Classification of Parasites

Introduction to Parasites - The Unwanted Guests

• Parasite: Eukaryotic organism living in/on a host, deriving benefit while causing harm.
• Host Types:
  • Definitive Host (DH): Harbors adult or sexual stage (e.g., human for Taenia solium adult).
  • Intermediate Host (IH): Harbors larval or asexual stage (e.g., pig for Taenia solium larvae).
  • Paratenic Host (Transport): Harbors parasite, no development, aids transmission.
  • Reservoir Host: Animal host maintaining infection, source for humans.
• Transmission Modes: Fecal-oral, vector-borne, direct contact, transplacental, skin penetration.

⭐ Facultative parasite: Can live independently or as a parasite (e.g., Naegleria fowleri).

Protozoa Classification - Single-Cell Invaders

Unicellular eukaryotes. Classified by motility:

• Amoebae (Sarcodina):
  • Entamoeba histolytica: Dysentery, liver abscess. Flask-shaped ulcers.
  • Naegleria fowleri: PAM (warm freshwater).
  • Acanthamoeba spp.: Keratitis, GAE.
• Flagellates (Mastigophora): 📌 "Leish Gi Try Trich"
  • Giardia lamblia: Giardiasis ("Falling leaf" motility).
  • Trichomonas vaginalis: Trichomoniasis (jerky motility).
  • Trypanosoma spp.: T. brucei (Sleeping sickness), T. cruzi (Chagas disease).
  • Leishmania spp.: Leishmaniasis (visceral, cutaneous).
• Ciliates (Ciliophora):
  • Balantidium coli: Dysentery (pigs reservoir). Only human ciliate pathogen.
• Sporozoa (Apicomplexa): Obligate intracellular.
  • Plasmodium spp.: Malaria.
  • Toxoplasma gondii: Toxoplasmosis (congenital, AIDS reactivation).
  • Cryptosporidium spp.: Cryptosporidiosis (watery diarrhea, AIDS).

⭐ Plasmodium falciparum causes malignant malaria; multiple ring forms and crescent-shaped gametocytes are characteristic.

Helminths: Nematodes - Roundworm Roundup

• Features: Cylindrical, unsegmented, complete gut, dioecious.
• Intestinal "Bad Boys":
  • Ascaris lumbricoides (Giant): Obstruction, Loeffler's.
  • Enterobius vermicularis (Pinworm): Perianal itch. 📌 Scotch tape test!
  • Trichuris trichiura (Whipworm): Rectal prolapse.
  • Hookworms (Ancylostoma, Necator): Anemia (blood loss).
  • Strongyloides stercoralis: Autoinfection, hyperinfection.
• Tissue Invaders:
  • Filarial worms: Wuchereria, Brugia (lymphatic filariasis); Loa loa (eye); Onchocerca (river blindness).
  • Dracunculus medinensis (Guinea worm).
  • Trichinella spiralis (pork worm): Muscle cysts.

⭐ Larva migrans (cutaneous/visceral) can be caused by non-human nematodes like Ancylostoma braziliense or Toxocara canis.

Helminths: Platyhelminths - Tapes & Flukes

• Cestodes (Tapeworms): Segmented, no gut. Scolex (attachment), proglottids (segments).
  • Taenia solium (Pork): Cysticercosis. Armed scolex (hooks).

  ⭐ Neurocysticercosis (T. solium larvae in CNS) is a major cause of acquired epilepsy.

  • Taenia saginata (Beef): Unarmed scolex.
  • Echinococcus granulosus: Hydatid cysts (larval).
  • Diphyllobothrium latum (Fish): Longest; Vit B12 deficiency. 📌 D. Latum = Dim.
  • Hymenolepis nana (Dwarf): Most common; direct cycle possible.
• Trematodes (Flukes): Unsegmented, leaf-like, blind gut (mouth, no anus). Snails often intermediate hosts.
  • Schistosoma (Blood flukes): Separate sexes. Cercarial dermatitis (swimmer's itch).
    • S. haematobium: Urinary tract (hematuria, bladder Ca risk).
    • S. mansoni / S. japonicum: Intestinal/hepatic (portal HTN).
  • Liver Flukes: Fasciola hepatica (ingest metacercariae on aquatic plants); Clonorchis sinensis (ingest in raw fish; cholangio Ca risk).
  • Lung Fluke: Paragonimus westermani (ingest in raw crabs/crayfish; hemoptysis).

Medically Important Arthropods - Vectors & Villains

• Insecta (Vectors & Ectoparasites):
  • Mosquitoes (Anopheles, Aedes, Culex): Malaria, Dengue, JE, Filaria
  • Flies (Sandfly, Housefly): Leishmaniasis, Myiasis, mechanical transmission
  • Lice (Pediculus): Pediculosis, Epidemic typhus
  • Fleas (Xenopsylla): Plague
• Arachnida (Vectors & Ectoparasites):
  • Ticks (Ixodid, Argasid): KFD, Rickettsioses
  • Mites (Sarcoptes): Scabies

⭐ Anopheles stephensi is a key urban malaria vector in India.

High‑Yield Points - ⚡ Biggest Takeaways

• Protozoa are unicellular eukaryotes; Metazoa (helminths, arthropods) are multicellular.
• Protozoa groups: Amoebae (pseudopods), Flagellates (flagella), Ciliates (cilia), Sporozoa (apical complex).
• Helminths: Nematodes (round, unsegmented), Cestodes (tapeworms, segmented, no gut), Trematodes (flukes, unsegmented).
• Definitive host harbors adult/sexual stage; Intermediate host harbors larval/asexual stage.
• Vectors (e.g., mosquitoes, ticks) transmit parasites; can be biological or mechanical.
• Many parasites cause zoonotic infections, transmitted from animals.