Helminths are parasitic worms that feed on a living host to gain nourishment and protection, while causing poor nutrient absorption, weakness and disease in the host. These worms and larvae live in the small bowel and are referred to as intestinal parasites.
The following groups of worms are classed as helminths:
- Nematodes or roundworms
- Trematodes, which includes flukes or flatworms
- Cestodes or tapeworms
- Monogenans, also members of the flatworm phylum
Schistosoma mansoni is a significant parasite of humans, a trematode that is one of the major agents of the disease schistosomiasis.
Characteristics of helminths
Helminths all share a similar morphology and are multicellular organisms that are visible to the naked eye. The worms are usually caught through treading on contaminated soil in warm, humid countries that have poor sanitation and hygiene.
If an infected person or animal has defecated on soil, helminth eggs present in their feces contaminate the soil. These eggs mature and hatch to produce larvae that grow into adult worms of up to 13 mm in length. These adult worms can penetrate human skin, which can happen if a person walks on contaminated soil. The worms then enter the bloodstream and migrate towards the lungs and also the throat where they are swallowed and transported to the gut.
Some of the features of the different groups of the helminths include:
- The trematodes or flatworms are flat, leaf-shaped and unsegmented. They are hermaphroditic, meaning they have the reproductive organs associated with both males and females.
- Nematodes are cylindrical and have lips, teeth and dentary plates. The worms are either male or female.
- The cestodes or tapeworm is segmented and hermaphroditic. They have a sucker and a projecting, hooked rostellum.
Some of the diseases caused by helminths are described below.
- Hookworm disease is a common worm infestation in the developing world caused by Ancylostoma duodenale or Necator americanus. The illness leads to anemia and malnutrition.
- Dracunculiasis is caused by the guinea worm or Dracunculus medinensis, which is transmitted through contaminated water. It lies burrowed within the skin and causes severe inflammatory reactions.
- Loiasis or African eye worm disease is caused by the filaria Loa loa worm, which is contracted through Deer fly or Mango fly bites. The adult worms move through subcutaneous tissue towards the subconjunctiva of the eye. The illness causes red, itchy swellings in the skin referred to as Calabar swellings.
- Cysticercosis is caused by the pork tapeworm or Taenia solium. Symptoms often do not present for years, but eventually painless bumps develop in the skin and muscles or cause neurological problems.
- Echinococcosis is caused by Echinococcus tapeworms.The liver is usually affected first, followed by the lungs and brain. Liver disease may cause abdominal pain and jaundice while lung disease leads to breathlessness and coughing.
Reviewed by Sally Robertson, BSc