Intestinal Parasites

Intestinal parasites are organisms that live in the human gut. They fall into two broad groups: single-celled protozoa such as Giardia, Blastocystis and Entamoeba, and worms (helminths) such as roundworm (Ascaris), whipworm (Trichuris), hookworm, pinworm and tapeworm. The World Health Organization estimates that soil-transmitted worms alone infect about 1.5 billion people, 24% of the world's population, mostly where sanitation and clean water are lacking. Pinworm is common even in wealthy countries: a German review puts infection at more than one billion people worldwide and close to 20% of kindergarten and primary-school children in Europe. Many infections cause no symptoms. WHO notes that people carrying only a few worms usually feel nothing, while heavier worm loads cause diarrhoea, abdominal pain, malnutrition, weakness and, in children, impaired growth; hookworm also drains blood and iron. Giardia typically causes diarrhoea, gas, greasy floating stools, cramps and nausea that start 1 to 2 weeks after infection and last 2 to 6 weeks, according to the CDC. Pinworm mainly causes itching around the anus, because the females lay their eggs on the skin there at night; tapeworm may show itself only as segments passed in the stool. Pork tapeworm matters more than its mild symptoms suggest: its eggs can cause cysticercosis, which can lead to seizures, so the CDC advises diagnosing and treating every tapeworm infection, with praziquantel as the drug of choice. Because these symptoms overlap with many other gut problems, the diagnosis rests on a stool test, and CDC advises examining three samples taken 2 to 3 days apart when the first is negative. Treatment is with antiparasitic drugs, which are effective and inexpensive; most need a prescription, though some pinworm medicines are sold over the counter. Albendazole and mebendazole are WHO's recommended medicines for roundworm, whipworm and hookworm; a Cochrane review of 19 trials found albendazole about as effective as metronidazole for clearing Giardia, with fewer side effects; and a pinworm review reports success rates of up to more than 90% for the approved drugs, adding that everyone in the household must be treated to stop it coming back. Supplement evidence is thin. The best of it is two small trials of the probiotic yeast Saccharomyces boulardii, used alongside or compared with metronidazole, and one pilot trial of dried papaya seeds in Nigerian children. Oil of oregano rests on an uncontrolled study of 14 adults, and we found no published clinical trials of the commercial 'parasite cleanse' products sold online.

• •No symptoms at all, especially with light infections
• •Diarrhoea, sometimes lasting weeks
• •Gas, bloating and stomach cramps
• •Greasy, smelly stools that float (typical of Giardia)
• •Nausea and loss of appetite
• •Night-time itching around the anus (typical of pinworm)
• •Fatigue, weakness and unintended weight loss
• •Iron deficiency anaemia from hookworm blood loss
• •Tapeworm segments passed in the stool
• •Poor growth and malnutrition in children with heavy infections

• •Drinking untreated or contaminated water
• •Eating raw produce that was not washed, peeled or cooked
• •Hand-to-mouth contact after touching contaminated soil
• •Walking barefoot on contaminated soil (hookworm)
• •Close contact with an infected household member (pinworm, Giardia)
• •Nail-biting and poor hand hygiene in children
• •Living in or travelling to areas with poor sanitation
• •Undercooked meat (tapeworm)

Most intestinal parasites spread by the faecal-oral route: eggs or cysts passed in an infected person's or animal's stool reach someone else's mouth. WHO describes three common paths for soil-transmitted worms: eggs on vegetables that are not carefully cooked, washed or peeled; contaminated drinking water; and children's hands after playing in contaminated soil. Hookworm larvae hatch in soil and burrow through the skin, so walking barefoot is the main risk. Giardia cysts spread through faecally contaminated water, food and surfaces, and can stay viable for 28 to 84 days in lake or river water. Roundworm, whipworm and hookworm do not multiply inside the body, so the number of worms, and the severity of illness, depends on repeated exposure. Strongyloides is the exception: it can reproduce in the host, and in people with weakened immunity its unchecked multiplication can be fatal. It also needs different tests from other worms and does not respond to albendazole or mebendazole. Pinworm is spread person to person; its main risk factors are being 4 to 11 years old, nail-biting, finger-to-mouth contact and poor hand hygiene, which is why it clusters in families and classrooms.