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Common Intestinal Parasitic Infection

Common Intestinal Parasitic Infection

15 August 2023

Causative agents and mode of transmission

There are many intestinal parasites affecting humans. Common ones include roundworms, pinworms, hookworms, tapeworms and liver flukes (usually Clonorchis sinensis in Hong Kong and neighboring areas).

Roundworms (Ascaris species) are round and long, and the length can reach 20 cm. They have pointed ends and are light brown or pink in colour. The larvae and adult worms live in the human small intestine and their eggs are passed out with stool. If the infected person defecates outside toileting facilities or the faeces is used as fertiliser, the eggs can be deposited in the soil. Parasitic infections are acquired through ingestion of food contaminated with their eggs. Hands when contaminated with soil can also be the transmission media.

Pinworms (also called threadworms) are small (about 1 cm in length) and whitish in colour, which resemble short pieces of thread. They inhabit the human large intestine and rectum. The female pinworm migrate to the anus at night to lay eggs. This may result in local itchiness. The patient's fingers may be contaminated during scratching. If the patient does not wash hands thoroughly before preparing food, ingestion of the contaminated food by other persons could result in infection. Pinworm infection usually affects young children who get infected by getting eggs on their hands (either directly from an infected person or indirectly through contaminated objects) and then putting their hands in their mouths.

Hookworms are about 1 cm long, round and curved. They inhabit the human small intestine and attach to the intestinal wall, resulting in blood loss and causing the host to become anaemic. If the infected person defecates outside toileting facilities or the faeces is used as fertiliser, the eggs can be deposited in the soil. The eggs can hatch and mature into larvae in soil. Human contact with contaminated soil, such as working barefoot in fields, may result in these larvae penetrating through the skin and causing infection.

Tapeworms are flat and could be up to 6 m or longer. They are segmented and white or pale yellow in colour. They inhabit the small intestine of the definitive host (e.g. Taenia solium in human) and their eggs are passed out with stool into the environment and ingested by an intermediate host (e.g. pig, cow and fish). Eggs hatch into larval cysts (cysticerci) in the muscles or other organs in the intermediate hosts. Ingestion of cysticerci in the undercooked and contaminated pork, beef or fish may then result in infection by the adult worm. In case a person ingests eggs of Taenia solium (e.g. by ingesting contaminated food or water), the eggs hatch into cysticerci that could infect the brain, muscle or other tissue (human cysticercosis) which can be a very serious disease.

Liver flukes (Clonorchis sinensis) are flat and about 1 to 2 cm long. They live in the human bile duct in the liver and their eggs are passed out with stool. Eggs are ingested by snails in freshwater pond and developed into the free-swimming form (cercariae). The cercariae in the pond will get into contact with fish skin and penetrate the flesh and change into an encysted form. Ingestion of the raw or undercooked infected fish may then result in infection.

Clinical features

Symptoms of parasitic infection depend on the type of parasite involved. Infection with a small number of parasites may be asymptomatic. For those who have symptoms, the following may occur:

  • Abdominal discomfort and distention, vomiting, diarrhoea
  • Weight loss
  • Anal itchiness (pinworm)
  • In case of tapeworm infection, passing tapeworm segments in stool
  • If infected with a large number of parasites in the body, they can result in more severe complications, e.g. intestinal obstruction, anaemia, obstruction and inflammation of bile ducts
  • Infection with Taenia solium larval parasite (human cysticercosis) can cause seizures and muscle or eye damage

Incubation period

The incubation period varies from a few weeks to more than three months, depending on the type of parasite involved.


Most parasitic infections can be completely cured by medication.


1. Maintain good personal hygiene

  • Perform hand hygiene frequently, especially before handling food or eating, and after touching soil or using the toilet.
  • Wash hands with liquid soap and water, and rub for at least 20 seconds; then rinse with water and dry with a disposable paper towel or hand dryer. If hand washing facilities are not available, hand hygiene with 70 to 80% alcohol-based handrub may be considered as an alternative hand hygiene measure.
  • Regular trimming of fingernails.
  • Discourage children from sucking fingers or scratching the anal area.
  • Wear footwear when working in fields.

2. Maintain good food hygiene

  • Drink only boiled water from the mains or bottled drinks from reliable sources.
  • Purchase fresh food from hygienic and reliable sources. Do not patronise illegal hawkers.
  • Avoid high-risk food like raw or undercooked meat, especially beef, pork and fish.
  • Clean and wash food thoroughly.
  • Adopt the 5 Keys to Food Safety in handling food, i.e. Choose (Choose safe raw materials); Clean (Keep hands and utensils clean); Separate (Separate raw and cooked food); Cook (Cook thoroughly); and Safe Temperature (Keep food at safe temperature) to prevent foodborne diseases.

* Please visit the website of Centre for Food Safety for more information on food safety.