Cat Flu

Pete Wedderburn
6th January 2020 - 4 min read


Cat flu is a “catch all” term that refers to one of two viral infections, both of which cause similar signs of upper respiratory infection. Everybody has heard of viral infections. But what is a virus? And why are viral infections so difficult to treat? The dictionary definition of a virus is ‘a combination of chemicals capable of multiplying rapidly inside a living cell, causing disease’.  

The two viruses that cause cat flu are Feline Calicivirus (FCov), and Feline Herpes Virus (FHV). Both viruses cause conjunctivitis with discharge from both eyes, sneezing, pyrexia (a high temperature) and coughing. Calicivirus in particular is prone to causing ulcers on the cat’s tongue.

Many kittens (especially those of a feral background) are infected with Herpes virus at a young age when they are still partially protected by their mother’s antibodies (which they have in their bloodstream still). They often recover, but are left with a low grade chronic viral infection. In a similar way to Herpes virus cold sores in humans, the virus is reactivated at times of stress for the cat in later life, leading to episodes of a mild form of cat flu. These cats are often referred to as “chronic snufflers” because of their tendency to have repetitive bouts of runny eyes and sneezing.

cat with yellow eyes 43

The science of viral infections

The aim of a virus, like all other forms of life, is to reproduce as many of its type as possible. A virus enters the body of its host, penetrates certain target cells of the host, and once, inside the cell, it begins to reproduce itself. Later, thousands of new viruses are released from this cell, and these infect other target cells. Finally, virus particles leave the body of the host and enter the body of other hosts, where the whole process starts again. 

In the process of entering the host cells and multiplying, viruses often cause ill health. Every different type of virus has different effects on a host, but some facts about viral infections are universally true.

Incubation Period

The first fact is that there is an ‘incubation period’ between when an animal picks up the infection and when the first symptoms are seen. The incubation period for different viruses is known, and so it is possible to predict how rapidly a viral epidemic will affect a population. For cat flu, the incubation period is about four or five days, so it can spread rapidly in a cat population.


The second fact is that it is very difficult to cure a viral infection. Apart from a few recently discovered exceptions, modern drugs have minimal effect on viruses. When treating animals, the aim is to give the animal supportive care to help the body’s immune system fight off the virus. In mild viral infections, the animal will recover rapidly. In serious infections, such as Feline Enteritis, the immune system of the animal may be overwhelmed by the virus, and they may even die. The cat flu viruses rarely cause death, but young kittens or older unvaccinated cats may succumb to a particularly extreme version of cat flu which may be fatal.


The third fact is that vaccines are now available which can completely prevent some viral infections. A vaccine is a tiny dose of a harmless version of the virus. A vaccine causes no ill health, but it does provoke the body’s immune system to produce antibodies against the virus, fighting off an otherwise dangerous viral infection.

Cat flu vaccines (given to kittens, then given to adult cats at intervals depending on their risk of exposure to teh cat flu virus) offer good protection against new viral infections, but they cannot “cure” long term infection of cats with an established cat flu virus infection (so-called “chronic snufflers”.

two cats one brown one orange

Common Viral Diseases

There are several other viral diseases commonly seen in cats. As well as cat flu, Feline Enteritis, and Feline Leukaemia are also a significant risk, and they can all be prevented by vaccinations. These need to be given to kittens, and regular booster vaccinations need to be given to adult animals to maintain immunity.

The World Small Animal Veterinary Association has issued global guidelines about the most appropriate way for vaccines to be given to protect cats: you can read them here.

Another viral infection, Feline Immunodeficiency Virus, which causes Cat AIDS, is also common in multi-cat households, and unfortunately, there is no vaccine available at the moment.

If you like cats, there is something cosy and welcoming about a multi-cat household. But don’t forget to keep their anti-virus vaccinations up-to-date!


  • Viral infections are common in unvaccinated cats.
  • Multiple cat households are particularly vulnerable.
  • Speak to your vet about a comprehensive vaccine programme for your pet cats.