Hookworms are parasitic worms that can infect humans in countries with poor sanitation and a warm, moist climate.
The hookworm larvae (immature worms) are found in soil that's been contaminated with human faeces. The larvae can infect people if their bare skin comes into contact with the soil - for example, if you're walking barefoot.
Most hookworm infections occur in Africa, the Americas, China and southeast Asia.
The hookworm life cycle
The larvae and adult worms live in the small intestine (small bowel) of an infected person or animal.
If an infected person defecates outside (for example, near bushes or in a field) or if their stools are used as fertiliser, the hookworm eggs in the stools will contaminate the soil.
The eggs hatch in the soil after one to two days and release larvae, which are too small to be visible. The larvae develop into a mature form after 5 to 10 days, which can penetrate human skin.
The main way people become infected with hookworm is through direct skin contact with contaminated soil, typically when walking barefoot. Children can also become infected after playing in the soil and putting their hands in their mouth.
Other ways to become infected include eating raw, unwashed vegetables that are contaminated with hookworm eggs or by drinking contaminated water.
Once the larvae have entered the body, they move through the bloodstream into the lungs, airways and throat, where they're swallowed and enter the small intestine.
After reaching the small intestine, the larvae mature into adult worms and attach themselves to the intestinal wall, where they can cause blood loss. Most adult worms are expelled from the small intestine after one to two years, although they can sometimes remain for longer.
The two main species of hookworm that infect people are Ancylostoma duodenale and Necator americanus.
Symptoms of a hookworm infection
Most people infected with hookworm don't have any symptoms.
The larvae of animal hookworms sometimes get into the skin, usually after lying on sand contaminated by animal faeces while on a beach holiday abroad.
These larvae are unable to go further into the body, but cause a slowly moving red line to appear on the skin. This is called cutaneous larva migrans and can last several weeks. It's a bit itchy, but doesn't usually cause any harm.
People infected with human hookworm larvae occasionally have a less obvious itchy rash for a few days around the area of skin where the larvae penetrated. Respiratory symptoms such as a cough and wheeziness may develop when the larvae reach the lungs, a few weeks after exposure.
Severe infections may cause abdominal (tummy) pain, diarrhoea, loss of appetite, weight loss, fatigue and anaemia. Blood loss leading to iron deficiency anaemia and protein loss are the most serious symptoms.
Diagnosing and treating a hookworm infection
Hookworm infections can be diagnosed by inspecting a stool sample under a microscope in a laboratory and looking for the eggs. If there's an infection, the number of eggs will be counted to see how severe it is.
An infection can be treated with anthelmintic (anti-worm) medication, such as albendazole or mebendazole, which your GP can prescribe. You'll need to take the medication for one to three days. The medications are usually effective and have few side effects.
Albendazole or another medication called ivermectin can be used to treat a cutaneous larva migrans infection of the skin.
If you have anaemia, you may need iron supplements. Folic acid may also be recommended to help replenish your red blood cells.
Preventing hookworm infections
If you're travelling to a tropical or sub-tropical region of the world where hookworm infections are common, avoid walking barefoot in areas where there may be contaminated soil, and don't touch soil or sand with your bare hands.
Good hygiene standards and effective sewage disposal systems are the reason hookworm infections aren't commonly seen in developed countries such as the UK, although they may still be a problem in some Mediterranean countries.
An estimated 576-740 million people in the world are infected with hookworm.