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Cat Flu

What Is Cat Flu?

Cat flu is a contagious upper respiratory tract infection commonly seen in unvaccinated cats. Unfortunately for those affected with the illness, they can become life-long carriers, having repeat symptoms brought out during times of stress.

Cat flu often does not have a single cause. A variety of viruses and bacteria  can lead to “flu-like” symptoms in cats. The two most common viral causes are feline calicivirus (FCV) and feline herpes virus (FHV). Bacterial causes, such as Mycoplasma felis and Chlamydophila felis, are less common (1).

Infections are often not as simple as one virus or bacteria causing the symptoms. Mixed infections can occur with a virus causing the initial damage which allows bacteria present in the nasal cavity to cause further damage and worsen the symptoms. For example, M. felis can be found in healthy cats as a normal bacterial inhabitant of the nose but can contribute to disease during FCV and FHV infections (2).

Cat Flu in Kittens

Cat Flu in Australia

Cat flu is an important cause of respiratory infections in Australian cats. There are a couple of studies which have examined which of the viruses and bacteria are responsible for cases of cat flu in Australia. Of the viruses, FCV is more commonly detected than FHV (2). M. felis is the most commonly detected bacteria and was often found alongside FCV infection. However, since M. felis can be found in healthy cats it’s difficult to interpret whether the M. felis detected in these sick cats is truly causing disease or whether it’s an innocent bystander in these cases (2). Just like human flu, infections are more common in winter (3). Most of the recorded infections were in cats younger than 2 years old (2,3).

Cat Flu Symptoms

Incubation Period

The incubation period for feline herpes virus is between two to six days while symptoms of feline calicivirus can take from two to 14 days to develop after infection (4).

Early Symptoms

Both FCV and FHV affect the upper respiratory tract (nasal cavity and the back of the mouth). Symptoms vary in severity depending on many factors including the virus dose and the cat’s vaccination status. The most common signs are sneezing and a runny nose (nasal discharge). The discharge from the nose might be clear initially before changing to yellow-white as infection sets in. Conjunctivitis (inflammation around the eye) is also common. Cats will often have a fever which, combined with the blocked nose and conjunctivitis, makes them lethargic and reluctant to eat. Some symptoms are specific to the particular virus causing the infection. FCV commonly causes ulcers on the tongue while FHV can cause ulcers on the cornea of the eye (5).

How long do the symptoms last?

The time to recovery depends on lots of factors including the severity of the symptoms and how well the cat’s immune system can fight off the infection. Mild cases should resolve in five to ten days but severe cases can last for six weeks.

Carrier Status

Unfortunately, infection with FHV and FCV is not a simple road to complete resolution. Just as the human herpes virus causes recurrent cold sores, the feline version establishes a lifelong infection in all affected cats by hiding from the immune system in nerve cells. Here, the virus remains in a dormant state known as latency. These “carrier” cats don’t have symptoms but the virus can be reactivated under the right conditions. Usually reactivation is triggered by stressful events which can include rehoming, introduction of a new cat into the household or other diseases. Once reactivated, the cat begins to shed virus allowing other cats to be infected. Shedding usually starts around a week after the stressful event and lasts for about two weeks (4).

Similarly, many cats infected by FCV become carriers. The good news is that carrier status for FCV is rarely lifelong. The bad news is that there doesn’t need to be a stressful event to start FCV shedding. Instead, virus shedding is nearly continuous providing a constant source of infection for new cats (4).

Cats that have been vaccinated can still become infected and develop carrier status for both viruses but they tend not to become unwell. When they do have symptoms, these are generally milder than in non-vaccinated cats.

Long-term Effects

While most cats will recover from FHV and FCV infection, in some cases infection can have long term effects. This is particularly important for FCV affected cats. Chronic inflammation and ulceration of the mouth and gums (stomatitis and gingivitis) have been associated with FCV infection.  It’s not clear how the virus causes these symptoms and what other factors are at play in the process. Some strains of FCV can cause lameness, usually with a fever. The lameness is not constant and can affect different limbs over the course of infection (4).

Infection by either FHV or FCV can lead to permanent damage to structures in the nose. This damage can impair the processes that help to prevent other infections. Cats with such changes are more susceptible to other types of respiratory infections. Combined with reactivation of the virus and periods of stress, some cats develop chronic, recurrent respiratory infections and are sometimes known as “snufflers” due to their persistently blocked noses.

Cat Flu Symptoms

Cat Flu Diagnosis

A diagnosis of cat flu is often made based on the patient history and symptoms alone. However, your vet might recommend finding out exactly which virus is causing the disease since a definitive diagnosis can inform the best treatment plan and how to manage the disease within a multi-cat household. Usually, the vet will take a swab from your cat’s mouth and send this to the laboratory. The laboratory can use a test called the polymerase chain reaction (PCR) to detect viral and bacterial genetic material. If your vet suspects that bacteria are playing an important role in the disease, they might recommend bacterial culture and sensitivity tests which will allow them to choose an appropriate antibiotic to treat the infection (1,4).

Cat Flu Treatment

Supportive Treatment

When you’ve got a cold all you want to do is curl up in bed and be nursed back to health with chicken soup and some sympathy. The same will be true for your cat. Cats will be reluctant to eat due to the fever, blocked nose and possible mouth ulcers. They can be tempted with their favourite foods. Smell is an important appetite stimulant in cats, so sardines or tuna are a good choice. Warming them in the microwave to release the fishy aromas could tempt a cat to eat.

Nasal discharge can be removed from around the nose by bathing with warm water. Further unblocking your cat’s nose will make them feel better. You may be offered a nebuliser by your vets or you can buy one from a pharmacy. Since cats will rarely tolerate a mask, you can use your cat carrier as a chamber for nebulisation therapy. Alternatively, you can shut your cat in the bathroom when you shower. The warm, wet air will help to loosen the nasal discharge inside the nasal cavity (4,5).

Cat Flu Treatment


Cats suffering from cat flu are often treated with antibiotics. Although the initial cause is usually viral, secondary bacterial infections are common and can complicate the symptoms. There are a wide variety of antibiotics that your vet could prescribe and these may be based on a culture and sensitivity test (1). A common antibiotic prescribed for cat flu is doxycycline.

Cat Flu Vaccine

Cat Flu Vaccine F3
F4 Cat Vaccination

Vaccination against FHV and FCV is usually included in any standard cat vaccination regime.

Kittens should be given an initial course of two to three vaccines. The first dose should be given at eight to nine weeks old with the second given three to four weeks later. The final dose, if required, should be given between 16 and 20 weeks old. Repeated vaccination ensures that kittens are properly protected against the viruses since antibodies absorbed from their mother’s milk can interfere with the vaccine (6).

Booster vaccines should be given every year starting from 12 months after the last vaccine in the initial course.

If vaccines have lapsed then an adult cat can be given two vaccine doses four weeks apart followed by boosters every year.

There are few recorded side effects of vaccines in cats and many are rare. Cats are at risk of developing a rare type of tumour known as feline injection-site sarcoma. About 1 in 10,000 cats will develop an injection-site sarcoma. The risk can be further minimised by choosing a different site to administer the vaccine each year (6).

Adopting a Cat With Cat Flu

Adopting a cat with cat flu is great but it’s important to assess the risk to other cats. Moving a cat to a new environment is a stressful event and will cause virus shedding. Some shelters recommend that cats with cat flu are adopted by homes without other cats or multi-cat households consisting of cat flu positive cats. If you’re planning on letting your cat go outdoors, it’s recommended to keep them indoors for at least four weeks to stop them from shedding virus around the neighbourhood.

Dealing with Cat Flu in Multi-cat Households

If you’re thinking of introducing a new cat into a multi-cat household which is free of cat flu, it’s important to check whether the new cat is infected. This can be extremely difficult as carriers of latent FHV will not shed virus and cannot be detected using diagnostic tests. To minimise the risk of transmission during introduction, a good protocol of hygiene including regular cleaning of food and water bowls, disinfection of surfaces and disinfection of litter trays are essential. Good hygiene will lower the viral load in the environment which will reduce the strain on the cats’ immune systems. Ultimately, the best protection is vaccination combined with good hygiene and measures to reduce stress (7).

Products than can be used during this introductory period to reduce stress include calming agents such as Feliway diffusers and sprays.

Cat Flu in Multi-Cat Households


Most frequent questions and answers about Cat Flu

In general, cat flu is a mild disease. However, some cats do experience severe symptoms and may need hospitalisation to help them recover.

This depends on the cause. Bacterial infections can be cured using antibiotics. If the cause is a virus, then over 98% of cats will recover from their symptoms. Although cats have recovered, they often remain permanently infected by the virus.

While a cat could recover from cat flu without treatment, the infection will be prolonged without good management. Without treatment the cat will feel worse and be reluctant to eat which may increase their risk of serious disease and hospitalisation.

None of the bacteria or viruses which cause cat flu can infect humans.

Most of the bacteria or viruses cannot be passed to dogs. One exception is Bordatella bronchiseptica, a bacteria which can cause kennel cough in dogs and cat flu in cats. However, there have been no documented cases of transmission and the risk is theoretical.

Indoor cats are at reduced risk of getting cat flu. You should consider whether your cat is likely to go to a cattery for boarding where they might be exposed. Since FHV and FCV can be spread by humans on their hands or clothes, you should consider whether you are likely to have contact with other cats. If you do, remember to wash your hands well to reduce the chance of transmission. However it is advisable to still vaccinate indoor cats.

Vaccinated cats can still develop symptoms of cat flu if they are exposed to lots of virus in the environment but their symptoms will be less severe compared to unvaccinated cats. Good hygiene can reduce viral particles in the environment.


Cat flu is a debilitating disease that can often have life long consequences for infected cats. By following the necessary protocols and keeping up to date with your cat’s vaccination, you can try to keep this feline illness at bay.


  1. Lappin MR, Blondeau J, Boothe D, Breitschwerdt EB, Guardabassi L, Lloyd DH, et al. Antimicrobial use guidelines for treatment of respiratory tract disease in dogs and cats: Antimicrobial guidelines working group of the International Society for Companion Animal Infectious Diseases. J Vet Intern Med. 2017;31(2):279–94.
  2. Nguyen D, Barrs VR, Kelman M, Ward MP. Feline upper respiratory tract infection and disease in Australia. J Feline Med Surg. 2019;21(10):973–8.
  3. Wong WT, Kelman M, Ward MP. Surveillance of upper respiratory tract disease in owned cats in Australia, 2009-2012. Prev Vet Med. 2013;112(1–2):150–5.
  4. Dawson S, Radford A, Gaskell R. Clinical update on feline respiratory pathogens. In Pract. 2004;26(6):320–3.
  5. Reed N, Gunn-Moore D. Nasopharyngeal disease in cats: 2. Specific conditions and their management. J Feline Med Surg. 2012;14(5):317–26.
  6. Day MJ. Small animal vaccination: A practical guide for vets in the UK. In Pract. 2017;39(3):110–8.
  7. Speakman A. Management of infectious disease in the multi-cat environment. In Pract. 2005;27(9):446–53.  
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