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, although virulent systemic disease appears more severe in adults. Like herpesvirus, calicivirus contributes significantly to the cat flu complex.
Typical infection causes upper respiratory signs and characteristic oral ulcers, which usually settle within a few days to two 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.
Swelling of the face or legs, skin sores, or yellowing
Rare virulent systemic strains cause swelling of the skin, mainly on the head and legs, ulcers on the skin and paws, high fever and dullness, and some cats become jaundiced (yellow gums, skin or eyes). Published mortality is about 30 to 70 per cent, and adults appear more severely affected than kittens.
Laboured breathing in kittens
In some severe cases, particularly in young kittens, calicivirus causes pneumonia: breathing becomes laboured, often with coughing, fever and dullness. Effortful breathing cannot be assessed safely at home.
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: Maintains hydration and energy during recovery; appetite stimulants may be used, and the ABCD guideline advises a feeding tube when a cat has not eaten for more than three days.
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 the virus; used when fever, lethargy or loss of appetite suggest secondary bacterial infection.
Interferon therapy
Recombinant feline interferon omega, an immune-modulating treatment. In the UK it is authorised for cats with FeLV or FIV, not for calicivirus, so its use here is under the veterinary medicines cascade.
Trade-offs: May help modulate the immune response; some response is reported in cats with chronic gingivostomatitis; the evidence base is limited; 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 usually protects cats from disease but does not stop them becoming infected or shedding the virus. Many strains exist and the virus changes readily, so protection against every strain is not complete.
"Calicivirus mouth ulcers are only a minor discomfort"
Calicivirus mouth ulcers are painful and often stop a cat eating. Pain relief is a standard part of care, and many cats then manage soft, warmed food while the ulcers heal.
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. Calicivirus is also found more often in cats with chronic gingivostomatitis than in healthy cats, so it may be involved in inflammation in a mouth with existing dental disease.
Pneumonia
Calicivirus pneumonia occurs mainly in young kittens. It is uncommon compared with the usual upper respiratory form of the infection.
Feline Asthma
Coughing is not a typical sign of calicivirus, but calicivirus pneumonia, mainly in young kittens, can cause coughing and laboured breathing that resemble asthma. The mechanisms differ: viral infection in calicivirus, and chronic airway inflammation in asthma, which is more often seen in adult cats. 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.
Cat owners noticing oral ulcers, drooling, or respiratory signs can recognise these as potential calicivirus infection. Most cases settle with supportive care. The signs that change this picture are a cat that stops eating, laboured breathing in a kitten, and swelling of the face or legs with skin sores, which point to the rare virulent systemic form. Hydration, the extent of ulcers at the back of the mouth and the cause of a fever cannot be judged reliably at home.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS