(Information provided with permission of Dr. Dennis Hacker, DVM, DACVO.)

One of the more common ophthalmic problems seen in our cat patients is infections caused by herpesvirus. This virus causes conjunctivitis (inflammation of the moveable white tissue surrounding the eye) and/or corneal ulcers. Occasionally sneezing and mouth ulcers occur in some patients. We hope this paper will give you information concerning this common condition.


A virus is not alive in the sense that you, I, your cat and even bacteria are alive. A virus is a capsule that contains only protein or nucleic acids known as DNA. This DNA is the building block that makes all of us who and what we are. As it is not alive, a virus particle cannot reproduce without a living cell to which it is able to attach itself. Once attached to a susceptible cell (a cell that will support the virus growth), the viral DNA (vDNA) is injected into the cell. The vDNA then continues to the cell nucleus that is the 'control center' of the cell. The vDNA then inserts itself into the cell's DNA. This causes the cell to start manufacturing new virus particles. To do this, the cell takes nucleic acids and proteins from the area surrounding itself and uses them to form new vDNA. This concept is important in how we must treat viruses that we will see below.


Feline herpesvirus is specific to cats. There are also dog herpesvirus, people herpesvirus, cow herpesvirus, chicken herpesvirus and horse herpesvirus. In fact, most animals have their own type of herpesvirus. These viruses will not infect other species, i.e., when a cat has herpesvirus, the owner has nothing to fear as far as getting the disease. Herpesvirus is a common respiratory pathogen (infectious agent) that causes an upper respiratory disease in most cats. The virus is everywhere and it infects most cats in almost every cattery in the country. It is our belief that almost every kitten is exposed to this virus following birth as the virus is often found in the birth canal of the queen. As a respiratory disease, the virus is acquired by aerosol, that is one cat sneezing around another cat. The virus is killed by drying and sunlight but can live for many hours in a moist, cool environment. The problems associated with herpesvirus depend on the age at which the cat first acquires the virus. Neonatal conjunctivitis occurs in kittens who have not yet opened their eyes. In young cats, 6 months to 4 years, conjunctivitis (redness of the white of the eye) A young kitten with neonatal conjunctivitis. The arrow indicates an infection which had developed behind closed eyelids.

neonatal conjuntivities

A young kitten with neonatal conjunctivitis. The arrow indicates an infection which had developed behind closed eyelids.

Herpes conjunctivitis

Herpesvirus conjunctivitis in a juvenile kitten.

In older patients, conjunctivitis is most often seen. Sneezing may or may not be seen in any of these patients. Most often, our patients have had a long-standing history of conjunctivitis and/or corneal ulcers that will not heal. We have seen cats who have had herpesvirus infections for as long as 12 years! When herpesvirus invades nerve tissues, a possibility of relapse exists. Perhaps only 15-to-20% of the cats with herpesvirus infections have relapses, yet this possibility must be kept in mind. As many of my clients know, human herpesvirus may cause the skin conditions known as 'cold sores' and 'shingles.' As with 'cold sores' and 'shingles, ' any stressful episode may cause a recurrence of the infection. In cats, a relapse may be triggered by the owner leaving town and a stranger coming in to feed the cats, being boarded, or strangers or new animals coming for a visit. We have one patient that has a new episode of herpesvirus every time the client leaves town for a business trip. Knowing this helps us understand the recurrence of redness following a stressful episode. This redness indicates a probable return of the herpesvirus infection and requires re-institution of the medication. Again, these relapses do not occur commonly.


Herpesvirus infection should be suspected anytime a cat has an eye problem that does not respond to antibiotics (which have no effect on viruses). To diagnose a herpesvirus infection, after applying a topical anesthetic, a scraping of cells is made from the eye and placed on a slide. The slide is submitted to a laboratory for a specific test procedure known as an Polimerase Chain Reaction (PCR) test. This test is quite specific when compared to other tests. A different staining test can be performed which is known as the Indirect Fluorescent Antibody Test (IFA). While the IFA test is not as precise as the PCR test, performing both tests simultaneously gives a better infection indicator as to whether your cat does or doesn't have herpes than either test alone. Another test that can be used is virus isolation that actually grows the virus in tissue cultures. This test takes up to 1 month and is quite costly.


If you or your child has a bacterial infection, a strep throat for example, penicillin may be given and the penicillin will kill the strep organism and the infection will go away. This is because the bacteria are alive and reproducing all the time. As mentioned earlier, a virus is active only when it gets into a cell. This is why antibiotics do not kill viruses. Since the living cells must bring in nucleic acids and proteins (protein building blocks) from the local cellular environment to make new viruses, the only way to kill susceptible viruses is to put abnormal proteins and nucleic acids in the environment. This is the way herpesvirus is killed. We use medications such as Herplex®, Viroptic®, or Vira-A® to introduce abnormal proteins into the environment. The infected cells draw these proteins into itself and use them to make new virus particles. These virus particles will then not be able to reproduce. Because we do not know how long it takes to kill all the virus particles this way, treatment must be continued for 4-to-6 weeks or longer! Occasionally, patients need to be treated longer. Herpesvirus can become resistant to these medications. This does not happen often, yet this fact should be kept in mind, especially if a patient initially improves and then relapses. If resistance occurs to one medication, a change to another medication can be made. If resistance occurs to all the commercial eye medications, an oral drug called Zovirax® (acyclovir) may be prescribed. This drug appears to work well in humans and rabbits, yet the full spectrum of its side-effects is not known in the cat. This drug is reserved for the most resistant cases. Eye medications must be applied often. This means 5 times a day! For clients who do not work or working clients on weekends, the medication is applied every 3 hours for 5 treatments. During the week for clients who work, I recommend treating when you awaken, before leaving for work, when you get home, half-way between arriving home and bedtime, and bedtime. This is 5 times a day although they are not equally spaced. One other aspect of these anti-herpes medications should be kept in mind. That is that any protein has the ability to cause an allergic response! You may know of someone who wears soft contact lenses. If they fail to clean the lenses to remove their body proteins, an allergic reaction occurs which may be quite irritating. As stated above, you are applying abnormal proteins directly into the eye of your pet. If you notice the eye and eyelid becoming quite red, you must call us immediately. Due to the possibility of the allergic response and the virus becoming resistant to the medication, re-examinations are critical. In addition to the medication previously discussed, we will probably dispense two others: Interferon and lysine. Interferon is a natural chemical produced by the body to fight off viruses. The application of this to the eyes five times daily induces the patient to produce more of their own interferon. We have also found that after the active infection is controlled, the use of interferon even once a day seems to help prevent relapses! Finally, lysine has been shown to help kill herpesvirus. Lysine is very safe in cats. If it is possible to give your pet tablets by mouth, lysine has been a definite help in cats with herpesvirus infections. If all three medications are prescribed for your pet, you will be using Herplex® (idoxuridine), interferon and lysine. The idoxuridine is applied to the eye(s) five times daily as stated above. Interferon is applied to the eyes five times daily and one minute after the idoxuridine. Lysine is given by mouth twice daily and may be mixed with food although the best way to give it is to simply pop it down the mouth. Although herpesvirus infections are treatable, the infections can be frustrating because not every cat can be treated the same as all others. Sometimes medication must be changed to provide results. Patience from everyone, you, your cat and your ophthalmologist is required.

Copyright © 1998 * Animal Eye Specialists, El Cerrito, CA. * All rights reserved.

Reprinted with permission from Dr. Dennis Hacker, DVM, DACVO