Classification of medical parasitology: Protozoa, Helminthes and Arthropods

Classification of medical parasitology: Protozoa, Helminthes and Arthropods

Parasites of medical importance come under the kingdom called Protista and Animalia. Protista includes the microscopic single-celled eukaryotes known as protozoa. In contrast, helminthes are macroscopic, multicellular worms possessing well-differentiated tissues and complex organs belonging to the kingdom Animalia. 

Classification of medical parasitology: Protozoa, Helminthes and Arthropods

Medical Parasitology is generally classified into:·

  •  Medical Protozoology – Deals with the study of medically important protozoa.
  •  Medical Helminthology – Deals with the study of helminthes (worms) that affectman. 
  • Medical Entomology – Deals with the study of arthropods which cause or transmitdisease to man.


Protozoan parasites consist of a single “cell-like unit” which is morphologically and functionally complete and can perform all functions of life. They are made up of a mass of protoplasm differentiated into cytoplasm and nucleoplasm. The cytoplasm consists of an outer layer of hyaline ectoplasm and an inner voluminous granular endoplasm. The ectoplasm functions in protection, locomotion, and ingestion of food, excretion, and respiration.


In the cytoplasm there are different vacuoles responsible for storage of food, digestion and excretion of waste products. The nucleus also functions in reproduction and maintaining life.The protozoal parasite possesses the property of being transformed from an active (trophozoite) to an inactive stage, losing its power of motility and enclosing itself within a tough wall. The protoplasmic body thus formed is known as a cyst. At this stage the parasite loses its power to grow and multiply.

The cyst is the resistant stage of the parasite and is also infective to the human host. Protozoa are divided into four types classified based on their organs of locomotion. These classifications are: amoebas, ciliates, flagellates, and sporozoans. Important human pathogens are; Entamoeba histolytica, Trypanosomes, Leishmanial, Trichomonas, Giardia, Plasmodium.spp, Balantidium coli


The heliminthic parasites are multicellular, bilaterally symmetrical animals having three germ layers

  • Cestodes: Tape like (segmented), Not separate sex (monoecious), Suckers: with hooks, alimentary canal and body cavity are absent   
  • Nematodes: Elongated (Cylindrical), Separate sex (diecious), No suckers andhooks, Alimentary canal and body cavity  present and complete 
  • Trematodes: Leaf like (Unsegmented), Sex not separate (monoecious) except blood flukeswhich are dioecious, suckers: no hooks,  Alimentary canal is present but incomplete, body cavity is absent


Arthropods, which form the largest group of species in the animal kingdom, are characterized by having a bilaterally symmetrical and segmented body with jointed appendages. They have a hard exoskeleton, which helps enclose and protect the muscles and other organs. An open circulatory system, with or without a dorsally situated heart pumps the blood (hemolymph) via arteries to the various organs and body tissues. Blood is returned to the heart through body spaces known as hemocoeles.


In addition, respiratory, excretory, and nervous systems are present. Arthropods affect the health of humans by being either direct agents for disease or agents for disease transmission. The arthropods of medical importance are found in Classes: Insecta, Arachnida, and Crustacia which have their own distinguishing features. In Class insecta the body is divided into head, thorax, and abdomen, with one pair of antennae. Diseases like malaria, yellow fever, onchocerciasis, and trypanasomiasis are primarily transmitted by insects.


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