Incubation period usually 2-4 days. Upper respiratory disease, acute or subacute, subclinical/mild/severe. Usually lethargy, anorexia, pyrexia, respiratory tract signs, vesicles & ulcers of the oral epithelium (2-5 mm). Also, dyspnoea, ocular & nasal discharges, conjunctivitis, rhinitis, tracheitis, pneumonia, stiff gait.

Strains differ in tropism & virulence; some cause upper & lower respiratory disease, others only upper respiratory disease, some associated with subclinical infections. Morbidity high, mortality low; often seen in kittens within 24-48 hours of infection. Mortality increased by concurrent viral or bacterial infections. Recovery followed by prolonged carrier state.