Feline calicivirus (FCV) is a virus of the family Caliciviridae that causes disease in cats. It is one of the two important viral causes of respiratory infection in cats, the other being feline herpesvirus. FCV can be isolated from about 50 percent of cats with upper respiratory infection .Cheetahs are the other species of the family Felidae known to become infected naturally

There are different strains of FCV that vary in virulence. Being an RNA virus, FCV has a high elasticity of its genome which makes it more adaptable to environmental pressures. This not only makes the development of vaccines more difficult, but allows for the development of more virulent strains.In persistently infected cats, it has been shown that the gene for the major structural protein of the viral capsid evolves through immune-mediated positive selection and allows the virus to escape detection by the immune system.

A form of FCV has been found to cause a particularly severe systemic disease in cats, similar to rabbit hemorrhagic disease (which is also caused by a calicivirus). This virus has been called virulent systemic feline calicivirus (VS-FCV) or FCV-associated virulent systemic disease (VSD). The disease is caused by direct viral invasion of epithelium and endothelium and secondary host immune response. It seems that strains of VS-FCV have risen independently of one another since first being described, meaning that not all cases of VS-FCV have spread from a single case.

Symptoms

Symptoms in cats infected with FCV may develop acutely, chronically, or not at all. Latent or subclinical infections often become symptomatic when the cat is stressed, such as at the time of adoption. Acute symptoms of FCV include fever, conjunctivitis, nasal discharge, sneezing, and ulceration of the mouth (stomatitis). Pneumonia may develop with secondary bacterial infections. In addition to stomatitis, some cats may develop a polyarthritis, both probably immune-mediated through immune complex deposition. Stomatitis and polyarthritis can develop without any upper respiratory infection symptoms, but fever and loss of appetite may occur. Less commonly, glomerulonephritis can develop in chronic cases secondary to immune complex deposition. The great variability of symptoms in individual cases of FCV is related to the relative virulence of different strains of the virus.Diagnosis of FCV is difficult without specific tests because the symptoms are similar to other feline respiratory diseases, especially feline viral rhinotracheitis. The presence of stomatitis may indicate FCV. Specific tests include virus culture, polymerase chain reaction (PCR), and immunohistochemical staining.

Treatment and prevention

There is no specific treatment for FCV. Antibiotics are used for secondary bacterial infections. Nursing care and rehydration are used for dehydrated and anorexic cats. Corticosteroids or azathioprine may be used for polyarthritis. Stomatitis is very difficult to treat. Antibiotics, corticosteroids, and tooth extractions all have been used with varying success. Cats on corticosteroids must be monitored carefully for worsening of any upper respiratory infection.

Natural immunity from maternal antibodies lasts in the kitten from three to nine weeks

After that, kittens are susceptible to FCV. Previous infection does not guarantee lifelong immunity, since an antigenically dissimilar FCV (such as VS-FCV) can cause infection. However, usually after the age of three years FCV infections are mild or asymptomatic.FCV vaccination will not always prevent disease but will reduce the severity. FCV vaccines come in two types, attenuated (live, but not virulent) and inactivated. They have been shown to be effective for at least three years.  Attenuated FCV vaccine has been shown to possibly cause mild upper respiratory infection. Inactivated vaccine does not, but it causes more local inflammation and possibly predisposes the cat to vaccine-associated sarcoma.  The only vaccine licensed for prevention of VS-FCV is CaliciVax, manufactured by Fort Dodge Animal Health, a division of Wyeth. It also contains a strain of the traditional FCV virus . Since VS-FCV has arisen from variant strains of FCV, it is not certain that a vaccine for one virulent strain will protect against all virulent strains.

Quarantine is best for control of FCV in catteries and kennels. However, FCV is very contagious and latently infected cats will continue to shed virus, so complete control is difficult. An outbreak of VS-FCV at a humane society in Missouri in 2007 led to the euthanasia of the entire cat population (almost 200 cats) in order to contain it.[11] FCV may survive several days to weeks in a dry environment and longer in a cooler, wet environment. Quaternary ammonium compounds are not thought to be completely effective, but a 1:32 dilution of household bleach used with a detergent and sufficient contact time does seem to kill the virus.