🌿 Hosts, vectors & life cycle forms
Parasites are eukaryotic organisms that live on or within a host. They are broadly divided into protozoa (single‑celled) and metazoa (multicellular worms and arthropods).
- Definitive host: sexual reproduction (adult stage) occurs.
- Intermediate host: asexual or larval development occurs.
- Vector: living transmitter (mechanical: no development; biologic: development occurs).
- Infectious form: cysts (protozoa), eggs/larvae (helminths).
- Diagnostic form: trophozoites (protozoa), eggs or adult worms (helminths).
🔬 Protozoan parasites
Intestinal & urogenital protozoa
Disease: amebic dysentery, liver abscess (flask‑shaped ulcerations). Transmission: fecal‑oral (cysts). Diagnosis: trophozoites/cysts in stool; serology. Treatment: metronidazole + paromomycin.
- Key feature: “flask‑shaped” colonic ulcers; extraintestinal spread to liver, lung, brain.
- Laboratory: trophozoites with central karyosome and fine chromatin.
Disease: foul‑smelling, fatty diarrhea (malabsorption). Transmission: cysts via water/food. Diagnosis: trophozoites (bilobed nuclei, falling‑leaf motility) or fecal antigen. Treatment: metronidazole.
- Cryptosporidium: transient diarrhea in immunocompetent; severe in AIDS. Acid‑fast oocysts in stool. Treatment: nitazoxanide.
- Cystoisospora belli: malabsorption, diarrhea in AIDS. Elliptical oocysts (2 sporocysts). Treatment: TMP‑SMX.
- Cyclospora: water/foodborne; acid‑fast spherical oocysts; UV fluorescence. Treatment: TMP‑SMX.
- Microsporidia: spores in stool; no proven treatment.
Urogenital flagellate. Often asymptomatic; frothy vaginal discharge, “strawberry” cervix. Diagnosis: motile trophozoites (corkscrew motility) on wet mount. Treatment: metronidazole.
Blood & tissue protozoa
Plasmodium species (malaria)
| Species | Fever pattern | Distinctive features | Liver stage | Treatment |
|---|---|---|---|---|
| P. vivax | 48h (tertian) | Enlarged RBC, ameboid trophozoites | Hypnozoites (relapse) | Chloroquine + primaquine |
| P. ovale | 48h | Oval, jagged RBC | Hypnozoites | Chloroquine + primaquine |
| P. malariae | 72h (quartan) | Bar/band forms, rosette | No hypnozoites (recrudescence) | Chloroquine |
| P. falciparum | Irregular | Multiple rings, crescent gametes | No persistent stage | Artemether‑lumefantrine, atovaquone‑proguanil, quinine |
| P. knowlesi | 24h | Similar to P. malariae | No hypnozoites | Chloroquine |
Hemoflagellates
Vector: reduviid bug (kissing bug). Transmission: feces, scratching. Acute: Romana sign, myocarditis. Chronic: dilated cardiomyopathy, megacolon, achalasia. Diagnosis: trypomastigotes in blood. Treatment: benznidazole or nifurtimox.
Vector: tsetse fly. Antigenic variation. Gambiense (West Africa) chronic; rhodesiense (East Africa) acute. Diagnosis: trypomastigotes in blood/CSF. Treatment: suramin (acute), melarsoprol (CNS).
- L. donovani → visceral (bone marrow, liver, spleen).
- L. braziliensis → mucocutaneous.
- Vector: sandfly. Amastigotes in macrophages. Treatment: stibogluconate.
Other Apicomplexa
Babesia: malaria‑like, Ixodes tick. “Maltese cross” tetrads. Treatment: clindamycin + quinine.
Toxoplasma gondii: cat definitive host. Congenital: chorioretinitis, hydrocephaly, calcifications. In AIDS: ring‑enhancing brain lesions. Treatment: pyrimethamine + sulfadiazine.
Free‑living amebae
- Naegleria fowleri: primary amebic meningoencephalitis (PAM) – warm fresh water, fatal. Amphotericin B.
- Acanthamoeba: keratitis (contact lens), granulomatous encephalitis. Star‑shaped cysts; treatment: miconazole, propamidine.
🪱 Metazoan parasites (worms)
Nematodes (roundworms)
Egg‑transmitted
- Enterobius vermicularis (pinworm): perianal itching, tape prep, treat family.
- Trichuris trichiura (whipworm): barrel‑shaped eggs, prolapse.
- Ascaris lumbricoides (largest roundworm): lung migration, bile duct obstruction.
- Toxocara (visceral larva migrans): pica, self‑limiting.
Larva‑transmitted & filarial
- Necator/Ancylostoma (hookworm): skin penetration, anemia.
- Strongyloides (threadworm): autoinfection, indefinite infection.
- Trichinella (trichinosis): encysted larvae in muscle (bear meat).
- Wuchereria, Brugia (elephantiasis): mosquito vector.
- Onchocerca (river blindness): blackfly vector, skin snips.
- Dracunculus (guinea worm): contaminated water, removal with stick.
Trematodes (flukes)
| Fluke | Acquisition | Disease / site | Ova / diagnosis | Treatment |
|---|---|---|---|---|
| Schistosoma (mansoni, japonicum, haematobium) | Skin penetration (water) | Intestinal/vesicular granulomas, bladder cancer (haematobium) | Non‑operculated eggs; terminal spine (haematobium) | Praziquantel |
| Clonorchis sinensis | Raw fish | Liver fluke; serum‑like sickness | Operculated eggs | Praziquantel |
| Fasciola hepatica | Watercress | Hepatitis | Operculated eggs | Praziquantel |
| Paragonimus westermani | Raw crab/crayfish | Lung fluke (hemoptysis, TB‑like) | Operculated eggs in sputum | Praziquantel |
Cestodes (tapeworms)
Saginata: cattle intermediate, humans definitive; intestinal tapeworm, mild symptoms. Solium: pigs intermediate; humans definitive OR intermediate (cysticercosis, especially brain → seizures). Diagnosis: proglottids/eggs in stool. Treatment: praziquantel; surgery for cysticercosis.
Two intermediate hosts (crustacean → fish). Human intestinal tapeworm; can cause megaloblastic anemia (B12 deficiency). Treatment: praziquantel.
E. granulosus (sheep raising areas) – liver/lung cysts with brood capsules. E. multilocularis – alveolar hydatid. Treatment: surgery + albendazole.
📊 Comparison & high‑yield summaries
- Protozoa: unicellular, cysts/trophozoites, symptoms days–weeks, neutrophils predominant.
- Metazoa: multicellular, eggs/larvae, symptoms >1 month, eosinophilia.
- Giardia: falling‑leaf motility.
- Plasmodium vivax/ovale: hypnozoites → relapse; primaquine required.
- Schistosoma haematobium: bladder carcinoma.
- Echinococcus: hydatid cysts in liver/lung.
- Toxoplasma: ring‑enhancing brain lesions in AIDS.