Cat Flu: Symptoms, Treatment, and When It Turns Serious

Cat flu is the common name for a feline upper respiratory infection, and in most cases it is caused by one of two viruses, feline herpesvirus or feline calicivirus, which trigger sneezing, runny eyes and nose, mouth ulcers, and a cat that goes quiet and off its food. Most otherwise healthy adult cats shake off the worst of it in a week or two, but in kittens and frail older cats it can turn serious quickly, and that is the difference this guide is built around.

This article sits within our wider cat health guide; read it alongside the signs your cat is sick if you are trying to work out how worried to be.

What cat flu actually is

Cat flu is not a single disease but a syndrome, and the large majority of cases come from just two viruses: feline herpesvirus and feline calicivirus. International Cat Care notes that these two agents together account for roughly 80 to 90 percent of cat flu, with a handful of bacteria such as Bordetella and Chlamydia making up most of the rest¹. Despite the name, it has nothing to do with human influenza and cannot pass to people.

The two main viruses behave a little differently. Herpesvirus tends to cause the more severe, snotty, eye-heavy illness, while calicivirus is the one more often linked to mouth ulcers, dribbling, and a sore, hoarse throat. In practice a vet cannot always tell them apart by eye, because the outward signs overlap so much, which is one reason the outward picture matters more than the exact virus for how you nurse a cat at home.

The symptoms to look for

The classic signs cluster around the nose, eyes, and mouth: sneezing, a runny nose, watery or gummy eyes, ulcers on the tongue or gums, dribbling, a hoarse meow, and a cat that is subdued and eating less. The incubation period is short, typically about 2 to 10 days from exposure to the first sneeze, so a new arrival can look fine on collection and unwell within a week¹. A mild fever, a snuffly voice, and reluctance to eat because the nose is blocked and the mouth is sore are all common.

Appetite is the sign I watch hardest. A cat that cannot smell its food because its nose is congested will often refuse to eat, and a cat that stops eating for more than a day or so is a separate problem in its own right, because cats tolerate fasting badly. If your cat is turning its nose up at food, warming it gently to lift the aroma sometimes tips the balance, and any refusal that lasts is a reason to phone the vet rather than wait.

How it spreads, and the lifelong carrier state

Cat flu spreads easily between cats through sneeze droplets, direct contact, and shared bowls, bedding, and hands, and most cats that recover from herpesvirus carry it silently for the rest of their lives. This carrier state is the part owners find most surprising. According to International Cat Care, the majority of cats infected with feline herpesvirus become lifelong carriers, and many will shed the virus again and flare up at times of stress, even though they seem perfectly healthy in between¹.

I fostered a scruffy rescue kitten called Nettle who arrived sneezing, cleared up beautifully in ten days, and then every single time something big changed in the house, a cattery stay, a new foster, once just the builders in, she would spend a few days with a runny eye and a snuffle before settling again. Nothing was wrong with my nursing; that is simply how the carrier state works. Knowing it saved me a lot of panic, and it is worth bearing in mind before you bring home a cat with a history of it, something I cover in bringing home a rescue cat.

When cat flu turns serious

Cat flu is at its most dangerous in kittens, elderly cats, and cats with weakened immune systems, where dehydration, pneumonia, and lasting eye damage become real risks rather than rare ones. The Cornell Feline Health Center lists upper respiratory infection among the common and potentially serious problems in young and unwell cats, precisely because a small kitten has so little reserve when it stops eating or drinking². In the worst cases the infection spreads to the lungs, or a severely ulcerated eye is permanently scarred.

Treat these as reasons to get to a vet promptly rather than nurse on at home:

• A young kitten or a frail senior cat with any signs of cat flu
• A cat that has stopped eating or drinking, or is becoming dehydrated
• Laboured or open-mouthed breathing, or fast breathing at rest
• Thick discharge crusting the eyes or nose shut, or an eye that looks cloudy, bulging, or very sore
• Marked lethargy, collapse, or a cat that is hiding away and unresponsive

If you are unsure which side of the line your cat is on, our guide to when to take a cat to the vet walks through the red flags in more detail, and a phone call to your own clinic is always reasonable.

Treatment and nursing at home

There is no drug that kills the viruses outright, so treatment centres on good nursing and on your vet tackling the complications: secondary bacterial infections, painful ulcers, sore eyes, and dehydration. A bright adult cat with mild signs can often be supported at home. Wipe discharge gently from the eyes and nose with cotton wool and warm water, sit the cat in a steamy bathroom for a few minutes to loosen congestion, and offer strong-smelling, warmed food little and often to keep it eating.

Your vet may prescribe antibiotics if a secondary bacterial infection has set in, eye drops or ointment for the eyes, pain relief for mouth ulcers, and in more serious cases fluids under the skin or a short hospital stay. Antibiotics do not touch the virus itself, but they matter when bacteria take hold on top of it. Keep an unwell cat warm, quiet, and separated from other cats while it recovers, and keep bowls, bedding, and trays for that cat apart.

Preventing cat flu with vaccination

The core FVRCP vaccine, which protects against feline herpesvirus and calicivirus as well as panleukopenia, is the main way to prevent serious cat flu, and it is recommended for every cat. The American Association of Feline Practitioners classes this combination as a core vaccine for all cats, indoor or outdoor, because the diseases it covers are widespread and can be severe³. Kittens usually start the course at about 6 to 8 weeks of age, with further doses every few weeks until around 16 weeks, then a booster at about a year old⁴.

It is worth being honest about what the vaccine does. It does not always stop a cat catching the viruses altogether, and it does not clear an existing carrier state, but it reliably makes the illness milder and less frequent, which in a kitten can be the difference between a snuffle and a hospital stay. International Cat Care recommends keeping vaccination up to date throughout a cat’s life on the schedule your vet sets⁵, and our full cat vaccinations guide explains how the core and non-core groups fit together.

This is general information, not a diagnosis. If your cat is sneezing, off its food, struggling to breathe, or you have a kitten showing any signs of cat flu, contact your own vet, who can examine your cat and knows its history.