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
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