Causes of Croup

Croup is a viral infection of the voice box (larynx) and windpipe (trachea), and it is almost always a childhood illness. Understanding what causes it explains both why it sounds so frightening and why it usually passes on its own.

The direct cause

• Viruses - almost always. The commonest culprit is parainfluenza virus (especially type 1), with RSV, influenza, adenovirus and ordinary cold viruses causing most of the rest
• The virus settles in the upper airway, right at the narrowest part of a child's windpipe - just below the vocal cords. Swelling there produces the famous barking, seal-like cough
• Why children and not adults: a toddler's airway is narrow to begin with, so a little swelling makes a lot of noise and obstruction. By age 6-8 the airway is wide enough that the same virus just causes a regular cold
• Typical age: 6 months to 3 years, most cases between 1 and 2 years; boys slightly more than girls
• Season: late autumn and winter, in waves

Who gets it more

• Children in daycare or with school-age siblings - these viruses spread easily through coughs, sneezes, hands and shared toys
• A family tendency toward spasmodic croup (the recurrent, sudden nighttime kind) - some children are simply wired to react this way
• Children with reflux or allergies may be more prone to recurrent episodes

Spasmodic croup - the odd cousin

Some children get sudden croup attacks at night with no fever and no cold - the child goes to bed fine and wakes at midnight barking. This is spasmodic croup, thought to be more of an airway over-reaction than an infection. It looks dramatic, responds to the same measures, and tends to recur until the child outgrows it.

When "croup" is not croup

Three rare but dangerous look-alikes: epiglottitis (a very ill, drooling child struggling to swallow - an emergency), an inhaled foreign body (sudden choking onset), and bacterial tracheitis (high fever, very sick child). Fever plus a barking cough in an otherwise bright child is typical croup; a drooling, toxic, silent child is not.

One-line summary

Croup is caused by common cold-type viruses settling in a small child's narrow upper airway - which is why it sounds dramatic, why it mostly affects toddlers, and why the great majority outgrow and recover from it without any specific treatment.