Feline herpesvirus type 1 (FHV-1) is one of the most common infectious diseases in cats worldwide. Exact prevalence numbers vary widely depending on how testing is done and which cats are studied, but PCR testing finds the virus in roughly 6% of healthy cats and up to 28% of cats showing respiratory symptoms. In shelter and cattery environments, infection rates climb even higher. Most veterinary sources consider it the single most common cause of eye disease in cats.
How Many Cats Are Infected
Pinning down a single number is tricky because detection rates swing dramatically based on the testing method and the population being sampled. Studies using PCR, the most sensitive available test, have reported detection rates ranging from about 1% to nearly 89% across different groups of cats. A large multicenter study in Spain found FHV-1 DNA in 6% of healthy cats with no symptoms at all, 28% of cats with upper respiratory disease, and 24% of cats with eye inflammation. Another study using both standard and advanced PCR methods found an overall detection rate between 6.7% and 9.2% across cats with and without symptoms, rising to about 12% when only cats with active conjunctivitis were tested.
These numbers almost certainly undercount true infection rates. FHV-1 goes dormant in nerve tissue after the initial infection, meaning a cat can carry the virus for life without it showing up on a swab taken during a quiet period. The virus is only reliably detectable when it reactivates and migrates back to the eyes, nose, or throat. So a negative test doesn’t mean a cat is virus-free.
Why Shelters and Catteries See Higher Rates
Crowding, stress, and constant turnover make shelters ideal environments for FHV-1 to spread. Cats shed the virus through nasal and eye secretions, and close quarters mean direct contact is nearly unavoidable. Stress is a major trigger for reactivation in cats already carrying a latent infection, so the shelter environment creates a cycle: stressed carriers start shedding, and newly arrived cats with no prior exposure pick it up quickly. While specific shelter prevalence numbers vary by facility, veterinary guidelines consistently identify multi-cat environments as the highest-risk settings for transmission.
What the Infection Looks Like
FHV-1 primarily targets the upper respiratory tract and the eyes. The classic signs include sneezing, nasal congestion, clear or colored discharge from the eyes and nose, swelling of the membranes around the eyes (conjunctivitis), mouth ulcers, fever, lethargy, and loss of appetite. Corneal ulcers are a hallmark complication, and FHV-1 is considered the leading infectious cause of these painful eye lesions in cats.
The initial infection tends to be the most severe, especially in kittens. After recovery, the virus retreats into nerve cells and stays there permanently. Most cats never fully clear it. Periodic flare-ups can occur throughout a cat’s life, typically triggered by stress, illness, or anything that suppresses the immune system. These recurrences are usually milder than the first episode but can still cause noticeable eye and respiratory symptoms.
Long-Term Damage From Severe Infections
In some cats, particularly those infected as young kittens, the initial bout of herpes causes lasting structural damage inside the nasal passages. The intense inflammation can destroy the delicate scroll-shaped bones (called nasal conchae) that line the inside of the nose. This bone loss disrupts the normal defense mechanisms of the nasal lining, leaving cats vulnerable to chronic bacterial infections and ongoing nasal congestion, a condition called chronic rhinosinusitis.
Cats that develop these chronic nasal problems before age two tend to have more severe bone destruction and worse inflammation than those affected later in life. Interestingly, the virus itself often can’t be detected during the chronic phase. The damage has already been done, and ongoing symptoms are driven by secondary infections and permanently altered anatomy rather than active viral replication.
How It Spreads
FHV-1 passes between cats through direct contact with infected secretions from the eyes, nose, or mouth. Shared food bowls, bedding, and litter boxes can also serve as transmission routes, though the virus is relatively fragile outside the body compared to some other pathogens. Sneezing creates infectious droplets that can reach nearby cats.
The trickiest aspect of transmission is that cats can shed the virus without looking sick. A carrier cat experiencing a stress-induced reactivation may appear perfectly healthy while actively spreading the virus for days. This silent shedding is a major reason the virus is so thoroughly embedded in the cat population.
What Vaccination Does and Doesn’t Do
The standard FVRCP vaccine, which most cats receive as kittens and in booster doses, includes a feline herpesvirus component. It does not prevent infection. No current FHV-1 vaccine produces sterilizing immunity, meaning vaccinated cats exposed to the virus will still become infected, still carry it, and still shed it. What vaccination does accomplish is meaningful: it significantly reduces the severity of clinical illness. Vaccinated cats that encounter the virus develop milder symptoms and may shed less virus, which could slow transmission in a household or shelter.
Studies comparing vaccinated and unvaccinated kittens after deliberate exposure to FHV-1 consistently show that vaccinated groups have less severe illness. Combining injectable and intranasal vaccine routes appears to offer better protection than either approach alone. But even with the best vaccination strategy, your cat can still pick up and carry the virus.
The L-Lysine Question
L-lysine supplements have been recommended for decades as a way to manage feline herpes, based on the idea that lysine interferes with viral replication. The evidence, however, is underwhelming. A controlled study of cats with latent FHV-1 infections found that 400 mg of oral lysine daily did not significantly reduce clinical signs of disease. Cats receiving lysine had a similar overall disease severity compared to the control group. There was a modest reduction in viral shedding after housing changes (a stress trigger), but not after drug-induced reactivation. The onset of symptoms was delayed by about a week on average in the lysine group, but the difference did not reach statistical significance.
Veterinary opinion has shifted in recent years, with many specialists no longer recommending routine lysine supplementation. It doesn’t appear to cause harm at standard doses, but the clinical benefit is marginal at best.
How Feline Herpes Is Diagnosed
Vets often diagnose FHV-1 based on symptoms alone, especially during an obvious upper respiratory episode. When confirmation is needed, PCR testing of swabs from the eyes, nose, and throat is the most reliable method. Older techniques like virus isolation are less sensitive and miss more cases. Taking samples from multiple sites, rather than just one, significantly improves the chance of detecting the virus.
A positive PCR result during symptoms confirms active infection or reactivation. A negative result is harder to interpret because the virus hides in nerve tissue during latency, where swabs can’t reach it. For cats with chronic or recurring symptoms, repeated testing during flare-ups gives the clearest picture.
Managing Flare-Ups
Most mild recurrences resolve on their own within a week or two with supportive care: keeping the nose clear, encouraging eating (warming food helps when congestion dulls a cat’s sense of smell), and reducing stress. For more severe episodes, especially those involving corneal ulcers or prolonged illness, vets may prescribe antiviral medication. Some common human antivirals are toxic to cats, so treatment selection matters. The goal of antiviral therapy is to shorten the episode and limit tissue damage, not to eliminate the virus permanently.
Stress management is arguably the most practical long-term strategy. Since reactivation is closely tied to immune suppression and stress, keeping a cat’s environment stable, minimizing disruptions like boarding or household changes, and addressing multi-cat tension can reduce how often flare-ups occur.