Feline Calicivirus and Herpesvirus: Cat Flu Causes and Care

By Tailwisp Editorial TeamUpdated October 20, 20264 min read

If you have ever adopted a kitten from a shelter, there is a good chance you have met cat flu. It is one of the most common feline illnesses. Understanding the two main viruses behind it helps you care for your cat, and explains why signs sometimes come back.

The two main viruses

Feline herpesvirus type 1 (FHV-1) causes feline viral rhinotracheitis. It mainly affects the nose, throat and eyes. After infection, the virus hides in nerve cells and stays for life, flaring up during stress or illness.

Feline calicivirus (FCV) affects the mouth and respiratory tract, causing ulcers on the tongue and palate, and can cause pneumonia. Some strains cause joint pain and limping, or rarely a severe systemic disease. Many cats clear the virus, but some shed it for months or years.

How they spread

Both viruses spread by direct contact, sneezing, shared bowls and bedding, and handling by people. Herpesvirus is fragile and does not last long outside the cat, but calicivirus can survive for days on surfaces. Outbreaks happen where many cats live together.

Signs

• Sneezing and nasal discharge.
• Watery or pus-like eye discharge, red eyes, squinting. See conjunctivitis in cats.
• Fever, lethargy and reduced appetite.
• Drooling, bad breath and mouth ulcers, especially with calicivirus.
• Cough and trouble breathing, in more severe cases.
• Limping, with calicivirus.
• Weight loss in cats that will not eat.

Herpesvirus often causes more eye disease, while calicivirus often causes more mouth disease. They can occur together.

Why some cats get repeated flare-ups

After recovery, herpesvirus remains dormant in nerves, and may reactivate during stress, such as a move, a new pet, a hospital stay or another illness. Signs are often milder the second time, but can recur through life. Calicivirus carriers may shed the virus constantly, and can keep spreading it to other cats. Some carriers develop chronic mouth inflammation. See gingivitis and stomatitis in cats.

Diagnosis

Vets usually diagnose cat flu from the signs and exam, and may use swabs for PCR tests, particularly in outbreaks or severe cases. They may test for FeLV and FIV in cats that are repeatedly ill. See FeLV and FIV in cats.

Treatment and care

Most cats recover within one to three weeks, with supportive care:

• Keep the cat eating. Congestion blunts smell, so warm, smelly wet food can help. Sick cats that stop eating may need appetite stimulants or feeding support. See cat not eating: causes.
• Keep hydrated.
• Clean discharge gently with a warm, damp cloth.
• Steam, such as sitting in a steamy bathroom, may ease congestion.
• Antibiotics, for secondary bacterial infection.
• Eye medication, for herpes eye disease, possibly antivirals.
• Pain relief, for mouth ulcers.
• Isolate the sick cat from others.
• Keep it warm and comfortable.

Never use human cold medicines, which can be toxic. See upper respiratory infections in cats and why is my cat sneezing.

L-lysine

L-lysine supplements are widely used for herpesvirus, but recent studies have not shown clear benefit, and some suggest they may not help. Ask your vet before using it.

Vaccination

FHV-1 and FCV are part of the core FVRCP vaccine. Vaccination does not prevent infection fully, but it lowers the severity. Kittens receive a series, and adults get boosters. See cat vaccination schedule and kitten first vet visit.

Preventing spread

• Isolate sick cats, and wash hands between cats.
• Disinfect bowls, toys and surfaces. See how to clean cat food bowls.
• Quarantine new cats for at least a couple of weeks.
• Reduce stress.
• Keep ventilation good in multi-cat spaces.

Nursing a sick cat at home

A cat with flu feels miserable, and a few simple measures help. Wipe the eyes and nose several times a day with a soft, warm, damp cloth, using a clean one for each eye. Keep the room warm and draft-free, and provide a cozy bed. Offer strong-smelling foods, warmed slightly, and keep water fresh. Watch for dehydration, such as sticky gums or sunken eyes, and for breathing effort. A humidifier can help, though keep it clean. Never leave a cat that has not eaten for a day or more at home without veterinary advice.

Kittens and seniors

Kittens can become very sick, very quickly, as congestion and fever lead to dehydration and low blood sugar. Seniors and cats with other conditions can also struggle. Seek help early. See kitten sneezing and eye discharge.

Multi-cat homes and shelters

In a household with several cats, isolate the sick cat in a separate room with its own bowls and litter box. Handle healthy cats first, then the sick one, and wash your hands and change clothes before returning. Most cats in the home will already have been exposed, but isolation reduces the dose and spread. Vaccinate the rest and keep stress low.

Long-term carriers

If a cat has repeated flare-ups, your vet may recommend management steps such as stress reduction, treatment of flares, and attention to dental health. Carriers can live normal lives, and many live happily in homes with other cats if their needs are met.

When to see a vet

See a vet if your cat will not eat, has labored breathing or thick eye discharge, or is a young kitten. Use the US vet directory to find a clinic.

This article is general educational information, not veterinary advice. Every cat is different. If your cat is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.