Feline herpesvirus keratoconjunctivitis

What it is

Feline herpesvirus type 1 (FHV-1) is one of the main agents of the feline respiratory disease complex and the most important cause of corneal disease in the cat. It spreads by direct contact with ocular, nasal and oral secretions. After the initial infection the virus becomes latent in the trigeminal ganglion of most infected cats for the rest of their lives. Stress (moving house, a new cat, giving birth, illness, corticosteroids) can reactivate it: the virus travels along the nerves to the eye and causes new episodes or is shed without signs.

Lesions arise through two mechanisms: the virus's direct cytopathic effect on the epithelium (conjunctivitis, dendritic ulcers) and the cat's immune response to viral antigen in the corneal stroma (stromal keratitis). It is also associated with other chronic eye diseases:

• Corneal sequestrum: a plaque of necrotic stroma, amber to black, typical of the cat, especially brachycephalic breeds such as the Persian.
• Eosinophilic (proliferative) keratitis: white-pink plaques with vascularisation, often on the temporal cornea.
• Symblepharon: adhesions between the conjunctiva and the cornea or between areas of conjunctiva, after severe infections in kittens.

Who it affects

Kittens and cats in shelters, catteries or colonies, where the virus circulates widely, have the primary infection, often with respiratory signs. Carrier adults have reactivations with recurrent keratitis, usually in one eye. Vaccination reduces the severity of signs but does not prevent infection or latency.

Anatomy involved

• Conjunctiva: hyperaemia, chemosis (oedema) and discharge, often in both eyes in primary infection.
• Corneal epithelium: dendritic ulcers (linear and branching), pathognomonic for herpesvirus, which can coalesce into geographic ulcers.
• Corneal stroma: oedema, vascularisation, cellular infiltrates, scarring; sequestrum in some cats.
• Corneal nerves and trigeminal ganglion: the latent virus resides in the ganglion and reaches the cornea along the ophthalmic branches; chronic infection reduces corneal sensitivity.

Clinical signs

Blepharospasm, epiphora or serous to mucopurulent ocular discharge, conjunctivitis with chemosis, third eyelid protrusion and photophobia. In kittens, together with sneezing, nasal discharge, fever and sometimes ophthalmia neonatorum with the eyelids still closed. In adults, keratitis with recurrent ulcers, vascularisation, corneal opacity or the dark spot of a sequestrum.

Diagnosis

• Fluorescein stain and, better still, rose bengal or lissamine green, which stain devitalised epithelium: dendritic ulcers are practically diagnostic.
• PCR of conjunctival or corneal samples: detects the virus, but also in healthy carrier cats, and can be negative in stromal keratitis; it is interpreted together with the signs.
• Cytology of the conjunctiva or cornea: eosinophils in eosinophilic keratitis.
• Slit-lamp examination and Schirmer tear test.

Treatment

• Antivirals: oral famciclovir in moderate to severe cases; topical antivirals such as cidofovir, ganciclovir, idoxuridine or trifluridine, several times a day.
• Topical antibiotics to prevent secondary bacterial infection of ulcers, and treatment for Chlamydia felis or Mycoplasma if suspected.
• Debridement of loose epithelium; grid or punctate keratotomy must not be performed in cats, because it promotes corneal sequestrum.
• Topical corticosteroids: contraindicated while an ulcer is present; in stromal and eosinophilic keratitis they are used with caution under antiviral cover.
• Corneal sequestrum is treated by keratectomy and, if deep, a graft.
• L-lysine was used for years, but a systematic review concluded that it is not effective.

Stress is reduced to limit reactivations. The infection is lifelong and episodes may recur.

Key studies

Antivirals

• Fontenelle et al. (2008), randomised controlled study in 12 cats experimentally infected with FHV-1. Topical 0.5% cidofovir twice daily for 10 days reduced clinical scores and ocular viral shedding compared with the vehicle alone.
• Thomasy et al. (2011), randomised placebo-controlled study in 16 experimentally infected cats. Oral famciclovir (90 mg/kg three times daily for 21 days) lowered disease scores from day 4 to day 18, reduced viral shedding and conjunctival inflammation and preserved conjunctival goblet cells. It modelled primary infection, not recurrent disease.
• Ledbetter et al. (2022), randomised placebo-controlled study in 16 experimentally infected cats. Topical 0.15% ganciclovir gel three times daily and oral famciclovir (90 mg/kg twice daily) reduced ocular disease scores and corneal inflammation to a similar degree; only ganciclovir significantly lowered viral load. The groups had 4 to 6 cats.
• Mironovich et al. (2023), masked placebo-controlled study in 132 shelter cats positive for FHV-1, treated for 7 days with compounded topical cidofovir, oral famciclovir or topical ganciclovir. Only cidofovir reduced viral load, and no drug improved ocular scores more than placebo (65–75% of cats improved in every group). All corneal ulcers in the placebo group failed to heal, whereas 77% healed with an antiviral. The course was short and the compounded drugs may vary in quality.
• Vernau et al. (2024), randomised, triple-masked, placebo-controlled trial in 373 kittens with ocular signs of infectious upper respiratory disease, all treated with doxycycline and half also with famciclovir. Fewer kittens developed corneal disease with famciclovir, kittens with mild disease reached clinical resolution 4–5 days sooner, and all 5 kittens withdrawn because they worsened were in the doxycycline-only group.

L-lysine

• Bol and Bunnik (2015), systematic review of 7 studies on lysine and FHV-1 (2 in vitro, 5 in cats). Lysine had no effect on viral replication and did not prevent or treat infection; some studies found more infection or more severe disease in supplemented cats. The authors recommend stopping its use.