Medically Important Parasites Overview

Medically Important Parasites

• Parasites exist in two forms: single-celled protozoa and multicellular metazoa (helminths or worms).
• Protozoa are subdivided into Sarcodina (amebas), Sporozoa (sporozoans), Mastigophora (flagellates), and Ciliata (ciliates).
• Metazoa are categorized into Platyhelminthes (flatworms) with Cestoda (tapeworms) and Trematoda (flukes) and Nemathelminthes (roundworms or nematodes).
• Hosts can be classified as definitive, intermediate, reservoir (carrier), and vector hosts.
• Definitive host: sexual maturity and reproduction occur here.
• Intermediate host: essential development of the parasite happens here.
• Reservoir (carrier) host: harbours a parasite in nature and acts as a source of infection to other hosts; shows no symptoms or signs.
• Vector: "carrier" of the parasite from one host to another (often insects).

Intestinal & Urogenital Protozoa (Entamoeba histolytica)

• Entamoeba histolytica causes amebic dysentery and liver abscesses.
• The parasite's lifecycle has two stages: trophozoite (motile ameba) and cyst (nonmotile).
• Trophozoite is pear-shaped with two nuclei, four pairs of flagella and a suction disc to attach to the intestinal wall.

Giardia lamblia

• Giardia lamblia's lifecycle involves two stages: trophozoite and cyst.
• The trophozoite is pear-shaped with two nuclei and four pairs of flagella, with a suction disk to attach to the intestinal wall.

Trichomonas vaginalis

• Trichomoniasis is caused by Trichomonas vaginalis.
• Trichomonas vaginalis is pear-shaped with a central nucleus and four anterior flagella.
• An undulating membrane extends about two-thirds its length.
• Exists only in trophozoite form (not as a cyst).

Blood & Tissue Protozoa (Plasmodium)

• Malaria is caused by four species of plasmodia: Plasmodium vivax, Plasmodium ovale, Plasmodium malariae and Plasmodium falciparum.
  • P. vivax and P. falciparum are prevalent.
• Malaria is a significant worldwide infectious disease and a leading cause of death.
• Malaria's vector and definitive host is the female Anopheles mosquito. The lifecycle involves two phases: sexual cycle (occurs in mosquitoes) and asexual cycle (occurs in humans).
• The sexual cycle is called sporogony and the asexual cycle is called schizogony.

Toxoplasmosis

• Toxoplasmosis is a zoonotic disease caused by the coccidian protozoan Toxoplasma gondii.
• It primarily infects animals and birds.
• Toxoplasmosis can be transmitted transplacentally to cause spontaneous abortion, stillbirth, or severely mentally and/or physically handicapped children.

Trypanosoma cruzi

• Trypanosoma cruzi causes Chagas' disease (American trypanosomiasis).
• The life cycle involves the reduviid bug (Triatoma, cone-nose, or kissing bug) as the vector.

Trypanosoma gambiense and Trypanosoma rhodesiense

• These organisms cause sleeping sickness (African trypanosomiasis).
• The vector for both is the tsetse fly.

Leishmaniasis

• Leishmaniasis is caused by unicellular, flagellate, intracellular protozoa belonging to the genus Leishmania.
• Leishmaniasis involves an obligatory intracellular parasite transmitted by female sandflies (Phlebotomus spp).
• The condition is classified into different forms, including visceral (VL, Kala-azar), cutaneous (CL), and mucosal (ML).

Helminths (Trematodes)

• Trematoda (flukes) and Cestoda (tapeworms) are large classes of parasites in the Platyhelminthes phylum.
• Important trematodes include Schistosoma species (blood flukes), Clonorchis sinensis (liver fluke), and Paragonimus westermani (lung fluke).
  • Schistosomiasis has significant impacts on morbidity and mortality.

Helminths (Cestodes)

• Tapeworms have two parts: a rounded head (scolex) and a flat body with multiple segments (proglottids).
• Humans acquire tapeworm infection from consuming undercooked flesh containing the larval form of the parasite.
• Medically important species include Taenia solium, Taenia saginata, Diphyllobothrium latum, and Echinococcus granulosus.
• The lifecycle and intermediate hosts vary between the parasite species.

Helminths (Nematodes)

• Nematodes are roundworms (also known as Nemathelminthes) with a complete digestive tract and a mouth and an anus.
  • The body is covered in a protective cuticle.
• Nematodes are sexually dimorphic (separate sexes, usually with the female larger than the male, and a coiled tail in males).
  • Intestinal nematodes and tissue nematodes are the primary divisions important for human infections.
• Important intestinal nematodes include Enterobius, Trichuris, Ascaris, Necator, Ancylostoma, Strongyloides, and Trichinella.
  • Ingesting eggs transmits Enterobius, Trichuris, and Ascaris.
  • Other intestinal nematodes are transmitted as larvae.
• Important tissue nematodes include Wuchereria, Onchocerca, and Loa (filarial worms).
  • Transmitted by bloodsucking mosquitoes or flies. Another important nematode is Dracunculus (guinea worm).

Explore the fascinating world of medically important parasites, including protozoa and helminths. This quiz covers classifications, hosts, and specific diseases caused by parasites such as Entamoeba histolytica. Test your knowledge on the lifecycle and impact of these organisms on human health.