Cidofovir shows promise in treatment of FHV-1 conjunctivitis
FORT COLLINS, COLO. — An antiviral drug used to treat humans for certain viral diseases is showing promise in the treatment of eye disorders associated with feline herpesvirus type 1 (FHV-1).
Cynthia Powell, DVM, MS, associate professor in ophthalmology at Colorado State University in Fort Collins, Colo., recently completed a study that examined the effectiveness of cidofovir as a treatment option for conjunctivitis caused by FHV-1.
Cidofovir is a parenteral antiviral agent used primarily to treat symptoms of cytomegalovirus (CMV) retinitis in humans with acquired immune deficiency syndrome (AIDS).Powell read of studies in rabbits using an eye drop solution containing cidofovir .5%-1% to successfully treat conjunctivitis caused by adenovirus or herpes. Then a colleague told Powell that she had obtained positive results using the same treatment in cats that were suffering from keratitis associated with FHV-1.
"She suggested that this would be a good subject for a study at a research institution," Powell states.
Powell, a diplomate of the American College of Veterinary Ophthalmologists (ACVO) worked with Michael Lappin, DVM, PhD, professor in small animal medicine at Colorado State University to develop protocols for the research.
The study was supported by the Morris Animal Foundation and completed in October 2005.
FHV-1 is the causative agent behind feline viral rhinotracheitis and associated eye disorders. Because the virus is highly contagious, FHV-1 infections are common in catteries and in households that have more than one cat.
The initial infection is generally seen in younger or adolescent cats. It is characterized by upper respiratory symptoms, oral ulcers, hyperemia and redness of the conjunctiva, serous or mucopurulent discharge from one or both eyes and, in some cases, corneal ulcers.
Following recovery from the initial episode, most cats — up to 80 percent — may become latently infected with FHV-1. The virus may lie dormant indefinitely or it may flare up when the cat is stressed. Stressors may be physiologic, such as illness, estrus or pregnancy, or lactation, or from external causes such as disruption of the cat's household or normal routine, surgery or systemic illness, or corticosteroid therapy.
Symptoms of recurrent FHV-1 conjunctivitis may appear intermittently, or the cat may display chronic symptoms that do not resolve with treatment or time. Unlike the initial infection, usually only one eye is affected. Respiratory symptoms are not present in most chronic cases.
Limitations of current options
Treatments of chronic FHV-1 infections are usually focused on providing symptomatic relief through the use of ointments and artificial tears. If corneal ulcers are present, antibiotics may also be used to prevent secondary bacterial infections of the eye. But neither of these general approaches has any effect on the specific viral causes underlying the infection.
Effective antiviral treatments in recurrent instances of chronic FHV-1 conjunctivitis are a challenge due, in part, to the number of doses normally required to reach a therapeutic level. The very short half-life of these agents in the ocular tissues necessitates as many as five or six applications daily to be effective.
But an even larger problem results from the inherently irritating nature of these medications.
"Pretty quickly, cats will either hide to avoid receiving another painful treatment or they will hurt the person who is trying to apply the medication," Powell says. "Either way, it's a treatment failure."