These protozoa are obligatory intracellular parasites: they must spend at least part if not all of their life cycle in a host animal.
Apicomplexans are characterized by the presence of special organelles (tiny organ-like structures) located at the tips (apices) of the cells. These organelles contain enzymes that punch through, slice open and otherwise penetrate host tissues.
The best known apicomplexan is Plasmodium
, the agent that causes malaria. Plasmodium
spends part of its life cycle in mosquitoes and the other part in human hosts where it ultimately infects and ruptures blood cells in large numbers.
Another familiar apicomplexan is Cryptosporidium parvum
. This water-borne parasite forms extremely durable cyst-like structures that enable it to survive UV radiation and sometimes chlorine in swimming pools and treated water. An outbreak of Cryptosporidium
in Milwaukee’s drinking water supply in 1993 killed 50 people and sickened more than 400,000. (See Water Quality: Pathogens and Diseases