Human Parainfluenza Viruses are a family of four distinct but related viruses (HPIV-1, HPIV-2, HPIV-3, and HPIV-4) that are a common cause of respiratory illness. It is critical to understand that HPIVs are not the same as the influenza virus that causes the seasonal flu. They belong to a different viral family (Paramyxoviridae) and are not covered by the annual flu vaccine.

While all four types can cause illness, HPIV-3 is one of the most clinically significant. Its primary importance lies in its role as a leading cause of severe lower respiratory tract infections in infants under one year of age, most notably bronchiolitis and pneumonia. While HPIV-1 is famous for causing outbreaks of croup in the fall, HPIV-3 has a different personality. It often circulates in the spring and early summer, although it can be detected year-round, and it tends to strike the youngest infants the hardest. Nearly all children will have been infected with HPIV-3 by the time they are two years old, but the first infection is typically the most severe.

I’ve often seen HPIV-3 misattributed to a mild cold or early flu, especially in infants. But when a child starts showing signs of wheezing or labored breathing, HPIV-3 quickly becomes a suspect.

The sole cause of this illness is infection with the Human Parainfluenza Virus Type 3, a single-stranded RNA virus. The virus primarily targets the epithelial cells that line our respiratory tract, from the nose and throat all the way down to the tiny air sacs (alveoli) deep within the lungs.

The virus invades these cells and uses the cell’s own machinery to replicate, creating thousands of new viral particles. This process causes direct damage to the airway lining and triggers a robust inflammatory response from the body’s immune system. It is this combination of viral damage and the body’s own inflammatory reaction that causes the symptoms of the illness. The inflammation causes airway walls to swell and increase mucus production. When this occurs in the large airways it causes laryngitis or croup; when it happens in the small airways of the lungs (the bronchioles), it causes bronchiolitis; and when it affects the air sacs, it results in pneumonia.

In hospitalized children with pneumonia who test negative for flu or RSV, I often find HPIV-3 as the culprit especially during spring months when it tends to spike.

HPIV-3 is highly contagious and spreads through close contact with an infected person or contaminated surfaces. The virus can be transmitted in several ways:

• Inhaling Respiratory Droplets: When an infected person coughs or sneezes, they release tiny, virus-containing droplets into the air. If a nearby person inhales these droplets, they can become infected.
• Direct Personal Contact: The virus can be spread through close contact, such as touching or shaking hands with an infected person who has contaminated their hands by coughing or sneezing into them.
• Touching Contaminated Surfaces (Fomites): HPIVs can survive on surfaces like doorknobs, countertops, and toys for several hours. A person can become infected if they touch a contaminated surface and then touch their own eyes, nose, or mouth before washing their hands.

The incubation period, the time from exposure to the virus to the onset of symptoms, is typically between 2 and 7 days. An infected person can spread the virus for a week or longer after their symptoms start. Because reinfections are common throughout life, older children and adults with a mild “cold” caused by HPIV-3 can easily transmit the virus to a more vulnerable infant.

I’ve often had parents tell me their child “just had a cold,” but within a few days, symptoms worsened into deep coughing and wheezing, classic for HPIV-3, especially in toddlers sharing toys and space.

The symptoms of HPIV-3 can resemble a common cold but may progress to lower respiratory illness in vulnerable patients.

In Older Children and Adults: In most healthy older children and adults, an HPIV-3 infection results in a mild to moderate upper respiratory illness that is indistinguishable from the common cold. Symptoms are usually self-limited and may include:

• Runny nose or nasal congestion
• Sore throat
• Cough
• A low-grade fever
• General malaise or feeling unwell

In Infants, Young Children, and Vulnerable Adults: In these groups, HPIV-3 is a major cause of more severe lower respiratory tract infections. The most common and serious presentations include:

• Bronchiolitis: This is an inflammation of the bronchioles, the smallest airways in the lungs. It is most common in children younger than two. The swelling and mucus in these tiny airways make it very difficult to breathe. Symptoms include a persistent cough, rapid breathing, and a characteristic wheezing sound (a high-pitched whistling sound, especially on exhalation).
• Pneumonia: This is an infection of the lung tissue itself. It is a more serious illness than bronchiolitis and is characterized by high fever, a severe cough that may produce phlegm, and significant difficulty breathing.
• Croup (Laryngotracheobronchitis): While more classically associated with HPIV-1, HPIV-3 can also cause croup. It is easily recognizable by its three hallmark symptoms: a harsh, “barking” cough that sounds like a seal, a hoarse voice, and stridor, which is a high-pitched, noisy sound heard during inhalation.

Individuals with weakened immune systems, such as organ transplant recipients or those undergoing chemotherapy, are also at high risk for developing severe pneumonia from HPIV-3.

What sets HPIV-3 apart is how quickly it can go from mild to serious, especially in babies. When I hear about rapid breathing, poor feeding, or wheezing in an infant, HPIV-3 jumps to the top of my differential.

When to See a Doctor

Parents should contact a healthcare provider if their infant or young child has a respiratory illness and seems to be getting worse, is not feeding well, or appears unusually irritable or lethargic.

It is crucial for parents and caregivers to recognize the signs of respiratory distress, which indicate that a child is working very hard to breathe and needs immediate medical attention. Go to the nearest emergency department if you observe any of these signs:

• Rapid, shallow breathing.
• Nasal flaring, where the nostrils widen with each breath.
• Retractions, where the skin is seen pulling in between the ribs, below the ribcage, or at the neck with each breath.
• A grunting sound with each exhalation.
• Cyanosis, a bluish or grayish color around the lips, on the tongue, or in the nail beds.
• Extreme lethargy, listlessness, or difficulty waking up.

The diagnosis of HPIV-3 respiratory disease is often clinically.

• Clinical Diagnosis: In most outpatient cases, a doctor will make a diagnosis based on the patient’s symptoms and a physical examination. For example, a doctor can diagnose bronchiolitis by listening to the lungs for wheezing or croup by identifying the characteristic barking cough.
• Laboratory Testing: For hospitalized or very ill children, a doctor can confirm the specific virus causing the illness. A nasal swab or a nasal wash/aspirate is collected, and a multiplex polymerase chain reaction (PCR) panel is run. This advanced molecular test can simultaneously check for a wide range of common respiratory viruses, including all four types of HPIV, RSV, influenza, etc.
• Chest X-ray: If pneumonia is suspected, a chest X-ray will be done to look for inflammation and fluid in the lung tissue.

If a child tests negative for RSV and flu but still has lower respiratory symptoms in spring, I often test for HPIV-3. It helps guide care and reduce unnecessary antibiotic use.

There is no specific antiviral treatment for HPIV-3. Management is supportive and aimed at relieving symptoms.

• Home Care for Mild Illness: For a mild upper respiratory infection, home care is focused on comfort. This includes:
  • Ensuring the child gets plenty of rest.
  • Providing adequate fluids to prevent dehydration.
  • Using a cool-mist humidifier to help soothe irritated airways.
  • Managing fever with over-the-counter medications like acetaminophen or ibuprofen (always following weight-based dosing instructions).
• Hospital Care for Severe Illness: Children with bronchiolitis or pneumonia who are in respiratory distress require hospitalization. Supportive care in the hospital includes:
  • Supplemental Oxygen to maintain safe oxygen levels.
  • IV Fluids for dehydration.
  • Nasal Suctioning to help clear mucus from the airways.
  • In the most severe cases of respiratory failure, the child may need to be placed on a mechanical ventilator (breathing machine).

I often reassure parents that most children recover fully with home care. But I’ve also had to admit infants with breathing issues for oxygen and monitoring, so knowing when to escalate care is key.

There is currently no vaccine to prevent infection with HPIVs. Therefore, prevention relies on standard, commonsense infection control measures, similar to those used for preventing the common cold or the flu.

• Handwashing: This is the most important precaution. Wash hands frequently and thoroughly with soap and water for at least 20 seconds.
• Avoid Touching Your Face: Try to avoid touching your eyes, nose, and mouth with unwashed hands.
• Clean and Disinfect Surfaces: Regularly clean frequently touched surfaces like doorknobs, toys, and countertops.
• Avoid Close Contact with Sick People: Try to keep a safe distance from individuals who are coughing or sneezing.
• Practice Respiratory Etiquette: Cover your mouth and nose with a tissue or your elbow when coughing or sneezing.
• Stay Home When Sick: Keeping sick children (and adults) home from daycare, school, or work is crucial to prevent the spread of the virus to others.

Human Parainfluenza Virus Type 3 is an extremely common respiratory virus that nearly every person encounters in early childhood. While it typically causes a harmless common cold in healthy adults, it stands as a major cause of serious and potentially life-threatening lower respiratory tract infections, like bronchiolitis and pneumonia, in infants. There is no specific cure or vaccine for HPIV-3; treatment is purely supportive. While there is no vaccine, the spread of HPIV-3 can be significantly reduced through simple yet powerful hygiene practices like handwashing. For parents, the most important tool is knowing the signs of respiratory distress and seeking prompt medical care for their child when needed.

Centers for Disease Control and Prevention (CDC). (2024). Human Parainfluenza Viruses (HPIVs). Retrieved from https://www.cdc.gov/parainfluenza/index.html

American Lung Association. (2023). Human Parainfluenza Viruses. Retrieved from https://www.lung.org/lung-health-diseases/lung-disease-lookup/parainfluenza