Understanding EHV-1 and the Role of Nutrition in Horse 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 to 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-esque "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. Specific equine nutrient requirements have yet to be established. Arginine plays a role in cell functions essential for normal immunity, such as M2 macrophage expression. M2 macrophages are crucial for anti-inflammatory responses. Research in other species shows improved immune markers after supplementing Arginine, highlighting its importance.

Researchers have fed 50g of L-Arginine to broodmares for five months during late pregnancy and foaling. No side effects were observed in supplemented horses, and no alterations in protein metabolism were found between supplemented and unsupplemented groups. A significant amount of supplemental Arginine does not fully enter circulation. Data from other studies indicate that 40% of dietary Arginine is degraded in the small intestine, which could explain why this amount of supplementation had minimal effects.

What About Lysine?

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

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

Make sure your horse has sufficient amounts in their diet, but don't feel pressured to add MORE. Many nutrition professionals do not recommend adding individual amino acids without professional guidance and a 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, hormones, cytokines, endotoxins, and other substances. Arginine and Lysine compete for entry into cells, but this interaction is not well studied in horses. Most forages contain more Lysine than Arginine, and as mentioned above, a lot of dietary Arginine does not reach circulation.

Studies feeding additional Arginine at levels of 50-100g to horses did not report negative side effects or effects on protein metabolism. One study that fed over 125g reported some amino acid differences in plasma one hour after feeding. These doses are higher than any supplement would provide, as amino acid supplements often contain only tiny, often insignificant amounts of amino acids.

How Does This Relate to EHV-1?

There are theories suggesting that supplementing L-Lysine can help inhibit herpesvirus activity, based on data from humans with Herpes simplex outbreaks. The idea is that since herpesviruses require Arginine for replication, and since Lysine and Arginine can interact, supplementing Lysine may compete with Arginine uptake into the body. This could prevent the virus from getting what it needs to replicate. However, 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). Some other forms of equine herpesvirus are classified under gammaherpesvirus subfamilies (EHV-2, EHV-5, EHV-7). Only EHV-1 to EHV-5 can produce disease in horses, while EHV-6 to EHV-8 can cause disease in donkeys. EHV-1 causes respiratory, reproductive, and neurological disease, whereas the human herpes complex, also an alphaherpesvirus, causes sores. As such, equine herpesviruses differ from those in other species, and findings from herpesvirus data in other species may not be applicable.

Currently, we lack scientific studies involving horses, so we don't have enough evidence to determine if amino acid manipulation can influence immunity. Until quality evidence is available, ensuring your horse’s protein and amino acid requirements are met gives your horse the best chance to perform necessary normal functions.

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Addressing Misconceptions About EHV-1

There are two aspects of this discussion that I find upsetting.

1. The Implied Blame on Owners

If 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 can remain latent, meaning the virus can reactivate at any time in an infected horse, leading to clinical disease and viral shedding. Reactivation of latent EHV-1 can occur due to stressful conditions (transport, handling, moving, weaning, etc.), which naturally suppress the immune system. It can also be experimentally induced by immunosuppressants. EHV-1 is highly contagious through contact exposure with objects and aerosols.

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

Anything else you read that lacks context, is not backed by quality evidence, and often takes advantage of a crisis by making horse owners feel like they’re not doing enough should be approached with caution.

2. “Cheaper Than One Temperature Check from the Vet”

Any horse care advice that dissuades you from consulting a vet raises an instant red flag. Whether it’s a supplement company claiming their product replaces a veterinary drug (which is illegal), an influencer suggesting a DIY remedy, or a Facebook group downplaying your horse’s condition, I’m usually painting that red. Bright red. Taylor Swift lipstick red. Are we clear? That's a big no-no.

Often, pieces of misinformation are shared by well-meaning sources. However, it’s crucial to treat new ideas with curiosity instead of accepting them as fact, especially when emotional factors make us more likely to believe these new sources.

Nutritional Support for Your Horse

So, you want to feel like you’re doing something nutritionally? The easiest way to support your horse’s immune health is to focus on their digestive health. This includes ensuring species-appropriate calories driven by 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, and 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, along with salt, free-choice water, and the recommended amount of a commercial product (vitamin/mineral 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 other ways to protect and support your horse’s immunity. 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.