Fact Or Fiction: Lysine, Arginine & Immune Health

I was sent this post by a fellow professional today, and my gosh there's a LOT going on here. I don't want to call anyone out, so I'll paraphrase some of the contents:

“EHV-1 Outbreak: You Might Be Accidentally Feeding the Virus”

“You can add as much lysine as you want, but if your horse is still getting a big dose of arginine from another supplement, the lysine won’t work.”

“EHV-1 needs arginine to replicate, and Arginine both blocks lysine, and weakens antiviral protection. You have to cut arginine, stop handing the virus its favourite fuel.”

“Buy a 10-lb bucket of pure L-lysine and supplement 10–20 g. It lasts a month and is cheaper than one temperature check from the vet.”

*insert Sabrina Carpenter-eqse "oh boy" here*

So, as always, what does the science say?

To get started:

What is Arginine? How much does my horse need?

Arginine is an amino acid involved in metabolic functions and immunity, and as is the case with most amino acids in the equine diet, specific equine nutrient requirements have yet to be established. Arginine is involved in cell functions required for normal immunity, such as M2 macrophage expression. M2 macrophages are involved in anti-inflammatory responses.  Research in other species reports improved immune markers after supplementing Arginine in other species, highlighting its important functions.

Researchers have fed 50g of L-Arginine to broodmares for 5 months during late pregnancy and foaling. No side effects in supplemented horses were seen, and no alterations were seen in protein metabolism between supplemented and unsupplemented groups. A large amount of supplemental Arginine does not entirely enter circulation, as data from other studies reports that 40% of dietary Arginine is degraded in the small intestine, which could be why this amount of supplementation did not do much.

What about Lysine?

Lysine is the top limiting amino acid in the equine diet, in addition to threonine and methionine. The average adult horse needs at least 27g of Lysine per day, with working horses and breeding horses requiring higher amounts.

If you’re feeding the appropriate amount of a quality ration balancer, performance feed, etc, that feed is formulated to provide at least 10g of Lysine per day, so if you are feeding an appropriate diet, your horse is already getting additional lysine.

Do make sure your horse has sufficient amounts in the diet, but don't feel pressured to add MORE. Many nutrition professionals do not recommend adding individual amino acids without professional guidance and complete diet review.

Do Arginine and Lysine interact?

Yes - just like every other dietary molecule.

Arginine metabolism is regulated by many nutrients in the diet, including Lysine, as well as hormones, cytokines, endotoxins and other substances. Arginine and Lysine do compete with each other for entry into cells, but this is not well studied in horses, and most forages contain more Lysine than Arginine, and as mentioned above, a lot of dietary Arginine does not reach circulation into the body.

Studies feeding additional Arginine at levels of 50-100g to horses did not report negative side effects or effects on protein metabolism, with one study feeding over 125g reporting some amino acid differences in plasma 1 hour after feeding. These doses are higher than any supplement would provide (amino acid supplements are notorious for ‘fairydusting’ ingredients and only providing tiny, often insignificant amounts of amino acids).

And how does this relate to EHV-1?

There are some theories that supplementing L-Lysine can help inhibit herpesvirus activity, based on data in humans with Herpes simplex outbreaks. The theory is that since herpesviruses require Arginine for replication, since Lysine and Arginine can interact, supplementing Lysine may compete and block Arginine uptake into the body, preventing the virus from getting what it needs to replicate. Other studies have reported that Arginine can suppress virus replication.

EHV-1 is classified as an alphaherpesvirus (along with EHV-3, -4, -6, -8, -9), while some other forms of equine herpesvirus are classified under gammaherpesvirus subfamilies (EHV-2, EHV-5, EHV-7), but only 1-5 can produce disease in horses, while 6-8 can produce disease in donkeys. EHV-1 causes respiratory, reproductive and neurological disease, whereas human herpes complex, while also an alphaherpesvirus, causes sores. As such, equine herpesviruses differ from that of other species, and findings from herpesvirus data from other species with different viruses may not be applicable.

As we do not have any scientific studies with horses, so we don’t have enough evidence either way to know if amino acid manipulation can influence immunity. Until quality evidence is available, ensuring your horse’s protein and amino acid requirements (in addition to remaining requirements) are met gives your horse’s body the best bet to perform necessary normal functions.

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There are 2 parts about this that I find upsetting.

1: The implied blame that’s placed on owners. Your horse contracted an infectious virus? Well, YOU were “feeding the virus” (sigh).

EHV-1 is a terrifying and heartbreaking disease, and it’s not something to make light of. EHV-1 is capable of latency, where the virus can be reactivated anytime in an infected horse, showing clinical disease course and viral shedding. Reactivation of latent EHV-1 is possible following stressful conditions (transport, handling, moving, weaning, etc) which naturally suppress the immune system, and has been experimentally induced by immunosuppressants. EHV-1 is highly contagious by contact exposure with objects and aerosols.

Yes, nutrition plays a role in immunity, but so does biosecurity protocols, regular vaccination and overall horse management practices, and in an outbreak, following biosecurity measures & appropriate veterinary guidance is the only surefire way to reduce your horse’s risk.

Anything else you read lacks context, is not backed by quality evidence, and often times, is taking advantage of a crisis by making horse owners feel like they’re not doing enough. 

2: “Cheaper than one temperature check from the vet” - any horse care advice that dissuades you from the vet is an instant red flag.

Whether it’s a supplement company telling you that their product replaces a veterinary drug (illegal claim, they are NOT allowed to even say this), an influencer telling you to try a DIY instead, or a Facebook group saying your horse’s condition is “not that serious”, I’m usually painting that red. Bright red. Taylor Swift lipstick red. Are we clear? Big no no.

A lot of the time, pieces of misinformation are shared by a well-meaning source, but it’s important that we treat new ideas with curiosity instead of taking them as fact, especially when there are emotional factors at play that make us more likely to want to believe these new sources.

So, you want to feel like you’re doing something nutritionally? 

The easiest way to support your horse’s immune health is to support their digestive health - species-appropriate calories driven with fiber, situation-appropriate NSC intake, gradual feed transitions and for added support, digestive health supplements with research-backed ingredients (at the right concentrations!). Prebiotics, (some) probiotics, postbiotics, antioxidants, anti-inflammatory omega-3s may be useful additions, but only on top of a balanced diet. 

If you’re feeding 1.5-2.0% of your horse’s body weight in hay with salt, free-choice water and the recommended amount of a commercial product (vit/min premix, ration balancer, performance/growth/senior feed), you’re on the right track. 

Work with your vet team and if you have one, your horse’s nutrition professional to identify any other ways your individual horse’s immunity can be protected and supported, but at the end of the day, biosecurity is key. 

Say hi to your horse for me, stay safe out there, and don’t let facebook get you down!

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References & Recommended Reading 

Hunka, M. M., Silva, E. R. R. da, Kutschenko, M., Nogueira, E. T., Manso, H. E. . C. da C. C., & Filho, H. C. M. (2016). Effects of L-Arginine Supplementation on Lactating Mares and the Development of Foals. Acta Scientiae Veterinariae, 44(1), 10. https://doi.org/10.22456/1679-9216.80904

Kelley DE, Warren LK, Mortensen CJ. (2014). Orally supplemented L-arginine impairs amino acid absorption depending on dose in horses. J Anim Sci, 92(12), 5560-5566. https://doi.org/10.2527/jas.2014-7690

Kelly, B., Pearce, El. 2020. Amino Acids: How Amino Acids Support Immunity. Cell Metabolism 32(2): 154-175. https://doi.org/10.1016/j.cmet.2020.06.010

Oladunni, FS., Horohov, DW., Chambers, TM. 2019. EHV-1: A Constant Threat to the Horse Industry. Front. Microbiol 10(2019): https://doi.org/10.3389/fmicb.2019.02668

Skelton, JE., Warren, LK., Kivipelto, J., Mortensen, CJ. 2011. Arginine Supplementation in Mares Does Not Augment Passive Transfer of Immunity to Foals. Journal of Equine Veterinary Science 31(5-6): 326-327.

Sun, Y., Zhang, S., Luan, X., He, J., Wang, J., Han, T. 2025. Evaluating the Impact of Arginine-to-Lysine Ratios on Growth Performance, Antioxidant Defense, and Immune Modulation in Juvenile Largemouth Bass (Micropterus salmoides). Animals 15(13):1947

Wu G., Bazer, F.W., Davis T.A., Kim S.W., Li P., Rhoads J.M., Satterfield M.C., Smith S.B., Spencer T.E. & Yin Y. 2009. Arginine metabolism and nutrition in growth, health and disease. Amino Acids. 37: 153-168.