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BLOOD AND TISSUE PROTOZOA: PLASMODIUM SPECIES
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caused by Apicomplexan parasites of the genus Plasmodium
1
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Falciparum
P
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Malariae

P
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Knowlesi

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tropical areas worldwide
both tropical and temperate zones
east horn of Africa, Central Asia and the
Indian Subcontinent, Southeast Asia,
and Latin America
occurs worldwide but to a much lesser
extent than P
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vivax
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malariae
microscopically

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Sporogony – sexual stage in anopheles

Exoerythrocytic Schizogony (Liver)
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Sporogony (sexual phase) in anopheles
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Sporozoites – motile spindle shaped form
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Anopheles inoculates sporozoite into the human host
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Sporozoites reaches the liver (in ~30 minutes)
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Sporozoites mature into schizonts (in the liver)
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Release of merozoites from ruptured hepatic schizonts initiates
the erythrocytic schizogony
Erythrocytic Cycle / Schizogony (Occurs in the RBC)
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Responsible for clinical manifestations
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Merozoites infects RBCs
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Ring/Trophozoite mature into schizonts – rupture into merozoites
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Some Merozoite differentiate into gametocytes (process of
maturation is called gametogeny)
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Gametocytes are ingested by Anopheles
Sporogonic Cycle – sexual production (within the mosquito)
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Micro and macrogametes fuse and transform into motile ookinete
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Ookinete invades midgut wall of mosquito → transforms into
oocyst (contains numerous spindle-shaped sporozoites)
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Oocysts grow, rupture and release sporozoites
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Sporozoite migrate to the salivary glands

Diagnostic Testing
NAATs
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detection of parasite-specific DNA - provide ↑ sensitivity and
specificity
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generally not appropriate or available for smaller laboratories
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not typically performed on a STAT basis, as is necessary for
detection of acute disease
Antigen Detection
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Most commercially available tests utilize a lateral flow format and
detect Plasmodium-specific antigens e
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lactate dehydrogenase,
aldolase, and/or P
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falciparum infections
RBC Infection
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P
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Ovale – younger RBCs
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P
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Falciparum – infects all ages of RBCs
Transfusion related Malaria
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P Falciparum - most common
Congenital Malaria
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Acquired via rupture of placental vessels with maternal-fetal
transfusion
*Neither Transfusion-related or Congenital Malaria is expected to
relapse because exoerythrocytic schizogony does not occur
P Vivax + P
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LIFE CYCLE
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Schizogony – asexual stage in humans

*8 to 21 days - time required for development in the mosquito

Relapse and Recrudescence
P
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ovale
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true disease relapse
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may occur weeks to months following subsidence of previous
attacks
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Renewed exoerythrocytic and eventual erythrocytic schizogony
from latent hepatic parasite forms known as hypnozoite
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Liver cells are infected only by sporozoites from the mosquito
o Transfusion-acquired P
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ovale infection does not
relapse
P
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malariae

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recrudescence
Arise from ↑ numbers of persisting blood stage forms to clinically
detectable levels, not from persisting liver stage forms
may occur weeks to months following subsidence of previous
attacks
result of renewed exoerythrocytic and, eventually, erythrocytic
schizogony from latent hepatic parasite forms known as
hypnozoites

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P
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4%) > P
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7%) > P
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1%) > P
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1%)
undetermined in 12
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CLINCIAL DISEASE
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P
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Vivax, P
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Ovale
o Quartian Fever – P
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Knowlesi
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Abdominal pain, headache, muscle aches and pain
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P
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EPIDEMIOLOGY
Endemic Transmission
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Endemic transmission of malaria requires
o reservoir of infection
o appropriate mosquito vector
o susceptible host
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Control of malaria is directed at
o elimination of mosquito hosts
o treatment of active cases
o prophylaxis of susceptible persons
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Challenges to prevention of endemic transmission
o emergence of mosquitoes resistant to insecticides
o development of resistance to prophylaxis and therapy by P
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vivax
o lack of adequate funding

Disease Course
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Highly dependent on infecting species
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Most fatal cases of malaria are due to P
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knowlesi
can also cause fatalities
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In nonfatal cases, the febrile paroxysms become ↓ severe with
time and the disease gradually subsides

Hereditary Phenotypes Providing protection for Malaria
Sickle Cell Trait
Hemoglobin AS
Protection against severe P Falciparum
Hereditary
malaria
Ovalocytosis
Thalassemia
Protection against P
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vivax to bind and invade erythrocytes
G6PD
Less protection from malaria
Deficiency
Transfusion-acquired malaria
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may occur when blood donors have subclinical malaria
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may be fatal for the recipient
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NOT expected to relapse because exoerythrocytic schizogony does
not occur
Congenital malaria
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may occur in infants born to mothers from endemic areas
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infant acquires the infection at birth as a result of rupture of
placental blood vessels leading to maternal-fetal transfusion
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NOT expected to relapse because exoerythrocytic schizogony does
not occur

Relapse and Recrudescences
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Relapses and recrudescences may be associated with
o changes in the host’s defense mechanisms
o possibly with antigenic changes in the infecting organisms
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P
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ovale infection may have relapses after many
months or, occasionally, years
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P
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malariae infection - may have symptom-free
periods
o suffer from sporadic recrudescences owing to persisting lowgrade parasitemia
Peripheral Blood
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(±) WBC’s that contain malaria pigment called hemozoin on
peripheral blood smears (PBS)
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↑ reticulocyte counts occur commonly and are associated with
rapid RBC turnover
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presence of greatly enlarged platelets may be noted on peripheral
blood films as a result of their rapid turnover secondary to splenic
sequestration
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Malarial infection may interfere with certain serologic tests,
producing false-(+) results, especially those for syphilis

Frequency of Disease Causing Plasmodium
5
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Vivax

Size of RBCs

Cytoplasmic inclusion

Parasite Pigment

# of merozoites

Stages Found

Golden brown
inconspi-cuous

12-24; ave: 16

All, wide range

Schuffner’s dots
Enlarged; can be up to 12x normal size of RBC

All stages except early
ring forms

P
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Falciparum

Normal

P
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Vivax and P
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Malariae and P
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Falciparum – young, ring forms are smaller than other forms;
~1/6 diameter of RBC, 1/3 in other species
o Applique or accole forms – most often seen in P
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Vivax – ameboid; growing trophozoite have irregular shape
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P
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falciparum infection
o can occur with the other species as well
Schizont Form
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P
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Falciparum – characteristic sausage shape (male =
microgametocyte)
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P
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falciparum in a person obviously
infected with P
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falciparum and P
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Falciparum
Trophozoite

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Gametocytes
Crescent or sausage shaped
Usually 1
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Malariae
Trophozoite

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Ring Forms with Maurer’s Cleft
Maurer’s cleft - can be seen in infection with older ring-form
trophozoites and asexual stages, depending on the quality of
the smear
Schizonts
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Have only one chromatin (rarely two) dot
Thicker than falciparum
Bird’s eye form may appear
No enlargement of the infected RBCs

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Schizonts
6 to 12 (usually 8) merozoites often arranged in rosettes or
irregular cluster
Mature schizonts nearly fill up the normal sized host RBCs
Coarse pigments, often peripheral
Can be common in the PBS
Gametocytes

Bird’s eye trophozoite

Trophozoite

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Gametocytes
Compact, tends to fill host RBC
No enlargement of RBCs, may be reduced in size
Blue cytoplasm with pink to red chromatin
P
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malariae, chromatin is
rounded or streaky
cytoplasm is usually compact with no vacuole
Pigment may be coarse and peripheral
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ovale
amoeboid cytoplasm
Schüffner's dots may appear as the trophozoites mature
Infected RBCs are often larger than uninfected RBCs
Multiply-infected RBCs common
band-like appearance of the trophozoite band-form
trophozoite of P
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Vivax will still have fine, light
brown pigment distributed throughout cytoplasm
Schizonts

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Motile zygotes formed by the combination of
macrogametocytes + exflagellated microgametocytes in the
mid-gut of the mosquito host
Invade epithelial cells of the mosquito’s mid-gut where an
oocyst is formed
not found in peripheral blood in the human host, very rarely
found on blood smears
Their presence on smears usually indicates a substantial delay
occurred between the time the blood was collected and the
time the slide was prepared
P
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vivax are round to oval
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usually fill the host cell
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infected RBC is usually noticeably larger than uninfected RBCs
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cytoplasm is usually a darker blue and contains fine brown
pigment throughout
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Schüffner's dots may be seen with proper staining
Microgametocytes
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usually the size of an uninfected RBC and have a paler blue,
pink or gray cytoplasm
Ookinete

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Usually contain a single chromatin dot, may contain double
chromatin dots
Multiply infected RBCs may be seen
More mature trophozoites → less amoeboid than vivax
May exhibit fimbriation and Schuffner’s dots

Developing trophozoites

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compact with little vacuolation
Infected RBCs are often slightly enlarged and may exhibit
fimbriation and schuffner’s dots
Pigment s less coarse and diffuse
Schizonts

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Similar to P vivax, tend to be smaller and contain fewer
merozoties (6-14, on average 8)
Elongation to an oval shape and fimbriation are common
Schuffner’s dots can be observed with proper staining
Pigment is lighter and less coarse
Gametocytes

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Can be difficult to distinguish from vivax
Generally less enlargement of the infected RBC
Mature macrogametocytes fills the host RBC
Microgametocyte is smaller
Schuffner’s dots may be seen with proper staining
May have fimbriation

References:
Henry's Clinical Diagnosis and Management by laboratory Methods
24th edition
CDC DPDx for Malaria: https://www
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gov/dpdx/malaria/index

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