Cat Flu

Cat flu is quite a bit like a human cold - it causes runny nose and eyes and a sore throat. Like humans, cats with flu will suffer from aches and pains in the muscles and joints. They may also get mouth ulcers, dribbling, sneezing, lose their voices and get a fever.

But symptoms are the only resemblance between cat flu and a human cold. In fact these two diseases are caused by completely different pathogens (types of germ) and human colds cannot infect cats or vice versa. Cat flu is not usually life-threatening in adult cats, although they can get quite ill. However, it can be deadly in kittens and older cats. Cat flu can also be associated with secondary bacterial infection. Therefore all cats with symptoms of cat flu should be seen by a vet.

The Science of cat flu

Cat flu is a upper respiratory tract disease which can be caused by a number of pathogens. These infectious agents are often referred to as feline upper respiratory infection (URI) complex pathogens. The the two main pathogens which cause cat flu are feline herpesvirus and calicivirus. Cat flu caused by these viruses is also referred to as Feline viral rhinotracheitis (FVR). The incubation time (the time between the cat catching the infection and showing symptoms) is 3 to 5 days. After the cat becomes symptomatic, the illness nornally runs its course within four to seven days, but secondary bacterial infections can cause the symptoms to persist for weeks.

Cat flu can also be caused by certain types of bacteria namely bronchiseptica and chlamydophila felis. All of these pathogens have slightly different symptoms.

  1. Feline herpesvirus produces the most severe symptoms and is more likely to result in eye ulcers.
  2. Calicivirus is characterized by upper respiratory symptoms, pneumonia and characteristically causes mouth ulcers and occasionally arthritis. It can cause young kittens to go temporarily lame. It is a fairly mild flu-like condition which rarely has serious complications.
  3. Bordetella bronchiseptica induces coughing in infected animals and affects the lungs. It more often affects dogs than cats but it is believed that it is possible for cats to catch this form of flu from infected dogs. This is not very common and cats in cat-only households are at minimal risk.
  4. Chlamydophila felis mainly produces sore, red, runny eyes, sometimes with a mild "cold".

About the virus

Since feline herpesvirus and calicivirus are jointly responsible for 85% to 90% of all cat flu cases we will focus here on these two viruses.

Feline calicivirus is a member of the genus Calicivirus. Calicivirus is a 20-30 nm size capsid virus. Inside the capsid there is a single positive-stranded RNA which is not segmented. The virus can be distinguished by the capsid size by electron microscopy and by virus specific surface antigens.

Feline herpesvirus-1 belongs to the family Herpesviridae. It is also known as the feline rhinotracheitis virus. The herpes virus iswhat is known as a DNA enveloped virus. It binds to fibroblasts and epithelial cells and fuses with the cell surface to enter the cells. Once inside the cell, the virus releases proteins which initiate viral gene transcription. Proteins called DNA binding proteins stimulate DNA synthesis and promote production of viral genes. Once the newly multiplied viral particles are assembled they are released via two major routes:exosytosis (A process by which secretory products are released from a cell via transport within vesicles to the cell surface and subsequent fusion with the plasma membrane, resulting in the extrusion of the vesicle contents from the cell.) and cell lysis (killing cells). Feline herpes virus often establishes latent infection in ganglion cells in the nervous system.


Because viral particles are present in mucous and eye/nose discharge these can be collected and tested using ELISA or radio-immunoassay or by isolating a virus directly. However, this is not done on a regular basis. Diagnosis of FVR is usually by the symptoms, especially corneal ulceration (eye ulcers).

Positive diagnosis may be helpful in catteries or if a cat is suffering from one of the long-term complications of infection. But since most cats recover from the infection relatively quickly the formal diagnosis is not essential.

How is cat flu spread?

Once infected, cats pass on the virus through nasal and eye discharges and in their saliva. Ill cats are therefore the biggest source of infection. However, some healthy cats carry the viruses and can pass it on even though they seem unaffected by it. That is, although the infected cat may not show clinical symptoms, she may still be infectious to other cats. Cat flu can be also passed from a mother to her litter and this is dangerous for the kittens since their immune system is not yet prepared to fight the infection.

The other important thing to remember is that the viral particles can survive for up to a week in the environment, which means that direct contact between cats is not essential for a cat to get infected. Infection can easily be spread by contact with infected litter trays, food bowls, toys and even human clothing or touch after they have been incontact with an infected cat.

How can I protect my cat from cat flu?

Cat flu is only a major problem in multi-cat households, in breeding studs or in catteries where the infection spreads easily due to continuous contact between animals. Also, outdoor cats are more likely to catch the flu than indoor cats. Bringing a new cat to a household which already has other cats always carries certain risks, among them that the cat might have an infection. The best method of protection is vaccination. Make sure that the cat you are bringing in has been vaccinated and shows no symptoms of infection.

If you get a cat suffering from cat flu in a multi-cat household the only way to prevent all the cats from getting infected is to quarantine the sick cat from the rest of the household. Make sure that the ill cat has its own food bowls and litter tray and has no contact with other cats for approximately two weeks. The food bowls and cat litter trays of the remaining cats should be disinfected with products which kill viruses but are safe for cats. The vet will be able to advise you on that. Where possible only one person in the household should care for the ill cat to minimize the chances of the infection spreading. It is a good idea to use disinfected cloths to wipe the carer's hands and face on leaving in the room where ill cat is kept (leave the cloths in the room as well). Make sure that all other cats have up-to-date vaccinations and watch carefully for any signs of illness among the remaining cats.

Breeding cats should be vaccinated before they are mated so that they produce high levels of antibody in their milk. These maternal antibodies only protect the kittens until they are about 4 - 8 weeks old, after which the levels of antibody gradually disappear. Kittens can only be vaccinated successfully when the levels of antibody have disappeared, usually at between 6 and 12 weeks of age.

Following infection, many cats are left as carriers, which means they do not have any symptoms, but are potentially infectious to others. This is particularly true in the case of cats which were infected with feline herpes virus. The virus often persists in the body in latent form and resurfaces from time to time resulting in the cat becoming infectious. The reactivation of the virus can often be caused by stress. Be careful when introducing a kitten to a household which has had a recent outbreak of flu. Kittens are very susceptible, and will be at risk in the presence of an apparently healthy carrier.


There are no specific anti-viral drugs which can be used for cat flu, though antibodies are often given to prevent secondary bacterial infections. Most important, however is supportive care for an ill cat, and in severe case this may require administration of intravenous fluid to prevent acute dehydration, or oxygen therapy in a bad case in pneumonia. Cats which do not want to eat may need to be force fed.

Conjunctivitis and corneal ulcers are treated with topical antibiotics for secondary bacterial infection. L-lysin is thought to suppress viral replication. This idea is however anecdotal, and lacks definite scientific proof.


Flu vaccines are available. They require yearly boosts. Although vaccines do not provide 100% protection, if vaccinated cats do become ill, the illness is normally much milder. The vaccine against feline herpesvirus-1 provides more protection than the vaccine against calicivirus, because the latter mutates continuously. This means that new vaccines have to be prepared on a regular basis to provide protection against the new strains of the disease.

It is particularly important to know that your cat will need to be fully up-to-date with vaccinations if going to a boarding cattery while you are on holiday. No reputable boarding cattery will take cats which are not fully vaccinated, and for your cat's sake, nor would you want them to.

Living with an ill cat

Cats with cat flu can be very ill and will require nursing care at home. A blocked nose, mouth ulcers and a sore throat may stop a cat eating and drinking, leading to dehydration. This is especially dangerous in kittens. Cats rely strongly on smell, not least in their selection of food, and with a blocked nose the sense of smell is far less acute. It is therefore important to tempt your cat with strong-smelling food to keep her eating. It eating is difficult because of mouth ulcers soothing food like cream or even ice cream can be offered.

Gently wipe away any discharge from nose and eyes. Hot steam helps to loosen catarrh, so steam inhalations can be beneficial. The best way to do this is to bring your cat into the bathroom when you have a hot bath or shower.

Note: This information is for guidance only. It is not intended to replace consultation with a licensed practitioner.

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