Chapter outline
A-Ameabiasis
• Entamoeba histolytica
• Other intestinal Amoebae
B-Flagellates
• Intestinal flagellates Giardia lamblia and Trichomonas
vaginalis
• Hemoflagellates Trypanosoma and Leishmania
C-Sporozoa
• Toxoplasma gondii
• Cryptosporidium parvum
• Plasmodium species
D- Ciliates
• Balantidium coli
Sporozoa
These protozoa have no locomotory extensions of the body and all
species are parasitic. Their motile stages move by bending, creeping
and gliding and the Apicomplexa have an APICAL COMPLEX formed by
micronemes, rhoptries, dense granules and microtubules at their
anterior end playing a role in the invasion
Coccidia constitute a very large group called Apicomplexa, some
members of which are intestinal parasites and others are blood and
tissue parasites.
All coccidia demonstrate typical characteristics, especially the
existence of asexual (schizogony) and sexual (gametogony)
reproduction.
Most members of the group also share alternative hosts; for example,
in malaria, mosquitoes harbor the sexual cycle and humans the
asexual cycle.
Micronemes
Rhoptries
Dense Granules
Cliché JF Dubremetz
Micronemes: adhesion
and motility
Rhoptry neck proteins
RONs+ MICs: Moving
junction formation
Rhoptry Bulb Content:
Parasitophorous vacuole
formation
Dense Granules:
Parasitophorous
vacuole
remodeling
• Most sporozoans are intracellular parasites or at least part
of their life-cycle takes place inside a host cell.
• The Apicomplexa are a large group of protists,
characterized by the presence of a unique organelle called
an apical complex. Some diseases caused by apicomplexan
organisms include:
• Malaria (Plasmodium)
• Coccidian diseases including:
– Cryptosporidiosis (Cryptosporidium parvum)
– Toxoplasmosis (Toxoplasma gondii)
Sporozoa
1- TOXOPLASMA GONDII
T. gondii is a typical coccidian parasite related
to Plasmodium, Isospora, and other members
of the phylum Apicomplexa.
T. gondii is an intracellular parasite, and it is
found in a wide variety of animals, including
birds and humans.
The essential reservoir host of T. gondii is the
common house cat and other felines.
Sporozoa
1- TOXOPLASMA GONDII
Physiology and Structure
Organisms develop in the intestinal cells of the cat, as well as during
an extraintestinal cycle with passage to the tissues via the
bloodstream.
The organisms from the intestinal cycle are passed in cat feces and
mature into infective cysts within 3 to 4 days in the external
environment.
Infection in cats is established when the tissues of infected rodents are
eaten.
Some infective forms, or trophozoites, from the oocyst develop as
slender, crescentic types, called tachyzoites.
These rapidly multiplying forms are responsible for the initial infection
and the tissue damage.
Slow-growing, shorter forms called bradyzoites also develop and form
cysts in chronic infections.
Life cycle of Toxoplasma gondii.
bradyzoite
Reactivation
20 µm
20 µm
BRADYZOITES
Slow multiplication
Cyst formation
TACHYZOITES
Fast multiplication
tachyzoite
carnivorism
Intermediate host
sporozoite
sporogony
a
schizogony
b
10 µm
OOCYSTES
a- non sporulated
b- sporulated
merozoite
zygote
gamogony
Final host
Toxoplasma gondii
Obligate Intracellular Parasite
 Responsible of toxoplasmosis
Neurotoxoplasmosis
Congenital
Toxoplasmosis
Chorioretinitis
Immunocompromised
individuals
Cliché JF Dubremetz
Sporozoa
1- TOXOPLASMA GONDII
Epidemiology
Human infection with T. gondii is ubiquitous; however, it is
increasingly apparent that certain immunocompromised individuals
(patients with acquired immune deficiency syndrome [AIDS]) are more
likely to have severe manifestations.
The wide variety of animals that harbor the organism-carnivores and
herbivores, as well as birds-accounts for the widespread transmission.
Humans become infected from two sources: (1) ingestion of
improperly cooked meat from animals that serve as intermediate
hosts and (2) the ingestion of infective oocysts from contaminated cat
feces.
Serologic studies show an increased prevalence in human populations
where the consumption of uncooked meat or meat juices is popular.
It is noteworthy that serologic tests of human and rodent populations
are negative in the few geographic areas where cats have not existed.
Outbreaks of toxoplasmosis in the United States are usually traced to
poorly cooked meat (e.g., hamburgers) and contact with cat feces.
Transplacental infection can occur in pregnancy, either from infection
acquired from meat and meat juices or from contact with cat feces.
Transfusion infection via contaminated blood can occur but is not
common.
Transplacental infection from an infected mother has a devastating
effect on the fetus.
Although the rate of seroconversion is similar for individuals within a
geographic location, the rate of severe infection is dramatically
affected by the immune status of the individual.
Patients with defects in cell-mediated immunity, especially those who
are infected with the human immunodeficiency virus (HIV) or who
have had an organ transplant or immunosuppressive therapy, are most
likely to have disseminated or central nervous system (CNS) disease.
It is believed that illness in this setting is caused by reactivation of
previously latent infection rather than new exposure to the organism.
Sporozoa
1- TOXOPLASMA GONDII
Clinical syndromes
Most T. gondii infections are benign and asymptomatic, with
symptoms occurring as the parasite moves from the blood to tissues,
where it becomes an intracellular parasite.
When symptomatic disease occurs, the infection is characterized by
cell destruction, reproduction of more organisms, and eventual cyst
formation.
Many tissues may be affected; however, the organism has a particular
predilection for cells of the lung, heart, lymphoid organs, and CNS,
including the eye.
Symptoms of acute disease include chills, fever, headaches, myalgia,
lymphadenitis, and fatigue; the symptoms occasionally resemble those
of infectious mononucleosis.
In chronic disease the signs and symptoms include lymphadenitis,
occasionally a rash, evidence of hepatitis, encephalomyelitis, and
myocarditis.
In some of the cases, chorioretinitis appears and may lead to
blindness.
Congenital infection with T. gondii also occurs in infants born to
mothers infected during pregnancy.
If infection occurs in the first trimester, the result is spontaneous
abortion, stillbirth, or severe disease.
Manifestations in the infant infected after the first trimester include
epilepsy, encephalitis, microcephaly (abnormal smallness of the head),
intracranial calcifications, hydrocephalus (Hydrocephalus is an abnormal
expansion of cavities (ventricles) within the brain that is caused by the accumulation of
cerebrospinal fluid), psychomotor or mental
retardation, chorioretinitis,
blindness, anemia, jaundice, rash, pneumonia, diarrhea, and
hypothermia (Hypothermia, a potentially fatal condition, occurs when body temperature
falls below 35°C).
Infants may be asymptomatic at birth, only to develop disease months
to years later.
Most often these children develop chorioretinitis with or without
blindness or other neurologic problems, including retardation,
seizures, microcephaly, and hearing loss.
A different spectrum of disease is seen in immunocompromised older
patients.
Reactivation of latent toxoplasmosis is a special problem for these
people.
The presenting symptoms of Toxoplasma infection in
immunocompromised patients are usually neurologic, most frequently
consistent with diffuse encephalopathy, meningoencephalitis, or
cerebral mass lesions.
Reactivation of cerebral toxoplasmosis has emerged as a major cause
of encephalitis in patients with AIDS.
Sporozoa
1- TOXOPLASMA GONDII
Laboratory Diagnosis
Serologic testing is required for the diagnosis of acute active infection;
the diagnosis is established by the finding of increasing antibody titers
documented in serially collected blood specimens.
Because contact with the organism is common, attention to increasing
titers is essential to differentiate acute, active infection from previous
asymptomatic or chronic infection.
Currently the enzyme-linked immunosorbent assay (ELISA) for
detecting immunoglobulin (Ig) M antibodies appears to be the most
reliable procedure because of its simplicity and rapidity in
documenting acute infections.
The test is not generally satisfactory in AIDS patients with latent or
reactivated infections, because they fail to produce an IgM response or
increasing IgG titer.
Demonstration of these organisms as trophozoites and cysts in tissue
and body fluids is the definitive method of diagnosis.
Biopsy specimens from lymph nodes, brain, myocardium, or other
suspected tissue, as well as body fluids, including cerebrospinal fluid,
amniotic fluid, or bronchoalveolar lavage fluid, can be directly
examined for the organisms.
Newer monoclonal antibody-based fluorescent stains may facilitate
direct detection of T. gondii in tissue.
Culture methods for T. gondii are largely experimental and not usually
available in clinical laboratories.
Cyst of T. gondii in tissue. Hundreds of organisms may be present in the cyst, which may
become active and initiate disease with decreased host immunity (e.g., immunosuppression
in transplant patients and in diseases such as AIDS).
The two methods available are to inoculate potentially infected
material into either mouse peritoneum or tissue culture.
Advances in developing polymerase chain reaction-based detection
methods are promising and may provide rapid and sensitive
approaches for detecting the organism in blood, cerebrospinal fluid,
amniotic fluid, and other clinical specimens.
Sporozoa
2- CRYPTOSORIDIUM SPECIES
Physiology and Structure
The life cycle of Cryptosporidium species is typical of coccidians, as is
the intestinal disease, but this species differs in the intracellular
location in the epithelial cells.
Cryptosporidium organisms are found just within the brush border of
the intestinal epithelium.
The coccidia attach to the surface of the cells and replicate by a
process that involves schizogony.
2- CRYPTOSORIDIUM SPECIES
Epidemiology
Cryptosporidium species are distributed worldwide.
Infection is reported in a wide variety of animals, including mammals,
reptiles, and fish.
Waterborne transmission of cryptosporidiosis is well documented as
an important route of infection.
The massive outbreak of cryptosporidiosis in Milwaukee
(approximately 300,000 individuals infected) was linked to
contamination of the municipal water supply.
Cryptosporidia are resistant to the usual water-purification procedures
(chlorination and ozone), and it is believed that runoff of local waste
and surface water into municipal water supplies is an important source
of contamination.
Zoonotic spread from animal reservoirs to humans, as well as personto-person spread by fecal-oral and oral-anal routes, is also common
means of infection.
Veterinary personnel, animal handlers, and homosexuals are at
particularly high risk for infection.
Many outbreaks have now been described in daycare centers where
fecal-oral transmission is common.
CLINICAL SYNDROMES
As with other protozoan infections, exposure to Cryptosporidium
organisms may result in asymptomatic carriage.
Disease in previously healthy individuals is usually a mild, self-limiting
enterocolitis characterized by watery diarrhea without blood.
Spontaneous remission after an average of 10 days is characteristic. In
contrast, disease in immunocompromised patients (e.g., patients with
AIDS), characterized by 50 or more stools per day and tremendous
fluid loss, can be severe and last for months to years.
In some patients with AIDS, disseminated Cryptosporidium infections
have been reported.
LABORATORY DIAGNOSIS
Cryptosporidium may be detected in large numbers in unconcentrated
stool specimens obtained from immunocompromised individuals with
diarrhea.
Specimens may be stained using the modified acid-fast method or by
an indirect immunofluorescence assay.
The number of oocysts shed in stool may fluctuate; therefore a
minimum of three specimens should be examined.
Serologic procedures for diagnosing and monitoring infections are
under investigation but are not yet widely available.
Acid-fast stained (red) Cryptosporidium oocysts
(approximately 5 to 7 μm in diameter).
Sporozoa
3- PLASMODIUM SPECIES
Plasmodia are coccidian or sporozoan parasites of blood cells, and as
seen with other coccidia, they require two hosts: the mosquito for the
sexual reproductive stages and humans and other animals for the
asexual reproductive stages.
Infection with Plasmodium spp. (i.e., malaria) accounts for 1 to 5
billion febrile episodes and 1 to 3 million deaths annually, 85% of
which are in Africa.
The four species of plasmodia that infect humans are Plasmodium
vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium
falciparum.
These species share a common life cycle.
Human infection is initiated by the bite of an Anopheles mosquito,
which introduces infectious plasmodia sporozoites via its saliva into
the circulatory system.
Human Malarial Parasites
Parasite
Plasmodium vivax
Disease
Benign tertian or vivax malaria
P. ovale
P. malariae
P. falciparum
Benign tertian or ovale malaria
Quartan or malarial malaria
Malignant tertian or falciparum malaria
Life cycle of Plasmodium species.
Asexual reproduction (Human Liver
and Red Blood Cells)
Sexual reproduction
(Anopheles midgut)
Salivary glands
Liver
Red Blood Cells
Midgut
Plasmodium falciparum: Blood Stage Parasites
A: Stages of P. falciparum in
thin blood smears. Fig.
1: Normal red cell; Figs. 218: Trophozoites (among
these, Figs. 2-10 correspond to
ring-stage trophozoites); Figs.
19-26: Schizonts (Fig. 26 is a
ruptured schizont); Figs.27,
28: Mature macrogametocytes
(female); Figs. 29, 30: Mature
microgametocytes (male).
Illustrations from: Coatney
GR, Collins WE, Warren M,
Contacos PG. The Primate
Malarias. Bethesda: U.S.
Department of Health,
Education and Welfare; 1971
The sporozoites are carried to the parenchymal cells of the liver, where
asexual reproduction (schizogony) occurs.
This phase of growth is termed the exoerythrocytic cycle and lasts 8 to
25 days, depending on the plasmodial species.
Some species (e.g., P. vivax, P. ovale) can establish a dormant hepatic
phase in which the sporozoites (called hypnozoites or sleeping forms)
do not divide.
The presence of these viable plasmodia can lead to the relapse of
infections months to years after the initial clinical disease (relapsing
malaria).
The hepatocytes eventually rupture, liberating the plasmodia (termed
merozoites at this stage), which in turn attach to specific receptors on
the surface of erythrocytes and enter the cells, thus initiating the
erythrocytic cycle.
Asexual replication progresses through a series of stages (ring,
trophozoite, schizont) that culminates in the rupture of the
erythrocyte, releasing up to 24 merozoites, which initiates another
cycle of replication by infecting other erythrocytes.
Some merozoites also develop within erythrocytes into male and
female gametocytes.
If a mosquito ingests mature male and female gametocytes during a
blood meal, the sexual reproductive cycle of malaria can be initiated,
with the eventual production of sporozoites infectious for humans.
This sexual reproductive stage within the mosquito is necessary for the
maintenance of malaria within a population.
Most malaria seen in the United States is acquired by visitors or
residents of countries with endemic disease (imported malaria).
However, the appropriate vector Anopheles mosquito is found in
several sections of the United States, and domestic transmission of
disease has been observed (introduced malaria).
In addition to transmission by mosquitoes, malaria can also be
acquired by blood transfusions from an infected donor (transfusion
malaria).
This type of transmission can also occur among narcotic addicts who
share needles and syringes ("mainline" malaria).
Congenital acquisition, although rare, is also a possible mode of
transmission (congenital malaria). .
Sporozoa
3-1-PLASMODIUM VIVAX
P. vivax is selective in that it invades only young, immature
erythrocytes.
In infections caused by P. vivax, infected red blood cells are usually
enlarged and contain numerous pink granules or Schüffner's dots, the
trophozoite is ring shaped but amoeboid in appearance, more mature
trophozoites and erythrocytic schizonts containing up to 24 merozoites
are present, and the gametocytes are round.
These characteristics are helpful in identifying the specific plasmodial
species, which is important for the treatment of malaria.
Plasmodium vivax ring forms and young trophozoites. Note the multiple stages of the
parasite (rings and trophozoite) seen in the peripheral blood smear, the enlarged
parasitized erythrocytes, and the presence of Schüffner's dots with the trophozoite form.
These are characteristic of P. vivax infections.
Epidemiology
P. vivax is the most prevalent of the human plasmodia, with the widest
geographic distribution, including the tropics, subtropics, and
temperate regions.
Clinical Syndromes
After an incubation period (usually 10 to 17 days) the patient
experiences vague influenza-like symptoms with headache, muscle
pains, photophobia, anorexia, nausea, and vomiting.
Clinical Syndromes
As the infection progresses, increased numbers of rupturing
erythrocytes liberate merozoites and toxic cellular debris and
hemoglobin into the circulation.
Together, these produce the typical pattern of chills, fever, and
malarial rigors.
These paroxysms usually reappear periodically (generally every 48
hours) as the cycle of infection, replication, and cell lysis progresses.
The paroxysms may remain relatively mild or progress to severe
attacks, with hours of sweating, chills, shaking, persistently high
temperatures (103° to 106°F [39° to 41°C]), and exhaustion.
Clinical Syndromes
P. vivax causes "benign tertian malaria," which refers to the cycle of
paroxysms every 48 hours (in untreated patients) and the fact that
most patients tolerate the attacks and can survive for years without
treatment.
If left untreated, however, chronic P. vivax infections can lead to brain,
kidney, and liver damage as a result of the malarial pigment, cellular
debris, and capillary plugging of these organs by masses of adherent
erythrocytes.
Laboratory Diagnosis
Microscopic examination of thick and thin films of blood is the method
of choice for confirming the clinical diagnosis of malaria and
identifying the specific species responsible for disease.
The thick film is a concentration method and may be used to detect
the presence of organisms.
With training, thick films may be used to diagnose the species as well.
The thin film is most useful for establishing species identification.
Blood films can be taken at any time over the course of the infection,
but the best time is midway between paroxysms of chills and fever,
when the greatest numbers of intracellular organisms are present.
Laboratory Diagnosis
It may be necessary to take repeated films at intervals of 4 to 6 hours.
Serologic procedures are available, but they are used primarily for
epidemiologic surveys or for screening blood donors.
Serologic findings usually remain positive for approximately a year,
even after complete treatment of the infection.
Sporozoa
3-2-PLASMODIUM OVALE
Physiology and Structure
P. ovale is similar to P. vivax in many respects, including its selectivity
for young, pliable erythrocytes.
As a consequence, the host cell becomes enlarged and distorted,
usually in an oval form.
Schüffner's dots appear as pale-pink granules, and the cell border is
frequently fimbriated or ragged.
The schizont of P. ovale, when mature, contains approximately half the
number of merozoites seen in P. vivax.
Epidemiology
P. ovale is distributed primarily in tropical Africa, where it is often
more prevalent than P. vivax.
It is also found in Asia and South America.
Clinical Syndromes
The clinical picture of tertian attacks for P. ovale (benign tertian or
ovale malaria) infection is similar to that for P. vivax.
Untreated infections last only approximately a year instead of the
several years for P. vivax. Both relapse and recrudescence phases are
similar to P. vivax.
Laboratory Diagnosis
As with P. vivax, thick and thin blood films are examined for the typical
oval host cell with Schüffner's dots and a ragged cell wall.
Serologic tests reveal cross-reaction with P. vivax and other plasmodia.
Sporozoa
3-3-PLASMODIUM MALARIAE
Physiology and Structure
In contrast with P. vivax and P. ovale, P. malariae can infect only
mature erythrocytes with relatively rigid cell membranes.
As a result, the parasite's growth must conform to the size and shape
of the red blood cell.
This produces no red cell enlargement or distortion, as seen in P. vivax
and P. ovale, but it does result in distinctive shapes of the parasite
seen in the host cell: "band and bar forms," as well as very compact,
dark-staining forms.
The schizont of P. malariae shows no red cell enlargement or distortion
and is usually composed of eight merozoites appearing in a rosette.
Occasionally, reddish granules called Ziemann's dots appear in the
host cell.
Unlike for P. vivax and P. ovale, hypnozoites of P. malariae are not
found in the liver, and relapse does not occur.
Recrudescence does occur, and attacks may develop after apparent
abatement of symptoms.
Epidemiology
P. malariae infection occurs primarily in the same subtropical and
temperate regions as the other plasmodia but is less prevalent.
Clinical Syndromes
The incubation period for P. malariae is the longest of the plasmodia,
usually 18 to 40 days, but possibly several months to years.
The early symptoms are influenza-like, with fever patterns of 72 hours
(quartan or malarial malaria) in periodicity.
Attacks are moderate to severe and last several hours.
Untreated infections may last as long as 20 years.
Laboratory Diagnosis
Observing the characteristic bar and band forms and the rosette
schizont in thick and thin films of blood establishes the diagnosis of P.
malariae infection.
As already noted, serologic tests cross-react with other plasmodia.
Sporozoa
3-4-PLASMODIUM FALCIPARUM
Physiology and Structure
P. falciparum demonstrates no selectivity in host erythrocytes and
invades any red blood cell at any stage in its existence.
Also, multiple sporozoites can infect a single erythrocyte.
Thus three or even four small rings may be seen in an infected cell.
P. falciparum is often seen in the host cell at the very edge or
periphery of the cell membrane, appearing almost as if it were "stuck"
on the outside of the cell.
This is called the appliqué or accolé position and is distinctive for this
species.
Ring forms of Plasmodium falciparum. Note the multiple ring forms within the
individual erythrocytes, which is characteristic of this organism.
Growing trophozoite stages and schizonts of P. falciparum are rarely
seen in blood films because their forms are sequestered in the liver
and spleen.
Only in very heavy infections are they found in the peripheral
circulation.
Thus, peripheral blood smears from patients with P. falciparum
malaria characteristically contain only young ring forms and,
occasionally, gametocytes.
The typical crescentic gametocytes are diagnostic for the species.
Erythrocytic Stages of P. falciparum
Ring
Mature gametocyte of P. falciparum. The presence of this sausage-shaped form is
diagnostic of P. falciparum malaria.
Infected red blood cells do not enlarge and become distorted, like they
do with P. vivax and P. ovale.
Occasionally, reddish granules known as Maurer's dots are observed in
P. falciparum.
P. falciparum, like P. malariae, does not produce hypnozoites in the
liver.
Relapses from the liver are not known to occur.
Sporozoa
3-4-PLASMODIUM FALCIPARUM
Epidemiology
P. falciparum occurs almost exclusively in tropical and subtropical
regions.
Clinical Syndromes
The incubation period of P. falciparum is the shortest of all the
plasmodia, ranging from 7 to 10 days, and does not extend for months
to years.
After the early influenza-like symptoms, P. falciparum rapidly produces
daily (quotidian) chills and fever, as well as severe nausea, vomiting,
and diarrhea.
The periodicity of the attacks then becomes tertian (36 to 48 hours),
and fulminating disease develops.
The term malignant tertian malaria is appropriate for this infection.
Because the symptoms of this type of malaria are similar to those of
intestinal infections, the nausea, vomiting, and diarrhea have led to
the observation that malaria is "the malignant mimic."
Although any malaria infection may be fatal, P. falciparum is the most
likely to result in death if left untreated.
The increased numbers of erythrocytes infected and destroyed result
in toxic cellular debris, adherence of red blood cells to vascular
endothelium and to adjacent red blood cells, and formation of
capillary plugging by masses of red blood cells, platelets, leukocytes,
and malarial pigment.
Involvement of the brain (cerebral malaria) is most often seen in P.
falciparum infection.
Capillary plugging from an accumulation of malarial pigment and
masses of cells can result in coma and death.
Kidney damage is also associated with P. falciparum malaria, resulting
in an illness called blackwater fever.
Intravascular hemolysis with rapid destruction of red blood cells
produces a marked hemoglobinuria (hemoglobinuria is a condition in which the
oxygen transport protein hemoglobin is found in abnormally high concentrations in the urine)
and can result in acute renal failure, tubular necrosis, nephrotic
syndrome, and death.
Liver involvement is characterized by abdominal pain, vomiting of bile,
severe diarrhea, and rapid dehydration.
Sporozoa
3-4-PLASMODIUM FALCIPARUM
Laboratory Diagnosis
Thick and thin blood films are searched for the characteristic rings of P.
falciparum, which frequently occur in multiples within a single cell, as
well as in the accolé position.
The distinctive crescentic gametocytes are also diagnostic.
Laboratory personnel must perform a thorough search of the blood
films because mixed infections can occur with any combination of the
four species, but most often the combination is P. falciparum and P.
vivax.
The detection and proper reporting of a mixed infection directly
affects the treatment chosen.