RSV

**Virology** - Family Paramyxoviridae (now Pneumoviridae in updated taxonomy); enveloped, nonsegmented, negative-sense ssRNA virus. Carries its own RNA-dependent RNA polymerase. - F (fusion) protein: mediates viral fusion with host cells and fusion of infected cells into multinucleated syncytia (hallmark). It is the main target of neutralizing antibody and of prophylaxis. - G (attachment) protein: attachment. Unlike parainfluenza, measles, and mumps, RSV lacks hemagglutinin and neuraminidase. - Seasonal: winter and early spring in temperate climates. Spread by respiratory droplets and fomites. Highly contagious, and reinfection is common because immunity is incomplete.

**Clinical syndromes** - Bronchiolitis (classic): infants <2 years, peak at 2-6 months. Begins with URI symptoms (rhinorrhea, cough, low-grade fever) and progresses to tachypnea, wheezing, crackles, nasal flaring, retractions, and hypoxia. Inflammation and necrosis of bronchiolar epithelium with mucus plugging cause air trapping, hyperinflation, and atelectasis. - Pneumonia, and sometimes apnea in very young infants or premature infants. - Older children and adults: mild URI. Older adults (≥60-75 years) and immunocompromised patients are at risk of severe disease, pneumonia, and exacerbations of COPD, asthma, and heart failure. - Associated with later recurrent wheezing.

**Risk factors for severe disease** - Prematurity, bronchopulmonary dysplasia, hemodynamically significant congenital heart disease, immunodeficiency, neuromuscular disease, daycare attendance, and tobacco smoke exposure.

**Diagnosis** - Mainly clinical. Rapid antigen testing or PCR of a nasopharyngeal swab can confirm. - CXR (not routinely needed): hyperinflation, peribronchial thickening, patchy atelectasis.

**Management** - Supportive: nasal suctioning, hydration, supplemental O2, and respiratory support (high-flow nasal cannula or ventilation if needed). - Bronchodilators, corticosteroids, and routine antibiotics are not recommended. - Ribavirin (a guanosine analog that inhibits IMP dehydrogenase and viral RNA synthesis) is rarely used, for severe disease in immunocompromised patients. It is teratogenic and can cause hemolytic anemia.

**Prevention** - Palivizumab: monoclonal antibody against the F protein, given monthly during RSV season to high-risk infants. - Nirsevimab (and clesrovimab): long-acting monoclonal antibodies against the F protein, given as a single dose to infants, now recommended for most infants entering their first RSV season. - Maternal RSV vaccine (RSVpreF) at 32-36 weeks' gestation gives passive antibody protection to the newborn. - RSV vaccines are recommended for older adults (including adjuvanted and mRNA formulations). - Infection control: hand hygiene and contact/droplet precautions.

**High-yield board associations** - Infant with winter wheezing and respiratory distress → RSV bronchiolitis. - Multinucleated syncytia in culture → RSV. - Palivizumab → F protein. - Differential: croup (parainfluenza, steeple sign, barking cough) versus bronchiolitis (RSV, wheezing).