FHV, or feline herpesvirus type 1 (FHV-1), is one of the most common viral infections in cats. It causes upper respiratory illness and eye problems, and it’s estimated to be behind 50 to 75% of respiratory infections in cats. Almost all cats that recover from an active infection become lifelong carriers of the virus, which can reactivate during periods of stress.
How FHV Spreads and Survives
FHV spreads through direct contact with an infected cat’s saliva, nasal discharge, or eye secretions. Cats that are actively shedding the virus, whether during a first infection or a flare-up, are the primary source of transmission. Shared food bowls, bedding, and litter boxes can also pass the virus between cats, though the window is short. FHV is a fragile virus outside the body. It has a lipid envelope that makes it highly susceptible to common disinfectants and can survive only up to 18 hours in a damp environment, even less in dry conditions. Standard household cleaners and bleach solutions will kill it on surfaces.
This fragility means FHV spreads most efficiently in environments where cats are in close contact. Shelters, catteries, and multi-cat households carry the highest risk. In one survey of cats with upper respiratory disease, about a third tested positive for FHV-1.
What FHV Looks Like in Cats
The classic signs of an active FHV infection involve the nose and eyes. Sneezing, nasal congestion, and discharge (often thick and yellowish if a secondary bacterial infection develops) are the hallmarks. Eye involvement is extremely common and can range from mild redness and watery discharge to more serious problems like corneal ulcers. A specific type of corneal ulcer called a dendritic ulcer, which has a branching, tree-like pattern, is considered a definitive sign of FHV eye infection. No other common feline pathogen produces this pattern.
Cats may also develop fever, lose their appetite, and become lethargic. Because nasal congestion dulls their sense of smell, many cats stop eating. Kittens and cats with weakened immune systems tend to have the most severe symptoms, and very young kittens can develop life-threatening illness. In most healthy adult cats, acute symptoms resolve within two to four weeks, though eye problems can persist or recur.
Latency: Why FHV Never Truly Goes Away
The defining feature of FHV is its ability to go dormant. After the initial infection clears, the virus retreats into nerve tissue, primarily the trigeminal ganglion (a cluster of nerve cells near the base of the skull that supplies sensation to the face). Research has also detected latent viral DNA in the cornea, nasal passages, olfactory bulbs, optic nerve, and brain tissue. The virus essentially hides in these locations where the immune system can’t reach it.
Reactivation happens when a cat’s immune system is stressed. Common triggers include moving to a new home, boarding, the introduction of a new pet, surgery, illness, or any significant change in routine. Corticosteroid medications, which suppress immune function, are another well-documented trigger. When the virus reactivates, the cat begins shedding infectious virus again and may develop a fresh round of nasal or eye symptoms. Some cats flare up only once or twice in their lives. Others deal with frequent recurrences.
How FHV Is Diagnosed
Vets often diagnose FHV based on clinical signs alone, especially when a cat presents with the typical combination of sneezing, nasal discharge, and eye inflammation. Dendritic corneal ulcers, visible with a special staining technique, are considered conclusive on their own.
When lab confirmation is needed, PCR testing (which detects the virus’s genetic material) is the preferred method. It’s significantly more sensitive than older virus isolation techniques. Taking swabs from multiple sites, such as the conjunctiva, throat, and nose, increases the chance of detection. Blood tests that look for antibodies to FHV are not particularly useful. Because the virus is so widespread and because vaccination also produces antibodies, a positive antibody test doesn’t distinguish between an active infection, a past infection, or a vaccinated cat.
Treatment for Active Infections
There’s no cure for FHV, but antiviral medications can shorten flare-ups and reduce severity. For cats with eye involvement, topical antiviral eye drops or ointments are commonly prescribed. Several formulations are available, and your vet will choose based on the type and severity of the eye disease.
For more widespread or severe infections, an oral antiviral (famciclovir) is the standard systemic treatment. It’s given multiple times daily and typically continued until about a week after symptoms resolve. Cats with secondary bacterial infections, which are common when the virus damages the lining of the nasal passages and airways, often need antibiotics as well.
Supportive care at home plays a big role in recovery. Gently clearing nasal discharge with a warm, damp cloth helps cats breathe more easily. Warming food slightly can make it more aromatic and encourage eating, since congested cats can’t smell their meals well. A humidifier or brief exposure to steam from a hot shower can help loosen thick nasal secretions. Keeping your cat hydrated is critical, especially if appetite is poor.
The L-Lysine Question
L-lysine supplements have been widely recommended for cats with FHV for years, based on early laboratory studies suggesting the amino acid could interfere with herpesvirus replication. The real-world evidence, however, has not supported this. Multiple studies in shelter cat populations found that dietary lysine supplementation did not reduce clinical signs of upper respiratory disease. One study actually found increased detection of FHV-1 in cats receiving lysine. Despite these findings, some veterinarians still consider it a low-risk option for individual cats with eye disease, but it should not be relied on as a primary treatment.
Vaccination and Prevention
The FVRCP vaccine, one of the core vaccines recommended for all cats, includes protection against FHV-1. Kittens should begin the vaccine series at 6 to 8 weeks of age, with boosters every three to four weeks until they’re at least 16 weeks old. A booster is given between 6 and 12 months of age, then every three years after that.
An important caveat: the FVRCP vaccine does not completely prevent FHV infection. What it does is significantly reduce the severity of a first infection and limit the impact of future flare-ups. A vaccinated cat can still catch and carry the virus, but is far less likely to become seriously ill from it.
For multi-cat households, keeping stress levels low is one of the most practical things you can do. Providing enough litter boxes, separate feeding stations, and vertical space reduces territorial tension. If you’re introducing a new cat, quarantine them for at least two weeks to watch for signs of respiratory illness before allowing contact with resident cats. And because FHV is so easily killed on surfaces, regular cleaning of shared items goes a long way toward limiting spread.