Feline calicivirus (FCV) is best known for its role in causing acute upper respiratory disease and oral ulceration in kittens and cats. Although morbidity in routine respiratory infections is high, these infections are rarely fatal. Most recovered kittens do develop a chronic carrier state characterized by continuous shedding of virulent virus from the oral cavity and respiratory tract that may last for months to years. The caliciviruses are comprised of a variety of small, non-enveloped, single-stranded RNA viruses that are regarded as being somewhat more resistant than feline herpesvirus-1 when outside the host. Unlike herpesvirus-1, however, over 40 strains of calicivirus have been identified and are known to vary significantly in virulence and antigenicity. Despite widespread vaccination against FHV-1 and FCV, these viruses are still known to occur in large numbers in the domestic cat population, particularly in multiple cat households and animal shelters.

Since 1998, however, at least 6 outbreaks of a virulent feline calicivirus have been reported. [REF #1] In these outbreaks, virulent, systemic strains of feline calicivirus (referred to as VS-FCV or hemorrhagic feline calicivirus) were recovered. Unlike FCV associated with routine, acute upper respiratory infection in kittens, infection with a virulent systemic strain of calicivirus has resulted in high mortality among previously healthy kittens and cats. Associated clinical signs include high fever, oculonasal discharge, alopecia, cutaneous ulcers involving the face and limbs, and subcutaneous edema. Infection causes epithelial cell cytolysis and systemic vascular compromise in susceptible cats. VS-FCV viruses appear to be highly contagious and are easily transmitted to other cats by way of eating utensils, hands, and even clothing.

Although the incidence of infection is quite low within the cat population, new vaccines are being developed to protect cats against the emerging strains of virulent FCV. This column is a 'first look' at the clinical aspects of hemorrhagic feline calicivirus and will facilitate decisions regarding the use of these new vaccines as they enter the market in 2007.

Virulent systemic calicivirus is a newly recognized feline calicivirus variant that causes severe systemic disease with up to 60% mortality in affected populations of cats. Clinical signs include upper respiratory disease (oculo-nasal discharge, oral ulcers), pneumonia, peripheral edema and skin sloughing (especially on the face and limbs) due to cutaneous vasculitis, and systemic vasculitis with DIC that causes multiple organ/system failure which may lead to death.

Which cats are most likely to be affected?

Virulent systemic calicivirus has occurred almost exclusively in populations of group housed cats. In each documented outbreak, the disease seems to have spontaneously appeared in the population, most likely by mutation from caliciviruses already circulating in the group of cats. This is supported by genomic analysis of the virulent strains - they are unique, rather than clonal, and have no common mutations that could easily explain the change in virulence. Outbreaks of virulent calicivirus have been isolated and are very rare. Shelter and rescue catteries are the most commonly affected populations. Several epizootics in veterinary facilities have occurred because of the introduction of a sick shelter cat into the veterinary hospital. Because this calicivirus variant is so pathogenic, the disease usually burns itself out in the affected population over several weeks and general spread to and among household pets in the surrounding community has not been reported. However, in at least one case, veterinary personnel handling cats in an affected population have carried the disease to their own pets at home.

How can I recognize this disease?

Virulent systemic calicivirus is a very rare disease that most often affects group housed cats. Typically, the disease emerges as an apparent epizootic of upper respiratory disease that causes severe respiratory symptomatology but progresses to produce cutaneous and systemic complications with a higher than expected mortality rate among affected cats. The incubation period for VS-FCV is 1-5 days. Older cats often have more severe disease than younger cats with this unusual calicivirus variant. Keep in mind that this is a very rare variant of feline calicivirus and that the vast majority of cats with clinical signs of upper respiratory disease will have infection caused by the much more common feline upper respiratory disease agents.