Croup is most commonly caused by viral infections, with the parainfluenza virus being the most frequent culprit. In fact, parainfluenza virus types 1, 2, and 3 account for roughly 50 to 75% of croup cases, with type 1 the most common. Parainfluenza viruses belong to the Paramyxoviridae family, the same family as the measles and mumps viruses, and they spread easily through respiratory droplets when an infected person coughs or sneezes. In addition to parainfluenza, other viral pathogens are also associated with croup, including influenza A and B, respiratory syncytial virus (RSV), adenovirus, human metapneumovirus, rhinovirus, enteroviruses, and coronaviruses (SARS-CoV-2 produced a wave of croup in young children during the 2021 to 2022 Omicron period), as well as, less commonly, measles virus and Mycoplasma pneumoniae.
Parainfluenza virus is the most prominent pathogen linked to croup and tends to follow a seasonal pattern, with outbreaks often occurring in the fall and early winter. Parainfluenza type 1 is the leading cause of epidemics of croup, which in North America recur every other year in the fall, while type 2 tends to cause more sporadic, milder cases and type 3 causes sporadic spring and summer cases that can be more severe. Other viruses, such as influenza and RSV, are also common during the colder months and can contribute to croup infections. Measles, once a more significant cause of croup, is now a rare cause in countries where the measles vaccine is routinely administered, but it remains a concern in regions with low vaccination rates.
The infection usually begins with viral replication in the nasopharynx, after which the virus spreads to the larynx and trachea, causing inflammation and edema of the mucosal lining of the airway. This inflammation leads to the hallmark symptoms of croup, including stridor and the barking cough. The subglottic region of the airway, located just below the vocal cords, is particularly prone to swelling in children because it is narrower compared to adults. Even a small amount of swelling in this region can lead to significant airway obstruction, which is why younger children are more severely affected by croup.
In some cases, viral croup can be complicated by secondary bacterial infection, which occurs when bacteria colonize the already inflamed and weakened airway. This can lead to more serious conditions, such as bacterial tracheitis, which is characterized by the presence of thick, purulent secretions and can cause severe airway obstruction. The most common bacterial pathogens associated with secondary infections include Staphylococcus aureus (including methicillin-resistant strains), Streptococcus pyogenes, Streptococcus pneumoniae, Moraxella catarrhalis, and Haemophilus influenzae. The typical picture is a child with several days of croup-like symptoms who then becomes toxic, with high fever and worsening stridor that fails to improve with nebulized epinephrine and corticosteroids; unlike a child with epiglottitis, this child usually tolerates lying flat and does not drool. Bacterial tracheitis requires more aggressive treatment, including antibiotics and often airway management with intubation and suctioning of the thick secretions.
Epiglottitis is not a complication of croup, but it is the most important differential diagnosis: inflammation of the epiglottis and supraglottic structures can mimic the symptoms of croup while being far more severe and rapidly progressive. This condition requires immediate medical attention, as it can lead to complete airway obstruction. Fortunately, with the widespread use of the Haemophilus influenzae type B (Hib) vaccine, the incidence of epiglottitis has drastically decreased, but it remains an important differential diagnosis in cases of severe upper airway obstruction in children.
| Category | Organisms | Clinical note |
|---|---|---|
| Most frequent cause | Parainfluenza virus types 1, 2, and 3 (50-75% of cases) | Type 1 drives the biennial fall epidemics; types 2 and 3 cause sporadic cases, type 3 often more severe |
| Other viral causes | Influenza A and B, respiratory syncytial virus, adenovirus, human metapneumovirus, rhinovirus, SARS-CoV-2 | Influenza and RSV are also common during the colder months |
| Now-rare viral cause | Measles | Rare where the measles vaccine is routine; still a concern in regions with low vaccination rates |
| Secondary bacterial infection | Staphylococcus aureus (including MRSA), Streptococcus pyogenes, Streptococcus pneumoniae, Moraxella catarrhalis, Haemophilus influenzae | Colonize the already inflamed airway; may cause bacterial tracheitis with thick purulent secretions, needing antibiotics and possibly intubation |
| Key differential | Haemophilus influenzae type B (epiglottitis) | Can mimic croup but is more severe and rapidly progressive; incidence has fallen sharply since the Hib vaccine |
