Calicivirus is, along with herpesvirus, one of the two main causes of what is commonly called cat flu. It is widespread, spreads easily between cats, and exists in many strains that vary in severity.

What it looks like

The characteristic sign is ulcers in the mouth: on the tongue, the hard palate and the lips. They are painful, and they are the reason an affected cat drools and stops eating.

Alongside them: sneezing, discharge from the nose and eyes, fever, lethargy and reduced appetite. Some strains cause lameness that shifts from leg to leg, particularly in kittens, which recovers on its own.

Most cats recover within one to two weeks.

Why eating matters so much

Two things stop a cat eating here: painful mouth ulcers, and a blocked nose that removes its sense of smell. A cat that cannot smell its food will not eat it.

This matters because a cat that goes without food for more than a couple of days risks hepatic lipidosis, a liver condition that can be fatal. So in cat flu the appetite is watched as closely as the infection.

How it spreads

Through direct contact, through sneezed droplets, and through shared bowls, bedding and hands. Recovered cats can continue shedding the virus for months and infect others while appearing entirely well.

Calicivirus is more resistant in the environment than herpesvirus and is not killed by every household disinfectant.

Treatment

There is no drug that eliminates it. Treatment supports the cat:

Pain relief, which is central, because the ulcers are the reason the cat is not eating
Keeping food going: warmed, strongly scented, soft food; assisted feeding where needed
Fluids for dehydration
Cleaning the eyes and nose
Antibiotics only where a secondary bacterial infection has developed
Isolation from other cats

The severe form

A small number of strains cause a systemic disease with high fever, swelling of the face and paws, jaundice and a high death rate. It is uncommon but it moves fast, and it is one reason a cat with flu that is deteriorating rather than improving should be reassessed rather than waited out.

Vaccination

Calicivirus is part of the core vaccine. Because the virus has many strains, vaccination does not prevent infection entirely; it reduces the severity considerably. That is a meaningful difference in a disease whose main danger is a cat that stops eating.

When to come in

Mouth ulcers or drooling. A cat that has stopped eating. Thick yellow or green discharge. Eyes held shut or a cloudy cornea, which suggests a corneal ulcer and needs treating promptly. Any change in breathing. Or a kitten, an older cat, or one already unwell.