Feline keratitis and conjunctivitis (Proceedings)
Conjunctivitis is probably the most common ocular disease of cats, and is often infectious is origin. The most common viral cause of conjunctivitis is cats is FHV-1, but Reovirus and Calicivirus conjunctivitis are reported. Other infectious causes include Chlamydophila, Mycoplasma, systemic salmonellosis, and other less common non-infectious causes such as allergic and hypersensitivity reactions, tear film disorders and eosinophilic conjunctivitis.
Acute herpes virus keratoconjunctivitis is usually a bilateral ocular disease with systemic signs (upper respiratory signs, fever, lethargy). It is a very common cause of neonatal ophthalmia and is supposedly the only infectious, upper respiratory disease causing keratitis (ulcerative or interstitial). Two forms of ulcerative keratitis are usually seen: dendritic ulcers and geographic ulcers. The diagnosis can be obtained by various tests, however each has their potential pitfalls. The IFA test is a low yield test; if comes back positive it probably is. There is a high number of "false negatives". False positives are seen when an animal's cornea is stained with fluorescein prior to doing scraping. PCR testing (looking for amplified viral DNA) can be performed, but false negatives are common as laboratory processing is important in obtaining a definitive diagnosis. Conjunctival scrapings or biopsies can be put into laboratory specific transport media. The most common diagnostic test of the past was antibody titers, but it is felt by most to be useless, as most cats are naturally exposed or vaccinated with FVRCP. Virus isolation is considered the "gold standard", but it is expensive and only useful when special media can be immediately inoculated. Acute disease usually runs its course in immune competent animals within 14 days. The presence of dendritic ulcerations is considered pathognomonic for FHV-1 infection. Screening for FeLV/FIV in FHV-1 infected cats is recommended as immunosuppression is a common underlying cause of this disease in cats. Systemic symptoms of FHV-1 infection are treated with supportive care.
The initial therapy for ocular symptoms include supportive care and treatment with topical broad spectrum antimicrobials. Other therapies may include the application of Betadine solution (not the scrub) diluted in saline 1:10 (1% final concentration) several times daily, oral Interferon 30-100 (up to 1000) IU PO daily one week on, one week off. When disease is progressive, severe, and corneal or conjunctival ulcerations are present, topical and/or oral antivirals are indicated.Various topical antiviral medications are available. The most commonly prescribed topical antivirals in cats include trifluridine, idoxuridine, vidarabine and cidofovir. Trifluridine, idoxuridine, and vidarabine need to be administered 6-8 times daily until effect and then slowly tapered thereafter, treating one week beyond resolution of clinical symptoms. Trifluridine is the only topical antiviral that is commercially available (Viroptic). Studies have shown this to be a very effective antiviral in cats, but is often associated with local irritation. Idoxuridine 1% can be obtained from compounding pharmacies and is usually well tolerated by cats. Cidofovir 0.5% is also available through compounding pharmacies and is the only topical BID antiviral, which makes it very appealing to clients (and their pets). In my experience, topical cidofovir is irritating to cats after 6 weeks or more of continuous treatment. Other topical antivirals available include a 3% compounded vidarabine (vira-A) and is well tolerated, but may be less effective at controlling the virus in cats.
Oral antivirals should be used with caution in cats. The only oral antiviral I currently recommend for cats is famcyclovir because high doses appear to be extremely well tolerated in cats. 125 and 250 mg tablets are available. Dose is 30mg/kg (anecdotal) to 90mg/kg (therapeutic -Maggs et al) PO BID-TID. Acyclovir 200mg PO BID-TID was previously recommended for cats, but most agree now this is not enough to maintain a therapeutic and tolerated dose. Consider ANY other oral antiviral (ie. Valacyclovir) to be hepato/nephro/myelo toxic and potentially lethal to cats!
L-lysine, an amino acid supplement, comes in 500 mg -1000mg tablets for humans which can used for cats. Various other veterinary specific flavored formulations are available as pastes/gels, powders, or Lysine can be compounded into flavored liquids by most compounding pharmacies. Currently I prescribe 250mg/day PO BID for cats <10# and 500 mg/cat PO BID for cats >10# in cases where recrudescence is common. Work at Purdue University showed that l-lysine therapy early in course of disease reduces duration of illness, viral shedding, and incidence of recrudescence. Various other studies are available in the veterinary literature. Lysine may work by binding arginine, an essential amino acid for FHV-1 replication.