In infants under three months, pertussis very often does not produce the classic paroxysmal cough with an inspiratory whoop. It produces apnoea, cyanotic episodes, or a feeding-associated colour change — and the whoop, when it comes at all, comes later. Waiting for it is how the diagnosis gets made in the intensive care unit rather than in the clinic.
The features that should raise it in a small infant are: apnoea or a colour change with no fever, a cough that comes in clusters and leaves the baby exhausted, post-tussive vomiting, and a household contact with a cough of more than two weeks. A markedly raised lymphocyte count on a full blood count is a supportive finding and is often the first laboratory clue, though it is neither sensitive nor specific.
Test with a nasopharyngeal PCR, start a macrolide on suspicion without waiting for the result, and admit any infant under three months with apnoea. The antibiotic is given largely to stop transmission — it has limited effect on the cough once paroxysms are established — which is exactly why starting early and tracing contacts both matter.
- Apnoea without fever in an infant under three months should put pertussis on the list before a whoop appears.
- Ask specifically about a prolonged cough in the mother, older siblings or a grandparent — the source is almost always a household adult or child.
- A lymphocyte-predominant leucocytosis is supportive; a very high white cell count in infant pertussis is a marker of severity, not just of infection.
- Check maternal pertussis vaccination in pregnancy — and use the consultation to recommend it for the next pregnancy.
- Start treatment on clinical suspicion; do not wait for PCR confirmation.
Why it matters
The classic sign that gives the disease its name is the one small infants are least likely to produce, and they are the ones who die of it.
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