Feline Calicivirus & Virulent Systemic Feline Calicivirus (VS-FCV)

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Top Highlights

• Outbreaks of severe FCV tend to be a focal population problem, not a community problem

• Feline calicivirus is extremely common in shelters, causing a range of effects from subclinical or mild disease to Virulent Systemic Feline Calicivirus (VSFCV)

• outbreaks of VSFCV are rare

• release of the first vaccine billed as a “virulent calicivirus vaccine”,

• Differentiation between virulent and less-virulent FCV (referred to here as “field strains”) is not always straightforward.

• FCV infection is common even in healthy-appearing cats: up to 25% of asymptomatic cats from multiple cat environments such as shelters and catteries, and up to 8% of pet cats, will be shedding FCV from the oral cavity at any given time

• As many as half of all upper respiratory infections are caused by FCV.

• FCV can be recovered from the oral cavity, conjunctiva, blood and/or feces

• FCV infection is often chronic: as many as 50% of cats will still be shedding FCV for 75 days after recovery from acute disease

• Some cats will shed for years or even for life.

• Chronically shedding cats may transmit infection, but risk is substantially decreased once all clinical signs have resolved, simply because there is less drool, snot, etc. to carry virus.

• FCV causes extremely variable signs

• some strains cause no signs at all

• Others manifestations of FCV include: mild upper respiratory infection without oral ulceration (indistinguishable from URI due to herpesviral infection); limping with or without URI signs; mild to very severe oral ulceration/faucitis/ulceration of the esophagus with or without URI signs; systemic signs such as fever, anorexia and lethargy in combination with one or more of the preceding signs; pneumonia; and virulent systemic calicivirus.

• Not all strains of FCV that cause severe disease are truly “virulent systemic FCV”

• FCV does not always act alone

• mild field strain FCV in combination with other pathogens – most notably Bordetella bronchiseptica and feline panleukopenia – can cause severe disease, including pneumonia and death.

• Not all cases of oral ulceration/URI followed by severe disease and death are VS-FCV

• it is much more common to diagnose panleukopenia with FCV or another respiratory pathogen than it is to find VS-FCV in outbreaks of URI with high mortality in incompletely vaccinated cats

• There is no genetic marker that can distinguish virulent systemic strains from any other strain