Feline Herpes Virus: Bad News for a Popular Treatment

By Ken Tudor, DVM on Aug. 29, 2014

Your veterinarian prescribes L-Lysine to treat the periodic runny eyes and runny nose in your cat, but you don’t think it really is helping all that much. You are probably right. Although inconsistent studies in cats have raised doubts about this medication, it continues to be a mainstay for the treatment of herpes virus in cats. A recent study published in the American Journal of Veterinary Research raises further doubt about the role of L-Lysine in the treatment of feline herpes virus.

What is Herpes Virus in Cats?

Feline Herpesvirus 1, or FHV-1, is the most common upper respiratory virus in cats worldwide. It causes a condition called Feline Viral Rhinotracheitis, or is sometimes called feline influenza. The virus is extremely common in shelters and catteries. Some studies have indicated positive blood titers for herpes virus as high as over 90 percent in feral and shelter cat populations.

Most kittens and cats exposed to herpes virus experience symptoms within two to four days. Coughing, sneezing, nasal discharge, and conjunctivitis (red swelling of the tissue that surrounds the eyeball) are the most common symptoms. Some animals may experience a high fever and decreased appetite. The condition generally runs its course in four to seven days. Some kittens may become extremely ill with secondary pneumonia or develop severe, sometimes permanent, scarring of the cornea of the eye.

The problem with the herpes virus family, as many humans with herpes know, is that it is the gift that keeps on giving. The immune system of humans and cats cannot clear the infection and rid the body of the virus. The virus lays dormant for periods of time and then starts reproducing causing flare-ups. Periodic “cold sores” on the lips of humans is a common herpes flare-up. In cats, seasonal sneezing and conjunctivitis coincide with seasonal changes in spring and fall or during stressful holiday periods like Christmas. Seasonal changes and stress result in an increase of corticosteroid hormone released into the blood, which suppresses immune function and promotes shedding of the latent herpes virus. It is during these periods of flare-ups that veterinarians recommend the use of L-Lysine to decrease viral reproduction and shedding.


What is L-Lysine?

L-Lysine is an amino acid. Its use in cats was predicated on human research that suggested that large amounts of the amino acid inhibit human herpes virus in cell cultures. Some studies with cat cells indicated the same findings. This led to the widespread use of oral L-lysine gels for the treatment of herpes symptoms in cats, especially those associated with the eyes and nose. But research in actual cats, not cat cells in a petri dish, has failed to show consistent success with the treatment.

The purpose of the recent study was to re-visit the early research done on cat cells. This research group corrected some of the technical flaws in the original research and then analyzed the effect of increased dosage levels of L-lysine on herpes virus reproduction. They found that increasing amounts of L-lysine in the cell cultures had little effect on suppressing herpes virus reproduction. These laboratory findings are consistent with the research performed in cats with herpes virus. Contrary to popular veterinary belief, these researchers found the results of their study, as well as the studies in cats, offer little scientific justification for the use of L-lysine in the treatment of feline herpesvirus 1 in cats.

If your cat is being treated for FHV-1, you might want to ask your vet about other treatment alternatives.

Dr. Ken Tudor


Cave, NJ et al: Effects of physiological concentrations of L-Lysine on in vitro replication of feline herpesvirus 1. American Journal of Veterinary Research; June 2014:Vol. 75, No 6; 572-80

Image: Goodluz / Shutterstock


Ken Tudor, DVM


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