by Noelle La Croix, DVM, Dip. ACVO
Ocular Herpes in Kittens
The feline herpesvirus (FHV-1) was first isolated in 1958. The virus can replicate within the conjunctival epithelia, upper respiratory tract epithelia, and sensory ganglia. Neuronal infection with FHV-1 establishes lifelong latency with intermittent re-activation and viral shedding. Virus transmission is commonly associated with exposure to acutely infected cats, or recrudescing latently infected cats. Environmental contamination with FHV-1 is not considered a significant route of transmission.
The feline herpesvirus can be transmitted via oral, nasal, and/or conjunctival routes. Kittens under 6 weeks of age do not typically show clinical signs of herpes as maternal antibodies confer passive immunity. The primary source of kitten infection is also their lactating mothers who shed the herpesvirus after queening. Clinical signs of FHV-1 infection typically last 2 weeks while virus is actively shed for 1 – 3 weeks. Cats that exhibit clinical signs of herpes will generally become lifelong carriers of this low mortality and mild morbidity virus. The herpesvirus is believed to be endemic within the cat population, so that any stressed or immunosuppressed cat has the potential for recrudescence. In one study, 70% of cats shed herpesvirus when administered glucocorticoids.
Immunity to FHV-1 is primarily cell-mediated. Vaccination is recommended for all cats between 9 and 12 weeks of age, followed by a booster 1 year later. Vaccinated indoor cats have a low risk of viral exposure and should be re-vaccinated every 3 years. Free-roaming cats should be re-vaccinated yearly. Importantly, vaccination decreases the likelihood of displaying the clinical signs of herpes, but it does not protect against initial or repeat infection with FHV-1.
Acutely FHV-1 infected kittens display fever, depression, anorexia, and serous oculonasal discharge. Secondary bacterial infection is associated with a purulent oculonasal discharge. Some infected kittens also develop viremia or pneumonia. The infection can be lethal. Less frequently oral, ocular, and/or skin ulcerations are associated with infection. Secondarily infected corneal ulcerations can lead to corneal rupture and subsequent globe loss.
A common secondary complication of corneal ulceration associated with herpes is symblepharon, in which the erosive surfaces of the conjunctiva and cornea adhere to each other (Figure 1). Adhesions lasting beyond a few days can permanently fibrose. Ideally, during an active herpes infection, any early conjunctival-corneal adhesions should be broken down every 2 – 3 days with a cotton applicator to prevent fibrosis. Fibrosis can also be inhibited by decreasing the factors (fibrinogen, thromboplastin, and other clotting factors) associated with fibrin production. Frequent lubrication, medication, and flushes of the eye can reduce these factors.
When a cat presents with fibrosed symblepharon and corneal scarring, veterinarians are often tempted to break down these adhesions to improve vision. However, mechanical breakdown of these fibrosed adhesions is highly inadvisable. Fibrosed symblepharon can mask corneal ruptures and removing conjunctival tissue may result in globe rupture. Conjunctival cells have also transferred to the cornea in symblepharon. The removal of conjunctival tissue will therefore not prevent re-scarification, since conjunctival cells will multiple to seal any newly created defects. Symblepharon resection will also not typically remove eyelid scar tissue that can interfere with eyelid function. All erosive epithelial surfaces tend to re-adhere, and so there is a high risk for symblepharon recurrence. Cats with symblepharon and limited vision may not be helped by attempting to break their adhesions by any means.
Scarification is often associated with symblepharon and herpes because of the phenomena known as corneal limbal stem cell exhaustion (Figure 2). Corneal epithelia are continuously lysed by herpesvirus, so that corneal stem cells become overburdened attempting to replace them. Conjunctival cells will then act as replacements, resulting in scarification.
The ocular manifestations of herpes are common in young cats, but few older cats show both ocular and respiratory signs at the time of presentation. Kittens presenting with bilateral oculonasal purulent discharge are commonly diagnosed with herpes. Primary treatment should include restoring fluid and pH balance (preferably via IV fluids). Food intake is extremely important in these kittens, and I recommend hand-feeding of a palatable diet. Nasogastric tube feeding can be initiated if an infected kitten does not eat for 3 consecutive days. Broad spectrum antibiotics, such as Clavamox (amoxicillin/clavulanic acid), will help prevent secondary bacterial infections. Topical ocular antibiotics, such as erythromycin or Terramycin, will also decrease the likelihood of Chlamydia, mycoplasma, and other secondary bacterial infections. Pain management should be initiated as the erosions associated with herpes are painful. The eyes and nostrils of kittens should be cleaned routinely, and nebulization of acetylcystine will increase comfort. As previously stated, any early conjunctival-corneal adhesions should be broken down every 2 – 3 days to prevent permanent fibrosis.
Antiviral therapy is also important in controlling the ocular manifestations of herpes. Herpesvirus replication is inhibited by 0.1% idoxuridine (Wedgewood Pharmacy) applied topically 6 times daily. Alternatively, Viroptic (trifluridine) can be used, but burning or stinging during ocular application is common. An ocular flush with a dilute (0.2%) iodine solution can also be effective.
A number of oral medications are also used to treat herpes in cats. Oral L-lysine (250 – 500 mg BID) alleviates the clinical signs of herpes, and decreases viral shedding when administered to indoor or hospitalized cats. However, recent studies indicate that oral L-lysine may not be effective in preventing herpes outbreaks within shelters. Oral famciclovir (¼ of 125 mg tablet BID or ½ of 125 mg tablet SID) inhibits herpesvirus replication. Other medications that have been used to treat herpes include acyclovir, ganciclovir, cidofovir, and feline or human interferon.
Kittens commonly present with ocular manifestations of FHV-1 infection. Simple treatments can often alleviate their clinical signs of herpes. If you have any questions about ocular herpes, please do not hesitate to contact a veterinary ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: The right eye of a kitten with symblepharon.
Figure 2: The left eye of a kitten with corneal scarification.