CONDITION
Feline Calicivirus
Feline calicivirus is a common cause of respiratory infection and oral ulceration in cats, with strains varying in severity from mild disease to rare but serious systemic illness.
Why this matters now
Feline calicivirus circulates widely in cat populations, particularly in shelters, breeding catteries, and multi-cat households. Most cats encounter the virus at some point, with kittens and immunocompromised cats typically showing more pronounced signs. Like herpesvirus, calicivirus contributes significantly to the cat flu complex.
Typical infection causes upper respiratory signs and characteristic oral ulcers over a course of one to three weeks. Most cats recover without complications, though some become persistent carriers, shedding virus for months or years. Occasionally, highly virulent strains cause severe systemic disease affecting multiple organs, though this presentation remains rare.
Signals & patterns
Early signals
Mouth ulcers
Painful sores on the tongue, gums, or palate are highly characteristic of calicivirus infection.
Drooling
Excessive salivation often accompanies oral ulceration due to pain and difficulty swallowing.
Sneezing
Upper respiratory involvement causes sneezing, though often less severe than with herpesvirus.
Mild fever
Elevated temperature may accompany acute infection, contributing to lethargy.
Later signals
Reluctance to eat
Oral pain makes eating difficult, leading to reduced food intake and potential dehydration.
Nasal congestion
Respiratory involvement may cause stuffiness and mouth breathing.
Limping
Some strains cause joint inflammation, producing lameness that may shift between legs.
Depression and high fever
Virulent systemic strains can cause severe illness with multi-organ involvement.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis typically relies on clinical presentation, particularly the characteristic oral ulcers, with testing available when confirmation is needed or outbreak investigation is warranted.
Clinical examination
Oral swab for PCR
Blood tests
Joint tap (arthrocentesis)
Virus isolation
Options & trade-offs
Management focuses on supportive care to maintain comfort and nutrition while the immune system clears the infection, with more intensive intervention for severe cases.
Pain management
Medications to reduce oral pain and enable eating
Trade-offs: Improves quality of life; supports nutrition; requires appropriate drug selection for cats.
Nutritional support
Soft, palatable, or warmed foods to encourage eating despite oral discomfort
Trade-offs: Essential for recovery; may require syringe feeding in severe cases; maintains hydration and energy.
Fluid therapy
Subcutaneous or intravenous fluids for dehydrated cats
Trade-offs: Corrects dehydration; supports kidney function; may require hospitalisation for IV fluids.
Antibiotics
Treatment for secondary bacterial infection if present
Trade-offs: Addresses bacterial complications; does not treat virus directly; prevents pneumonia in compromised cats.
Interferon therapy
Immune-modulating treatment used in some countries for severe cases
Trade-offs: May help modulate immune response; limited availability; variable evidence base; cost considerations.
Intensive care
Hospitalisation with aggressive support for virulent systemic disease
Trade-offs: May be lifesaving in severe cases; expensive; requires specialist facilities; guarded prognosis with VS strains.
Common misconceptions
"Calicivirus and herpesvirus cause identical disease"
While both cause cat flu, calicivirus characteristically produces oral ulcers and occasionally limping, while herpesvirus more prominently affects the eyes.
"Vaccinated cats cannot get calicivirus"
Vaccination reduces disease severity but does not prevent infection with all strains; the virus mutates, and multiple strains exist.
"Cats with mouth ulcers are always in severe pain"
While oral ulcers cause discomfort, many cats continue to eat soft food and recover well with supportive care.
Related conditions
Feline Herpesvirus (FHV-1)
Feline herpesvirus and calicivirus are the two most common viral causes of upper respiratory infection in cats, often producing overlapping signs of sneezing, nasal discharge, and ocular involvement. In some cases, cats may be infected with both viruses simultaneously, and distinguishing between them clinically can be difficult without laboratory testing.
Dental Disease in Cats
Calicivirus frequently causes painful oral ulceration, particularly affecting the tongue and hard palate, which can lead to reluctance to eat and altered grooming behaviour similar to signs seen in dental disease. Chronic oral inflammation associated with some calicivirus strains may also complicate existing dental pathology in affected cats.
Pneumonia
In rare cases, particularly with virulent systemic strains of calicivirus, viral damage to the respiratory tract can predispose to secondary bacterial pneumonia. Calicivirus may also occasionally cause primary viral pneumonia as part of severe systemic disease, though this is uncommon compared to typical upper respiratory presentations.
Feline Asthma
Both feline calicivirus and asthma can present with coughing and respiratory signs, though the underlying mechanisms differ—viral inflammation in calicivirus versus chronic airway hypersensitivity in asthma. Distinguishing between acute viral respiratory infection and an asthma exacerbation may require consideration of the cat's history, pattern of signs, and response over time.
Inflammatory Bowel Disease in Cats
Chronic calicivirus infection has been associated with persistent oral inflammation in some cats, and there is ongoing interest in potential links between calicivirus and broader inflammatory conditions affecting the gastrointestinal tract. Both conditions involve chronic immune activation and may present with reduced appetite and weight loss, though the primary sites of inflammation differ.
Cat owners noticing oral ulcers, drooling, or respiratory signs can recognise these as potential calicivirus infection. Understanding that most cases resolve with supportive care while remaining alert to signs of more serious illness helps guide appropriate responses to this common feline infection.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS