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

Purpose: Identifies characteristic oral ulcers and respiratory signs
Considerations: Pattern of mouth lesions highly suggestive; joint involvement supports calicivirus over herpesvirus.

Oral swab for PCR

Purpose: Detects viral genetic material, confirming active infection
Considerations: Useful for definitive diagnosis; cannot distinguish strain virulence; carrier cats may test positive.

Blood tests

Purpose: Assesses overall health and identifies complications of severe disease
Considerations: Important in systemically unwell cats; monitors organ function during virulent strain infection.

Joint tap (arthrocentesis)

Purpose: Analyzes joint fluid if limping is prominent
Considerations: Can support diagnosis of calicivirus-associated lameness; invasive procedure.

Virus isolation

Purpose: Grows virus from samples for detailed characterisation
Considerations: Specialist procedure; useful for outbreak investigation; identifies strain characteristics.

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

Misconception:

"Calicivirus and herpesvirus cause identical disease"

Reality:

While both cause cat flu, calicivirus characteristically produces oral ulcers and occasionally limping, while herpesvirus more prominently affects the eyes.

Misconception:

"Vaccinated cats cannot get calicivirus"

Reality:

Vaccination reduces disease severity but does not prevent infection with all strains; the virus mutates, and multiple strains exist.

Misconception:

"Cats with mouth ulcers are always in severe pain"

Reality:

While oral ulcers cause discomfort, many cats continue to eat soft food and recover well with supportive care.

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