Feline Herpesvirus (FHV-1) Keratitis: Antivirals and Steroid Caution

Bottom line

Feline herpesvirus-1 (FHV-1) is the most common infectious cause of feline ocular surface disease, producing conjunctivitis, branching dendritic epithelial ulcers, stromal keratitis, eosinophilic keratitis, and, in neonates, symblepharon. The single most important management rule is to withhold corticosteroids in any fluorescein-positive herpetic keratitis and treat with antivirals instead: oral famciclovir (~90 mg/kg PO BID–TID) and/or topical cidofovir 0.5% BID [1][3][4]. The evidence base also shows L-lysine supplementation is ineffective, and that even the attenuated modified-live vaccine (MLV) strain can replicate in the corneal epithelium and produce a dendritic ulcer [2][5]. Diagnosis is clinical, supported by history; PCR confirms exposure but rarely confirms causation.

Condition facts

Definition. FHV-1 (an alphaherpesvirus) causes acute upper respiratory and ocular disease and establishes lifelong latency, chiefly in the trigeminal ganglion. More than 80% of exposed cats become persistently infected, and reactivation drives recurrent conjunctivitis and keratitis [1][2].

Signalment. Any cat; disease is over-represented in kittens (primary infection), multi-cat and shelter environments, brachycephalic breeds, and immunocompromised or stressed individuals. Recrudescence is provoked by stressors — rehoming, concurrent illness, parturition, and systemic corticosteroids [1].

Pathophysiology. Primary infection targets conjunctival and corneal epithelium. Cytolytic replication produces punctate then dendritic (branching) epithelial ulcers — the lesion considered pathognomonic for FHV-1 [1]. After primary infection the virus travels retrograde to the trigeminal ganglion and becomes latent; reactivation returns virus to the ocular surface. Immune-mediated sequelae — stromal keratitis, eosinophilic keratitis, and corneal sequestrum — reflect the host response as much as active viral cytolysis, which is why immunomodulation is tempting yet hazardous when the epithelium is ulcerated [1].

Clinical spectrum.

• Conjunctivitis — often the sole sign; chemosis, hyperemia, blepharospasm, serous-to-mucopurulent discharge.
• Dendritic ulcerative keratitis — linear branching fluorescein-retaining defects; pathognomonic and the clearest indication for antivirals [1].
• Geographic ulcers — coalesced dendrites forming larger irregular epithelial defects.
• Stromal keratitis — deeper, immune-mediated corneal inflammation with vascularization; the highest-risk setting for a misdirected "steroid trial" [1].
• Eosinophilic (proliferative) keratitis — raised, pink-white, cottage-cheese plaques with peripheral vascularization; FHV-1 DNA is recovered from a majority of cases [8].
• Symblepharon — adhesion of conjunctiva to conjunctiva or cornea, typically a neonatal sequela of severe ulcerative disease.
• Corneal sequestrum — a brown-black necrotic stromal plaque, associated with chronic FHV-1 irritation and provoked by topical corticosteroids [1].

Diagnosis

Diagnose clinically; use PCR to support, not to crown. A branching dendritic ulcer on fluorescein (or rose bengal, which highlights devitalized epithelium and early dendrites better) in a cat with a compatible history is enough to commit to antiviral therapy [1]. Laboratory confirmation is imperfect because latency is near-universal: quantitative real-time PCR will detect FHV-1 DNA in the ocular fluid of many clinically normal carriers, and a positive qualitative result can reflect low-level shedding rather than the cause of the presenting signs [6]. Reported detection rates in clinically suspect cats vary widely across laboratories, so a negative PCR does not exclude FHV-1 and a positive PCR does not prove it is the culprit [6].

Practical points: sample before instilling stains or topical anesthetics; recent MLV vaccination can yield a transiently PCR-positive result; and conjunctival cytology showing eosinophils supports eosinophilic keratitis. Reserve PCR for atypical, refractory, or research contexts rather than routine dendritic disease.

Antiviral treatment and efficacy

For fluorescein-positive herpetic keratitis, antivirals are the appropriate, evidence-supported alternative to steroids [3].

Oral famciclovir is the workhorse. It is the prodrug of penciclovir; feline pharmacokinetics are nonlinear with wide inter-individual variability, which is why doses are high. Approximately 90 mg/kg PO every 8–12 hours achieves therapeutic penciclovir concentrations and reduces clinical signs and viral shedding [3]. In a 59-case clinical series, famciclovir was associated with clinical improvement across ocular, respiratory, and dermatologic FHV-1 disease and was well tolerated, with the caveat that compounded formulations vary in potency — prescribe reputable products and monitor response [4]. Continue for at least 1 week beyond resolution of clinical signs.

Topical antivirals are virostatic and require frequent dosing:

• Cidofovir 0.5% — twice daily; a masked, placebo-controlled study showed reduced viral shedding and clinical disease, and its long tissue half-life allows the least frequent dosing of the topical agents. Monitor for nasolacrimal stenosis, a recognized adverse effect [3].
• Idoxuridine 0.1% — compounded; typically every 4–6 hours (up to 5–6× daily).
• Trifluridine 1% — every 4–6 hours; effective but often stings and is poorly tolerated by cats.

Topical agents suit surface epithelial disease; oral famciclovir is preferred when there is concurrent respiratory or dermatologic involvement, poor topical compliance, or deeper disease.

The L-lysine question

L-lysine is not recommended. A systematic review of in-vitro, laboratory, and clinical studies concluded lysine supplementation does not prevent FHV-1 infection, does not reduce the risk of clinical signs, and does not improve the course of disease — with some studies reporting worse outcomes in supplemented cats [5]. The mechanistic rationale (lysine–arginine antagonism suppressing viral replication) does not hold in cats: dietary lysine does not lower feline plasma arginine, and lowering arginine would be dangerous because cats cannot synthesize it and become hyperammonemic when deficient [5]. The evidence-based position is to stop recommending lysine and redirect owners to antivirals and stress reduction [5].

The corticosteroid caution

Do not use corticosteroids — topical or systemic — in fluorescein-positive herpetic keratitis. Corticosteroids are a recognized trigger for FHV-1 recrudescence: roughly 70% of latently infected cats shed virus after corticosteroid administration [1]. In experimentally infected cats, topical corticosteroids have induced corneal sequestrum formation and worsened stromal disease, and in chronic stromal keratitis they "run the very real risk of inducing viral recrudescence" [1]. The clinical trap is the "red, sore eye" reflex to reach for a topical steroid — in a herpetic cornea this is exactly wrong. Confirm the fluorescein status of the cornea before contemplating any anti-inflammatory, and treat inflammation, when unavoidable, under ophthalmology guidance with antiviral cover.

Vaccine-strain caveat. Vaccine strains are attenuated, not inert. One whole-genome-sequenced isolate has confirmed the F2 MLV vaccine strain of FHV-1 recovered from a dendritic corneal ulcer in a cat vaccinated 17 days earlier, distinguished from field strains by single-nucleotide variants in ORF28 and ORF44 [2]. This is a single, mechanistic proof-of-concept — the vaccine strain can replicate in the corneal epithelium — not a population risk estimate, and it does not argue against MLV vaccination. Three of the four cats in that report had a prior corticosteroid history. The practical takeaways: take a recent-vaccination history in a cat with a new dendritic ulcer, and avoid corticosteroids in the window immediately after a first MLV vaccination in a cat without prior allergy [2].

Immune-mediated and structural sequelae

Eosinophilic keratitis is a paradox: FHV-1 DNA is detected in the majority of cases (reported in the range of ~55–76% of corneal samples), yet the disease is inflammatory and typically responds to immunomodulation — but only under antiviral cover [8]. Topical 1.5% ciclosporin resolved or controlled disease in the large majority of a 35-case series [7]. Many ophthalmologists initiate or continue oral famciclovir alongside immunomodulation, and reserve topical corticosteroids for refractory cases with an intact epithelium, never over an ulcer [6][7].

Symblepharon management is largely preventive — aggressive early treatment of neonatal ulcerative disease and lubrication limit adhesion formation; established adhesions often require surgical division, though recurrence is common. Corneal sequestrum frequently requires keratectomy; avoid provoking topical steroids [1].

Prognosis and monitoring

Most acute herpetic keratitis resolves with appropriate antiviral therapy and stress reduction, but the underlying latency means recrudescence is expected over a cat's lifetime — owners should be counseled that FHV-1 is managed, not cured [1]. Monitor with serial fluorescein staining to confirm epithelial healing before considering any anti-inflammatory, watch cats on topical cidofovir for nasolacrimal signs, and reassess non-responders for eosinophilic keratitis (cytology), sequestrum, or a misdiagnosed non-herpetic ulcer [1][3]. Minimizing environmental stress is a genuine therapeutic lever, not an afterthought [1].

Frequently Asked Questions

What is the treatment of choice for feline FHV-1 dendritic keratitis?

Antiviral therapy, not corticosteroids. Oral famciclovir at approximately 90 mg/kg PO every 8-12 hours achieves therapeutic penciclovir levels and reduces clinical signs and shedding, and topical cidofovir 0.5% twice daily reduces shedding and disease in placebo-controlled study. Corticosteroids are contraindicated in any fluorescein-positive herpetic cornea [1][3][4].

Why are corticosteroids contraindicated in FHV-1 keratitis?

Corticosteroids reactivate latent FHV-1: roughly 70% of latently infected cats shed virus after corticosteroid administration, and topical steroids have induced corneal sequestrum and worsened stromal disease in experimentally infected cats. A steroid trial for an unexplained feline red eye is a recognized trap [1].

Does L-lysine work for feline herpesvirus?

No. A systematic review found lysine supplementation does not prevent infection, does not reduce the chance of clinical signs, and does not improve disease course, with some studies showing worse outcomes. The lysine-arginine antagonism rationale does not hold in cats, and lowering arginine risks fatal hyperammonemia. The recommendation is to stop lysine supplementation [5].

How is FHV-1 ocular disease diagnosed?

Diagnosis is primarily clinical, based on a branching dendritic fluorescein-positive ulcer and compatible history. PCR supports but does not confirm causation because latency is near-universal: quantitative PCR detects FHV-1 DNA in many clinically normal carriers, and reported detection rates in suspect cats vary widely across laboratories. A negative PCR does not exclude FHV-1 and a positive result does not prove it is the cause [6].

What is the famciclovir dose for cats with FHV-1?

Approximately 90 mg/kg PO every 8-12 hours. Feline penciclovir pharmacokinetics are nonlinear with wide inter-individual variability, which is why the dose is high. It is generally well tolerated across ocular, respiratory, and dermatologic FHV-1 disease; use reputable formulations because compounded product potency varies, and continue for at least a week past resolution of signs [3][4].

Can the modified-live FHV-1 vaccine strain cause a corneal ulcer?

Yes, rarely. A whole-genome-sequenced isolate has confirmed the F2 modified-live vaccine strain recovered from a dendritic corneal ulcer in a cat vaccinated 17 days earlier, distinguished from field strains by ORF28 and ORF44 variants. This is a single mechanistic proof-of-concept, not a population risk estimate, and does not argue against MLV vaccination — but it supports taking a recent-vaccination history and avoiding steroids in that window [2].

How is feline eosinophilic keratitis related to FHV-1 and how is it treated?

FHV-1 DNA is detected in the majority of eosinophilic keratitis cases, yet the disease responds to immunomodulation under antiviral cover. Topical 1.5% ciclosporin controlled disease in the large majority of a 35-case series, and many clinicians continue oral famciclovir alongside it. Topical corticosteroids are reserved for refractory cases with an intact epithelium and never used over an ulcer [6][7].

Which topical antiviral is best for feline herpetic keratitis?

Topical cidofovir 0.5% twice daily is often preferred because its long tissue half-life allows the least frequent dosing while reducing viral shedding and clinical disease; monitor for nasolacrimal stenosis. Idoxuridine 0.1% and trifluridine 1% are alternatives but require dosing every 4-6 hours and trifluridine often stings. Topical agents suit surface epithelial disease; oral famciclovir is preferred for deeper or multisystem involvement [3].