Antiviral Medications in Cats

Feline Herpesvirus-1

Feline herpesvirus-1 (FHV-1) is the causative agent of feline viral rhinotracheitis (FVR), a common cause of upper respiratory tract disease, and considered the most common cause of feline ocular disease^1,2; it is highly contagious. Initial infection in kittens leads to severe upper respiratory and ocular disease, often with fever, sneezing, and purulent nasal and ocular discharge. After primary infection, the virus becomes dormant in the trigeminal ganglion, and cats become latent carriers. Stressful events, such as concurrent illness, travel, introduction of a new pet, or immunosuppressive therapy (eg, steroids, chemotherapy), can cause recrudescence of the virus and clinical flare-ups.

Figure 1. Conjunctivitis in a 6-week-old kitten, with adhesions of the conjunctiva caused by fibrinous membranes

Clinical Signs

Conjunctivitis is the most common ocular manifestation of FHV-1.^1,2 The inflammation may be severe in kittens, causing formation of fibrinous membranes and adhesions (eg, symblepharon; Figure 1). In adult cats, the disease is often more mild and unilateral.

Figure 2. Axial, malacic, deep stromal corneal ulcer in an adult cat

Corneal ulceration (Figure 2) is the second most common ocular manifestation of FHV-1.^1,2 Early on, it is confined to the epithelial cell layer, but secondary bacterial infection leads to increasing ulcer depth and stromal malacia (ie, corneal melt).

Other clinical signs include corneal sequestrum (Figure 3) and eosinophilic keratoconjunctivitis^1,2 (Figure 4).

Figure 3. Large axial corneal sequestrum with a rim of nonadherent corneal epithelium in an adult cat

Antiviral Therapy

Several antiviral medications are available (Table), and although none has been specifically developed for cats, most have been tested for safety and efficacy. Antiviral potency is reported as the IC50 (ie, concentration of a drug that suppresses viral replication by 50%). Of importance, all antivirals are virostatic and thus require frequent administration and rely on the host’s immune system to effectively clear the virus.<sup3  sup>  

Kittens with upper respiratory disease benefit greatly from systemic and/or topical antiviral therapy. Supportive care with supplemental fluids and nutrition, as well as antibiotics to address secondary bacterial infections, may be necessary.<sup2 sup> 

Figure 4. Eosinophilic keratoconjunctivitis in an adult cat. Note the pink vascularized tissue with multifocal white foci encroaching from the lateral limbus.

Antivirals are the most appropriate medication for adult cats with recrudescent disease. In addition, all cats with moderate-to-severe conjunctivitis or with corneal ulcerations should be treated with antivirals. The less common clinical syndromes of corneal sequestrum or eosinophilic keratoconjunctivitis may also benefit from antiviral therapy.^1,2

Trifluridine 1%, the only commercial topical antiviral available for cats, is very effective against FHV-1; however, it must be administered 5 to 6 times per day, the drops may cause a stinging sensation, and many cats appear to be bothered by the topical application.³  

Idoxuridine was previously available in the United States but now must be compounded. A 0.1% solution or 0.5% ointment is recommended. Although this drug also requires frequent application (ie, 5–6 times per day), it is well tolerated by most cats.

Cidofovir is a relatively new topical antiviral that is compounded into a 0.5% solution. Recent studies show that cidofovir administered q12h is associated with reduced viral shedding and lower clinical disease scores.⁴ The decreased dosing frequency is attributed to the long half-life of its active metabolites.

Famciclovir is an oral antiviral shown to be quite efficacious for FHV-1.^6,9,10 It is available in 125- and 250-mg tablets and may be compounded into smaller tablets or a liquid formulation for kittens. Numerous studies have attempted to elucidate the appropriate dose and treatment frequency.^5-9 The metabolism and pharmacokinetics appear to be very complex, with the true antiviral activity attributed to penciclovir, a metabolite of famciclovir. Experimentally, 90 mg/kg PO q8h produces appropriate plasma concentrations. However, anecdotally, significantly lower doses (62.5 mg per cat q24h to 40 mg/kg q8–24h for 21 days) are associated with clinical improvement. In early studies, client-owned cats demonstrated clinical improvement with dosages as low as 8 mg/kg q24h.<sup10 sup> 

Table. Available Antiviral Medications, Their Potency, and Dose Recommendations^1-3

*Reported IC50 is for penciclovir, the active metabolite of famciclovir