HPIVs are common respiratory pathogens causing a wide range of illnesses affecting the nose, throat, and lungs in people of all ages. They cause both upper and lower respiratory tract infections, often resembling the common cold. While most infections are mild, HPIVs can lead to serious complications and are a major cause of hospitalization. This risk is highest in infants, young children, older adults, and those with weakened immune systems. Nearly all children are infected by at least one type of HPIV by age five.
The Parainfluenza Virus Family
HPIVs are enveloped, single-stranded RNA viruses belonging to the family Paramyxoviridae. This family also includes measles and mumps, but HPIVs are entirely separate from the influenza virus. The HPIV family is genetically divided into four distinct types: HPIV-1 through HPIV-4, with HPIV-4 further separated into subtypes 4a and 4b.
HPIV types exhibit different seasonal patterns and disease associations. HPIV-1 outbreaks often occur in the fall, typically peaking every other year (biennial pattern). HPIV-2 also circulates in the fall but is generally less common. In contrast, HPIV-3 tends to peak annually during the spring and early summer months. HPIV-4 is less frequently detected, associated with milder disease, and often appears in late summer and fall.
How Parainfluenza Spreads
HPIV transmission occurs primarily through respiratory secretions from an infected person. When an individual coughs or sneezes, the virus is released in large respiratory droplets that can be inhaled by others. Direct contact with these infectious droplets, such as touching a sick person’s face, can also lead to transmission.
The virus is highly contagious and spreads indirectly by touching contaminated objects (fomites) and then touching the mouth, nose, or eyes. HPIVs remain infectious on surfaces for several hours and in airborne droplets for over an hour. The incubation period typically ranges from two to seven days. Infants, young children, older adults, and immunocompromised individuals are at the highest risk for severe illness.
Diseases Caused by Parainfluenza
HPIV infection ranges from mild cold symptoms confined to the upper respiratory tract to severe lower respiratory tract infections. The location and specific HPIV type often determine the clinical presentation. Reinfections are common throughout life, but existing immunity usually results in milder, cold-like symptoms.
Croup (Laryngotracheobronchitis)
Croup is strongly associated with HPIV infection, particularly HPIV-1 and HPIV-2. This condition involves inflammation and swelling of the larynx and trachea. The resulting narrowing of the airway produces the hallmark symptom: a loud, harsh, “barking” cough that sounds like a seal.
Children with croup may also exhibit stridor, a high-pitched, noisy sound heard when breathing in, which indicates significant airway obstruction. Croup is most common in children between six months and five years old, and symptoms can often be worse at night. Most cases of croup are managed with supportive care at home.
Bronchiolitis and Pneumonia
HPIVs, especially HPIV-3, are a significant cause of lower respiratory tract infections in young children. Bronchiolitis is an inflammatory infection of the smallest airways in the lungs (bronchioles). This condition is most prevalent in infants under two years old and can cause wheezing, rapid breathing (tachypnea), and difficulty breathing.
Pneumonia, an infection causing inflammation in the air sacs of the lungs, can also be caused by HPIVs. HPIV-3 is the second most common cause of pneumonia and bronchiolitis in infants and young children, second only to Respiratory Syncytial Virus (RSV). These illnesses pose a serious threat to infants and may necessitate hospitalization for respiratory support.
Mild Upper Respiratory Infections
In older children and healthy adults, HPIV infection usually remains confined to the upper respiratory tract. The illness often presents as a common cold with symptoms like a runny nose, sneezing, sore throat, and cough. Fever may be present, but the overall symptoms are typically mild and self-limiting.
Testing and Treatment
Diagnosis of HPIV infection is often based on clinical symptoms, especially during seasonal outbreaks. Laboratory testing is available and typically involves collecting respiratory secretions using a nasal swab. Molecular tests, such as Polymerase Chain Reaction (PCR), are the most common method used to detect the virus’s genetic material.
Testing is usually reserved for hospitalized patients, infants with severe respiratory distress, or outbreak investigations. There is currently no specific antiviral medication licensed for HPIV infection. Treatment focuses entirely on supportive care to manage symptoms until the virus runs its course.
Supportive measures include ensuring adequate hydration and using acetaminophen or ibuprofen for fever and discomfort. For children with croup, supportive care often involves using a cool mist humidifier or exposing the child to cool night air to soothe the inflamed airways. Prevention relies on basic non-pharmaceutical interventions, as no vaccine is currently available.