Intial Symptoms: The infant 1st develops a mild upper respiratory tract infection with sneezing and clear rhinorrhea.
Fever: This may be accompanied by diminished appetite and fever of 38.5–39°C (101–102°F), although the temperature may range from subnormal to markedly elevated.
Respiratory symptoms: Gradually, respiratory distress ensues, with paroxysmal wheezy cough, dyspnea, and irritability. The infant is often tachypneic, which may interfere with feeding. The child does not usually have other systemic complaints, such as diarrhea or vomiting. Apnea may be more prominent than wheezing early in the course of the disease, particularly with very young infants (<2 mo old) or former premature infants.
The physical examination is characterized most prominently by wheezing. The degree of tachypnea does not always correlate with the degree of hypoxemia or hypercarbia, so the use of pulse oximetry and noninvasive carbon dioxide determination is essential. Work of breathing may be markedly increased, with nasal flaring and retractions. Auscultation may reveal fine crackles or overt wheezes, with prolongation of the expiratory phase of breathing. Barely audible breath sounds suggest very severe disease with nearly complete bronchiolar obstruction. Hyperinflation of the lungs may permit palpation of the liver and spleen