Key Points in Parasitology

Leishmania spp.

• Disease: Leishmaniasis, a spectrum of diseases including cutaneous, mucocutaneous, and visceral forms.

• Infective stage: Promastigotes are injected into the mammalian host by infected female sandflies during a blood meal. These transform into amastigotes within macrophages.

• Diagnostic stage: Amastigotes (non-flagellated forms) are found intracellularly within macrophages in host tissues. Promastigotes (flagellated forms) are found in the gut of the sandfly vector.

• Life Cycle: Involves both a sandfly (vector) and a human/mammalian host. Sandflies ingest amastigotes during a blood meal, which develop into promastigotes in the sandfly gut and migrate to the proboscis.

Cryptosporidium spp.

• Disease: Cryptosporidiosis, primarily causing diarrheal disease.

• Infective stage: Thick-walled oocysts, which are environmentally resistant and immediately infectious upon excretion.

• Host: Major hosts include cattle ($C. parvum$ is a significant cause of calf scours), but humans can be infected by several species, especially $C. hominis$ and $C. parvum$.

• Transmission: Fecal-oral route via contaminated water (often outbreaks in community water supplies), food, or direct contact with infected individuals or animals. Can be severe and prolonged in immunocompromised individuals.

Schistosoma spp.

• Disease: Schistosomiasis (bilharzia), a chronic parasitic disease.

• Infective stage: Cercariae (fork-tailed larvae) are released from freshwater snails and actively penetrate intact human skin.

• Diagnostic stage: Eggs are shed via feces ($S. mansoni, S. japonicum$) or urine ($S. haematobium$), depending on the species. Eggs have characteristic spines.

• Life Cycle: Involves an intermediate host (specific freshwater snails) and a definitive human host. Adult worms reside in blood vessels around the intestines or bladder.

Dracunculus medinensis

• Disease: Dracunculiasis (Guinea worm disease).

• Infective stage: Larvae ($L_3$) ingested within copepods (water fleas) when unfiltered water is consumed.

• Pathology: The mature female worm (up to 1 meter long) migrates through subcutaneous tissues and emerges from a painful blister, usually on the lower limb, about one year after infection.

• Transmission: Strictly through drinking contaminated water containing infected copepods.

Trypanosoma brucei

• Disease: African Trypanosomiasis (African sleeping sickness).

• Infective stage: Metacyclic trypomastigotes are injected into the mammalian host by the tsetse fly (Glossina species) during a blood meal.

• Forms: There are two main subspecies affecting humans: $T. b. rhodesiense$ (acute, East African sleeping sickness) and $T. b. gambiense$ (chronic, West African sleeping sickness).

• Life Cycle: Involves the tsetse fly vector and human/animal hosts. Parasites multiply as trypomastigotes in the bloodstream and lymph, eventually invading the central nervous system, leading to neurological symptoms.

Giardia intestinalis

• Disease: Giardiasis, a common intestinal infection causing diarrheal disease (often called "beaver fever").

• Infective stage: Cysts, which are highly resistant to environmental conditions and common water disinfectants, are ingested from contaminated food or water or via the fecal-oral route.

• Diagnostic stage: Cysts and trophozoites can be found in feces, though trophozoites are less stable and degrade rapidly outside the host.

• Clinical: Trophozoites colonize the small intestine, causing malabsorption, flatulence, and steatorrhea.

Echinococcus spp.

• Disease: Hydatid disease (echinococcosis).

• Infective stage: Eggs are ingested from contaminated food, water, or direct contact with definitive host feces (e.g., dogs, canids).

• Forms: $E. granulosus$ causes cystic echinococcosis, forming large, single hydatid cysts, typically in the liver or lungs. $E. multilocularis$ causes alveolar echinococcosis, characterized by destructive, tumor-like lesions.

• Life Cycle: Involves a definitive host (dogs, foxes, wolves) where adult worms reside, and an intermediate host (sheep, rodents; humans are accidental intermediate hosts) where larval cysts develop.

Trichuris trichiura

• Disease: Trichuriasis (whipworm infection).

• Infective stage: Embryonated eggs are ingested from soil contaminated with human feces.

• Development: Eggs require an incubation period in warm, moist soil (usually 2-4 weeks) to become infective.

• Diagnostic stage: Unembryonated eggs are found in feces.

• Pathology: Adult worms reside in the large intestine (cecum and appendix), where their anterior ends are embedded in the mucosa. Heavy infections can lead to chronic dysentery, anemia, growth retardation, and rectal prolapse.

Trypanosoma cruzi

• Disease: Chagas disease (American Trypanosomiasis).

• Infective stage: Metacyclic trypomastigotes are found in the feces of infected triatomine bugs (Kissing bugs). Infection occurs when these feces contaminate mucous membranes or a skin lesion (e.g., the bug's bite wound).

• Life Cycle: Involves a triatomine bug vector (intermediate host) and mammalian hosts (including humans, dogs, reservoir animals).

• Clinical: Acute phase can be asymptomatic or involve local swelling (chagoma) or Romaña's sign (periorbital swelling). Chronic phase can lead to cardiomyopathy, megaesophagus, and megacolon.

Entamoeba histolytica

• Disease: Amoebiasis, ranging from asymptomatic infection to dysentery and extra-intestinal abscesses.

• Infective stage: Mature quadrinucleate cysts are ingested via the fecal-oral route from contaminated food, water, or hands.

• Pathology: Trophozoites emerge in the small intestine, colonize the large intestine, and can invade the intestinal mucosa, leading to amoebic colitis (dysentery). They can also disseminate hematogenously to extra-intestinal sites, most commonly the liver (amoebic liver abscess).

• Diagnostic stage: Cysts and trophozoites (often containing ingested red blood cells) in feces.

Taenia solium/saginata

• Disease: Taeniasis (intestinal tapeworm infection) or Cysticercosis (larval infection with $T. solium$).

• Infective stage: Cysticerci (larval cysts) are ingested from undercooked pork ($T. solium$) or beef ($T. saginata$).

• Cysticercosis: Occurs when humans ingest $T. solium$ eggs from contaminated food/water or fecal-oral transmission, leading to larval development in human tissues (e.g., brain, muscle, eye).

• Diagnostic stage: Gravid proglottids or eggs are found in feces. $T. saginata$ proglottids are actively motile; $T. solium$ proglottids are less motile but can be differentiated by uterine branch counts.

Hookworm

• Disease: Hookworm infection, primarily causing iron-deficiency anemia.

• Infective stage: Filariform larvae ($L_3$) in contaminated soil actively penetrate intact human skin.

• Species: Major human hookworms are $Ancylostoma duodenale$ and $Necator americanus$.

• Life Cycle: Larvae migrate through the bloodstream to the lungs, are coughed up and swallowed, then mature into adult worms in the small intestine. Adult worms attach to the intestinal wall and feed on blood.

• Diagnostic stage: Unembryonated eggs are found in feces.

Toxoplasma gondii

• Disease: Toxoplasmosis, usually asymptomatic but severe in specific populations.

• Infective stage: Oocysts are ingested from contaminated food, water, or cat feces. Tissue cysts (bradyzoites) are ingested from undercooked meat (pork, lamb, beef).

• Definitive Host: Domestic cats and other felines are the only definitive hosts, shedding oocysts in their feces.

• Clinical: Can cause congenital toxoplasmosis in infants if acquired during pregnancy, leading to hydrocephalus, intracranial calcifications, and chorioretinitis. Severe disease (encephalitis, chorioretinitis) can occur in immunocompromised individuals through reactivation of latent cysts.

Dientamoeba fragilis

• Disease: Dientamoebiasis, often asymptomatic but can cause abdominal pain, diarrhea, and nausea.

• Infective stage: Trophozoites are the only stage found; there is no recognized cyst stage. Transmission is believed to be direct fecal-oral or potentially through co-infection with helminth eggs (e.g., Enterobius vermicularis or Ascaris lumbricoides) acting as protective vectors.

• Diagnostic stage: Trophozoites are identified in fresh or preserved stool samples; they are fragile and difficult to detect.

Ascaris lumbricoides

• Disease: Ascariasis, "giant roundworm" infection.

• Infective stage: Embryonated eggs are ingested from soil contaminated with human feces.

• Life Cycle: Ingested eggs hatch in the small intestine; larvae migrate through the intestinal wall, via the bloodstream to the lungs (causing "Ascaris pneumonitis"), where they mature further, are coughed up and swallowed, returning to the small intestine to develop into adult worms.

• Pathology: Heavy adult worm burdens can lead to malnutrition, abdominal pain, and intestinal or biliary obstruction. Migrating adult worms can exit through various orifices.

• Diagnostic stage: Unembryonated eggs are found in feces.

Strongyloides stercoralis

• Disease: Strongyloidiasis.

• Infective stage: Filariform larvae ($L_3$) in contaminated soil penetrate intact human skin.

• Unique Feature: This nematode has a unique capacity for autoinfection (larvae mature within the host and reinfect) and a free-living life cycle phase in the soil, besides the parasitic one.

• Clinical: Can cause "larva currens" (serpiginous skin lesions due to migrating larvae). In immunocompromised individuals, it can lead to potentially fatal strongyloides hyperinfection syndrome or disseminated strongyloidiasis.

• Diagnostic stage: Rhabditiform larvae are found in fresh feces (eggs are usually not seen in feces).

Plasmodium falciparum

• Disease: Malaria, specifically the most severe and often fatal form of human malaria.

• Infective stage: Sporozoites are injected into the human host by an infected female Anopheles mosquito during a blood meal.

• Life Cycle: Involves an Anopheles mosquito vector and a human host. Sporozoites rapidly travel to the liver, where they develop into merozoites. Merozoites are then released and infect red blood cells (erythrocytic stage), leading to cyclic fever, chills, and anemia. Some merozoites develop into gametocytes, which are taken up by mosquitoes to continue the cycle.