Introduction to Parasitology

Introduction to Parasitology

• Introduction to parasitology is presented by Chua Candy (Ph.D.), from the Unit of Microbiology, Faculty of Medicine, AIMST University.

Topic Outcomes

• Define terms related to basic parasitology.
• Describe the classification of parasites.
• Discuss the transmission modes of medically important parasitic infections.
• Discuss the life cycles of medically important parasites (Entamoeba, Giardia, Plasmodium, Ascaris, Ancylostoma, filarial worms).
• Apply knowledge of life cycles to laboratory diagnosis.

Parasites

• A living organism that obtains its needs from another organism (host), harming the host.
• Parasites have special structures/features including adult and developmental stages
  • Classification based on physical characteristics, morphology, phylogenetics, and location of infestation

Terminology

Ectoparasite

• A parasite living outside the host's body.
  • Example: Lice on human scalp

Endoparasite

• A parasite living inside the host's body.
  • Example: Ascaris lumbricoides in the human intestine

Definitive Host

• Host where the parasite reaches sexual maturity or the most important host if no reproduction occurs.
  • Example: Human is the definitive host for Schistosoma mansoni.

Intermediate Host

• Host where the parasite undergoes some development or asexual reproduction but not sexual maturity.
  • Example: Snail is the intermediate host for Schistosoma mansoni. Many mammals like mice and birds.

Schistosoma spp. Life Cycle

• Includes stages like free-swimming cercariae, penetration of human skin, circulation, migration to portal blood in the liver, maturation into adults, and migration to the mesenteric venules of the bowel/rectum, laying of eggs that circulate to the liver, and shedding of eggs in stools/urine.

Classification of Parasites

• Protozoa (single-celled eukaryotes)
  • Intestional Protozoa: Entamoeba, Giardia, Trichuris
  • Urogenital Protozoa: Trichomonas
  • Blood and Tissue Protozoa: Plasmodium, Toxoplasma
• Helminths (parasitic worms):
  • Nematode: Ascaris, Trichuris, Ancylostoma
  • Cestode: Taenia, Echinococcus
  • Trematode: Schistosoma, Fasciola
• Ectoparasites:
  • Ticks, fleas, lice, mites

Protozoa

General Characteristics

• Microscopic, unicellular eukaryotes.
• Perform complex metabolic activities (respiration, excretion, digestion).
• Most are free-living; some are parasitic.

Morphology of Protozoa (A) Nucleus

• Surrounded by a nuclear membrane.
• Contains a karyosome, and chromatin granules.
  • Karyosome position varies (center or periphery).
• May or may not contain DNA (depending on the species).
• Most have a single nucleus; exceptions exist.

Morphology of Protozoa (B) Cytoplasm

• Surrounded by a plasma membrane.
• Divided into ectoplasm and endoplasm.
  • Ectoplasm: Outer, transparent layer; functions include protection, locomotion, and sensation.
  • Endoplasm: Inner granular layer; functions include nutrition and reproduction.

Characteristics of Protozoa (1) Stages

• Cyst: A protective stage for survival in unfavorable conditions.
  • Cysts transform into trophozoites in favorable environments.
• Trophozoite: Active feeding and growing stage.
  • Food obtained via phagocytosis/pinocytosis or cytostomes.

Characteristics of Protozoa (2) Life Cycle

• Direct Life Cycle: One host.
  • Hand-to-mouth infection.
  • Examples: Ascariasis, and trichuriasis.
• Indirect Life Cycle: Two or more hosts.
  • Examples: Malaria and schistosomiasis.

Entamoeba histolytica

• Third leading parasitic cause of death in developing countries.
• Causes amoebiasis, intestinal, or amoebic dysentery.
• A pathogenic amoeba associated with intestinal and extraintestinal infections (e.g., liver, brain, lung).
• Risk factors include contaminated food/water and contaminated hands from food handlers.
• Trophozoites found in the large intestinal lumen, mucosa, and submucosa

Morphology of E. histolytica

• Trophozoite: Ingested red blood cells
• Cyst: Pseudopodia, Chromatoidal body, karyosome

Giardia lamblia

• Causes giardiasis
• Common in young children (<10 years old)
• Common transmission: Contaminated water/food/surfaces.
• Infects the small intestine

Morphology of G. lamblia

• Cyst: Cyst wall, axonemes
• Trophozoite: Nucleus, Karyosome, parabasal bodies, ventral flagella, lateral flagella, posterior flagella, sucking disc

Ciliophora (Ciliates)

• Movement: Cilia for movement and feeding.
• Two nuclei: Macronucleus (metabolism) and Micronucleus (reproduction)
• Most free-living, but one species infects humans: Balantidium coli

Balantidium coli

• Only ciliate protozoan parasite of humans.
• Causes balantidiasis.
• Infects the large intestine.
• Distribution: Worldwide; natural host: pigs.

Apicomplexa (Sporozoa)

• Obligate parasites.
  • Absent flagella and cilia but exhibit gliding motility
  • Have a complex life cycle with asexual and sexual stages, typically involving two hosts
• Examples: Plasmodium , Babesia, Toxoplasma, Cryptosporidium, Isospora

Plasmodium spp.

• Cause malaria, a leading tropical disease.
• Includes: P. falciparum, P. vivax, P. ovale, P. malariae.
• Life cycle involves humans, and female Anopheles mosquitoes.
• Morphology of P. falciparum: Sporozoites, ring, trophozoites, schizonts, merozoites, and gametocytes.

Hookworms

• Four species: Ancylostoma duodenale, Necator americanus, Ancylostoma braziliense, and Ancylostoma caninum
• Cause hookworm infection.
• Transmission: Contaminated soil.
• Adult worms are bloodsuckers; heavy infection can lead to significant blood loss.
• Life cycle: Filariform larvae penetrate skin, leading to infection

Filarial Worms

• Cause lymphatic filariasis (elephantiasis).
• Transmission: Filarial parasites are transmitted to humans through mosquitoes.
• Three main types: Wuchereria bancrofti, Brugia malayi, and Brugia timori

Helminths (Nematoda)

• Unsegmented, elongated, and cylindrical worms.
• Covered with complex cuticle.
• Dioecious (separate sexes).
• Examples: Ascaris lumbricoides, Trichuris trichiura, Ancylostoma spp, Necator americanus, Enterobius vermicularis, Strongyloides stercoralis, Wuchereria bancrofti, Brugia spp

Helminths (Cestoda)

• Flattened, segmented tapeworms.
• No alimentary canal.
• Adults live in the intestines of humans and animals.
  • Example: Taenia solium, Hymenolepis nana, Diphyllobothrium latum Echinococcus granulosus

Helminths (Trematoda)

• Flattened, leaf-shaped flukes.
• Use suckers for attachment.
• Mostly hermaphrodites (except Schistosoma)
  • Examples: Fasciolopsis buski, Fasciola hepatica, Clonorchis sinensis, Paragonimus westermani, Schistosoma species.

This quiz covers the fundamentals of parasitology, including definitions, classifications, and transmission modes of medically significant parasites. You'll also explore the life cycles of key parasites such as Entamoeba, Giardia, and Plasmodium, and apply this knowledge to laboratory diagnosis.