Herpesvirus Infections - Overview
Ocular disease due to feline herpesvirus (FHV) is common. It is estimated that 80% of cats are latently infected with the
virus, and approximately 40% of these cats will suffer recrudescent infection in later life. These estimates are based on
data that is several decades old, and the actual percentages may be higher. FHV affects cats of all ages, but the initial
(or primary) infection usually occurs in neonatal and adolescent cats. Symptoms include bilateral conjunctivitis, respiratory
disease, and fever. Most cats recover from the primary infection in 7-10 days without specific antiviral treatment, but neonatal
cats are more likely to suffer serious corneal and conjunctival scarring. Recrudescent infection can be unilateral or bilateral
and with or without respiratory signs. Unilateral conjunctivitis or ulcerative keratitis in the absence of respiratory signs
is common in adult cats with recrudescent infection. Stress appears to be an important factor in precipitating FHV. "Stressful"
events may include topical or systemic corticosteroids, concurrent systemic disease, anesthesia, hospitalization, acquisition
of a new cat, or extended owner absences (e.g., vacation). Other conditions that may be associated with FHV include non-ulcerative
(or stromal) keratitis, conjunctival and corneal scarring (symblepharon), corneal sequestrum, eosinophilic keratitis, keratoconjunctivitis
sicca (KCS), blocked nasolacrimal duct, and possibly uveitis. The remainder of this discussion will focus on corneal diseases.
FHV keratitis can be ulcerative or non-ulcerative, but the ulcerative form is most common and is due to direct cytopathic
effects of the virus. The non-ulcerative form (stromal keratitis) is primarily immune-mediated, occurs in response to viral
antigen, and is characterized histologically by lymphocytic infiltrates. Corneal ulcers are most often superficial and irregular
or geographic (map-like). However, faint linear to tree-branching ulcers, or dendritic ulcers, are considered pathognomonic
of FHV. Dendritic ulcers can be detected with fluorescein stain using magnification and a cobalt blue light, but they may
be easier to detect using rose Bengal stain. Ulcers resulting from especially virulent strains of FHV, or secondarily infected
with bacteria, can quickly deteriorate. Conjunctival graft or corneo-conjunctival transposition surgery may be required
for a deep stromal ulcer or descemetocele. Any cat with a corneal ulcer of undetermined cause should be promptly treated
with an antiviral agent.
The non-ulcerative or stromal keratitis is characterized by circumcorneal to diffuse stromal opacity and blood vessels. Because
the stromal keratitis is immune-mediated, improvement may occur with judicious application of topical steroids. However,
topical steroids should be used only if the cornea is negative to fluorescein stain and if prior antiviral treatments have
been ineffective. Then, they should only be used in conjunction with an antiviral agent. Caution is advised when contemplating
steroid use in a cat with suspected FHV, and frequent examinations (at least initially) are prudent if they are used.
Diagnostic tests such as immunofluorescent antibody (IFA) and polymerase chain reaction (PCR) to detect FHV can be performed
on samples obtained by conjunctival or corneal scraping. However, the incidence of false-negative test results is significant.
Furthermore, normal cats with no evidence of ocular disease can be positive for FHV. The diagnostic value of such testing
is therefore questionable, and the author rarely performs these tests. However, conjunctival or corneal scrapings for cytologic
examination can be useful to eliminate other disease conditions from consideration (e.g., eosinophilic keratitis). If a cat
has ocular disease consistent with FHV and has not received antiviral treatment, the best approach is to initiate such treatment.