Phylum Apicomplexa

The phylum Apicomplexa (AY-PI-kuhm-PLEKS-uh) includes at least 4,000 species that have adult forms with no means of locomotion. Many apicomplexans were once known as sporozoans (SPOH-ruh-ZOH-uhnz). All apicomplexan species are animal parasites.

Apicomplexans have complex life cycles involving both asexual and sexual reproduction. The reproduction of apicomplexans often results in sporozoites, infectious cells that are protected by resistant coats. Apicomplexan sporozoites are characterized by an apical complex, or a group of organelles specialized for entering host cells and tissues. Because of their complex life cycles, many apicomplex-ans require two or more different hosts to complete their life cycle. For example, Plasmodium, the parasite that causes malaria, needs both mosquitoes and humans to complete its life cycle.

As apicomplexans inhabit a host's cells and tissues, the apicom-plexans absorb nutrients and destroy host cells. Apicomplexans may have killed more people than any other group of pathogens on Earth. Each year, millions of people die as a result of Plasmodium infection. Another apicomplexan, Toxoplasma gondii, causes few symptoms in healthy adult humans but can be dangerous to developing fetuses and newborn babies. Cryptosporidium, which is shown in Figure 25-7, is often transmitted by contaminated drinking water or contact with the feces of infected animals.

figure 25-7

Cryptosporidium (stained purple) is a genus of apicomplexans that cause diarrhea, abdominal pain, and vomiting. They sometimes spread to humans through the feces of infected animals, such as cats.

figure 25-7

Cryptosporidium (stained purple) is a genus of apicomplexans that cause diarrhea, abdominal pain, and vomiting. They sometimes spread to humans through the feces of infected animals, such as cats.

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