My cat has chronic congestion and sneezing (aka feline viral rhinopneumonitis (FVR) or feline herpesvirus type 1 (FHV-1)). During a recent flareup (the congestion worsened so that he stopped eating) my vet sold us a bag of L-Lysine treats and instructed us to feed 6 treats a day normally, or 12 during a flareup.
Fortunately, Hunter thinks they are the best food ever so he is eating again, but is there any scientific evidence that L-Lysine can help a cat with chronic sinus congestion?
The gold standard for evaluating a treatment is to look at meta-analyses. This type of research paper looks at multiple studies and judges the experimental design, sample size, and other factors that may influence results to determine the overall outcome of multiple studies.
A meta-analysis for lysine supplementation as a treatment for FHV-1 was published in 2015.
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 Veterinary Research. 2015;11:284. doi:10.1186/s12917-015-0594-3. link
This paper reviewed 7 studies on lysine and FHV-1 (5 on cats, 2 on cell cultures) and 10 studies on lysine and human HV-1 (7 on people, 3 on cell cultures). They concluded:
There is evidence at multiple levels that lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats. Lysine does not have any antiviral properties, but is believed to act by lowering arginine levels. However, lysine does not antagonize arginine in cats, and evidence that low intracellular arginine concentrations would inhibit viral replication is lacking. Furthermore, lowering arginine levels is highly undesirable since cats cannot synthesize this amino acid themselves. Arginine deficiency will result in hyperammonemia, which may be fatal. In vitro studies with feline herpesvirus 1 showed that lysine has no effect on the replication kinetics of the virus. Finally, and most importantly, several clinical studies with cats have shown that lysine is not effective for the prevention or the treatment of feline herpesvirus 1 infection, and some even reported increased infection frequency and disease severity in cats receiving lysine supplementation.
We recommend an immediate stop of lysine supplementation because of the complete lack of any scientific evidence for its efficacy.
Although I do not know the answer specifically for feline herpes, L-lysine is helpful for human herpes (HSV-1 and HSV-2). Lysine and arginine are both amino-acids that pair with each other. The herpes virus feeds on arginine, so keeping the ratio of lysine higher than arginine is quite helpful in preventing and/or reducing the severity of outbreaks. Lysine is an essentional amino-acid, meaning we cannot create it ourselves. This may or may not apply to felines.
For humans, do not omit arginine from the diet, simply make sure you are ingesting a higher amount of lysine. However, I have read that taking lysine supplements daily is bad for your liver and kidneys. In moderation it is fine as long as there are no other contraindications. Basically, if you know you're stressed (or about to be stressed), you should start taking some lysine. You can also be more mindful as to what foods you eat. A simple websearch will list out all the foods that are higher in lysine vs argenine content. You may want to do some research on whether lysine suppliments are safe for cats to take daily, unlike for humans.
