RSV

Respiratory syncytial virus (RSV) is an RNA virus that is the most common cause of respiratory tract infections in infants and young toddlers (Dawson-Caswell & Muncie, 2011). It is the most commonly associated pathogen that causes bronchiolitis in children (McCance & Huether, 2019). RSV can also present with different clinical syndromes, including acute lower respiratory tract infection, pneumonia, and asthma (Barr, Green, Sande, & Drysdale, 2019). It was estimated that there were 33.1 million cases of RSV with acute lower respiratory tract infection globally in 2015 and 3.2 million hospital admission in children under five years of age (Barr et al., 2019).

RSV is an enveloped, nonsegmented, negative-stranded RNA virus found within the genus Orthopneumovirus, a member of the Paramxoviridae family (Dawson-Caswell & Muncie, 2011). When there are viral infections, RSV causes inflammation and necrosis of the bronchial epithelium with the destruction of ciliated epithelial cells (McCance & Huether, 2019). The bronchial epithelium’s normal function is to act as a defensive barrier and maintain normal airway function. The bronchial epithelial cells form the interface between the external environment and the internal milieu and serve as effector chemokines and cytokines that activate inflammatory cells (Gao et al., 2015). However, when there is an inflammation of the bronchial epithelium, the inflammation causes the submucosa becoming edematous, and cellular debris and fibrin form plugs within the bronchioles (McCance & Huether, 2019). As a result, there is edema of the bronchial wall, accumulation of mucus, and cellular debris, which lead to possible bronchospasm narrow or occlude many peripheral airways (McCance & Huether, 2019). In more severe cases, children can have decreased lung compliance and increased breathing due to airway resistance and hyperinflation (McCance & Huether, 2019). The symptoms present in infants under one-year and children up to 2 years old usually have a low-grade fever, cough, coryza, difficulty breathing, and reduced feeding (Barr et al., 2019).

By two years of age, most children will have had an RSV infection with the peak seasons from November to April (Dawson-Caswell & Muncie, 2011). There is also an estimation of over 95% of children who have been infected with RSV by two years of age (Griffiths, Drews, & Marchant, 2017). The main reason for the high risk of RSV infection in children is the high surface-area-to-volume ratio of developing airways (Griffiths et al., 2017). In children, the lumen of the airway’s bronchioles is smaller than in adults and thus more prone to airway obstruction (Griffiths et al., 2017). When there is an RSV infection, it causes smooth muscle hyperreactivity, which coincides with airway inflammation and further constricts already small airways (Griffiths et al., 2017). Thus, the inflammation of developing airways makes them more susceptible to increased airway resistance (Griffiths et al., 2017).

Besides, in infants and young children, they have immature immune systems of young naïve hosts and lack of functionally RSV protective maternal antibody (Pickles & DeVincenzo, 2015). Infant’s lungs have smaller airway dimensions to compare with adults and are more likely to develop severe distal airway diseases (Pickles & DeVincenzo, 2015). Infant’s lung has poor development of collateral ventilation of alveolar regions, which in adult lungs enables well with the ventilation of the lung regions (Pickles & DeVincenzo, 2015). There is also no increase in infection susceptibility or replication by RSV in adults to compare with children (Griffiths et al., 2017). Therefore, infants and young children with an immature immune system and the airway’s smaller physical dimensions are more likely to develop distal airway such as RSV than adults.