Hookworms are parasitic worms (helminths) that are mainly caught by treading on contaminated soil in developing countries with a warm, moist climate.
The larvae and adult worms live in the small bowel of an infected person or animal.
If an infected person defecates outside (near bushes or in a field), or if their stools are used as fertiliser, the eggs of the hookworm in their stools will contaminate the soil.
Infected pets (especially puppies and kittens) can also deposit hookworm eggs.
The eggs then mature and hatch to release larvae (immature worms), which grow into a mature form (5-13mm in length) that can penetrate the skin of humans.
The main way people become infected with hookworm is by walking barefoot on contaminated soil. The hookworm larvae are too small to be visible in the soil.
Once inside the blood circulation, the worms reach the lungs and throat and are then swallowed, entering the gut.
Children are particularly at risk if they spend lots of time running around and playing with contaminated soil.
The two main species of hookworm infecting adults are Anclostoma duodenale and Necator americanus.
Who is at risk?
People living in areas with warm, moist climates and in countries with poor sanitation and hygiene are at risk of hookworm infection.
The box on the left of this page lists the high-risk countries.
What are the symptoms?
Most people infected with hookworm won't have any symptoms.
Some people have itching and a rash around the area of skin where the larvae penetrated. Sometimes, people can develop a cough and wheeze a few weeks after exposure, when the larvae reach the lungs.
If the infection is heavy, it may cause tummy pain, diarrhoea, loss of appetite, weight loss, fatigue and anaemia.
The most serious symptoms are blood loss leading to iron-deficiency anaemia and protein loss. In children, this can also be associated with slow growth and mental development.
How can I prevent becoming infected?
If you are travelling to an area where hookworm is common (see box on this page), do not walk barefoot where there may be contaminated soil and don't touch the soil or sand with your bare hands.
Good hygiene standards and effective sewage disposal systems are the reason hookworm infections are not commonly seen in developed countries like the UK, although they may still be a problem in some Mediterranean countries.
How is it diagnosed?
Hookworm infection can be diagnosed by inspecting a stool sample under a microscope in a laboratory, and looking for the eggs.
If there is an infection, the number of eggs will be counted to see how severe it is.
How is it treated?
A hookworm infection is treated with antihelminthic (anti-worm) medication, such as albendazole or mebendazole, which your GP can prescribe. You will need to take medication for one to three days. The drugs are effective and appear to have few side effects.
If you have anaemia, you may need iron supplements.
In developing countries
In developing countries, the following vulnerable groups of people are given preventative medicine without even inspecting their stools:
pre-school and school-aged children
women of childbearing age
adults in occupations where there is a high risk of heavy infections
This is normally done through population-wide treatment with albendazole or mebendazole.
However, children and adults often become reinfected shortly after treatment, which is why a vaccine is currently being developed. The vaccine would avoid repeated and frequent use of the drugs.
How common is it?
Hookworm was once widespread in developed countries, but improvements in living conditions have greatly reduced hookworm infections.
An estimated 576-740 million people in the world are infected with hookworm.
A hookworm under the skin
Areas where hookworm is common
The two species of hookworm that cause human infection are found in:
North and South America
Islands of the South Pacific
Iran, India and Pakistan