Apicomplexans !
- Parasites
Intracellular protozoa with an apical complex for invading cells: Plasmodium, Toxoplasma, Cryptosporidium, Cyclospora, Cystoisospora and Babesia.
Updated 2026-11-01
What they are
Apicomplexans (formerly «sporozoans») are obligate intracellular parasites that use a set of organelles at their front end to enter host cells. They alternate asexual (schizogony) and sexual (gametogony) reproduction, and many form oocysts. The group includes the agents of malaria, toxoplasmosis and several diarrheal infections of people with HIV.
What they share
- Apical complex (rhoptries, micronemes, conoid) for cell invasion.
- Life cycle with asexual and sexual phases; in Plasmodium the sexual phase happens in the mosquito.
- Intestinal coccidia (Cryptosporidium, Cyclospora, Cystoisospora) show up on modified acid-fast stain.
- Plasmodium and Babesia infect red blood cells and are diagnosed on thick and thin blood films.
Microorganisms in this group
- Plasmodium vivaxThe most common malaria parasite in Colombia. Spread by Anopheles mosquitoes, it can sleep in the liver and cause relapses months later.Exam priority: high
- Plasmodium falciparumThe deadliest malaria species. In Colombia it predominates on the Pacific coast and can cause cerebral malaria and organ failure.Exam priority: high
- Plasmodium malariae and Plasmodium ovaleTwo less common malaria species: P. malariae gives fever every 72 hours and can harm the kidney; P. ovale forms hypnozoites like vivax.Exam priority: medium
- Toxoplasma gondiiA parasite caught from undercooked meat, water or soil with cat feces. Usually silent, it can harm the fetus, the eye and the brain in people with AIDS.Exam priority: high
- Cryptosporidium spp.A tiny gut parasite spread by water, even chlorinated, and by contact. It causes watery diarrhea, which can be severe in people with AIDS.Exam priority: medium
- Cyclospora cayetanensis and Cystoisospora belliTwo intestinal coccidia that cause long-lasting watery diarrhea; Cyclospora comes with fresh produce and Cystoisospora mostly affects immunocompromised people.Exam priority: medium
- Babesia spp.A red-cell parasite spread by ticks. It mimics malaria and is severe in people without a spleen or with weak immunity.For completeness
Test yourself
Think of the answer, then tap the question to see it.
What structure do apicomplexans share, and what is it for?
The apical complex, a group of organelles at the front end that lets the parasite invade the host cell.
What is the definitive host of Toxoplasma gondii?
Felids, especially the domestic cat: sexual reproduction happens in its gut and it sheds oocysts in its feces.
Which stain shows Cryptosporidium oocysts in stool?
The modified acid-fast stain (modified Kinyoun or Ziehl-Neelsen): oocysts look red on a blue-green background.
Which blood-film finding separates Babesia from P. falciparum?
Babesia makes no malaria pigment and may form tetrads; there are no gametocytes.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: CDC - DPDx - Malaria (CDC, 2024); CDC - DPDx - Toxoplasmosis (CDC, 2024); CDC - DPDx - Cryptosporidiosis (CDC, 2024).
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Representative model, for educational purposes only.