Feline calicivirus (FCV) is one of the most common and contagious infectious diseases affecting domestic cats worldwide. Whether you are a cat owner, breeder, or veterinary professional, understanding FCV—its clinical signs, how it spreads, and how to manage it—is essential for protecting feline health.

What Is Feline Calicivirus?

Feline calicivirus is a non-enveloped, single-stranded, positive-sense RNA virus and one of the two primary causes of “cat flu” in domestic cats, the other being feline herpesvirus type 1 (FHV-1). The two viruses can infect cats independently or simultaneously, and their clinical signs are frequently indistinguishable without laboratory testing.

Exposure to FCV does not always result in clinical illness. However, infections can range from mild, self-limiting conditions to severe, life-threatening systemic disease. The virus is notable for its extraordinary infectivity, high morbidity, and significant mortality potential in vulnerable populations.

How Common Is Feline Calicivirus?

Calicivirus is among the most frequently encountered infectious pathogens in cats and poses a particular challenge in multi-cat environments such as catteries, shelters, and rescue facilities, where transmission is rapid and containment is difficult. Despite widespread vaccination programs, available vaccines offer incomplete protection due to the virus’s high rate of genetic mutation. At least six distinct outbreaks of virulent, high-mortality FCV disease have been documented in the United States alone.

Clinical Signs of Feline Calicivirus

FCV has a strong preference for the epithelial tissues of the oral cavity and the deep tissues of the lungs, which explains its most characteristic presentations.

Upper Respiratory Infection

The most common manifestation of FCV is upper respiratory tract infection. After the virus enters via the nasal and oral mucosa through aerosol exposure, viremia follows, and viral shedding occurs for approximately two weeks post-infection. In cattery and shelter settings, shedding may persist for months. Concurrent lymphopenia and neutropenia are common laboratory findings.

Typical signs include nasal discharge (serous to mucopurulent), conjunctivitis, sneezing, oral ulceration of the tongue, hard palate, or nostrils, drooling, inappetence, fever, lethargy, and depression. While conjunctivitis and rhinitis can occur with FCV, these signs are more characteristic of FHV-1 infection.

Oral Ulceration and Stomatitis

FCV characteristically causes vesicular stomatitis involving the tongue (glossitis), throat (faucitis), and palate (palatitis). The vesicles rupture within hours to days, leaving small, painful erosions. Most cats regain appetite 2–3 days after symptom onset, and the overall clinical course usually lasts 7–10 days. In some cats, FCV has been associated with lymphocytic-plasmacytic stomatitis (LPS), a severe chronic form of gingivitis requiring aggressive long-term management.

Limping Syndrome

Two specific FCV strains produce a distinctive limping syndrome without typical oral or respiratory signs. Affected cats develop sudden-onset fever, alternating leg lameness, and pain on palpation of affected joints. This syndrome occurs most commonly in kittens aged 8–12 weeks and typically resolves without treatment. It can also appear following recent FCV vaccination, as no currently available vaccine protects against both strains responsible for this syndrome. Localized joint infection can occasionally progress to septic synovitis.

Virulent Systemic Disease (FCV-VSD)

The most severe form of FCV is virulent systemic disease (FCV-VSD), most frequently seen in overcrowded multi-cat households with suboptimal husbandry. Signs extend beyond the respiratory tract and include facial and limb edema, ulcerative dermatitis, severe respiratory distress, and progressive immune complex-mediated glomerulonephritis. This leads to chronic renal disease characterized by persistent proteinuria and elevated urine protein-to-creatinine ratios. FCV-VSD carries a serious prognosis and has been associated with significant mortality in reported outbreaks.

Chronic Infection and Viral Shedding

Approximately 25% of FCV-infected cats develop chronic infection, while up to 50% continue shedding virus after apparent recovery. This chronic shedding sustains viral circulation within cat populations, particularly in multi-cat environments.

Differences in clinical severity reflect both genetic variation among circulating viral strains and individual immune responses. FCV isolates in endemically infected populations consistently display high genetic diversity, with some strains clustering genetically with vaccine strains—a consequence of the virus’s rapid evolutionary capacity.

Diagnosis

Diagnosis is based on clinical evaluation combined with laboratory testing. Clinical indicators include ocular and nasal discharge, conjunctivitis, oral ulcers, stomatitis, and upper respiratory distress. In FCV-VSD, facial and limb edema and ulcerative dermatitis are additional findings.

Laboratory confirmation involves excluding FHV-1 (the most important differential diagnosis) and detecting FCV via reverse transcriptase PCR (RT-PCR), now commercially available worldwide at relatively low cost and considered the current standard for definitive diagnosis.

Treatment

No specific antiviral cure exists for routine FCV infection. Management centers on supportive care and secondary complication control.

Broad-spectrum antibiotics address secondary bacterial infections. Mucolytic agents help manage respiratory secretions, and nutritional support is critical for anorexic cats. Lysine supplementation—useful in FHV-1 management—is not effective against FCV.

Cats with chronic LPS or FCV-associated glomerulonephritis may require antiviral therapy such as acyclovir or related agents; prognosis in these cases is guarded. Isolating infected cats is an essential infection control measure, and breeding from chronically infected cats is not recommended.

Prevention and Vaccination

FCV vaccination is a core vaccine for cats. The primary course should be administered at 8 weeks and 12 weeks of age for highest efficacy. Even so, vaccination provides approximately 75% protection at best, reflecting the virus’s capacity for rapid genetic change.

Vaccination reduces severity of clinical signs in exposed cats but does not reliably prevent infection. Vaccinated cats can shed vaccine-strain virus orally, and kittens born to vaccinated queens may have only minimal protection due to genetic divergence of circulating strains. No vaccine currently covers all FCV strains, including both strains responsible for the limping syndrome. Three-year duration-of-immunity data support routine booster vaccination as part of a core preventive health program.

Key Takeaways for Cat Owners and Breeders

• FCV is highly contagious and particularly prevalent in multi-cat environments
• Clinical presentations range from mild mouth ulcers to potentially fatal systemic disease
• Chronically infected cats can spread the virus even without visible symptoms
• Vaccination reduces disease severity but does not guarantee protection
• Early veterinary evaluation is important for accurate diagnosis and management

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References

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