HELMINTHS
Helminth means worm (Greek helmins – worm). They are multicellular, bilaterally symmetrical, elongated, flat or round and three germ layered (triploblastic) metazoan. They are divisible into the following phyla.
- Phylum platyhelminthes: They are flat, leaf-like or tape-like segmented, mostly hermaphrodite. Their alimentary canals incomplete or entirely absent and do not possess a body cavity. Human pathogenic helminths of this phylum are of two classes: the cestodes and the trematodes. They infact are the helminths of low organization.
- Phylum nemathelminthes: They are elongated, cylindrical, unsegmented bodies with separate sexes, alimentary canal is complete and body cavity is very well present. There is no circulatory system. The body is of an outer layer of longitudinal muscles and a simple excretory system. They are better organized than platyhelminthes.
Top Helminths According to the WHO
- Ascaris lumbricoides
- Hookworms
- Trichuris trichiura
- Filarial worms
- Schistosomes.
FEATURES OF HELMINTHS
- Helminths do not multiply in the body of humans. However, Strongyloides stercoralis and Echinococcus granulosus are exceptions.
- The outer layer or the tegument is quite tough. It may resist intestinal digestion of living worm.
- Helminths move by muscular contraction and relaxation. Suckers present in some helminths, may help in their movement. However, helminths have no special organs of movement.
- True body cavity is absent.
- In some helminths, the digestive system is either rudimentary or altogether absent. They get predigested nutrients from humans.
- The reproductive system is highly developed.
- The excretory system is just sufficient.
- The nervous system is quite primitive.
- Many helminths may need more than one intermediate host to complete their life cycle.
- Environmental factors, food habits, practices or customs, etc. influence and modify the transmission of helminthic diseases.
- Preparation of vaccine is difficult as helminths are multicellular.
FEATURES OF CESTODES
- They are dorsoventrally flattened, segmented and tape-like.
- Their size varies from a few millimeters to many meters.
- Habitat of adult worms is intestinal canal of the host.
- The body of adult worm comprises head (scolex), neck and strobilla.
- The strobilla contains a large number of proglottids (segments).
- Cestodes are hermaphrodite.
- Alimentary canal is absent.
- Excretory and nervous systems are present.
- Suckers (cup-like) are present on the scolex. Scolex may bear hooks and rostellum.
- Reproductive system is highly developed.
- Examples of cestodes are Diphylobothrium latum, Echinococcus granulosus, Tenia solium, Tenia saginata and Hymenolepis nana.
FEATURES OF TREMATODES
- They are dorsoventrally flattened like a leaf.
- They are also called fluke.
- They have segmented body which is covered with tegument.
- The size varies from 1mm to 3 cms sucker or more.
- They have two suckers: (a) anterior sucker also called oral sucker (b) posterior sucker with which parasite is attached to the host.
- Trematodes are hermaphrodite except schistosomes.
- Body cavity is not present.
- They lay eggs (oviparous).
- The alimentary tract is incomplete.
- The excretory system and nervous system are present.
- Reproductive system is highly developed.
- Eggs are operculated except in schistosomes.
- Examples are Schistosoma haematobium, Paragonimus westermani, Fasciolopsis buski, Clonorchis sinensis, etc.
FEATURES OF NEMATODES
- They are cylindrical and elongated which tapers at both ends.
- Sexes are separate and males are smaller than females.
- Size varies from 1 mm (Trichinella spiralis) in length to more than 1 m (Dracunculus medinensis).
- The cuticle is generally smooth. Striation, ridges, spines, bristles, etc. may be present which are utilized for the identification of worm.
- Alimentary canal is usually complete.
- In the body cavity, digestive, genital and other systems float.
- Circulatory system is absent.
SIGNIFICANCE OF NEMATODES
- More than a billion people are infected with one or more species of intestinal nematodes all over the world.
- They are more common in areas with poor faecal sanitation especially in developing countries.
- Nematodal infection may cause malnutrition and diminished working stamina. They are not fatal.
- Zoonotic nematodal infection do occur, e.g. trichostrongyliasis, capillarias abdominal angiostrongyliasis and anisakiasis.
- The life cycle is quite varied. Strongyloides stercoralis and Enterobius vermicularis may be transmitted from person to person. On the other hand, Ascaris lumbricoides, Ancylostoma duodenale and Necator americanus may have to pass through soil for development.
- Heavy burden of adult worms require repeated exposure to parasites in its infectious form (larva or egg). It needs so because helminths fail to replicate themselves. Hence, clinical diseases are seen only in those patients who are prolonged residents in endemic areas.
- IgE and eosinophilia in abnormal levels must prompt the clinicians to look for occult helminthiasis.
- It appears as if significant immunity to intestinal nematodes does not develop.