Lysine For Cats

Chelsea McGivney, DVM, Nestle Purina Pet Care

Reading Time: 4 mins, 17 secs.

In the past, administering L-lysine for cats was thought to be helpful in fortifying the immune system, while warding off respiratory infections - specifically, feline herpes virus, also known as FHV-1.

L-lysine for cats was thought to help alleviate: 

• Respiratory issues: panting, coughing, difficulty breathing
• Conjunctivitis (inflammation of the conjunctiva) 
• Sinus issues: sneezing, congestion, sinus discharge
• Lethargy
• Fever
• South throat
• Cold sores, fever blisters, and mouth sores
• Appetite loss

Lysine for Cats Dosage

Oral L-lysine (250mg twice a day) was one treatment option for recurring FHV-1 flare-ups. Veterinarians typically prescribed L-lysine for cats to eliminate respiratory infections because of the assumption that the amino acid reduces the symptoms and prevents future flare-ups. Previously, veterinarians thought L-lysine interfered with the FHV-1 virus replication by interfering with the uptake of the amino acid, arginine.

Why is L-lysine beneficial for cats?

• Supports the immune system by helping treat the symptoms of a respiratory infection.
• Naturally treats illness by alleviating symptoms and preventing future flare-ups.
• L-lysine is an organic compound, which every cat has, but then again, some especially don’t have enough of it.
• The feline herpes virus needs the amino acid arginine to reproduce. Lysine is an amino acid, which was thought to ‘trick’ the virus by not letting it reproduce and replicate as efficiently.

Lysine for Cats: Questionable Benefits

Recent research is contesting that L-lysine may not be beneficial for treating cats with feline herpes virus like previously believed.

An industry leader, The Merck Veterinary Manual stated, “Previously lifelong oral L-lysine (250/500 mg per day) was recommended to help prevent or reduce the severity of feline herpes virus infections. However, recent work has shown that oral L-lysine can exacerbate the feline herpes virus.”

In 2009, an article published in the American Journal of Veterinary Research examined 261 cats, and determined that dietary lysine supplementation bore no influence on controlling or preventing infectious upper respiratory disease among cats. This study went on to demonstrate that L-lysine was an ineffective management tool for treating symptoms or flare-ups of FHV-1, and may even make the infection worse.

Then in 2015, the BMC Veterinary Research Journal published a study contesting L-lysine for cats is ineffective.

Why L-lysine for cats does not support FHV-1 treatment/prevention:

• Trials have failed to prove the efficacy of L-lysine supplements.
• L-lysine does not have antiviral properties.
• L-lysine undesirably lowers arginine levels, and since cats can't synthesize the amino acid themselves, an arginine deficiency can result in hyperammonemia.
• No studies show that intracellular arginine is ever at low enough levels to actually inhibit the synthesis of FHV-1.
• In vitro studies with FHV-1 do not show an inhibitory effect of excess L-lysine on viral replication.
• A L-lysine allergy may occur causing uncomfortable side effects.

What are Lysine's Side Effects?

Along with the conflicting L-lysine research, a L-lysine allergy can be a threat when the immune system overreacts, causing the skin to become inflamed and itch. The root of a  L-lysine allergy is quickly addressed and resolved by removing the L-lysine from the cat’s diet altogether.

Possible side effects of L-lysine in cats:

• Nausea, vomiting
• Scratching, Itchiness, licking, biting of the skin
• Diarrhea
• Seizures
• Pale gums

Texas feline practitioner Gary D. Norsworthy, DVM, Dipl ABVP, who co-edited ‘The Feline Patient, 4th Edition,’ said, “Some cats have a convincing response despite what science says. I do not recommend it as a first-line treatment for feline herpes virus, but I also do not discourage it… Its use should be on the cat’s response, whether scientific or not.”

Lysine for Cats FAQs

QUESTION: Are there other illnesses that could be confused with the feline herpes virus?

ANSWER: Veterinarypartner.com estimates 90 percent of cat respiratory infections are a result of the herpes virus, or a similar virus known as, Calicivirus.

QUESTION: What causes FHV-1 to flare-up?

ANSWER:  According to The Textbook of Veterinary Internal Medicine, “...After a stressful event, the virus reactivates in carriers, and they can affect other animals. Stresses can induce virus shedding, including a change of housing, kittening, etc. Some cats show clinical signs during a reactivation episode, which is a useful indicator they're likely to be infectious.”

QUESTION: Does L-lysine suppress FHV-1?

ANSWER: Authors David Maggs, BVSc, Mark P. Nasisse, and Philip H. Kass, DVM, PHD state their study finds, “Fewer cats and eyes were affected by conjunctivitis. The onset of clinical signs of infection was delayed on average by 7 days in cats receiving L-lysine. Compared with cats in the control group. However, significant differences between groups were not demonstrated.”

Lysine Supplements for Cats

QUESTION: Do studies fail to demonstrate that L-lysine effectively treats FHV-1?

ANSWER: The Textbook of Veterinary Internal Medicine states, “The two main causes of viral respiratory disease in cats are feline herpesvirus FHV-1 or feline calicivirus FCV. FHV-1 generally induces a more severe disease than FCV…"

"FHV-1 causes upper respiratory tract disease [URT] with oculonasal discharge, conjunctivitis, sneezing, and sometimes hypersalivation and coughing….The primary treatment involves supportive therapy… Robust evidence that orally administered L-lysine ameliorates the signs of FeHV-1 infection is lacking…”

QUESTION: Does L-lysine have no side effects?

ANSWER: False. L-lysine lowers arginine levels, can cause allergic reactions, and has research negating its effects.

Authors Sebastiaan Bol and Evelien M. Bunnik wrote in their study, "We recommend an immediate stop of lysine supplementation because of the complete lack of any scientific evidence of its efficacy."

QUESTION: Is L-lysine no longer the forerunner in treating FHV- 1?

ANSWER: Dr. Jennifer Coates of PetMd wrote, “It will be hard for me to continue to endorse the use of lysine in the face of this recent article. I guess, I’ll now have to lean more heavily on my other recommendations:

1. Practice aggressive preventative medicine and treat health conditions that do develop quickly to avoid “distracting” the immune system.
2. Treat especially severe flare-ups with antiviral medications and secondary bacterial infections with antibiotics.
3. Reduce exposure to whatever is stressful for the infected cat.
4. Most importantly, provide excellent overall nutrition to support the immune system.

 FHV-1 and Lysine Alternatives

When it comes to alternatives to L-lysine for the management of the feline herpes virus, consider the following tips.

1. Use Probiotics: Upkeep a feline’s digestive system by incorporating Fortiflora into the cat's meal.
2. Mix in Healthy Toppers: Ensure all the minerals and vitamins a cat needs to maintain their health are a part of their diet.
3. Consider Bovine Lactoferrin: Known for its antibacterial, antiviral, and anti-allergic properties.
4. Try Famciclovir: Helps stop the replication of the feline herpes virus with attention to reducing symptoms.
5. Blend Catnip into a Dish: Relax your cat's mood by incorporating catnip into its regiment.
6. Create a Calm Home Environment: Maintain a stress-free home environment, to support your cat's health.