11/22/2025
More research backed info on the nutrition side of EHV-1.
Short version - a time trying to sell you lysine as a preventative or “treatment” is not current on the research. Marketing products that are not likely to help is kinda shady.
Short version part 2 - rather than focus on a single element of the diet, focus on an overall balanced diet that meets your horses requirements based on work load and weight as shown in the National Research Council’s "Nutrient Requirements of Horses", published by the National Academies Press.
Need help?
Legacy Equine Nutrition advisors, PhD equine nutritionists like Dr Rachel Mottet and Dr Clair Thunes can guide you.
Above all, maintain solid biosecurity practices, monitor horses and if the risk feels like too much, stay home and avoid travel.
𝐇𝐞𝐫𝐩𝐞𝐬𝐯𝐢𝐫𝐮𝐬 & 𝐋𝐲𝐬𝐢𝐧𝐞: 𝐖𝐡𝐚𝐭 𝐭𝐡𝐞 𝐑𝐞𝐬𝐞𝐚𝐫𝐜𝐡 𝐒𝐚𝐲𝐬
There’s some chatter in the equine world right now that’s created just enough chaos to wake me up from my fall hiatus. And unless you’ve been living under a rock, you probably know exactly what I’m talking about – the equine herpesvirus outbreak in the U.S. and Canada.
There’s already a ton of solid info out there about the virus, the strains, what to watch for, and the biosecurity steps everyone should be taking. So that’s not why I am here. What I want to talk about are some of the claims floating around about (you guessed it) lysine.
As soon as the outbreak popped up on my feed, I immediately saw tips to help protect your horse against the virus, but one in particular jumped out: supplemental lysine. I knew right away that I wanted to investigate it further to see if there was any truth to these claims. So I went down the rabbit hole of research, and here is what I found!
𝐖𝐡𝐚𝐭 𝐢𝐬 𝐥𝐲𝐬𝐢𝐧𝐞 𝐚𝐧𝐝 𝐰𝐡𝐲 𝐝𝐨𝐞𝐬 𝐢𝐭 𝐦𝐚𝐭𝐭𝐞𝐫?
Lysine is an essential amino acid for horses and a fundamental building block of protein. So how does this connect to herpesvirus? In humans and cats, early studies suggested that herpesviruses require arginine, another amino acid, to replicate. Some researchers proposed that lysine might interfere with arginine availability, which led to the idea that feeding extra lysine could outcompete arginine and reduce viral replication or shedding to ultimately reduce the severity or recurrence of the disease.
𝐖𝐡𝐚𝐭 𝐝𝐨𝐞𝐬 𝐭𝐡𝐞 𝐫𝐞𝐬𝐞𝐚𝐫𝐜𝐡 𝐬𝐚𝐲?
This is where it gets interesting.
The research on this topic has primarily taken place in humans and cats. So let’s take a deeper dive into what's out there!
𝑬𝒂𝒓𝒍𝒚 𝑰𝒏 𝒗𝒊𝒕𝒓𝒐 (𝑪𝒆𝒍𝒍 𝑪𝒖𝒍𝒕𝒖𝒓𝒆) 𝑾𝒐𝒓𝒌
It all began in 1960’s with an in vitro study by Tankersley (1964) evaluating human herpesvirus 1 (HHV-1/HSV-1) in cell culture. This study found that this virus required arginine for replication in cell culture while lysine was not required. By the 1980’s, Griffith et al. (1981) reported similar findings and suggested that lysine could potentially inhibit HSV-1 replication in specific cell culture conditions. By the 1990’s, this research had expanded into the veterinary field.
𝑪𝒍𝒊𝒏𝒊𝒄𝒂𝒍 𝑻𝒓𝒊𝒂𝒍𝒔
The first feline clinical study evaluating feline herpesvirus 1 (FHV-1) was in 2002 by Stiles et al., using only eight cats fed 500 mg of lysine. Those cats had higher plasma lysine and slightly less severe conjunctivitis, but no difference in viral shedding and their plasma arginine levels didn’t change.
The next study (Maggs et al., 2003) used 14 cats and found lysine supplementation (400 mg) resulted in elevations in plasma lysine but no change in plasma arginine levels. While clinical signs did not differ across treatment geoups, the authors found that the lysine-treated group had reduced viral shedding.
In 2007, the research became more robust as Maggs et al. used 50 cats with FHV-1 and fed half 51 g lysine per kg diet (lysine) and the other half a diet of 11 g lysine per kg diet (control). While the lysine supplemented cats had higher plasma lysine and lower plasma arginine, they also had greater disease severity than the control group.
Another study (Rees et al., 2008) looked at 291 cats from the day they were brought into the shelter until they were adopted. Cats were randomly assigned to a lysine (250 or 500 mg based on size) or control group. In this study, 37% of lysine cats developed clinical signs compared to 34% of cats in the control group. These authors also highlighted the importance of stress reduction and proper sanitation to reduce illness in cats which they believed would be a better use of money and resources than lysine supplementation.
In 2009, Drazenovich et al. used 261 cats which were fed either a control diet (17 g lysine/kg food) or a lysine-enriched diet (57 g lysine/kg food). While plasma lysine was higher in the lysine group, these cats were also more likely to develop moderate to severe signs of the disease and test positive when compared to the control group.
Based on these studies, lysine consistently increased plasma lysine but did not reliably change plasma arginine, did not consistently reduce viral shedding, and, in a surprising turn of events, two of these clinical trials even suggested that providing supplemental lysine in the diet could make the clinical signs of FHV-1 worse. As research on this topic has continued to unfold, limitations and potential sources of bias in the earlier studies have become more apparent. While some of the smaller trials showed promising results, those findings couldn’t be replicated in larger, more controlled studies, which really challenges the original claims. Most recent reviews conclude that there is no reliable evidence to support lysine supplementation as a management tool for herpesvirus.
𝑯𝒐𝒓𝒔𝒆 𝑹𝒆𝒔𝒆𝒂𝒓𝒄𝒉?
To my knowledge (and I dug hard), there is ZERO research that has been completed in horses to evaluate the impact of lysine on equine herpesvirus. Recommendations floating around are based almost entirely on early human HSV studies and later cat FHV-1 studies, many of which have since been challenged or contradicted.
Even though EHV and FHV are both alphaherpesviruses, they behave very differently in their respective hosts. EHV can cause severe respiratory disease, abortions, neonatal foal death, and neurological disease (EHM). None of these manifestations have ever been studied in relation to lysine. Horses also have entirely different digestive systems, amino acid metabolism, and dietary requirements than the species these studies were based on. So even if lysine showed positive effects elsewhere, there is no reason to assume those results would apply to horses or to equine-specific herpesviruses.
𝐒𝐨 𝐰𝐡𝐞𝐫𝐞 𝐝𝐨 𝐰𝐞 𝐠𝐨 𝐟𝐫𝐨𝐦 𝐡𝐞𝐫𝐞?
This situation is stressful and scary, and it's natural to want to do everything in our power to protect our horses. And if I am being honest, I am a bit disappointed that many supplement and feed companies are using this outbreak as a marketing strategy. In reality, if your horse is eating a quality, balanced feed at or above the recommended levels, their lysine requirements are almost certainly being met - which is what truly matters.
Rather than reaching for extra lysine, focus on what genuinely supports your horse’s health: a balanced diet. Many horses I evaluate are deficient in key nutrients like copper, zinc, vitamins A and E, and selenium. Ensuring overall nutrient balance will always do more for your horse than supplementing one unproven ingredient.
But most importantly, utilize proper biosecurity measures and work closely with your veterinarian if there are any concerns. Remember, an ounce of prevention is worth more than a pound of a cure!
Stay safe and healthy, everyone!
Cheers!
Dr. DeBoer
(a Ph.D., not a vet)
Tankersley Jr RW. Amino acid requirements of herpes simplex virus in human cells. Journal of bacteriology. 1964 Mar;87(3):609-13.
Griffith RS, DeLong DC, Nelson JD. Relation of arginine-lysine antagonism to herpes simplex growth in tissue culture. Chemotherapy. 1981 Sep 8;27(3):209-13.
Stiles J, Townsend WM, Rogers QR, Krohne SG. Effect of oral administration of L-lysine on conjunctivitis caused by feline herpesvirus in cats. American Journal of Veterinary Research. 2002 Jan 1;63(1):99-103.
Maggs DJ, Nasisse MP, Kass PH. Efficacy of oral supplementation with L-lysine in cats latently infected with feline herpesvirus. American Journal of Veterinary Research. 2003 Jan 1;64(1):37-42.
Maggs DJ, Sykes JE, Clarke HE, Yoo SH, Kass PH, Lappin MR, Rogers QR, Waldron MK, Fascetti AJ. Effects of dietary lysine supplementation in cats with enzootic upper respiratory disease. Journal of Feline Medicine and Surgery. 2007 Apr;9(2):97-108.
Rees TM, Lubinski JL. Oral supplementation with L-lysine did not prevent upper respiratory infection in a shelter population of cats. Journal of Feline Medicine and Surgery. 2008 Oct;10(5):510-3.
Drazenovich TL, Fascetti AJ, Westermeyer HD, Sykes JE, Bannasch MJ, Kass PH, Hurley KF, Maggs DJ. Effects of dietary lysine supplementation on upper respiratory and ocular disease and detection of infectious organisms in cats within an animal shelter. American journal of veterinary research. 2009 Nov 1;70(11):1391-400.
