C. Pathogenesis Parainfluenza viruses enter the host through the inhalation of infected droplet nuclei. Virus multiplication occurs throughout the tracheobronchial tree, inducing the production of mucus. The vocal cords of the larynx become grossly swollen, causing obstruction to the inflow of air, which is manifested by inspiratory stridor. In adults, the virus is usually limited to causing inflammation in the upper parts of the respiratory tract. In infants and young children, the bronchi, bronchioles and lungs are occasionally involved, which may reflect on the small size of the airways and the relative immunological immaturity.
Viraemia is neither an essential nor a common phase of infection.
D. Clinical Features Croup is the commonest clinical manifestation of parainfluenza virus infection in children. Typically, the children exhibit a croupy cough, inspiratory stridor, hoarse voice or cry and respiratory difficulty on inspiration. Patients are usually afebrile. Respiratory rhonchi are heard frequently throughout the lung fields. Radiological examination is usually normal.