Information on the causes of croup, including flu viruses. The infection causes the larynx (voice box) to become swollen and the trachea (windpipe) to become blocked.

Croup usually develops as the result of a viral infection. The infection causes the larynx (voice box) to become swollen and the trachea (windpipe) to become blocked, and may affect the tubes in the lungs (bronchi). It is sometimes called laryngo-tracheo-bronchitis

Parainfluenza virus

The parainfluenza virus is the most common cause of croup. Four strains of the virus are responsible for most croup cases. These are:

  • parainfluenza I
  • parainfluenza II
  • parainfluenza III
  • parainfluenza IV

Parainfluenza I is responsible for most cases of croup.

The virus can be transmitted through close contact with infected people, as well as contaminated objects and surfaces. As with many airborne viruses, the parainfluenza virus is often spread by breathing in droplets from infected coughs or sneezes.

Other viruses

A number of other viruses can also cause croup. These include:

  • influenza A and B (flu viruses)
  • the measles virus, in children who have not been immunised against measles
  • the rhinovirus (common cold virus)
  • enteroviruses
  • the respiratory syncytial virus (RSV), which can cause severe breathing problems and pneumonia in babies

Other causes

Less common causes of croup include:

  • inhaling a small object such as a peanut or a pen cap (inhaled foriegn body)
  • epiglottitis – inflammation of the epiglottis, (the flap at the base of the tongue that keeps food from going into the windpipe)
  • an allergic reaction to substances such as pollen or dust mites
  • inhalation of irritants, such as chemicals
  • acid leaking back out of the stomach and into the throat (acid reflux)

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