Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is highly infectious and easily transmitted from person to person by close contact. The primary modes of transmission include direct contact with large droplets of secretions (small particle aerosol is not a significant mode of transmission) and self-inoculation of eyes and nose by hands made infectious by touching contaminated objects. For example, RSV can be isolated from countertops more than 6 h after contact with an infected source such as nasal secretions. Other infectious media include rubber gloves (90 min), gowns (30 min), and hands (25 min). Good hand washing and proper disposal practices are, therefore, important as infected individuals shed the virus for 1-21 d (mean of 6.7 d) even if asymptomatic (52,53).

Epidemiology

RSV infections are distributed worldwide and are the leading cause of lower respiratory tract infections (50-90% of bronchiolitis cases are due to RSV) in infants and young children. In the United States the season generally begins in November peaks from January to March and then continues through April to mid-May. Approximately 90,000 children are hospitalized and 4500 deaths occur annually in the United States due to the complications of RSV.

Fifty to sixty-nine percent of all children develop primary RSV infection by the age of 12 mo, with 15-22% having lower respiratory involvement. One-half to two percent will be hospitalized, with mortality ranging from 0.5% to 3.5%. By 2 yr of age 95% have been infected with RSV. Immunity following infection is short lived; therefore, reinfection occurs throughout life. Reinfection rates in preschool-aged children range from 40% to 70%, with approx 20% recurrences in school-aged children, adolescents, and adults.

Increased disease severity is associated with low socioeconomic status, ethnicity, male gender, young age, body mass of < 5 kg, prematurity, chronic lung disease, congenital heart disease especially in association with pulmonary hypertension, and T-cell immunodeficiency. Increased risks of acquiring the disease occur with a maternal education level of grade 12 or less, crowding (two or more individuals sharing a bedroom), school-age siblings, multiple births, lack of breastfeeding, passive smoke exposure, day-care attendance, and birth within 6 mo prior to an anticipated RSV season (52,54).

Disease Process

The average incubation period of RSV is 5 d. Symptoms can range from coldlike symptoms to severe bronchiolitis or pneumonia with symptoms after reinfection typically being milder. The hallmark of infection involves the small intrapulmonary airways, with bronchiolitis being the most distinctive clinical syndrome. Lower respiratory tract infections are due to transfer of the virus from the upper respiratory tract and result in sloughing of the bronchiolar epithelium, hypersecretion of mucus, peribronchiolar mononuclear infiltration, and submucosal edema. The plugs of mucus and cellular debris lead to partial or complete airway obstruction, especially in the small lumens of infants. Following infection, the immunity to the RSV virus is transient and imperfect. Recurrent upper respiratory infections are probably due to the transitory nature of immunity of immunoglobulin A (IgA). In contrast, lower respiratory tract resistance appears to be more durable (52).

Neonate

Newborns typically acquire RSV via contact with visitors and healthcare personnel. The clinical variation and rare clinical evidence of lower respiratory tract involvement in individuals <3 wk of age is probably due to the presence of maternally derived neutralizing antibodies. Signs of upper respiratory tract infections occur in fewer than 50% of infected neonates and are highly variable and nonspecific. These clinical signs include poor feeding, lethargy, and irritability.

Young Infants

Infants younger than 1 yr of age who have low cord serum RSV antibody titers and are not breast fed have an increased risk of lower respiratory tract disease in the first 5 mo of life. One of the early manifestations in this age group following RSV infection is apnea, occurring more readily in infants who are <6 wk of age, are born prematurely, or have low arterial oxygenation saturations. Mechanical ventilation, if required, is necessary for approx 48 h with postextubation apnea being uncommon.

Infants with severe disease, but without an underlying medical condition, can be identified with six independent clinical and laboratory findings to predict those who would benefit from hospitalization: (1) Oxygen saturation of <95%, determined by pulse oximetry, is the best single objective predictor. (2) Atelectasis on chest radiograph. Typically, diffuse interstitial pneumonitis and hyperexpansion are apparent with this latter process being the hallmark of RSV infection. It occurs in 50% of hospitalized patients and may be the only radiographic finding in 15% of the cases. Twenty-five percent of children, especially younger infants, have subsegmental consolidation, typically in the right upper or middle lobe. (3) Respiratory rates > 70 breaths per minute. (4) Gestational age <34 wk. (5) Age < 3 mo. (6) "Ill" or "toxic" appearance. If hospitalization is required, the length of stay is typically 4-7 d with full recovery at about 2 wk. Major complications include respiratory failure, apnea, and secondary bacterial infections. Long-term complications are minimal, with recurrent episodes of wheezing being the major clinical sequelae. Recurrences diminish after the first few years with no increased risk for airway hyperreactivity or pulmonary function abnormalities by the age of 8-12.

Young Children

Initial infections with RSV are typically symptomatic and range from a mild coldlike illness to severe bronchiolitis or pneumonia. These latter syndromes occur in 30-70% of cases following the initial exposure to RSV and can be difficult to differentiate, but the classic signs of bronchiolitis are wheezing and hyperexpansion of the lung. Typically, children have fever ranging from 38°C to 40°C during the first 2-4 d of the illness, nasal discharge, pharyngitis, and cough. Hoarseness and laryngitis are uncommon. By the time these children present to their local healthcare facility, lower respiratory tract symptoms are more prominent. These signs and symptoms are based on the severity of disease and can include increased cough, increased respiratory rate up to 80 breaths per minute with substernal and intercostal retractions during inspiration, a prolonged expiratory phase, hypoxemia typically without cyanosis, hyperex-panded and hyperresonant chest, and intermittent rales and wheezes.

Older Children and Adults

Recurrent infection with RSV after the age of 2 yr most commonly manifests as upper respiratory tract infections or tracheobronchitis. Asymptomatic infections and lower respiratory disease are uncommon in this age group. Typically, symptoms

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