Introduction

What is “croup”?

Croup is a common airway infection found in children under 5 years old. This illness is generally not severe and can be self-recovered. It results from an infection of bacteria or viruses, which is typically a viral infection named parainfluenza. After viral infection, the airways will be swollen and lead to a barking cough (sounds like a seal’s bark).¹

What is “parainfluenza virus”?

Parainfluenza virus or human parainfluenza virus (HPIV) causes both upper airway infections (upper respiratory tract infection) and lower airway infection (lower respiratory tract infection) in infants, young children typically under 5 years, immunocompromised adults, and the elderly.²

Understanding Croup

Symptoms and diagnosis

Characteristic symptoms

After infection, the windpipe (trachea), the airways to the lungs (the bronchi), and the voice box (larynx) are swollen due to the fight of the human immune system against the infection. The swelling partially blocks the airways, resulting in breathing difficulty and a unique, turbulent, noisy airflow called stridor.¹ Children make a harsh or stridorous sound while they are breathing in. Seal-like barking cough is one of the characteristics of patients with croup. With difficulty breathing, they will develop a hoarse voice. In general, these symptoms become worse at night. Other symptoms are cold-like symptoms occurring a couple of days before having croup symptoms. Patients will have croup symptoms for a few days or occasionally up to 2 weeks.³

Diagnostic methods

Croup is usually diagnosed based on signs and symptoms. GP will check symptoms, especially the characteristic sound of a cough and run more tests to rule out other similar conditions such as epiglottitis, an airway foreign body, and bacterial tracheitis.¹

Typical Age Group and Incidence

Commonly affected age groups

Croup is commonly found in young children at the age of 6 months to 5 years, though it can affect younger or older. Depending on the seasons, it becomes common during the late autumn and early winter seasons. However, croup in adults is rarely found.³

Parainfluenza virus overview

Types of parainfluenza viruses

Parainfluenza virus is one type of RNA virus and belongs to the Paramyxoviridae family. The RNA or genetic substance in the virus will produce the necessary proteins, including nucleocapsid protein (NP), the phosphoprotein (P), the matrix protein (M), the fusion glycoprotein (F), the hemagglutinin-neuraminidase glycoprotein (HN), and RNA polymerase (L). These proteins are essential to facilitate the virus to attach and replicate itself in human cells.⁴ It is divided into 4 types depending on the types of proteins encoded and these proteins will reduce the effectiveness of the human immune response. The four types of HPIV consist of:

• HPIV-1
• HPIV-2
• HPIV-3
• HPIV-4, which is sub-categorised into HPIV-4a and HPIV-4b

Differences and similarities among types

HPIV-1 and HPIV-3 RNA encode short C proteins and belong to the genus of Respirovirus, whereas HPIV-2 and HPIV-4 are in the genus of Rubulavirus.^2,4 HPIV-2 encodes a V-protein. These proteins suppress the human immune response.⁴

Transmission and infection

How parainfluenza spreads

Parainfluenza virus typically spreads through direct contact with droplets or when a person having the virus breathes, coughs, or sneezes. The virus can still live and be active in airborne droplets for over an hour and on surfaces for a few hours depending on the conditions. Therefore, if some people touch any droplets or infectious surfaces within a viral active time, they may be infected easily.⁵ Summary of transmission routes of HPIVs:⁶

• The air via coughing or sneezing
• Direct contact such as touching, shaking hands
• Contact objects or surfaces that have the virus then touch your mouth, nose, or eyes

Seasons of Parainfluenza virus

Infection with the parainfluenza virus can occur anytime throughout the year, however, common seasons are spring, summer, and autumn.⁶ Additionally, seasonal trends of parainfluenza virus depend on each type and its subtypes.⁶

• HPIV-1 usually causes croup in children and commonly infects in the autumn every two years
• HPIV-2 can cause croup in the autumn every two years when HPIV-1 infection is low
• HPIV-3 commonly occurs in spring and early summer each year
• HPIV-4 patterns of each season are unclear but seem to cause infection during autumn and winter each year

Link Between Parainfluenza and Croup

Pathophysiology

In the late 1950s, the parainfluenza virus was first isolated from children having croup and then it was called croup-associated viruses.² More than 75% of croup infections come from parainfluenza virus.^1,2

How parainfluenza causes croup

When people contact the viruses via the nose, mouth, and eyes, they spread to the airways, bind, and replicate themselves in the cells of the upper and lower airways, respectively. The locations of infection are related to the symptoms; if infected in the upper airways, it causes cold symptoms but if infected in the windpipe and the voice box, it causes croup symptoms.⁴

Mechanisms of airway inflammation 

After being infected around areas of the windpipe and the voice box, these areas become inflamed and swollen, resulting in obstruction of the airways, which leads to characteristic symptoms of croup including hoarseness, cough, and stridor.⁴

The obstructed airflow creates the high-pitched sound of vibration when breathing and increases the effort of breathing, leading to fatigue (feeling extremely tired) and lower oxygen levels.⁴ In severe cases, these can lead to respiratory failure.⁴

When to speak to your GP

You need to make an appointment to your GP practice when:³

• Your child is younger than 3 months and has a temperature of 38^oC or above
• Your child is older than 3 months and has a temperature of 39^oC or above
• Having a barking cough
• Having a harsh sound when breathing

Diagnosis and treatment

Diagnostic criteria

Clinical examination

A doctor usually diagnoses croup by examining signs and symptoms.^1,7 Typical clinical presentations:

• Seal-like barking cough
• Hoarse voice
• A mild fever may present between 12 and 72 hours
• For more severe, stridor and respiratory distress can be observed

Treatment options

Supportive care

• Oxygen will be given as blow-by administration, which directly flows oxygen to the patient’s nose and mouth from a close distance^1,8
• Hot steam does not have a significant benefit for croup symptoms. Hot steam includes inhaled hot steam and humidified air¹

Medications 

• Steroids such as dexamethasone will be given in mild croup after being defined by the Westley croup score. Steroids (dexamethasone) help recover croup symptoms faster, reduce return rate to medical care and reduce the duration of staying in hospital⁹ 
• Epinephrine is given as an inhaled mist (nebulizer) for moderate to severe cases. It can reduce the swelling in the airway and work faster than steroids and in severe symptoms. However, symptoms can return, it usually occurs within two or four hours after treatment¹⁰

Hospitalisation criteria

In serious cases will be required to stay at the hospital. Go to the emergency if your child has:¹¹

• Has difficulty in breathing or swallowing
• Has blue, grey, pale, or irregular spots on skin, lip, or tongue
• Feel sleepier than usual
• Can’t stop coughing

Prevention and management

Preventive measures

Hygiene practices to reduce transmission

As viruses easily spread via contact and airborne, reducing virus transmission is important to take care of your hygiene, particularly your child's hygiene. Tips for preventing the contact or spreading of croup:¹⁰

• Wash hands regularly and correctly
• Remind your children to wash their hands every time before eating or touching their mouth, nose, or eyes
• If someone in your house has croup, stop sharing dishware, avoid any direct contact, and stay at home to reduce the spread of viruses to others
• Use alcohol-based hand rubs

Vaccination status

Unfortunately, there is no vaccine to protect against parainfluenza virus infection. Vaccines are in the developing process at the moment. Therefore, the best way to protect against viral infection is to take good care of your hygiene.¹²

Management strategies

Managing mild vs. severe cases

Croup is usually self-recovery, but treatment should be considered in mild cases and severe cases. For mild cases, it is recommended that a single dose of dexamethasone is sufficient. In moderate to severe croup, a combination of epinephrine and dexamethasone is given.¹³ Oxygen supplements will be required in patients with low oxygen.¹

Summary

Croup is one of the airway infection diseases and commonly affects young children under the age of 5. It is typically caused by the parainfluenza virus. Parainfluenza virus can cause infection in both the upper airway and lower airway. The types of this virus relevant to croup are HPIV-1 and HPIV-2, which can cause croup during autumn. After infection, the airway will be swollen and lead to a seal-like barking cough and respiratory stridor, which are characteristic symptoms of croup. Croup usually self-recovers within 7 days, however, in more severe cases, will require medication or a stay in the hospital. Currently, there is no vaccine to protect against the parainfluenza virus. Therefore, the best approach is to pay careful attention to your hygiene, particularly your child’s.