Virulent Systemic Feline Calicivirus: Recognising and Investigating a High-Mortality Outbreak

A cluster of cats with fever, depression and severe respiratory disease demands a different level of concern when several animals deteriorate rapidly and begin developing oedema, ulcerative skin lesions or bleeding abnormalities. In a multicat setting, this combination raises suspicion of virulent systemic feline calicivirus (VS-FCV), a severe form of FCV infection associated with high mortality^[1]. 

When FCV Disease Becomes Systemic 

VS-FCV differs clinically from the more typical oral and upper respiratory presentation of FCV. Affected cats can develop a severe systemic inflammatory response, followed by disseminated intravascular coagulation (DIC), multiorgan failure and death. 

The reported mortality is approximately 30–70%^[1]. In one outbreak, 11 of 14 affected cats died or were euthanised, corresponding to a mortality of 79%^[2]. 

The clinical progression can be rapid. Incubation has been reported as 1–5 days following hospital exposure and up to 12 days in household settings. More severe disease is reported in adult cats^[1]. 

Which Signs Should Raise Suspicion?^[1] 

The initial presentation may include severe acute upper respiratory signs, fever, depression, and anorexia. What makes VS-FCV particularly concerning is the development of systemic and vascular abnormalities. 

Look for combinations of: 

• Cutaneous oedema or ulcerative lesions involving the skin, paws, head or mouth 
• Jaundice 
• Severe respiratory distress 
• Petechiae or ecchymoses 
• Epistaxis or bloody faeces 
• Signs compatible with thromboembolism or coagulopathy 

These findings, particularly when occurring in several cats, should prompt consideration of a systemic FCV outbreak rather than an uncomplicated upper respiratory infection. 

Diagnosis Depends on the Outbreak Pattern 

One of the most important diagnostic points is that there is no consistent genetic marker that can independently identify VS-FCV. RT-PCR and sequencing cannot, by themselves, confirm or exclude the VS-FCV phenotype^[3]. 

Clinical and epidemiological information therefore becomes central. Suspicion is strengthened when there is: 

• Compatible systemic disease with multiple-organ involvement 
• Occurrence in a multicat environment 
• Rapid, highly contagious spread 
• High mortality among affected cats 
• Isolation of the same FCV strain from several diseased cats 

Virus detection can be performed using blood, oropharyngeal swabs, and samples from ulcerated lesions, with sequencing of hypervariable regions of the capsid used to compare viruses recovered from affected cats. 

Importantly, a single negative RT-PCR result does not rule out VS-FCV. Diagnostic interpretation needs to account for the clinical presentation and outbreak pattern rather than relying on one molecular result. 

Act on Suspicion Early^[1] 

Because VS-FCV can spread rapidly through multicat environments, hygiene and infection-control measures should be initiated as soon as compatible clinical signs appear, rather than waiting for definitive laboratory confirmation. 

This is particularly important when several cats develop severe disease within a short period, and the clinical picture includes systemic inflammation, cutaneous lesions, or bleeding abnormalities. 

Clinical Bottom Line 

VS-FCV should be suspected when severe systemic disease, rapid spread and high mortality occur together in a multicat setting. Unlike uncomplicated FCV infection, affected cats may develop oedema, ulcerative lesions, coagulopathy, respiratory distress, multiorgan failure and death. No single genetic test confirms the syndrome; diagnosis relies on the combination of clinical findings, outbreak characteristics and viral investigation across affected cats. Early infection-control measures are warranted when the clinical pattern is compatible.

Reference:

1. Hofmann-Lehmann R, Hosie MJ, Hartmann K, Egberink H, Truyen U, Tasker S, Belák S, Boucraut-Baralon C, Frymus T, Lloret A, Marsilio F. Calicivirus infection in cats. Viruses. 2022 Apr 29;14(5):937. https://www.mdpi.com/1999-4915/14/5/937
2. Deschamps JY, Topie E, Roux F. Nosocomial feline calicivirus-associated virulent systemic disease in a veterinary emergency and critical care unit in France. Journal of Feline Medicine and Surgery Open Reports. 2015 Dec 15;1(2):2055116915621581. https://journals.sagepub.com/doi/pdf/10.1177/2055116915621581
3. Bordicchia M, Fumian TM, Van Brussel K, Russo AG, Carrai M, Le SJ, Pesavento PA, Holmes EC, Martella V, White P, Beatty JA. Feline calicivirus virulent systemic disease: Clinical epidemiology, analysis of viral isolates and in vitro efficacy of novel antivirals in Australian outbreaks. Viruses. 2021 Oct 9;13(10):2040. https://www.mdpi.com/1999-4915/13/10/2040