Here is a fact that surprises most cat parents: your cat very likely carries feline herpesvirus, and so do most cats they will ever meet. It is one of the most common infections in the species, it stays for life once it takes hold, and for years the standard piece of advice for managing it was to sprinkle an L-lysine supplement into the food. That last part has not aged well. The current evidence says lysine mostly does not do what everyone hoped, and if you have been dutifully dosing it, this is the article that explains what actually helps instead.
First, the reassuring part: for all its permanence, feline herpes is usually a manageable, cold-like nuisance, not a crisis. Let’s cover what it is, then get to what has changed.
What feline herpes actually is
Feline herpesvirus 1, or FHV-1, is sometimes called feline viral rhinotracheitis. Despite the shared family name, it has nothing to do with the human sexually transmitted virus. It is a cat-only virus that mainly attacks the upper respiratory tract, producing the feline equivalent of a bad head cold, often with sore, runny eyes on top.
Like all herpesviruses, it has two traits that define how it behaves. It replicates fast, which is why it spreads so easily between cats. And it hides in nerve cells, which is why it never fully leaves. A cat’s symptoms clear, but the virus goes dormant and can wake up again later in life.
The symptoms to know
An active FHV-1 flare usually looks like a respiratory infection plus conjunctivitis (inflamed eyes). The common signs:
- Sneezing, often in frequent fits
- Nasal discharge and congestion, from clear to yellow or green mucus
- Fever, which can leave a cat tired and withdrawn
- Eye changes: redness, swollen lids, squinting, excess tearing or discharge
- Loss of appetite, partly because congestion dulls the sense of smell cats rely on to want food
- Lethargy, from the combination of fever, discomfort, and not eating
- Coughing or labored breathing in more serious cases
Most flares run their course in about three weeks. One symptom overrides the “wait it out” approach entirely: any difficulty breathing is an emergency and needs a vet right away, because severe infections can progress to pneumonia.
How cats catch it, and why nearly all of them do
The virus travels in the fluid from an infected cat’s eyes, nose, and mouth. It spreads through direct close contact and through contaminated objects: shared food bowls, bedding, toys, and even a person’s clothing after handling a sick cat.
This is why kittens, shelter cats, and cats from crowded or high-stress environments are hit hardest. An infected cat can shed the virus for up to three weeks, so a cat showing active respiratory signs should be kept away from other cats for at least that long. The flip side is genuinely good news: when the virus is dormant and the cat has no symptoms, they are not contagious.
Diagnosis: mostly a judgment call
Vets usually diagnose herpes from the pattern of symptoms and the cat’s history rather than a lab test, especially in a young or recently adopted cat with classic signs. Several viruses cause similar respiratory illness in cats, and you often will not know exactly which one is responsible. That is usually fine, because most of these infections are self-limiting and resolve with the same supportive care regardless of the cause. For flare-ups that are severe or refuse to clear, a vet may run a PCR respiratory panel to pin down the culprit and target treatment.
The lysine reversal
For a long time, the reflexive recommendation was L-lysine, an amino acid supplement believed to interfere with the virus’s ability to replicate. It was cheap, easy, and everywhere.
The problem is that the evidence never really backed it up, and as better studies came in, the picture got worse. A systematic review concluded that lysine is not effective for either preventing or treating FHV-1 in cats. Worse, in crowded settings like shelters, some research found lysine supplementation was associated with more viral shedding, not less, meaning it could backfire in exactly the environments where herpes spreads most. As a result, many veterinarians have stopped recommending it, or now treat it as optional at best.
“Lysine was the thing everyone reached for, and I understand why, it felt harmless and proactive,” says Dr. Lena Park. “But feeling proactive isn’t the same as helping. I’d rather a cat parent put that energy into lowering their cat’s stress and calling me early in a bad flare, because those are the things that actually move the needle.”
What actually helps
If not lysine, then what? A few things, and they are more about management than cure.
Supportive care during a flare. Keep the cat eating, hydrated, and comfortable. Wipe away eye and nose discharge gently, and warm up food to boost its smell so a congested cat still wants it. Running a humidifier can ease congestion.
Antivirals for the serious cases. When a flare is bad, especially with painful eye involvement, there is now solid evidence behind the antiviral famciclovir, a drug adapted from human herpes treatment that has shown real efficacy in cats. It is prescription-only and dosed by your vet, but it represents an evidence-based tool that lysine never was.
Stress reduction as prevention. Because reactivation is so tightly linked to stress, illness, and other immune hits, the most effective long-term strategy is a calm, stable life: predictable routines, minimal upheaval, and prompt care for other health problems. Reducing stress is not a soft suggestion here; it is the closest thing to real flare prevention we have.
Vaccination. The core FVRCP vaccine includes protection against FHV-1. It does not prevent infection outright, but it reduces the severity of illness, which is why keeping kittens and adults up to date matters.
When to call the vet
- Symptoms drag on beyond about two weeks, or keep coming back
- Your cat stops eating, since congestion-driven appetite loss can spiral
- The eyes look very painful, cloudy, or the discharge is heavy
- Any labored or open-mouth breathing, which is an immediate emergency
What’s new since this was written
The headline change is the shift away from L-lysine. Where it was once a near-automatic recommendation, current veterinary reviews find it does not reliably prevent or treat FHV-1, and it may increase viral shedding in group-housing settings. Many clinics have quietly retired it.
In its place, the evidence has strengthened around the antiviral famciclovir for significant flares, and around stress management and vaccination for keeping the virus dormant. If your cat was put on lifelong lysine years ago, it is worth a conversation with your vet about whether that is still the best use of your effort, or whether your cat would be better served by focusing on a low-stress routine and a clear plan for the next flare.
References
- Bol, S., Bunnik, E.M. “Lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats: a systematic review.” BMC Veterinary Research. ncbi.nlm.nih.gov
- Thomasy, S.M., Maggs, D.J. “Treatment of feline herpesvirus-1 associated disease in cats with famciclovir and related drugs.” Journal of Feline Medicine and Surgery. sciencedirect.com
- MSPCA-Angell. “Feline Herpesvirus: Therapeutic Review.” mspca.org
- American Association of Feline Practitioners. “Feline Vaccination Guidelines (FVRCP).” catvets.com
