Biomedical Engineering Reference
In-Depth Information
merozoites, that travel (extracellularly) to other host cells, where they
invade, feed, multiply, and grow into trophozoites within specific target
cells. The trophozoite
merozoite cycle continues for a time, producing
large numbers of infectious cells within the host organism. Eventually, the
merozoites enter a new stage of life cycle stage, as male or female gametes.
Depending on the species of Class Apicomplexa, fertilization may take place
in the original host or in another organism (the female Anopheles mosquito
in the case of malaria). The fertilized egg develops into an oocyst (that can
survive outside the host). Under favorable conditions, the oocyst will
undergo schizogony to produce small sporozoites that infect cells within a
new host. Like the trophozoite, the sporozoite produces merozoites.
Members of Class Apicomplexa have life cycles that vary somewhat from
this description, and interested readers are encouraged to study the life cycles
of the individual species. Nonetheless, knowledge of the most generic api-
complexan life cycle will help students understand the basic biology of the
many important diseases produced by the thousands of different apicomplex-
an species that infect animals.
Seven genera of Class Apicoomplexa cause diseases in humans: Plasmo-
dium, Babesia, Cryptosporidium, Cyclospora, Cystoisospora, Sarcocystis,
and Toxoplasma. Plasmodium and Babesia are subclasses of Class Aconoida-
sida (apicomplexans that lack a cone at the tip). The other apicomplexans
are subclasses of Class Conoidasida (apicomplexans that have a cone at
the tip).
-
Apicomplexa
Aconoidasida
Haemosporida
Plasmodiidae
*Plasmodium
Piroplasmida
Babesiidae
*Babesia
Genus Plasmodium is responsible for human and animal malaria. About
300
500 million people are infected with malaria, worldwide. About 2 million
people die each year from malaria [4,76]. Malaria accounts for more human
deaths than any other vector-borne illness. There are several hundred species
of Plasmodium that infect animals, but only a half dozen species are known
to infect humans [76]. Newly discovered species, causing human disease,
may arise from animal reservoirs. For example, Plasmodium knowlesi causes
malaria in macaque monkeys. It has emerged as a human pathogen in
Southeast Asia, where it currently accounts for about two-thirds of malarial
cases.
In humans, malaria is contracted from sporozoites injected by female
Anopheles mosquitoes. The sporozoites eventually develop into merozoites.
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