Famciclovir is an oral antiviral medication used in cats for the treatment of feline herpesvirus type 1 (FHV-1) infections. FHV-1 is one of the most common infectious causes of upper respiratory disease, conjunctivitis, and corneal ulceration in cats worldwide. Famciclovir is a prodrug that is converted to its active form, penciclovir, which inhibits viral DNA replication and helps reduce the severity and duration of herpesvirus flare-ups.
Overview
Feline herpesvirus type 1 infects the vast majority of cats at some point in their lives. After initial infection, the virus establishes lifelong latency in the trigeminal ganglia and can reactivate during periods of stress, immunosuppression, or concurrent illness. Clinical signs range from mild sneezing and nasal discharge to severe ulcerative keratitis, corneal sequestra, and herpetic dermatitis. While antiviral treatment cannot eliminate the latent virus, famciclovir can significantly reduce viral shedding, improve clinical signs, and shorten the duration of active disease episodes.
Famciclovir is favored over other antivirals in cats because it has good oral bioavailability, achieves therapeutic penciclovir levels in feline tissues, and is well tolerated at the doses used. Older antivirals such as acyclovir are poorly bioavailable and potentially toxic in cats, making famciclovir the preferred systemic antiviral for FHV-1.
Uses
- Acute FHV-1 upper respiratory infection: Reduces severity and duration of sneezing, nasal discharge, and conjunctivitis.
- Herpetic keratitis: Treatment of corneal ulcers and conjunctivitis caused by FHV-1, often alongside topical ophthalmic antivirals.
- Herpetic dermatitis: FHV-1 can cause facial or nasal skin ulceration in some cats.
- Chronic or recurrent FHV-1 flare-ups: Used during recurrent episodes or as short-term prophylaxis during high-stress periods (boarding, introduction of new cats).
- Severe stomatitis associated with FHV-1: Occasionally used as part of multimodal treatment.
Dosage
| Parameter | Detail |
|---|---|
| Dose (cats) | 40–90 mg/kg orally every 8–12 hours |
| Common practical dose | 125–250 mg per cat every 12 hours; 250 mg per cat every 8 hours for severe cases |
| Route | Oral |
| Frequency | Every 8–12 hours |
| Duration | 7–21 days for acute flare-ups; longer courses may be needed for severe cases |
Important dosing notes:
- Famciclovir tablets (125 mg, 250 mg, 500 mg) are human formulations. Your veterinarian may prescribe quartered tablets or compounded formulations.
- Higher doses (90 mg/kg every 8 hours) are often recommended for severe infections based on pharmacokinetic studies in cats.
- Do not crush enteric-coated tablets; compounded formulations should be used if the cat cannot swallow whole or quartered tablets.
- Treatment should be started as early as possible during a flare-up for best results.
- Famciclovir does not eliminate latent virus; recurrences are possible.
Side Effects
| Common Side Effects | Serious Side Effects (Contact Your Vet) |
|---|---|
| Generally very well tolerated | Significant GI upset (persistent vomiting, anorexia) |
| Occasional mild GI upset (soft stool) | Signs of allergic reaction (facial swelling, hives) |
| Mild decrease in appetite at high doses | Marked lethargy or behavioral changes |
Famciclovir has an excellent safety profile in cats. Side effects are uncommon, even at higher doses.
Drug Interactions
- Other nephrotoxic drugs: Penciclovir (the active metabolite) is renally excreted. Use with caution alongside other drugs that may impair renal function.
- Probenecid: May increase penciclovir levels by reducing renal tubular secretion.
- No significant interactions with most commonly used feline medications have been documented. However, always inform your veterinarian of all medications your cat receives.
Contraindications
- Do not use in cats with known hypersensitivity to famciclovir or penciclovir.
- Use with caution in cats with renal impairment, as the active metabolite is renally excreted. Dose adjustment may be needed.
- Safety in pregnant or lactating cats has not been established, though the drug is generally considered to have low teratogenic risk.
- Do not use acyclovir as a substitute for famciclovir in cats, as acyclovir has poor oral bioavailability and can cause bone marrow suppression in cats.
Storage
- Store at room temperature (59–86 degrees F / 15–30 degrees C).
- Keep in original packaging or tightly closed container.
- Compounded formulations may require refrigeration; follow your pharmacy’s directions.
- Keep out of reach of children and pets.
FAQ
Q: Will famciclovir cure my cat’s herpesvirus? A: No. FHV-1 establishes lifelong latent infection. Famciclovir reduces viral replication during active flare-ups and improves clinical signs, but it does not eliminate the latent virus. Recurrences are possible, especially during stressful periods.
Q: Can I use human acyclovir or valacyclovir instead? A: No. Acyclovir has very poor oral bioavailability in cats and is less effective. Valacyclovir (Valtrex) is toxic to cats and can cause severe, life-threatening bone marrow suppression, hepatic necrosis, and renal failure. Never give valacyclovir to a cat. Famciclovir is the only safe and effective oral antiviral for systemic FHV-1 treatment in cats.
Q: How long does it take for famciclovir to work? A: Many cats show improvement in clinical signs within 2 to 5 days of starting treatment. More severe infections may take longer to respond. If no improvement is seen after 7 to 10 days, consult your veterinarian.
Q: Should I also use L-lysine for my cat’s herpesvirus? A: L-lysine supplementation was previously widely recommended for FHV-1, but recent studies have failed to demonstrate a clear clinical benefit, and some evidence suggests it may not be effective. Discuss evidence-based treatment options with your veterinarian.
This content is for informational purposes only and is not a substitute for professional veterinary advice. Always consult your veterinarian before starting, changing, or stopping any medication for your cat.