Useful information on Schistosomiasis and STH
What are Soil Transmitted Helminths
“Helminth” is a technical word for a worm.
Soil-transmitted helminths (STHs) are also known in many places simply as common intestinal worms.
Causative agents
In terms of public health, three species are important,
(see table below)
Transmission
A person infected with STH has parasite eggs in their faeces.
In areas where there is no latrine system, the soil (and water) around the village or community becomes contaminated with faeces containing worm eggs.
In the soil, the eggs mature – a process that takes between 2 and 4 weeks, depending on the type of worm:
- about 2 weeks for roundworms and hookworms;
- about 3 weeks for whipworms.
The STHs can then infect humans in the following ways.
The eggs are ingested:
- The eggs stick to vegetables grown in the area. If the vegetables are not carefully cooked, washed or peeled, the eggs are ingested and infect the person.
- The eggs are ingested from water sources which have become contaminated.
- Young children who play on the ground and often put their hands in their mouths without washing them, ingest the eggs and become infected.
The larvae penetrate the skin:
- For hookworms, the eggs also hatch into larvae which rest in the soil. If a person walks on the contaminated soil, the larvae can penetrate the skin, usually between the toes.
There is no direct person-to-person transmission or infection from fresh faeces because eggs passed in faeces need about 3 weeks in the soil before they become infective.
Symptoms of infection and Impact on health:
The symptoms of STH infections are nonspecific and only become evident when the infection is particularly severe.
The nonspecific symptoms include nausea, tiredness, abdominal pain and loss of appetite.
STHs cause morbidity through various different mechanisms; three of particular significance are listed below.
- :: Anaemia. Hookworms live in the intestine. They attach and re-attach themselves to the intestine wall every few hours as they feed on blood from the cut vessels and mucosal tissues. This blood loss contributes to anaemia, especially in countries where the dietary intake of iron is already marginal and malaria is widespread.
- :: Vitamin A deficiency. Vitamin A deficiency. STHs need vitamin A to live. In many countries, a person's vitamin A-intake is already marginal and in these situations, STH infections can compete for the limited amount of vitamin A which is absorbed.
- :: Loss of appetite. One of the most noticeable changes after a child has been dewormed is that his/her appetite returns.
The impact of STH infections on an infected person’s life can be significant. Some examples are listed below.
- :: Restricted growth. Loss of appetite, increased levels of vitamin A deficiency and higher anaemia levels all interfere with a child's ability to grow healthily and to his or her full potential. Moreover, there is evidence to suggest that a child with heavy worm infections is less resistant to other infections. Altogether, this means a child infected with STHs will be sickly and, if untreated, will grow up to be an unhealthy adult.
- :: Need for surgery. When the number of worms in the body becomes extremely high, they build up in the child's intestines, stopping the normal flow of blood and eventually blocking the intestine entirely. The only solution in this situation is surgical intervention and in many cases, this is not possible in remote areas and the child can die.
- :: Reduced ability to learn. Worm-infected children are less able to concentrate or memorize information. They score less well in school tests and therefore their only chance to gain a few years of schooling in their lives is compromised.