Current Diagnosis & Treatment in Infectious Diseases

Section IV - Viral Infections

30. Parainfluenza Virus

Gregory Sonnen MD

Nancy Henry MD, PhD

Essentials of Diagnosis

  • Ubiquitous viral agent.
  • Usually diagnosed on clinical findings.
  • Can infect upper and lower respiratory tract in all ages.
  • Most common etiology of acute laryngotracheobronchitis (croup) in infants and toddlers.
  • Frequent cause of lower respiratory tract infection in children.
  • Yearly reinfection is common.

General Considerations

  1. Epidemiology.Parainfluenza is a ubiquitous virus. It is the primary cause of acute laryngotracheobronchitis (croup) in children aged 6 months to 3 years. It is capable of infecting the lower respiratory tract as well by manifesting as bronchiolitis or pneumonia (Box 30-1). Outbreaks can follow regular epidemic patterns or be sporadic. Certain antigenic types (described below) do follow epidemic patterns.
  2. Microbiology.Parainfluenza virus is a spherical, enveloped RNA virus of the Paramyxoviridaefamily. It ranges in size from 100 to 300 nm. The virus has six structural proteins: HN (hemagglutinin-neuraminidase), F (fusion), M (matrix), NP (nucleoprotein), P (phosphoprotein), and L (large). Parainfluenza is not related to influenza virus. The virus has been divided into four types (1, 2, 3, and 4) and two subtypes (4A and 4B) based on antigenic composition. Antigenic shift does not occur as is common in the influenza virus. The virus is labile and sensitive to low pH, heat, and lipid solvents.
  3. Pathogenesis.The virus is spread person to person commonly via respiratory droplet nuclei. Other routes include direct contact with respiratory secretions and fomites. The virus adheres to and invades the respiratory epithelium and is transferred from cell to cell. Viremia is uncommon. Pathologic epithelial changes have been noted from the nasal mucosa to the alveoli.



Clinical Findings

  1. Signs and Symptoms.Acute laryngotracheobronchitis, often referred to as croup, is a respiratory illness commonly seen in children 6 months to 3 years of age (Table 30-1). Symptoms, usually nocturnal, include a harsh “seal bark” cough, inspiratory stridor, and, in severe cases, respiratory distress. Rarely the airway may be compromised. Low-grade fever and rhinorrhea may be present. Respiratory symptoms usually recur for 3–5 nights and lessen in severity as the illness progresses.
  2. Laboratory Findings.Viral isolation is usually not necessary for appropriate clinical management. If an etiology is sought, culture may be taken from respiratory secretions. The preferred medium is monkey kidney cell medium. Viral antigen may be isolated from respiratory secretions by either enzyme immunoassay or immunofluorescence assay. Serologic evidence is of little clinical value.
  3. Imaging.Radiography is usually not necessary for the diagnosis of croup. If the diagnosis is in question, an anterior-posterior radiograph of the neck will demonstrate subglottic narrowing. This finding is commonly called the “steeple sign.” Lateral films should also be obtained to rule out other processes such as epiglottitis.
  4. Differential Diagnosis.The differential diagnosis of croup includes any situation resulting in upper airway narrowing. Included in this category would be epiglottitis, bacterial tracheitis, foreign body aspiration, retropharyngeal abscess, diphtheria, allergic angioedema, and spasmodic croup.
  5. Complications.Children with a medical history involving narrowing of the upper airway are at higher risk of airway compromise from croup. Diagnoses such as subglottic stenosis and tracheomalacia would fall into this category.


BOX 30-1 Parainfluenza Syndromes




More Common

· Acute laryngotracheobronchitis (croup)

· Upper respiratory tract illness

· Upper respiratory tract illness

Less Common

· Bronchiolitis

· Pneumonia

· Pneumonia (primarily the elderly)


Croup is usually diagnosed based on history and physical examination alone. Radiography can help confirm the diagnosis but is often not needed.


Treatment is aimed at reducing upper airway edema and inflammation to optimize the upper airway (Box 30-2). In the vast majority of cases, cool, humidified air is optimal. Home use of a humidifier in the child's room is an optimal home therapy. If the child is in distress or fails to respond to humidified air, nebulized racemic epinephrine is indicated. If racemic epinephrine is used, the patient should be closely observed for an appropriate amount of time (usually 4 h). Some patients will experience a “rebound” swelling of the laryngeal mucosa after epinephrine administration. Dexamethasone, administered either orally or intramuscularly, will help reduce inflammation and prevent recurrence of symptoms. Dexamethasone is preferred for its long half-life. An inhaled helium-oxygen mixture can be helpful in refractory cases. Nebulized budesonide is very efficacious, but is not yet FDA approved for use in the United States. Intubation is indicated for severe distress or airway collapse.

Table 30-1. Parainfluenzaclinical findings.



Clinical Findings

Acute laryngotracheobronchitis (croup)

· Usually 6 months to 3 years

· Symptoms usually nocturnal

· Initially harsh cough, “seal bark cough”

· Inspiratory stridor

· Respiratory distress

· Neck radiograph will show subglottic narrowing, “steeple sign”

· Symptoms repeat for 3–5 nights


· Usually 6 weeks to 2 years

· Cough

· Clear rhinorrhea

· Tachypnea

· Expiratory wheezing

· Hypoxia in severe cases


· Any age

· Primarily infants and the elderly

· Dyspnea

· Rales on exam

· Febrile

Upper respiratory tract infection (URI)

· Any age

· 2–5% of all adult URIs

· Cough

· Rhinorrhea

· Usually afebrile

· Serous otitis media


Clinical Findings

  1. Signs and Symptoms.Bronchiolitis is an acute respiratory illness in children aged 6 weeks to 2 years (see Table 30-1). Respiratory syncytial virus is the most common cause (see Chapter 31), with parainfluenza being a much less common cause. Patients present with clear rhinorrhea, fever, tachypnea, cough, and expiratory wheezing. Auscultation of the lungs will also reveal a prolonged expiratory phase. Severe cases will be hypoxemic. Some patients advance to respiratory failure.
  2. Laboratory Findings.Laboratory findings are the same as for croup (above).
  3. Imaging.Chest radiography will reveal hyperexpansion. Patchy infiltrates and atelectasis may be present.
  4. Differential Diagnosis.The differential diagnosis of bronchiolitis includes pneumonia, asthma exacerbation, foreign body aspiration, pulmonary edema, and noxious chemical inhalation.
  5. Complications.Children with underlying cardiac or pulmonary disease (eg, congenital heart disease, pulmonary hypertension, bronchopulmonary


dysplasia, asthma, or cystic fibrosis) are at significant risk of serious morbidity from bronchiolitis.

BOX 30-2 Treatment of Parainfluenza Syndromes


· Cool, humidified oxygen

· Racemic epinephrine, 2.25%, 0.05 cc/kg nebulized in 3 cc saline (max = 0.5 cc)

· Dexamethasone, 0.6 mg/kg IM or 0.15 to 0.6 mg orally (max = 10 mg)

· Helium-oxygen mixture can reduce the work of breathing in severe respiratory distress

· Nebulized budesonide is useful, but is not yet FDA approved

· Intubation if airway is severely compromised


· Supportive care

· Humidified oxygen, if needed

· Bronchodilators remain controversial

· Steroids provide no benefit in uncomplicated bronchiolitis

· Mechanical ventilation for severe disease


· Supportive care

· Humidified oxygen, if needed

· Parenteral antibiotics if bacterial etiology cannot be ruled out


· Supportive care

Host with Severe Combined Immunodeficiency

· Aerosolized ribavirin may modify the infection


Bronchiolitis is usually diagnosed based on history and physical examination alone (see Table 30-1). Air trapping will result in hyperexpansion as revealed by chest radiography. Hypoxia and hypercarbia are poor prognostic indicators and warrant aggressive therapy.


Therapy is aimed at maintaining oxygenation and adequate air exchange. Supportive care is effective in the vast majority of cases. Tachypneic infants will have increased insensible fluid losses. Intravenous fluids may be needed if dehydration is present. Nasal suctioning to maintain clear nasal passages is warranted in all infants. Oxygen should be provided immediately if hypoxia is present. Bronchodilators may provide some benefit. Mechanical ventilation is indicated if signs of respiratory failure are present. Corticosteroids will be of benefit if the patient has asthma.


Clinical Findings

  1. Signs and Symptoms.Pneumonia from parainfluenza may occur at any age (see Table 30-1). Typical presenting symptoms may include fever, dyspnea, tachypnea, purulent sputum, and hypoxemia. Diffuse or localized rales may be auscultated.
  2. Laboratory Findings.Laboratory findings are the same as for croup (above).
  3. Imaging.Chest radiography may demonstrate patchy or localized infiltrates.
  4. Differential Diagnosis.A bacterial or mycoplasmal pneumonia often cannot be excluded.
  5. Complications.Patients with underlying pulmonary, immunologic, and cardiac diseases are at risk for serious morbidity.


Diagnosis is based on physical and radiographic evidence (see Table 30-1). The etiology of pneumonia may be difficult to differentiate, especially in children. A sputum sample is often difficult to obtain from infants and toddlers. If a bacterial source cannot be ruled out, antibiotics are warranted.



Antibiotics should be used to cover all appropriate community acquired pneumonias unless a specific etiology can be isolated. Oxygen should be used for hypoxia. Mechanical ventilation should be utilized for respiratory failure.

Prevention & Control

Thorough hand washing will limit the spread of parainfluenza (Box 30-3). No vaccine is currently available. Droplet isolation of the hospitalized patient is indicated. Symptomatic children should be excluded from school or daycare.

BOX 30-3 Control of Parainfluenza Infection

Prophylactic Measures

· Handwashing

· No vaccine currently available

Isolation Precautions

· Droplet isolation for hospitalized symptomatic patients

· Children should be excluded from school and daycare until symptoms resolve


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