Human flatworms



Blood flukes Schistosoma haematobium; S. mansoni; S. japonica; S. indicum

This is a dioecious fluke that occurs in tropical and subtropical areas of the world. The adults are found in portal and pelvic veins of man. Female is thin while male is broad, 6-10 mm long and has a gynaecophoric canal in which it captures the female until copulation. Eggs are laid in blood from where they are excreted through urine or stools. Miracidium larva enters the freshwater snail in which larval stages are completed. There are no redia and metacercaria stages. Cercaria swims in water and enters human blood either by penetrating through the skin or along with drinking water.

Intestinal flukes – Fasciolopsis fuelleborni; F. buski

Found in Asian countries, this fluke is 5-7.5 cm long and is found in small intestine of man and animals. Eggs are excreted through faeces and washed into water where they enter fresh water snail. Metacercaria is formed on water plants that are eaten by man. Infection occurs by eating uncooked vegetables.

Liver fluke Opisthorchis sinensis

This fluke also occurs in Asia. Size is about 2 cm and it is found in liver. Male has two testes and female has a single ovary. Eggs are excreted through faeces and swallowed by freshwater snail. Miracidium larva hatched inside snail body and changes into sporocyst, redia and cercaria larvae. Cercaria swims freely and is eaten by fish and encysts in the muscles of fish. Infection takes place in man by eating improperly cooked fish.

Lung fluke – Paragonimus westermani

This is about one cm long fluke found in the lungs of man in Asian and African countries. It causes cough and pneumonia-like symptoms. Eggs are excreted through sputum and reach freshwater bodies where larvae grow in snails, crabs and crayfish. Infection in humans takes place by eating these animals.


Taenia saginata – The beef tapeworm

This worm is about 5 metre long found in the intestine of man and the secondary host is cow or other cattle. The adult releases gravid proglottids in intestine, from where they are excreted into the environment through the feces. The eggs are ingested by the cattle through the contaminated vegetation. The enzymes in the gastrointestinal tract break down the protective layer of oncospheres and release hexacanths in the intestine of cow. The hexacanth larvae that break through the intestinal epithelium and travel through the circulation of the bovine end up in the muscles and become a cysticercus. When undercooked beef is eaten by humans, the cysticercus metamorphoses to become adult by evagination of proscolex and attaches to the host intestine. 

Diphyllobothrium latum – The fish tapeworm

This is the largest tapeworm that infects humans and averages 30 feet in length.

Primary hosts are humans, dogs and bears. Eggs pass in the faeces of humans or animals and find their way into ponds or lakes. In water Coracidia larvae hatch from the eggs and are eaten by small crustaceans, in whose body the coracidia become procercoid larvae.

Small freshwater fishes then eat those crustacean hosts and the procercoid larvae are released from the crustaceans to become plerocercoid larvae in fish muscles. Humans and animals are infected by eating those fishes that harbour plerocercoid.

Echinococcus granulosis – The dog tapeworm or Hydated worm

The tapeworm is only six millimetres long. Thousands can inhabit the gut of an infected animal. Each mature worm grows and sheds the last segment of its body about every two weeks. This last segment contains immature eggs. The eggs are passed from the animal’s body in faeces and may stick to the animal’s hair or contaminate the vegetable garden. The eggs are highly resistant to weather conditions and can remain viable for months. The eggs have to be swallowed by an animal (intermediate host) to form hydatid cysts.

People usually become infected by accidentally swallowing the tapeworm eggs passed in dog faeces. A human acts as an intermediate host in the same way as a sheep, horse or kangaroo. The eggs travel through the bloodstream, lodge in organs and form watery cysts full of tapeworm heads. This is known as hydatid disease or echinococcosis.

Hymenolepis Nana – Dwarf Tapeworm

The dwarf tapeworm is the most common tapeworm in humans and is only 15–40 mm long. The eggs are passed with the stools of an infected human, contaminate the environment and then get ingested by rodents, humans or insects (intermediate hosts). If man ingests eggs, hexacanth larvae hatch in small intestine and penetrate intestinal wall and develops into a cysticercoid and matures into an adult.

If eggs are ingested by an insect, beetle or flea, the larvae develop inside the insect. If the insect is ingested by a rodent or accidentally by a human, the cysticercoid larvae get inside the intestine and develop into adults. Infected rodents can contaminate water and food with their faeces and infection can reach humans.