VetMedFeline herpesvirus-1: oral L-lysine vs placebo — MAYBE (weak evidence)
In cats with, or at risk of, feline herpesvirus-1 (FHV-1) infection, does oral L-lysine supplementation reduce viral shedding or clinical episodes compared with placebo or no treatment, in shelter or household settings?
Shelf #337 · gate-verified and approved by the developer
MAYBE (weak evidence): The evidence splits sharply by setting. In two tiny experimental bolus studies, lysine showed partial signals: less severe conjunctivitis than placebo after experimental inoculation in 8 cats (Stiles 2002, with no difference in virus isolation), and significantly fewer viral shedding episodes after rehousing — but not after corticosteroid reactivation — in 14 latently infected cats, whose clinical-sign differences were not significant (Maggs 2003). In the settings where lysine is actually used at scale, it did nothing or worse: 144 lysine cats versus 147 untreated shelter cats showed no effect on any measure (Rees & Lubinski 2008), and 261 shelter cats on a 5.7%-lysine diet developed more moderate-to-severe disease in week 4 and had FHV-1 DNA detected more often in week 2 than basal-diet cats (Drazenovich 2009). A 2015 systematic review of this literature (Bol & Bunnik, PMID: 26573523 — context only, not carded as a claim-carrier) concluded lysine supplementation is not effective for prevention or treatment of FHV-1 infection and recommended an immediate stop. Gaps: no carded trial tests an individual oral bolus in household cats with naturally occurring recurrent FHV-1; the only placebo-controlled card is an 8-cat experimental-inoculation study, and no carded record establishes blinding. Drafted Nov 10, 2026 (VetMed build, topic #337); all figures from abstracts fetched and verified at drafting.
Stiles 2002: lysine-treated cats had less severe conjunctivitis than placebo cats after experimental FHV-1 inoculation, but virus isolation did not differ
Clinical Trial (publication type); placebo-controlled experimental-infection study — the record does not state randomisation or blinding
Who: Cats: 8 healthy young adult cats in an experimental setting. Cats received oral lysine monohydrochloride (500 mg, q 12 h) or placebo (lactose) beginning 6 hours prior to inoculation of virus; the left conjunctival sac received a 50-microl suspension of FHV-1 on day 1, clinical signs were scored daily for 21 days, samples for virus isolation were collected from eye and throat every third day, and plasma lysine and arginine were measured before the study and on days 3, 14 and 22 (per-group numbers are not given in the record).
Result: Cats that received lysine had less severe conjunctivitis than cats that received placebo. Virus isolation results did not differ between the groups. Plasma lysine concentration was significantly higher in cats that received lysine, compared with control cats, whereas plasma arginine concentrations did not differ between groups. The authors concluded oral lysine was well tolerated and may be helpful in early treatment by lessening severity of disease.
Watch out: Only 8 cats, experimentally inoculated and treated from before exposure — not cats with naturally occurring or recurrent disease. The record gives no numeric severity scores, does not state randomisation or blinding, and a placebo (lactose) arm alone is not stated blinding, so none is claimed here.
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;63(1):99-103. PMID: 16206789. Registry ID: none in the record. MeSH: Administration, Oral · Animals · Arginine/blood · Cat Diseases/drug therapy · Cat Diseases/virology · Cats · Conjunctivitis, Viral/drug therapy · Conjunctivitis, Viral/veterinary · Female · Herpesviridae Infections/drug therapy · Herpesviridae Infections/veterinary · Lysine/administration & dosage · Lysine/blood · Lysine/therapeutic use · Male
Maggs 2003: lysine cats shed virus less often after rehousing, but clinical-sign differences were not significant and the shedding benefit vanished after corticosteroid reactivation
Experimental reactivation study; the record's MeSH includes Random Allocation, but the abstract says only that cats were 'assigned' — no blinding stated in the record
Who: Cats: 14 young adult, FHV-1-naive cats in an experimental setting. Five months after primary conjunctival inoculation with FHV-1, the latently infected cats were rehoused and assigned to receive 400 mg of L-lysine in food once daily for 30 days or food only; on day 15, all cats received methylprednisolone to induce viral reactivation, with clinical signs graded and viral shedding assessed by polymerase chain reaction assay throughout (per-group numbers are not given in the record).
Result: Fewer cats and eyes were affected by conjunctivitis, and 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. Significantly fewer viral shedding episodes were identified in the treatment group cats, compared with the control group cats, after rehousing but not following corticosteroid-induced viral reactivation. Mean plasma L-lysine concentration was significantly increased at 3 hours but not at 24 hours after L-lysine administration, and plasma arginine concentration was not significantly altered.
Watch out: This is the anchor trial the draft pointed to, and it is not a placebo trial: the control group received food only. The one significant result (fewer shedding episodes after rehousing) did not persist through corticosteroid-induced reactivation, and the clinical-sign outcomes the shelf question cares about were not significant. Fourteen cats only.
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;64(1):37-42. PMID: 12518876. Registry ID: none in the record. MeSH: Administration, Oral · Animals · Arginine/blood · Cat Diseases/metabolism · Cat Diseases/virology · Cats · Conjunctivitis/drug therapy · Conjunctivitis/veterinary · Conjunctivitis/virology · Dietary Supplements/standards · Glucocorticoids/pharmacology · Herpesviridae/physiology · Herpesviridae Infections/drug therapy · Herpesviridae Infections/veterinary · Herpesviridae Infections/virology · Lysine/blood · Lysine/pharmacology · Methylprednisolone/pharmacology · Polymerase Chain Reaction/veterinary · RNA, Viral/analysis · Random Allocation · Specific Pathogen-Free Organisms · Virus Activation · Virus Shedding/drug effects
Rees & Lubinski 2008: in 291 shelter cats, L-lysine did not prevent upper respiratory infection or conjunctivitis on any measure
Shelter prevention study with a no-treatment control (record's own wording); randomisation and blinding are not stated in the record
Who: Cats: 291 cats in an animal shelter. One hundred and forty-four cats were treated with L-lysine in a small amount of canned food once daily; a 'no treatment' group of 147 cats received no lysine during the course of the study; the development of conjunctivitis or upper respiratory infection (URI) was tracked between the two groups.
Result: In all measures, there was no effect between the two groups, suggesting that lysine was not able to prevent URI or conjunctivitis in this shelter situation. The authors suggested shelters may better use their resources by finding ways to decrease stress among their feline population, focusing on proper infection control measures and limiting fomite transmission of disease.
Watch out: The comparator was no treatment, not placebo — secondary sources that describe this trial as randomised and placebo-controlled go beyond its PubMed record, which states neither. Shelter URI is not all FHV-1, so this tests lysine against shelter URI as it presents, not against confirmed FHV-1 episodes; within that real-world question, the result was null on every measure.
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;10(5):510-3. PMID: 18547855. Registry ID: none in the record. MeSH: Administration, Oral · Animal Husbandry/methods · Animal Welfare · Animals · Cat Diseases/epidemiology · Cat Diseases/prevention & control · Cat Diseases/transmission · Cat Diseases/virology · Cats · Conjunctivitis/prevention & control · Conjunctivitis/veterinary · Conjunctivitis/virology · Dietary Supplements · Female · Herpesviridae Infections/epidemiology · Herpesviridae Infections/prevention & control · Herpesviridae Infections/transmission · Herpesviridae Infections/veterinary · Lysine/administration & dosage · Male · Respiratory Tract Infections/prevention & control · Respiratory Tract Infections/transmission · Respiratory Tract Infections/veterinary · Respiratory Tract Infections/virology · Risk Factors
Drazenovich 2009: shelter cats on a 5.7%-lysine diet developed more moderate-to-severe disease in week 4 and had FHV-1 DNA detected more often in week 2 than basal-diet cats
Shelter dietary-intervention study; the record states no randomisation or blinding
Who: Cats: 261 adult cats within an animal shelter, fed a diet containing 1.7% lysine (basal diet; control cats) or 5.7% lysine (supplemented diet; treated cats) for 4 weeks, with plasma lysine and arginine assessed at baseline and study end, clinical scores for nasal and ocular disease assigned three times a week, and conjunctival and oropharyngeal swab specimens tested weekly for FHV-1, feline calicivirus (FCV) and Chlamydophila felis nucleic acids; data were collected from 123, 74, 59 and 47 cats during study weeks 1, 2, 3 and 4 respectively.
Result: By study end, plasma lysine concentration in treated cats was greater than that in control cats and had increased from baseline. There was no difference between dietary groups in the proportion of cats developing mild disease. However, more treated cats than control cats developed moderate to severe disease during week 4, and during week 2, FHV-1 DNA was detected more commonly in swab specimens from treated versus control cats. The authors concluded dietary lysine supplementation in the amount used was not a successful means of controlling infectious upper respiratory disease within a cat shelter, and rather led to increases in disease severity and FHV-1 DNA detection at certain time points.
Watch out: Lysine was delivered in the diet (5.7% vs 1.7% basal), not as an individual oral bolus or against a placebo, and the record states no randomisation or blinding — descriptions of this trial elsewhere as randomised, blinded and placebo-controlled are not established by its PubMed record. Attrition was heavy: only 47 of 261 enrolled cats still contributed data in week 4, and the adverse differences were time-point-specific (weeks 2 and 4), not uniform across the study.
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;70(11):1391-400. PMID: 19878022. Registry ID: none in the record. MeSH: Animal Feed/analysis · Animals · Calicivirus, Feline/isolation & purification · Cat Diseases/microbiology · Cat Diseases/prevention & control · Cats · Chlamydophila Infections/prevention & control · Chlamydophila Infections/veterinary · Dietary Supplements · Eye Infections/microbiology · Eye Infections/prevention & control · Eye Infections/veterinary · Female · Herpesviridae/classification · Herpesviridae Infections/veterinary · Lysine/pharmacology · Male · Respiratory Tract Infections/microbiology · Respiratory Tract Infections/prevention & control · Respiratory Tract Infections/veterinary
Synthesis: bolus lysine shows small, partial signals in tiny experimental studies; in real shelter populations it does not prevent disease and may worsen it
Synthesis
Who: Synthesis of the cards above: Stiles 2002 (8 healthy cats, placebo-controlled experimental FHV-1 inoculation), Maggs 2003 (14 latently infected cats, lysine in food vs food only, rehousing and corticosteroid reactivation), Rees & Lubinski 2008 (291 shelter cats, lysine vs no treatment) and Drazenovich 2009 (261 shelter cats, 5.7% vs 1.7% dietary lysine).
Result: (1) Severity: less severe conjunctivitis on lysine in one 8-cat experimental study (Stiles), but clinical-sign differences not significant in the latent-infection study (Maggs). (2) Shedding: significantly fewer shedding episodes on lysine after rehousing only, not after corticosteroid reactivation (Maggs); virus isolation did not differ in Stiles; and FHV-1 DNA was detected more often in lysine-fed shelter cats at week 2 (Drazenovich). (3) Prevention in shelters: no effect on any measure in 291 cats (Rees & Lubinski), and more moderate-to-severe disease at week 4 on the lysine diet (Drazenovich). The pattern is consistent: any benefit is confined to small experimental bolus settings, and the large field studies are null or adverse.
Watch out: No carded study tests the common household use case — an individual bolus dose given to a pet cat with naturally occurring recurrent FHV-1 — and none of the carded records establishes blinding. Shelter studies used dietary or once-daily-in-food administration and shelter URI is multi-pathogen, so their null results answer the shelter question directly but the household bolus question only indirectly.
Synthesis of Stiles J et al. PMID: 16206789; Maggs DJ et al. PMID: 12518876; Rees TM et al. PMID: 18547855; Drazenovich TL et al. PMID: 19878022. Registry IDs as on the individual cards.
Back to the library index · VetMed home
VetMed is an educational resource, not veterinary advice.