Introduction to Protozoan Infections
2.2 Amoebozoa
This supergroup consists of thirteen clades of amoebae and amoeboid flagel- lates. The Amoebozoa contains free-living, commensal, and parasitic species.
Free-living species inhabit both terrestrial and aquatic environments. Most move via directed cytoplasmic flow and extension pseudopodia, although a few groups utilise flagella during particular points in their life cycle. Organisms within this group also show a great deal of diversity in their organelle comple- ment. Some species have branching mitochondria with irregular cristae, de- generate or no mitochondria, or harbour a variety of endosymbionts. Several members of this group are parasites. of whichEntamoeba histolyticais the most important to human health.
2.2.1 Entamoebidae
This family is comprised of species that are commensals or parasites of the digestive systems of arthropods or vertebrates. Genera and species have
Table 2.1 There are 30 species of protozoa that commonly infect humans.Many of these species are commensals or opportunistic pathogens that only develop into medically important parasites in immunocompromised individuals. These species belong to five supergroups which are listed in underlined text, along with their mode of transmission, tissue tropism and global distribution. Species which were formally considered protozoa but are now known to be fungi are not listed.
Transmission Tissue tropism Distribution
Amoebozoa Archamoebae
Entamoeba histolytica environmental cysts or
direct transmission
small intestine worldwide
coli environmental cysts or
direct transmission
small intestine non-pathogenic
worldwide dispar environmental cysts or
direct transmission
small intestine non-pathogenic
worldwide
Endolimax nana environmental cysts cecum
non-pathogenic
worldwide
Iodamoeba b¨utschlii environmental cysts large intestine
non-pathogenic
worldwide Centramoebida
Acanthamoeba castellanii and other spp.
environmental cysts and trophozoites
respiratory and CNS worldwide Balamuthia mandrillaris environmental cysts and
trophozoites
respiratory and CNS North America Flabellinea
Sappinia diploidea environmental cysts and
trophozoites
respiratory and CNS North America Excavata
Metamonad Diplomonadida
Giardia lamblia environmental cysts or
direct transmission
small intestine worldwide Retortamonadida
Chilomastix mesnili environmental cysts cecum and colon
non-pathogenic
worldwide Trichomonadida
Trichomonas vaginalis direct transmission vagina, prostate, urethra and seminal vesicles
worldwide
tenax direct transmission oral cavity
non-pathogenic
worldwide
Pentatrichomonas hominis cecum and large
intestine non-pathogenic
worldwide
Dientamoeba fragilis unknown large intestine worldwide
(Continued)
Table 2.1 (Continued)
Transmission Tissue tropism Distribution Discoba
Kinetoplastida
Leishmania braziliensis Sandfly causes mucocutaneous
disease
Brazil
donovani sandfly causes visceral disease India and
China
infantum sandfly causes infantile visceral
Leishmaniasis
old world
major sandfly causes cutaneous
disease
old world
mexicana sandfly causes cutaneous
disease
new world
tropica sandfly causes cutaneous
disease
Middle East and India
Trypansoma cruzi reduviid bug systemic Central and
South America rangeli reduvid bug or faecal systemic
non-pathogenic
Central and South America
Trypanosoma brucei brucei systemic
non-pathogenic
Africa
b. gambiense systemic chronic
disease
Africa
b. rhodesiense systemic acute disease Africa
Heterolobosea
Naegleria fowleri environmental cysts and
trophozoites
respiratory and CNS worldwide Harosa
Aveolata Apicomplexa
Babesia microti and other
spp.
tick systemic generally
non-pathogenic
USA
Cystoisospora belli environmental oocytes small intestine worldwide
Cryptospordium parvum environmental oocysts small intestine worldwide
homis environmental oocysts small intestine worldwide
Plasmodium falciparum mosquito systemic worldwide
knowlesi mosquito systemic South East
Asia
malariae mosquito systemic worldwide
ovale mosquito systemic old world
vivax mosquito systemic worldwide
Toxoplasma gondii ingestion of
environmental oocysts or tissue bradyzoites
systemic worldwide
Table 2.1 (Continued)
Transmission Tissue tropism Distribution Ciliophora
Balantidium coli environmental cyst large intestine Common in the
Philippines, worldwide Stramenophiles
Blastocystis sp. environmental cyst intestine worldwide
Opisthokont Ichthyosporea
Rhinosporidium seeberi environmental nasal mucosa Asia
Archeaplastida Chlorellales
Prototheca wickerhamii environmental dermis worldwide
traditionally been differentiated based on nuclear morphology. The most im- portant species, Entamoeba histolytica, is the causative agent of amoebic dysentery and infects millions of people annually, causing at least 100,000 deaths per year. While it is primarily transmitted via contaminated water or food, sexual transmission is also common in certain communities.
2.2.2 Entamaoba histolytica
The life cycle of this parasite is relatively simple and comprised of five stages:
trophozoites, precysts; cyst; metacyst; metacystic trophozoites. Trophozoites inhabit the crypts of the large intestine and are highly motile, feeding on lu- minal content as well as mucus and material scavenged from the destruction of epithelial cells. While vacuoles containing digesting material are readily ap- parent in E. histolytica trophozoites, they have a number of unusual ultra- structural characteristics (Figure 2.2A and 2.2B). The cell is generally divided into two sections: a granular central mass called the endoplasm, that contains the nucleus; and a clear outer zone called the ectoplasm, that surrounds the endoplasm. The ectoplasm trails behind endoplasm as the protozoa moves.
The nuclei of these organisms have a prominent endosome at the centre, with achromatic fibrils radiating to the inner surface of the nuclear membrane.
Typical endoplasmic reticulum and Golgi apparatus are not readily apparent in these organisms, although recent studies indicate that they have mem- brane compartments that perform similar functions. They also lack conven- tional mitochondria, instead having structures called mitosomes or cryptons.
The function(s) of the mitosome is still unclear. Like conventional mitochon- dria, they possess a double membrane, and nuclear-encoded proteins are tar- geted to them with mitochondrial-like signal sequences. They do not have an organellular DNA, nor do they perform metabolic functions seen in aerobic or- ganisms such as oxidative phosphorylation. Currently, they are believed to be
Mitosome/crypton Ectoplasm Endoplasm
Endocytosed material in food vacuole
Nucleus
Nucleus
Karyosome Chromatoid Body
(A) (B)
(C) (D)(D)
cyst
trophozoite
Figure 2.2 Entamoeba histolyticabasic anatomy.(A) A phase contrast image of anE. histolyticatrophozoite. (B) The illustration shows an idealised image of a trophozoite and a cyst. The major organelles and other subcellular features are labelled. (C) A phase contrast image of anE. colicyst showing the characteristic multinucleate structure. (D) A transmission electron microscopy (TEM) image of the chromatoid body within theE. histolyticacyst.
degenerate mitochondria which play a role in Fe-S cluster assembly. However, additional metabolic functions will undoubtedly be identified.
Trophozoites multiply by binary fission. Under certain conditions, trophozoites invade the submucosa, causing inflammation and ulceration that spreads to the underlying muscularis mucosae and serosa. These ulcers can develop into serious lesions, via a combination of tissue destruction by the invading para- sites and immune reactions to amoebas and luminal bacteria carried into the wound. In the most extreme cases, there is perforation of the colon and dissem- ination of parasites to other organs, leading to life-threatening conditions.
In normal, asymptomatic infections trophozoites pass in stool formations to the lower sections of the colon. Dehydration of the stool stimulates them to condense into a precyst sphere that secretes a hyaline wall around itself. The cyst initially has a single nucleus, but these rapidly divide into four nuclei as the cyst matures (Figure 2.2B and 2.2C). Chromatoid bodies containing large amounts of RNA are prominent during the development of the cyst (Figure 2.2B and 2.2C). This quadrinucleate cyst (or metacyst) is then passed into the ex- ternal environment, where it can then infect a new host. After ingestion and
excysting in the small intestine, the cytoplasm and nuclei divide, forming eight small metacystic trophozoites.