Apicomplexa

• Part of Infrakingdom Alveolata (withDinoflagellates, Ciliates).– Characterized by presence of small vesicles

(alveoli) just beneath the Plasma membrane.

• All Apicomplexa are parasitic andintracellular.– characterized by a unique structure, the Apical

complex

Host cell invasion and Gliding usesame mechanism.

• Parasite driven mechanism of internalisation.

Apicomplexa

• Gregarines: parasites of invertebrates. Undergosyzygy and have gametocyst stage. Typicallyhomoxenous.

• Coccidia: parasites of vertebrates; someheteroxenous transmitted by haematophagousarthropods; other heteroxenous involvingvertebrate intermediate.

• Haemosporidia: heteroxenous vector transmittedparasites of vertebrates

Monocystis

• Parasites in seminal vesicle ofearthworms.

• Spores containing 8 sporozoitesingested; sporozoites enter asperm mother cell, formtrophozoites. Maturetrophozoites associate in pairs,secrete wall around themselvesand undergo syzygy followed bysporogony.

Coccidia

• Monoxenous: e.g., Eimeria, Isospora• Heteroxenous: e.g., Sarcocystis, Toxoplasma• Sporozoites leave resistant oocyst after ingestion by

host, enter host cell: trophozoite.

• One or more rounds of schizogony followed bygamogony (micro and macrogametocytes)

• Macrogamete fertilized by microgamete: oocyst, aresistant infective stage.

Eimeria • Parasites of intestinefish, lizards, birds andmammals.

• Some (E. tenella)cause importantdisease.

• Trophozoite inintestinal mucosa;schizogony, gamogonyin intestine. Sporogonyin oocyst in externalenvironment.

Photograph by S.J. Upton, Kansas State

Isospora• Biology similar to

that of Eimeria butoocyst with 2sporocysts eachwith 4 sporozoites.

photographs by S.J. Upton, Kansas State

Sarcocystis• Heteroxenous:

dependent on predatorprey realtionships.– In predator:

development very muchlike that seen in Eimeriaor Isospora

– In the prey: a successionof schizogonic stagesresult in establishment ofpseudocysts throughoutthe body.

Sarcocystis spp.

• E.g. Sarcocystis neurona: oppossum/prey.Horses are abnormal hosts in which theparasites cause meningitis.

• E.g. Sarcocystis cruzi: cattle, dogs

• E.g. Sarcocystis rileyi: ducks/skunk

• E.g. others in vipers etc.

Toxoplasma spp.

• Heteroxenous dependent on predator/preyrelationships.

• T. gondii in cats and rodents: intraintestinal stages incats produce oocysts; mice eat oocysts and parasitesundergoes extraintestinal development (schizogony)producing pseudocysts in the tissues.

• Humans can act as intermediates host; pathogeniceffects depend on where pseudocysts develop.

Premunitive immunity

• Toxoplasma undergoes successive waves ofschizogony in host cells. As host developsimmunity, infecting merizoites reproducemore and more slowly eventually resulting inschizonts that are near quiescent but remaininfective.

Plasmodium4 species of Plasmodium infect man worldwide:

P. falciparum: malignant tertian form; perhaps 50% ofreported cases; 90% of deaths

P. vivax: benign tertian form; 43% of cases; death rare.

P. malariae: 7% of cases; malignant quartan.

P. ovale: mild tertian form. Rare.

Pathogenesis

• Plasmodium pathogenesis devolves from 2factors: inflammatory response of host(driven by TNF) and anemia (throughdestruction of host RBC.

• P. falciparum the most common cause ofdisease and death. Synchrony lesspronounced than in other species: feversmay be extended.

Epidemiology

• Reservoirs unimportant.• Transmission may be seasonal (coinciding

with rainy season)• Longitudinal studies show that individuals

are repeatedly exposed; immunity developsover time. Adults often immune in endemicareas (naïve adults remains highlysusceptible).

Immunopathology

• Symptoms flu like initially with intermittentand recurring fever.

• Alternating hot and cold flushes with profusesweating, head ache.

• Almost all death related to P. falciparum