disease, no treatment may be the best treatment of them all.

Figure legends

1. Conjunctival hyperemia, chemosis, and purulent secretions in a cat with feline herpesvirus conjunctivitis. Courtesy of L. Sebbag.
2. Conjunctival hyperemia, chemosis, and purulent secretions in a cat with Chlamydophila felis conjunctivitis.
3. Hypertrophy of lymphatic follicles in a cat with herpetic conjunctivitis.
4. Corneal ulceration confirms the diagnosis of feline herpesvirus, as Chlamydophila felis does not cause corneal disease. (A) A superficial geographical ulcer stained by fluorescein. (B) A Rose Bengal positive stain.
5. Symblepharon, or adhesion between the corneal and conjunctiva, is a common complication of herpetic keratoconjunctivitis. Courtesy of L. Sebbag.

Ron Ofri, DVM, PhD, DECVO, is a professor of comparative ophthalmology at the Koret School of Veterinary Medicine at Hebrew University of Jerusalem in Rehovot, Israel.