Lysine for Cats With Feline Herpesvirus: What the Trials Actually Found

L-lysine is the supplement most cat owners meet first, usually at the counter of a veterinary clinic after a bout of sneezing and weepy eyes. It comes as a gel, a powder, a chewy treat and, in some cases, already mixed into the kibble. It is also the most openly disputed supplement in feline medicine, and the dispute is unusually easy to follow because the evidence is small enough to read in an afternoon.

The short version is that the researchers do not agree on whether a measured daily dose does anything, and they do agree that lysine added to food does not help. Two shelter trials found it went the wrong way.

The virus it is sold against

Feline herpesvirus type 1 (FHV-1) is the cause of feline viral rhinotracheitis, the classic cat cold of sneezing, discharge from the eyes and nose, and inflamed conjunctiva. According to the Cornell Feline Health Center, up to 97% of cats are exposed to it in their lifetime, the virus establishes a lifelong infection in up to 80% of those exposed, and up to 45% of carriers periodically shed it again, usually when stressed. Young cats are the most susceptible. In older cats it tends to show up as recurring conjunctivitis or keratitis, which is inflammation and sometimes ulceration of the cornea.

That lifelong, stress-triggered pattern is what makes a daily supplement attractive. An owner with a cat who flares every time a new cat arrives or the household moves wants something to give in between. Lysine was the answer that arrived first, and it has stayed.

How common the recommendation is

When two researchers at the University of California, Riverside surveyed cat hospitals in the United States, Australia and the United Kingdom for their 2015 review, 21 of the 23 hospitals that replied (91%) said their veterinarians recommend lysine for cats infected with FHV-1, and 87% sold it on site. The sample is small. A supplement recommended that widely should have a solid evidence base behind it. The same review set out to find it.

The theory, and where it breaks

Lysine has no antiviral activity of its own. The idea borrowed from human cold-sore research in the 1960s and 1970s is indirect: herpesviruses need the amino acid arginine to build their proteins, lysine was thought to compete with arginine, so flooding the body with lysine should starve the virus.

Each step has a problem in cats, and the Bol and Bunnik systematic review walks through them.

Lysine does not appear to lower arginine in cats. In a feeding study the review describes, adult cats were fed diets containing up to 131 g of lysine per kilogram of food, against 11 g in the control diet, and plasma arginine did not change at any level. The one change the study did record was that cats ate less on the higher-lysine diets.

Lowering arginine in a cat would be dangerous anyway. Cats cannot make enough arginine themselves and depend on it to clear ammonia through the urea cycle. The review cites feeding studies in which cats on arginine-free or low-arginine diets lost weight rapidly, refused food, and died of ammonia intoxication. The AAFCO cat food nutrient profiles set arginine minimums of 1.24% of dry matter for growth and 1.04% for adult maintenance, both higher than the matching lysine minimums of 1.20% and 0.83%. Of the two, arginine is the one a cat food is required to supply in the larger amount, and it is the same table behind our kitten food nutrient comparison.

The feline virus is not especially arginine-hungry. Using the published genome sequences, the review calculated that FHV-1's proteins are about 6.7% arginine against about 5.7% for the cat's own proteins. The human cold-sore virus sits at 8.5% against 5.4% for human proteins. The gap the theory relies on is much smaller in cats.

The lab result that started it used starved cells. The 2000 cell-culture study behind the feline recommendation tested lysine only in media with very low arginine, conditions under which, the review reports, cell viability was about 10% of controls. When a later study repeated the work at concentrations closer to a real cat's blood, lysine did not change viral replication at all.

What the cat trials found

Five clinical studies in cats form the entire evidence base. Here they are in the order they were published.

Stiles and colleagues, 2002: 8 cats. Cats given 500 mg of lysine twice daily, starting six hours before they were deliberately infected, had less severe conjunctivitis than cats given a placebo. The amount of virus recovered from the eye and throat did not differ between the groups, and neither did plasma arginine. The Bol review notes that the difference in eye scores held for days 5 to 15 of the 21-day study and was gone by the third week.

Maggs and colleagues, 2003: 14 cats. In latently infected cats given 400 mg once daily in food for 30 days, the lysine group had fewer viral shedding episodes after being moved to new housing. After the researchers reactivated the virus with a corticosteroid, that difference disappeared, and differences in conjunctivitis between the groups were not statistically significant.

Maggs and colleagues, 2007: 50 cats. Cats housed under shelter-like conditions were fed a lysine-enriched diet or a standard diet for 52 days. The Bol review records that the lysine-rich diet did not reduce the frequency or severity of disease, and a later review by Thomasy and Maggs reports that disease was more severe and viral shedding higher in the lysine-fed cats.

Rees and Lubinski, 2008: 291 cats. At an Indianapolis shelter, 144 cats received lysine in a small amount of canned food once daily and 147 received none. There was no difference between the groups on any measure of upper respiratory infection or conjunctivitis. The authors concluded that shelters would do better to put their resources into reducing stress and controlling transmission.

Drazenovich and colleagues, 2009: 261 cats. In the largest trial, shelter cats were fed a diet containing 5.7% lysine or a basal diet containing 1.7% for four weeks. Mild disease was equally common in both groups. More cats on the lysine diet developed moderate to severe disease in week 4, and FHV-1 DNA was found more often in the lysine group's swabs during week 2. The authors wrote that lysine supplementation "was not a successful means of controlling infectious upper respiratory disease within a cat shelter."

Two small experimental studies leaned positive on one measure each. Three studies in larger groups of cats found nothing or found harm.

Where the experts land

This is the part owners rarely hear, because the two most-cited reviews reach different conclusions from the same five studies.

Bol and Bunnik (2015) conclude that there is evidence at multiple levels that lysine does not prevent or treat FHV-1, and recommend "an immediate stop of lysine supplementation because of the complete lack of any scientific evidence for its efficacy."

Thomasy and Maggs (2016), from the UC Davis veterinary school, with Maggs an author on several of the studies above, read the same data more narrowly. Their summary is that lysine appears safe when given by mouth, and that given as a separate measured dose (a bolus) it may reduce shedding in latently infected cats and clinical signs in cats meeting the virus for the first time. They add two caveats that matter to anyone with a cat at home. In a shelter, the stress of dosing may well cancel out any benefit, and "data do not support dietary supplementation." They also point out that no clinical trial has ever been run in the group that gets lysine most often: pet cats with recurrent herpes flare-ups.

Cornell takes the cautious middle. Its guidance calls lysine "quite controversial" and says several studies suggest it is not effective and may worsen symptoms and promote shedding. The same page notes that the stress of giving medications may itself worsen bouts of keratitis and conjunctivitis that would otherwise settle on their own.

What that means when you are standing at the shelf

Three things are well enough supported to act on.

Lysine built into the diet has the worst record of any form. A lysine-enriched diet was the form tested in two of the three larger studies, and both found cats did worse. The third, a once-daily dose in a spoonful of canned food, found no effect. Even the more favourable review says the data do not support adding lysine to the diet. The theory also runs into a practical problem here: a congested cat that cannot smell its food eats less of it, and a supplement delivered through a meal the cat is skipping is not being delivered.

If your vet has recommended lysine, the useful conversation is about stress. The only positive signals came from measured doses in two small experimental studies. Cornell and Thomasy and Maggs both flag that the process of dosing a stressed cat can work against the goal. If pilling or syringing your cat turns into a daily fight, that is worth raising at the next appointment. The decision about what to give a cat with an active infection, or with recurring eye disease, belongs with the veterinarian examining the eye.

The supplement is not where the proven levers are. Cornell recommends herpesvirus vaccination for all cats, noting that it reduces the severity of disease and the amount of virus shed even though it does not prevent every infection. For cats with recurring flare-ups, its guidance points to antiviral medication and to limiting the triggers it names: crowding, surgery, the arrival of a new cat, and moving house. Stress is the thread running through that list, and making an unavoidable event calmer is the part an owner controls. Getting a cat used to its carrier long before a vet trip is one example.

If you do buy a lysine product, read it the way you would read any supplement. Our guide to what the NASC seal means covers how to check what is actually in the tub, and why a label that claims to treat or prevent a disease is making a claim no supplement is allowed to make. The pattern is the same one that shows up with probiotics for dogs and cats: a widely used product, a plausible mechanism, and a trial record much thinner than the shelf space suggests.

Questions people actually ask

Does lysine work for cats with herpes?

The evidence says probably not. Five clinical studies exist. Two small experimental studies found a benefit on one measure each, and three studies in larger groups of cats found no effect or found that cats given lysine got sicker and shed more virus. A 2015 systematic review recommended stopping its use, while a 2016 review by researchers who ran several of the trials concluded a measured dose may help in narrow circumstances but that adding lysine to food is not supported.

Can lysine make a cat's herpes worse?

In two shelter trials, cats fed a lysine-enriched diet developed more severe disease and shed more virus than cats on a standard diet. One explanation offered is that sick cats ate less, so they took in less lysine exactly when it was meant to matter. Cornell's guidance says several studies suggest lysine may worsen symptoms and promote shedding.

Is lysine safe for cats?

The trials reported it was generally well tolerated at the doses studied, with mild digestive upset in some cats in one study. Feeding studies found high-lysine diets did not lower arginine in adult cats, although cats ate less on them. The practical risk the experts flag is stress: Cornell notes that the stress of giving medications can itself worsen herpes eye flare-ups.

Is a lysine-enriched cat food a good way to give it?

It is the form with the weakest record. In the two trials that fed cats a lysine-enriched diet, disease was more severe and viral shedding higher than in cats on a standard diet. The only positive findings came from a measured dose given to the cat, and even the more favourable 2016 review states that the data do not support dietary supplementation.

What does help a cat with feline herpesvirus?

Cornell recommends herpesvirus vaccination for all cats because it reduces disease severity and viral shedding. For flare-ups, the options it describes are veterinary ones, including antiviral drugs, alongside supportive care that keeps the cat eating and drinking and limiting stress triggers such as crowding, a new cat in the house, or a move.

Sources

• Cornell Feline Health Center: respiratory infections
• Cornell Feline Health Center: hepatic lipidosis
• Bol S, Bunnik EM. Lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats: a systematic review. BMC Vet Res 2015, 11, 284
• Thomasy SM, Maggs DJ. A review of antiviral drugs and other compounds with activity against feline herpesvirus type 1. Vet Ophthalmol 2016, 19(Suppl 1), 119-130
• Stiles J, Townsend WM, Rogers QR, Krohne SG. Effect of oral administration of L-lysine on conjunctivitis caused by feline herpesvirus in cats. Am J Vet Res 2002, 63(1), 99-103
• Maggs DJ, Nasisse MP, Kass PH. Efficacy of oral supplementation with L-lysine in cats latently infected with feline herpesvirus. Am J Vet Res 2003, 64(1), 37-42
• Rees TM, Lubinski JL. Oral supplementation with L-lysine did not prevent upper respiratory infection in a shelter population of cats. J Feline Med Surg 2008, 10(5), 510-513
• Drazenovich TL, Fascetti AJ, Westermeyer HD, et al. Effects of dietary lysine supplementation on upper respiratory and ocular disease and detection of infectious organisms in cats within an animal shelter. Am J Vet Res 2009, 70(11), 1391-1400
• AAFCO Dog and Cat Food Nutrient Profiles, revision document with the cat food nutrient table