What is parainfluenza?

Parainfluenza Virus: Clinical Overview

Parainfluenza viruses are a major cause of respiratory illnesses including croup, bronchiolitis, and pneumonia, accounting for up to 11% of all hospitalizations in children under 5 years old. 1

Virus Characteristics and Types

Parainfluenza viruses (PIVs) are single-stranded, enveloped RNA viruses belonging to the Paramyxoviridae family. There are four antigenically distinct types:

• PIV-1 and PIV-2: Most commonly associated with croup syndrome 1
• PIV-3: Most frequently associated with bronchiolitis and pneumonia 1
• PIV-4: Less commonly identified but can cause significant respiratory illness 2

PIV-3 is the most commonly detected type in both children and adults with hematological malignancies and stem cell transplants (80-90% of cases), followed by PIV-1 and PIV-2 1.

Epidemiology and Transmission

• PIVs cause respiratory infections throughout the year, but show type-specific seasonal patterns:

  • PIV-1: Circulates primarily in odd-numbered years 3
  • PIV-2 and PIV-3: Circulate annually 3
  • PIV-3: Most common during spring and summer
  • PIV-1 and PIV-2: Most common during fall and winter

• Transmission occurs through direct contact with infectious respiratory secretions or fomites

• Incubation period is approximately 2.6 days (95% CI, 2.1-3.1) 1

• Asymptomatic shedding occurs in 17.9% of cases 1

Clinical Manifestations

PIVs cause a spectrum of respiratory illnesses ranging from mild upper respiratory infections to severe lower respiratory disease:

Upper Respiratory Tract Infections

• Rhinorrhea
• Pharyngitis
• Congestion
• Cough
• Fever
• Otitis media (occurs in approximately 25% of children under 5 years) 1

Lower Respiratory Tract Infections

• Croup (Laryngotracheobronchitis): Characterized by barking cough, inspiratory stridor, and respiratory distress

  • PIV croup is often more severe than croup caused by other viruses 1
  • More likely to be complicated by bacterial tracheitis 1

• Bronchiolitis: PIV ranks second only to respiratory syncytial virus as a cause 1

• Pneumonia: Can be viral or complicated by bacterial superinfection

Severe Manifestations

• Apnea (particularly in infants) 2
• Febrile seizures (more likely to be recurrent with PIV than with other causes of fever) 1
• Encephalopathy/encephalitis (rare) 1

Special Populations

In immunocompromised patients, particularly hematopoietic stem cell transplant recipients:

• Higher risk of progression to lower respiratory tract infection (13-37%)
• Higher mortality (10-30%) 1
• Risk factors include corticosteroid exposure, neutropenia, lymphopenia, early post-transplant infection, and coinfections 1

Diagnosis

Respiratory secretions or nasopharyngeal swabs placed in appropriate viral transport medium are the specimens of choice 1. Diagnostic methods include:

1. Nucleic Acid Amplification Tests (NAATs):

   • Now common in commercial respiratory panels
   • Most sensitive method
   • Can identify all four PIV types 1

2. Viral Culture:

   • May take 4-7 days for detection
   • Less commonly used now 1

3. Direct Antigen Detection (DAD):

   • Rapid but less sensitive than NAATs
   • Being replaced by molecular methods in many centers 1

Treatment

Treatment for parainfluenza virus infections is primarily supportive:

• Mild to moderate disease: Symptomatic care including hydration, antipyretics, and monitoring
• Croup: Corticosteroids have been found beneficial 4
• Severe disease: Respiratory support as needed

For immunocompromised patients with severe disease:

• Some centers consider treating with ribavirin and/or intravenous immunoglobulin (IVIG), though evidence from randomized controlled trials is limited 1
• Novel drugs like DAS181 show promise for treating severe disease in immunocompromised patients 4

Prevention

• Standard infection control measures are crucial, especially in healthcare settings
• No licensed vaccines are currently available, though several are in development 4
• When vaccines become available, immunization would be most effective if administered within the first year of life 3

Clinical Impact and Burden

• PIVs account for 40% of pediatric hospitalizations for lower respiratory tract illnesses and 75% of croup cases 4
• Annual charges for PIV-associated hospitalizations in the U.S. for children under 5 years are estimated at:
  • Bronchiolitis: $43 million
  • Croup: $58 million
  • Pneumonia: $158 million 3

The majority of PIV-associated hospitalizations occur in children aged 0-2 years, highlighting the importance of early diagnosis and appropriate management in this vulnerable population 3.

Human Parainfluenza virus is an important respiratory pathogen that should be considered in the differential diagnosis of respiratory illnesses, particularly in young children and immunocompromised patients.