The infestation of various organs with roundworm larvae, which may cause fever

  • More common in children
  • Owning pet dogs or cats may be a risk factor
  • Gender and genetics are not significant factors

The adult forms of the roundworms Toxocara canis and Toxocara cati normally infest dogs and cats, which excrete faeces containing worm eggs. However, the larvae (immature forms) of the worms can infest humans, usually as a result of ingestion of soil contaminated with worm eggs. Children are particularly susceptible because they are more likely to play in soil and then put their fingers in their mouths.

Once the eggs have been swallowed, they hatch in the intestine into tiny larvae, which may then migrate to other parts of the body, including the lungs and liver. In rare cases, the larvae can migrate to the eyes and the brain.


A larva (immature form) of the worm that causes the disease toxocariasis can be seen in this highly magnified image. The larva has just hatched from the egg.

What are the symptoms?

In most cases of toxocariasis, symptoms do not develop. However, in some cases, symptoms may include:

  • Mild fever.

  • Feeling unwell.

A heavy infestation of the larvae in the lungs may cause wheezing and a dry cough. If larvae reach the brain, they may cause epilepsy, and infestation of the eyes may damage the retina (the light-sensitive membrane at the back of the eye), causing blindness that is likely to be permanent.

What might be done?

A diagnosis of toxocariasis can usually be made from blood tests, although a tissue biopsy (removal of a small sample of tissue for microscopic examination) may be necessary in some cases. Most people recover completely from the infestation without needing treatment. However, your doctor may prescribe an anthelmintic drug to kill the worms. In severe cases in which the brain is affected, anticonvulsant drugs will probably be given. Brain or eye damage due to toxocariasis is usually permanent.

It is important to deworm pets regularly to prevent toxocariasis, and dogs or cats should be prevented from defecating in places where children are likely to play, such as in a sandpit.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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