Lab 3: Phylum Apicomplexa
This is the most diverse group of parasites in terms of host distribution, life cycle and
tissue site inhabited in the host. The characteristic morphological feature of the group is
the apical complex, a combination of structures at the anterior end of the infective stages
that allow the parasites to enter cells.
Generalized Apicomplexan Life Cycle
Apicomplexan life cycles include both sexual and asexual development. Apicomplexans
develop asexually by a process called schizogony. Schizogony is a process of nuclear
replication, during which the full complement of nuclei is first formed and then the
daughter cells are formed one at a time through membrane formation around the nuclei.
There are 4 general components to apicomplexan development:
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Sporogony: the product of a large number of sporozoites
Merogony: production of merozoites
Gamogany: formation of gametes
Syngamy: fertilization of gametes to produce a zygote
The sporont, meront, and gamont multiply asexually by schizogony.
The following is a generalized plan of the apicomplexan life cycle. Note that this is only
a generalized plan. An understanding of the general plan will make the more specific
plans associated with each of the different organisms much easier to understand.
Differentiation
Gamonts/
Gametes
Micro
gametocytes
(male)
Zygote
SYNGAMY(sexual)
-fertilization
-fusion of
gametocytes
Macro
gametocytes
(female)
GAMOGONY (more
schizogony, asexual)
Cyst/Oocyst
SPOROGONY
(asexual)
Trophozoite/
Meront
-intracellular
Merozoite
-break out
and reinfect
cells
Zygote encysts
MEROGONY /
SCHIZOGONY
asexual
Schizont is a
cell with
schizogony
going on
inside it
Once in host they
excyst and
differentiate
intracellularly
Sporozoites
-Infective stage
-Usually in
Oocyst
Infection typically involves a cyst or oocyst containing sporozoites, the infective stage in
all apicomplexan life cycles. Sporozoites move through the host and invade host cells.
Inside the cells the sporozoites develop into meronts or trophozoites. The trophozoites
undergo merogony, a form of schizogony resulting in the production of merozoites.
Merozoites then break out of the cell and re-infect new cells. In the new cell, they again
transform into meronts /trophozoite, and initiate schizogony again.
The second-generation meront/trophozoites may either go through more additional
generations of merogony or it may differentiate into a gamont (gamogony). Gamonts
develop into microgametocytes (male) or macrogametocytes (female). Fertilization of
the gametes (syngamy) results in the production of a zygote. The zygote then develops a
thick outer wall to become a cyst. In the cyst, the zygote undergoes meiosis, followed by
a variable number of mitotic divisions (sporogony) to produce sporozoites. The
apicomplexan life cycle always occurs in the direction indicated in the diagram above.
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Class Gregarina
Slide: Monocystis
parasitize the seminal
vesicles of earthworms.
There is no schizogonic
phase in the life cycle;
sporozoites form trophozoites
in sperm mother cells in the
earthworm; these grow and
eventually associate as pairs in a common cyst and develop into gamonts (sygyny).
Hundreds of zygotes are formed in this common cyst and these become thick-shelled
oocyts, each containing 8 sporozoites. These are released when the worm dies or via the
genital pore.
Class Coccidia: Order Eimeriida
Species: Eimeria stiedae is a parasite of rabbits. Parasite development occurs in the
bile ducts of the liver. The life cycle is direct and transmission occurs when the rabbit
accidentally ingests food contaminated with oocysts, each containing 4 sporozoites. Once
ingested, the sporozoites break out and travel to the bile duct and penetrate epithelial
cells. Here they undergo schizogony forming a schizont that eventually forms many
merozoites. Schizonts will rupture and release individual merozoites that go on to infect
new epithelial cells. Some of these merozoites will undergo gamogomy and become
microgametocytes or macrogametocytes. Microgametocytes will continue on to develop
into microgametes. Microgametes will break out of the cells, penetrate cells containing
macrogametes, and fertilize the macrogametes to produce a zygote. The zygote will then
develop a thick wall and becomes an oocyst. Oocysts break free from the cells and pass
through the digestive tract and out with the feces.
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Slide: Liver tissue section infected with
Eimeria stiedae.
Study the liver section under low power to appreciate
the architecture of the liver. Learn to distinguish the
liver parenchyma from the bile ducts. Then focus in
on the bile ducts and search for stages of the parasite.
Trophozoites are round inclusions in the biliary
epithelium. Macrogametocytes are large oval bodies
with peripheral red-staining granules.
Microgametocytes are few in number and more diffusely stained than
macrogametocytes. Schizonts (contain merozoites) are more difficult to find. They
contain 6-20 banana-shaped merozoites.
Genus: Sarcocystis spp:
There are a number of species of Sarcocystis, all of which have obligate two host life
cycles. In most cases, an herbivore is the intermediate host, and many reptiles, birds and
mammals serve as the definitive host. The life cycle is indirect generally involving a
carnivore and herbivore host. The intermediate host (herbivore) is infected when it
ingests oocysts that
contaminate its food or water
supply. The oocysts excyst
in the small intestine
releasing bradyzoites (akin
to sporozoites), which move
from the intestine to the
circulatory system. The first
generation of merogony
occurs in the arteries in
several organ systems of the
body. The second-generation
merogony takes place in
small capillaries in the host
tissues. Second generation
merozoites then move
through the circulatory
system to striated muscle
tissues where they eventually
develop back into bradyzoites
within mother cells called
pseudocysts or sarcocysts. Carnivores (definitive host) then become infected when they
ingest the flesh of an infected prey. In the definitive host the parasite infects the
epithelium of the small intestine, reproducing asexually, eventually forming oocysts that
are passed in the feces.
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Slide: Sarcocystis tissue section. This image shows a
cross section of a tongue showing three sarcocysts.
Genus: Toxoplasma
Toxoplasma gondii has very low
host specificity, and will infect
almost any mammal, and most
nucleated cell types within a
given tissue. Felines are the only
known definitive hosts for
Toxoplasma gondii. The
Toxoplasma life cycle includes
two phases called the intestinal
(or enteroepithelial) and
extraintestinal phases. The
intestinal phase occurs in cats
only (wild as well as
domesticated cats) and produces
oocysts. The extraintestinal
phase occurs in all infected
animals (including cats) and
produces tachyzoites and,
eventually, bradyzoites or
zoitocysts. The disease
toxoplasmosis can be transmitted
by ingestion of oocysts (in cat
feces) or bradyzoites (in raw or
undercooked meat)
In most humans infected with Toxoplasma, the disease is asymptomatic. However, under
some conditions, toxoplasmosis can cause serious pathology, including hepatitis,
pneumonia, blindness, and severe neurological disorders. This is especially true in
individuals whose immune systems are compromised (e.g., AIDS
patients). Toxoplasmosis can also be transmitted transplacentally resulting in a
spontaneous abortion, a stillborn, or a child that is severely handicapped mentally and/or
physically.
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Slide: Toxoplasma smear:
The following slide contains
both intra- and extra-cellular
tachyzoites. Although this is
typically an intracellular stage,
the cells containing the
tachyzoites were broken open
when the slide was prepared.
Order Haemospora: Genus: Plasmodium
This group includes the most pathogenic parasites of man. The word malaria literally
means “bad air” reflecting the ancient belief that the disease was contracted by breathing
bad air (swamp gas). Human malaria parasites belong to one of four species;
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Plasmodium vivax,
P. falicparum,
P. malariae,
P. ovale.
You do not have to learn to distinguish these species based on the blood smear! Be
aware, however, that this can be done and know the sort of characters that are used! You
should also become familiar with the epidemiology of each of the human Plasmodiums.
Life cycles: Transmission to humans occurs when infected mosquitoes take a blood
meal. Sporozoites in the salivary glands of the vector are introduced into the vertebrate
host during feeding. Sporozoites remain in the blood for 30 minutes. These travel via the
lymphatics and blood system to the liver where they enter a liver cell, transform into
meronts, and undergo a series of schizogonic cycles (this cycle is called the
exoerythrocytic phase, or EE phase). Eight days after sporozoite infection, merozoites
derived from the EE cycle begin circulating in red blood cells (RBCs). The merozoites in
the RBCs undergo another series of schizogonic cycles (this phase is called the
erythrocytic phase). Inside RBCs, merozoites “round-up” to form the well-known
“ring-form” meront (or ring-form trophozoite). Each meront will divide to produce
12-18 “daughter” merozoites. Schizogonic cycles eventually become synchronized so
that the release of merozoites resulting from the bursting of red blood cells occurs at
regular intervals (every 48 hours) causing a recurrent succession of fever and chills.
Eventually, merozoites enter new RBCs, forming trophozoites that develop into microand macrogametocytes. These must be picked up by the vector to continue development.
In the vector, the microgametocyte exflagellates, and the macrogametocyte is fertilized,
developing into a motile zygote, the ookinete. The ookinete penetrates into the stomach
of the vector and develops into an oocyst in which thousands of sporozoites develop.
These travel to the salivary glands where they await transmission to the next host.
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Be able to identify the different stages of Plasmodium. Keep in mind the types of slides
(particular preparation) you are examining and where in the life cycle the parasite is
likely to be when inside these tissues.
Slide: Blood smear of Plasmodium vivax: Plasmodium vivax can be recognized by
its variable ring stage. Schizonts contain about 16 merozoites and the infected cell is
enlarged and contains Schuffner’s dots. The disease caused by this Plasmodium is mild
and known as benign tertian malaria (fever paroxysms typically every 48 hours).
Schizont
Schizont with Schuffer’s Dots
Ring stage
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Slide: Blood smear of Plasmodium falciparum: has a very neat ring stage
trophozoite. Multiply infected cells are common. Schizonts are rare in the peripheral
blood. Gametocytes are crescent shaped. The disease caused by this organism is severe
and known as malignant tertian malaria (fever paroxysms every 48 hours). It is this
species that kills the vast majority of humans that die of malaria.
Ring stage
Schizont
Macrogametocyte
Slide: Human malaria in liver tissue:
Hemozoin (digested hemoglobin) deposited
in the cells of the spleen in a human infected
with malaria.
Slide: Human malaria sporozoite.
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Genus: Haemoproteus are parasites of birds whose life cycles are similar to that of
Plasmodium except that the asexual stages do not appear in the circulating blood cells,
they remain in the tissues. The gamonts remain in the erythrocytes.
Haemoproteus sp. has a two obligate host life cycle with the intermediate hosts being
birds, generally pigeons or other columbiform birds and the definitive hosts being
Hippoboscid flies and Culicoides. Birds become infected when bitten by Hippoboscid or
Culicoides flies. Sporozoites enter the blood and invade the endothelial cells of blood
vessels, lungs, liver, and spleen to form schizonts. The schizonts then undergo multiple
fission and form cytomeres. Cytomeres grow within the endothelial cells while their
nucleus divides. When the endothelial cells breakdown, they release multinucleate
cytomeres, which accumulate in capillaries and release merozoites. Merozoites enter
erythrocytes (red blood cells) and become microgamonts and/or macrogametes. Insects
ingest the microgamonts and macrogametes with their blood meal. In the stomach of the
insect, the gamonts fertilize and form ookinetes (zygotes). The ookinetes then crawl to
the midgut wall and form oocysts. The oocysts mature and produce sporozoites, which
break into the body cavity and pass to the salivary glands to be injected into a new host.
Slide: Haemoproteus columbiae gametocytes. This
preparation is a blood smear collected from birds. Note
that bird erythrocytesare nucleated. You will find
Haemoproteus gametocytes in the cytoplasm of the red
blood cells.
Genus: Haemogregarina These are parasites of the blood of frogs, lizards and
turtles, and are transmitted by leeches or mites. Related species occur in fish.
Slide: Haemogregarine gametocytes in frog
blood.
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Learning Objectives
1. Phylum Apicomplexa
- General characteristics
- Apical complex!!
- 4 general components of Apicomplexan development
2. Class Gregarina – Monocystis spp.
- Visual Id – common cysts, oocysts, sporozoite
- Trophozoite stages feeding on sperm
- There is no schizogonic phase
- Host + tissue infected
3. Class Coccidia – Eimeria stiedae
- Host, tissues infected, transmission
- Life cycle
- Visual id trophozoite, macrogametocyte, microgametocyte, liver/bile ducts
4. Class Coccidia – Genus Sarcocystis
- Host, tissues, life cycle, transmission
- Visual id
5. Class Coccidia – Genus Toxoplasma
- Life cycle, transmission, hosts, tissues infected
- Intestinal vs. extraintestinal phase (where they occur, in what species…)
- Pathogenecity
- Visual id – tachyzoites
6. Class Haemosporidia – Genus Plasmodium
- Life cycle – what is definitive host
- Transmission,
- Visual id different stages and how they fit in life cycle
- Pathology vivax and falciparum
- Hemozoin in liver tissue
7. Genus Haemoproteus
- Visual id gametocytes
- Life cycle, host , transmission, etc.
8. Genus Haemogregarina
- Visual id, host, transmission
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Vocabulary
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Apical complex
Schizogony
Syngamy
Sporogony
Merogony
Gamogony
Oocyst
Sporozoite
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Meront/Trophozoite
Merozoite
Microgametocyte
Macrogametocyte
Zygote
Bradyzoite
Sarcocyst
Tachyzoite
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Exoerythrocytic cycle
Erythrocytic cycle
Sporogonic cycle
Ring-form trophozoite
Ookinete
Schuffner’s dots
Hemozoin