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A roundworm infection does not usually cause any noticeable symptoms.

If there are symptoms, they're usually in 2 stages:

  • early-phase symptoms – caused by the larvae (newly-hatched worms) moving from the small intestine to the lungs; it's unclear why some people develop these symptoms while others do not 
  • late-phase symptoms – caused by adult worms living in the intestine; the worms cannot reproduce inside you - there will only be as many worms as the eggs you first ingested (see causes of roundworm infection for more details about the life cycle of the roundworm)

Early-phase symptoms

The early-phase symptoms of a roundworm infection only affect a small proportion of people and are rare in the UK.

Symptoms usually begin 4 to 16 days after swallowing the eggs and last up to 3 weeks. They include:

Late-phase symptoms

The late-phase symptoms of a roundworm infection are even rarer in the UK, particularly in adults. If there are a lot of mature worms in the intestine, they can cause blockages. This will not happen before 6 weeks after the eggs have been swallowed.

Symptoms associated with mature worms include:

  • passing a worm in your poo
  • mild stomach pain
  • nausea and vomiting
  • diarrhoea (you may also notice blood in your poo)

However, most people have no symptoms.

If your intestines are blocked by a large number of worms, you'll probably vomit and have severe abdominal pain.