Antiviral therapy in cats

While there are an expanding number of antiviral therapies for humans, care should be taken when extrapolating use of these drugs in cats.

Most antiviral agents are virostatic (i.e. stop the virus from replicating) and therefore must be used frequently.

Topical antiviral agents

1. Trifluridine - (1% solution) - e.g. Viroptic - superior potency and corneal penetration but cats commonly hate the drug being applied, presumably because it  is irritating. No longer available in Australia

2. Idoxuridine - high clinical efficacy, cheaper and is less irritant than trifluridine. It is no longer available commercially but can be made by compounding pharmacists as a 0.1% ophthalmic solution or 0.5% ointment.

3. Vidarabine - available as a 3 % ointment. May be useful when keratoconjunctivitis sicca (dry eye) is present. Most cats tolerate this drug well. Not readily available in Australia

4. Acyclovir (Zovirax) 3% ophthalmic ointment - generally the response is not as good as seen in humans. Treatment needs to be started early in the course of the disease before cytolytic effects in the cornea become severe. Used for ocular lesions or for cold sores on skin lesions. Acyclovir should not be used as a systemic antiviral agent in cats. It has poor efficacy against FHV-1 and its por bioavailability limits its use in cats. The systemic dose required to be effective is toxic in cats. Valacyclovir is an acyclovir prodrug that has superior bioavailability in cats but does not reduce viral replication and causes fatal bone marrow suppression, liver and kidney damage and therefore should never be used in cats.