Infections caused by Pneumoviruses – Pathogenesis, Clinical Findings and Immunity
Human respiratory syncytial virus is the prime cause of lower respiratory tract diseases in newborns and infants under the age of 2 years.
Respiratory syncytial infection is transmitted by airborne (aerosol) route. Incubation period varies for 2-8 days. Virus multiplies in nasopharyngeal epithelial cells with subsequent descending to lower respiratory tract (bronchioli, alveoli).
Cell-mediated immunity limits the viral spread, thus viremia doesn’t occur.
The adults usually display the symptoms of common cold infection, but in patients above 50 years with chronic obstructive pulmonary disease (COPD) or immunocompromised the course of infection might be severe.
RS virus in infants often results in serious damage of lower airways, primarily affecting bronchioli and alveoli. The life-threatening complication of RSV infection is severe bronchiolitis with acute bronchial obstruction and pneumonia.
Virus disintegrates respiratory epithelium with syncytium formation; the damage is intensified by immune cell-mediated reactions. Excessive cytokine production promotes virus-induced inflammation. However, RS virus blocks interferon synthesis by host epithelial cells.
As RS infection provokes bronchial hyperreactivity, it predisposes to the development of bronchial asthma.
Usually the patients recover completely after the RSV infection. Bronchiolitis and pneumonia in infants can cause lethal outcome without adequate therapy.
Specific humoral and cellular immunity is unstable and of moderate grade. Mucous tissue IgAs are assumed to be responsible for protection. Nevertheless, the repeated exacerbations of RSV infection occur easily. Maternal antibodies defend the newborns against RSV infection during several months after birth.
The first outbreak of respiratory infection caused by human metapneumovirus was registered in Netherlands, where the virus was primarily isolated in 2001. Since that time, metapneumoviral respiratory infections became regularly diagnosed in many countries. The disease affects the susceptible persons of all ages.
Pathogenesis and clinical manifestations of these disorders are generally similar to RSV infection – human metapneumovirus damages lower respiratory tract resulting in bronchiolitis and pneumonia.