According to the way of development, parasites are classified into biochelminthes and geohelminthes. Geohelminthes develop without intermediate hosts. Soil is the best environment for their egg’s development. Humans are infected through dirty fruits and vegetables which contain geohelminthe’s eggs (for example, Ascaris lumbricoideus). Biohelminthes have complete life cycle with intermediate hosts. There are tropical connections between definitive and intermediate hosts (for example, Taenia solium).
General characteristics of Flatworms (Phylum Plathelminthes)
The flatworms (phylum platyhelminthes) consists of some 12,200 species. These ribbon-shaped, soft-bodied animals are also called because they are flattened dorsoventrally, from top to bottom. Flatworms are among the simplest of bilaterally symmetrical animals, but they have a definite head at the anterior end. Their bodies are solid: the only internal space consists of the digestive cavity. Flatworms are acoelomates. The acoelomates are some bilaterally symmetrical animals that have no body cavity at all other than the digestive system.
Phylum Plathelminthes includes two classes:
1. Class Trematoda
2. Class Cestoda
General characteristics of class Trematoda (Flukes):
1. Flattened dorsoventrally (leaf-like).
3. Body is covered by cuticle
4. Organs of fixation: Oral sucker, ventral sucker
5. Organs and systems of flatworms: digestive system, excretory system, nervous system, genital system: Trematodes are hermaphrodites except genus schistosoma.
6. The Life cycle is passed in two hosts (alternation of hosts) and has sexual and asexual stages.
Genus Schistosoma (S.haematobium, S.mansoni, S.japonicum), which affect population of Asia, Africa, Latin America, and the Middle East. Three species of the Schistosoma cause the disease called schistosomiasis.
Venous vessels of bowel, liver, bladder. Schistosoma mansoni causes Hepatosplenic Schistosomiasis, Schistosoma japonicum causes Hepatosplenic Schistosomiasis and Schistosoma haematobium causes Urinary Schistosomiasis. Infection is transmitted through the skin of larvae (cercariae) from snail hosts. Its infective stage is Cercariae, definitive host is man, intermediate host is the snail and mode of transmission is by penetration of skin.
In Hepatosplenic Schistosomiasis, Eosinophilia, granulomatous polyps in colon, fever, anorexia, weight loss, anemia, portal hypertension; dysentry and cirrhosis of liver; prurit skin rash. Eggs go back through portal circulation to liver, causing hepatomegaly, liver tenderness. The clinical manifestations of Urinary schistosomiasis: eosinophilia, hematuria, terminal dysuria (pain, difficulty at the end of urination); Obstructed urine flow.
The laboratory diagnosis of Hepatosplenic Schistosomiasis is the determination of eggs with lateral spine in feces. And for Urinary Schistosomiasis is eggs with terminal spine in urine.
Treatment is by administering Praziquantel. But if the Brain is involved or the illness becomes severe, corticosteroids may be given.
Prevention involves proper disposal of human waste and eradication of the snail host when possible. Swimming in endemic areas should be avoided.