Calicivirus (FCV) is a common viral infection found in cats that is characterised by the presence of flu-like symptoms such as upper respiratory infection. It is a member of the Caliciviridae family.
Around 80 – 90% of all feline respiratory disease complex are caused by either feline calicivirus or feline rhinotracheitis virus (feline herpesvirus). Dual infection with both feline calicivirus and feline herpesvirus are fairly common.  Calicivirus usually affects the throat, eyes, nasal cavity and oral cavity in cats although sometimes the lungs, musculoskeletal system, and intestines can also be affected.
In the healthy adult cat, the mortality rate is relatively low, however, kittens, older cats, and immunosuppressed cats are at greater risk. Feline calicivirus is most often seen in shelters and environments with overcrowding, although it can occur in any household. The geographical distribution of calicivirus in cats is worldwide.
How do cats become infected with calicivirus?
The most common mode of infection is via another infected cat. Infection can be passed from eye, nasal and mouth discharges. The incubation period of calicivirus is between 2-6 days. The virus replicates in the respiratory tract and oral tissues and is shed in oral, eye and nasal secretions as well as urine and feces. Transmission may be oral, aerosol or fomites (objects such as floors, food bowls etc).
Other ways infection is spread is via indirect contact or carriers.
Indirect contact: Contaminated food bowls, litter trays, flooring, bedding etc. Calicivirus is resistant to many disinfectants and can live in the environment for several weeks.
It is possible for cats to remain carriers for years after infection. This means even though they have contracted the virus, become sick and recovered, the virus is still being shed in excretions and it is possible to infect other cats.
Once the cat recovers from calicivirus he will either stop shedding the virus completely after two to four weeks or go on to become a chronic carrier, and shed the virus from time to time, particularly during stressful periods or when he is sick. These carrier cats may or may not display symptoms when they are shedding.
What are the symptoms of feline calicivirus?
There are several strains of calicivirus which infect cats, and symptoms differ depending on the virulence of the particular virus causing infection. Some strains may cause mild symptoms only, others severe.
Symptoms of calicivirus are often similar to that of a head cold in humans, typically affecting the respiratory tract and oral cavity and may include:
Ulceration of the tongue and palate are most common, although the lips and nose may also be affected
Gingivitis (inflamed gums)
Pneumonia can develop with more virulent strains of calicivirus
Calicivirus can cause transient arthritis in the joints of some infected cats leading to limping (known as limping syndrome). This has been seen in not only naturally occurring calicivirus but also after vaccination. 
Feline calicivirus can be complicated by secondary bacterial infections. It is not uncommon for a cat to develop anorexia (loss of appetite) and dehydration which can further weaken an already sick cat.
Virulent systemic feline calicivirus:
There is a particularly virulent form known as ‘virulent systemic feline calicivirus or VS-FCV‘ with a mortality rate of 67%. This strain, which has occurred in the United States The virus is spread in much the same way as other strains of calicivirus.
Many symptoms also include the upper respiratory tract, as well as ocular discharge, facial and limb swelling, hair loss and ulceration of the ears, face and feet, jaundice and eventually multiple organ failure. An immune response is believed to be responsible for the multiple organ failure.
How is feline calicivirus diagnosed?
A presumptive diagnosis is usually based on clinical signs in the cat. As stated earlier in the article, feline herpesvirus, and feline calicivirus account for 80 – 90% of all feline respiratory disease complex. If oral ulcers are present, calicivirus is most likely.
If corneal ulcers are present (eyes), herpesvirus is most likely.
X-Rays may be performed to check for inflammation of the lungs.
Specific tests may be necessary to identify the pathogen involved. Not all veterinarians will recommend these tests as the treatment for viral upper respiratory infections is mostly the same.
Polymerase chain reaction and/or viral isolation. These test will be performed at a specialist laboratory.
Serology to detect coronavirus antibodies. However, this poses two problems. Firstly, if your cat has been exposed to the coronavirus (and remember, there are many strains) in the past, he will have antibodies. Secondly, it can take up to seven days for antibodies to be produced after infection, so the test may return a false negative.
How is calicivirus treated?
Treatment is generally supportive while your cat’s own immune system fights the infection. There are currently no antiviral drugs available for use with coronavirus. A severely sick cat will need to be hospitalised, however, cats with milder symptoms may be treated at home. Supportive care at hospital may include:
Broad-spectrum antibiotics to treat secondary infections.
Anti-inflammatories to reduce fever and relieve symptoms of lameness and ulcers.
Corticosteroids may be prescribed if your cat has developed transient arthritis.
Fluids may also be required to treat dehydration.
Oxygen may be provided if your cat is having difficulty breathing.
Other supportive care may be required to make your cat more comfortable, including:
Removal of discharge from the nose and eyes with a damp gauze soaked in saline solution will make your cat more comfortable. Discard immediately.
Antibiotic eye drops may be prescribed to treat eye infections.
If your cat has oral ulcers, soft food should be offered. As congested cats often can’t smell, which decreases their appetite, gently warming the food can help to stimulate the appetite. If the cat has become anorexic, force-feeding may be necessary.
Increasing humidity can be of assistance to the congested cat.
Prevention of feline calicivirus:
Routine vaccination of your cat. It used to be recommended that cats receive the F3 or F4 vaccination (which covers three or four types of cat flu) annually, however, many veterinarians are now recommending a longer time frame between vaccinations, speak to your own veterinarian to discuss this issue. Vaccines can help to decrease the severity of the disease but haven’t been able to decrease its prevalence. The vaccine should be given to kittens at 8, 12 and 16 weeks of age.
Avoid overcrowding of feline populations and keep stress down. The more cats in an environment, the greater the risk of calicivirus occurring.
Always wash your hands when handling cats, it is easy to transfer the virus on to other cats.
It is recommended that new cats to the household be put in quarantine for one to two weeks prior to introducing him to household cats.
Sick cats should be isolated while they are recuperating.
Can I catch feline calicivirus from my cat?
No, it is not possible for humans to become infected with feline calicivirus.