Cat Herpesvirus FHV-1: Eye Ulcers and Respiratory Flares

Feline herpesvirus-1 (FHV-1) causes the most common eye and upper respiratory infections in cats. Once infected, cats carry the virus for life — stress or illness can trigger flares of corneal ulcers, eye discharge, and sneezing. Antiviral therapy and L-lysine support can reduce flare severity and frequency.

Last reviewed: June 2026

What Is Feline Herpesvirus (FHV-1)?

Feline herpesvirus-1 (FHV-1, also called feline rhinotracheitis virus) is a DNA herpesvirus responsible for approximately 50% of all upper respiratory infections in cats. Like human herpes simplex virus, FHV-1 establishes lifelong latency in the trigeminal nerve ganglia after the primary infection — and can reactivate months or years later when the cat's immune system is stressed.

Primary infection most commonly occurs in kittens via contact with infected mothers or littermates. After the acute illness resolves, approximately 80% of recovered cats become latent carriers. Roughly 45% of latent carriers experience reactivation episodes triggered by stress events: rehoming, introduction of new pets, veterinary visits, illness, boarding, surgery, or corticosteroid administration.

FHV-1 causes both upper respiratory disease (sneezing, nasal discharge) and, importantly, ocular disease — including corneal ulcers that can lead to scarring if untreated. This dual tropism makes it one of the most clinically significant infections in pet cats.

Signs of FHV-1 Flare in Cats

Ocular signs:

• Watery to thick yellow-green eye discharge
• Conjunctivitis — red, swollen conjunctival membranes
• Squinting or holding the eye shut (blepharospasm) — indicates pain
• Corneal ulcers — visible as a hazy spot on the cornea; requires fluorescein stain to detect
• Corneal cloudiness, scarring, or vascularization in chronic cases
• Symblepharon — adhesion of conjunctiva to cornea, a complication of severe kitten infections

Upper respiratory signs:

• Sneezing — often sudden clusters
• Clear to mucopurulent nasal discharge
• Nasal congestion with open-mouth breathing in severe cases
• Decreased appetite from anosmia (inability to smell food)
• Mild fever and lethargy during active flares

Approximately 65–80% of FHV-1 reactivation episodes in adult cats primarily involve ocular signs rather than upper respiratory signs, reflecting the virus's preference for the ocular epithelium during reactivation. Research published in the Journal of Feline Medicine and Surgery identified that mucociliary-disrupting stress events most reliably trigger reactivation (Helps et al., 2005, JFMS).

Diagnosis

A clinical diagnosis is typically made based on history (known carrier, recent stress) and characteristic signs. Specific diagnostic tests include:

• Ocular fluorescein stain — identifies corneal ulceration (corneal epithelial cells do not take up stain; ulcerated areas do). Essential before any topical corticosteroid is applied
• PCR for FHV-1 on conjunctival swab — detects viral shedding; a positive PCR confirms active replication. Note: PCR can be negative during latent phase, so negative does not rule out FHV-1 as the underlying cause
• Schirmer tear test — rules out concurrent keratoconjunctivitis sicca (KCS / dry eye)
• Tonometry — checks intraocular pressure if uveitis or glaucoma is suspected

For recurrent eye disease in an adult cat with a known history of respiratory illness as a kitten, empirical antiviral treatment is often started before PCR confirmation.

Treatment: Active Flare

Antiviral therapy:

• Cidofovir 0.5% eye drops — topical; applied every 12 hours; most effective topical antiviral for FHV-1 corneal disease in cats; well-tolerated
• Idoxuridine or trifluridine eye drops — alternatives; typically require every 4–6 hour dosing which is less practical for owners
• Famciclovir (oral) — as described in Plumb's Veterinary Drug Handbook, famciclovir 40–90 mg/kg twice daily is the preferred systemic antiviral for cats with severe or non-responsive ocular/respiratory FHV-1. It converts to penciclovir (the active inhibitor) and achieves therapeutic levels in feline tissue. Note: acyclovir, the human herpes drug, should NOT be used in cats — cats cannot metabolize it effectively and it causes bone marrow suppression.

Supportive care:

• Saline nebulization (twice daily for 10–15 minutes) loosens nasal secretions
• Appetite stimulant (mirtazapine) if the cat has stopped eating due to anosmia
• Warm, wet food to improve smell detection
• L-lysine (250–500 mg/day) — competes with arginine for viral replication; evidence in cats is modest but it is safe and inexpensive

Secondary infections: Broad-spectrum antibiotics (doxycycline, amoxicillin-clavulanate) treat secondary bacterial conjunctivitis or rhinitis but do not affect the herpesvirus directly.

Avoid corticosteroids topically or systemically during active FHV-1 flares — they can dramatically worsen viral replication and corneal ulceration.

Vaccination against FHV-1 (FVRCP vaccine) does not prevent latent infection or reactivation, but substantially reduces the severity of primary infection and subsequent reactivation episodes. The AAFP-AAHA Feline Life Stage Guidelines (2021) classify FVRCP as a core vaccine for all cats (AAFP-AAHA Feline Life Stage Guidelines, 2021).

Long-Term Management of Chronic Carriers

Cats with frequent reactivations benefit from:

• Minimizing known stressors (stable environment, predictable routine, Feliway pheromone diffusers)
• L-lysine supplementation year-round
• Pre-emptive famciclovir for 5–7 days before anticipated stressors (veterinary visits, boarding, introduction of new pets)
• Annual FVRCP boosters to maintain vaccine-primed immunity

When to See a Vet

Call your vet today if:

• Your cat has a red, weeping, squinting eye — especially if there is any cloudiness or white spot on the cornea
• Sneezing and nasal discharge have persisted more than 3–4 days
• Your cat has stopped eating due to nasal congestion

Go to the ER immediately if:

• Both eyes are severely ulcerated and your cat cannot open them
• Your cat is struggling to breathe due to severe nasal congestion
• Your cat has not eaten or drunk for 36+ hours and is lethargic

Frequently Asked Questions

Can I catch herpesvirus from my cat? No. FHV-1 is species-specific and cannot infect humans. You are at no risk of infection from a cat with herpesvirus.

How much does FHV-1 treatment cost? A vet visit with fluorescein stain and tear test costs $100–250. Cidofovir eye drops (compounded) typically run $30–70 per bottle. Oral famciclovir costs $80–200 for a two-week course. PCR testing adds $80–200 if needed. Total flare episode cost: $200–600 in most uncomplicated cases.

Will my cat have herpesvirus flares forever? Many cats have a few episodes in younger life then become subclinical carriers with rare reactivations. Some cats have frequent flares — these cats benefit most from pre-emptive antiviral therapy, stress reduction, and L-lysine supplementation. Flare frequency and severity typically decrease as cats age, unless they become immunosuppressed.

Is L-lysine proven to work in cats? The evidence is mixed. Some studies show reduced shedding and milder flares; others show no significant benefit. However, L-lysine is safe and inexpensive, so most vets recommend it as a benign adjunct. It should not replace antiviral drugs for an active flare.

Can FHV-1 cause permanent eye damage in cats? Yes — untreated or repeatedly inflamed corneas can develop scarring (corneal fibrosis), vascularization, and in severe cases, a condition called corneal sequestrum (a dark, dead patch of corneal tissue requiring surgical removal). Prompt treatment of each flare significantly reduces long-term ocular damage.

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Related reads

• Cat Conjunctivitis Symptoms
• Cat Upper Respiratory Infection
• Cat Eye Discharge Treatment