Parasitic life cycles

Intro to Life Cycles - Meet the Players

• Definitive Host: Site of parasite sexual reproduction.
• Intermediate Host: Site of asexual development or larval stages.
• Reservoir Host: Harbors the parasite, serving as a source of infection for others.
• Vector: An organism (e.g., arthropod) that transmits the infective form.
• Infective vs. Diagnostic Stage: Form that infects a host vs. form detected in samples.

⭐ Autoinfection: Reinfection by a parasite already in the body, leading to massive organism loads (e.g., Strongyloides stercoralis, Hymenolepis nana).

Ingestion Cycles - Don't Eat That!

• Protozoa (Cysts/Oocysts)

  • Entamoeba histolytica: Cysts in contaminated water → bloody diarrhea, liver abscess (flask-shaped ulcer).
  • Giardia lamblia: Cysts in unfiltered stream water → bloating, fatty diarrhea (non-bloody).
  • Cryptosporidium parvum: Oocysts in water → profuse, watery diarrhea; severe in AIDS (CD4 < 100).
  • Toxoplasma gondii: Cysts in undercooked meat; oocysts in cat feces → chorioretinitis, brain abscess.

• Helminths (Eggs/Larvae)

  • Ascaris lumbricoides: Eggs from contaminated food/water → intestinal/biliary obstruction.
  • Trichinella spiralis: Larvae in undercooked pork or bear meat → myalgia, periorbital edema.

⭐ Ingestion of Taenia solium eggs (from human feces) causes neurocysticercosis. Ingesting larvae (in undercooked pork) causes intestinal taeniasis. This distinction is a classic exam question.

Vector-Borne Cycles - Bitey Critter Crew

• Plasmodium (Malaria): Anopheles mosquito vector. Causes cyclical fevers, chills, and sweats due to synchronous RBC lysis.
• Babesia microti: Ixodes tick. Often a co-infection with Lyme disease. Look for the "Maltese cross" tetrad on blood smear.
• Trypanosoma cruzi (Chagas): Reduviid ("kissing") bug. Transmitted via feces rubbed into the bite wound.
• Leishmania: Sandfly. Causes cutaneous ulcers or visceral disease (kala-azar) with hepatosplenomegaly and pancytopenia.
• Trypanosoma brucei (Sleeping Sickness): Tsetse fly. Presents with a painful chancre and posterior cervical lymphadenopathy (Winterbottom sign).
• Wuchereria bancrofti (Filariasis): Mosquito. Larvae block lymphatics, leading to chronic lymphedema (elephantiasis).

⭐ Chronic Chagas disease can manifest decades later with megaesophagus, megacolon, and apical cardiac atrophy. Romana's sign (unilateral periorbital swelling) is a key acute finding.

Penetration Cycles - They Get Under Your Skin

• Certain parasites invade by actively penetrating the skin, typically as larvae.
• 📌 SANd gets in your feet: Strongyloides, Ancylostoma, Necator are nematodes found in soil contaminated with feces. Larvae penetrate bare skin.
• Schistosoma (trematode): Cercariae in freshwater penetrate the skin of swimmers.

⭐ Strongyloides stercoralis can cause autoinfection. Filariform larvae mature in the gut, penetrate the intestinal wall or perianal skin, leading to a persistent internal cycle and potential hyperinfection in the immunocompromised.

High‑Yield Points - ⚡ Biggest Takeaways

• Malaria: Anopheles mosquito (definitive host) injects sporozoites; humans are intermediate hosts.
• Schistosoma: Snails are the intermediate host; skin penetration by cercariae from contaminated freshwater.
• Taenia solium: Ingesting eggs causes cysticercosis; ingesting larval cysts in pork causes taeniasis.
• Toxoplasma gondii: Transmitted by cysts in undercooked meat or oocysts in cat feces.
• Trypanosoma cruzi: The reduviid bug defecates trypomastigotes onto the skin after a bite.
• Ascaris lumbricoides: Follows ingestion of eggs, featuring a crucial lung migration phase.