Protozoa, particularly Toxoplasma gondii, are parasitic organisms that can cause significant health issues in humans, including toxoplasmosis, which often goes unnoticed in immunocompetent individuals but can lead to severe problems in immunocompromised patients and pregnant women. T. gondii has a complex life cycle involving both sexual and asexual reproduction, primarily within felids, and infection can occur through various routes, including consumption of contaminated food or water. Diagnosis typically relies on serological tests, while treatment usually involves sulfonamides and pyrimethamine.

1. Protozoa-Sporozoa
Asa Masese
KIUT
Department of Microbiology Parasitology and
Immunology.

2. Protozoa
Protozoa are microscopic, one-celled organisms that can
be free-living or parasitic in nature. They are able to
multiply in humans, which contributes to their survival
and also permits serious infections to develop from just a
single organism. Transmission of protozoa that live in a
human's intestine to another human typically occurs
through a fecal-oral route (for example, contaminated
food or water or person-to-person contact).

3. Protozoa that live in the blood or tissue of humans are
transmitted to other humans by an arthropod vector (for
example, through the bite of a mosquito or sand fly).

4. The protozoa that are infectious to humans can be
classified into four groups based on their mode of
movement:
•Sarcodina – the ameba, e.g., Entamoeba
•Mastigophora – the flagellates, e.g., Giardia, Leishmania
•Ciliophora – the ciliates, e.g., Balantidium
•Sporozoa – organisms whose adult stage is not motile e.g
.,

5. Characters of Sporozoans:
(i) All sporozoans are endoparasites.
(ii) Some sporozoans such as Eimeria cause severe
diseases like coccidiosis in the birds,
(iii) Locomotory organelles (cilia, flagella, pseudopodia,
etc.) are absent.
(iv) Nutrition is parasitic (absorp­
tive). Phagotrophy is
rare.
(v) The body is covered with an elastic pellicle or cuticle.
(vi) Contractile vacuoles are absent.

6. (vii) Asexual reproduction occurs through multiple
fission.
(viii) Sexual reproduction takes place through
syngamy (fusion of two cells or their nuclei in
reproduction).
(ix) Life cycle consists of two distinct asexual and
sexual phases. They may be passed in one
(monogenetic) or two different hosts (digenetic).

7. The genus Toxoplasma consists of only one
species, T. gondii and, based on multilocus
restriction fragment length polymorphism (RFLP)
techniques(technique that uses restriction enzymes
to cut DNA into fragments which are then separated
and analyzed to detect genetic variation), there are
three genotypes (genotypes I, II and III) which
correlate with different patterns of human disease
(Howe and Sibley, 1995).

8. Toxoplasma gondii
Toxoplasma gondii is a parasitic protozoan (specifically an
apicomplexan) that causes toxoplasmosis. It is the leading cause
of death from foodborne illness in the United States.
-The parasite occurs worldwide and can last for long periods of
time (up to a lifetime) in human bodies (and other animals).
-People who are pregnant or people who are
immunocompromised (have a weakened immune system) can
take steps to reduce their risk of infection due to Toxoplasma
infection causing more serious health problems.

9. Life cycle of Toxoplasma gondii

10. The life cycle of Toxoplasma is somewhat
complicated, containing two distinct cycles, a
sexual cycle and an asexual cycle. The definitive
hosts are members of the cat family (Felidae); thus,
the sexual cycle occurs only within the intestinal
epithelial cells of felids. Oocysts are excreted
unsporulated (uninfectious) in cat feces.

11. Oocyst sporulation in the environment usually takes
around three days, depending on factors such as
temperature and humidity. Oocysts are environmentally
robust and can retain infectivity in a cool damp
environment for months.

12. The asexual cycle occurs when consumption of tissue
cysts (see succeeding text) or oocysts results in infection
of the intestine, and the tachyzoite form of the parasite
multiplies asexually in the cells of lamina propria until the
cells rupture, releasing tachyzoites into the surrounding
tissues and resulting in systemic infection. Circulating
tachyzoites infect new cells throughout the body, and after
several more rounds of asexual division, tissue cysts are
formed.

13. Tissue cysts of T. gondii are microscopic (5–100 μm) and
contain bradyzoites, which are infectious when ingested
with the surrounding tissue. If ingested by a felid, then
the sexual cycle occurs and the life cycle is complete; if
ingested by any other host, then the asexual cycle occurs,
as previously described. If a female host is pregnant
when first infected, then circulating tachyzoites may
move to the fetus (intrauterine or congenital
transmission).

14. The only known definitive hosts for Toxoplasma
gondii are members of family Felidae (domestic cats and
their relatives).
Unsporulated oocysts are shed in the cat’s feces 1.
Although oocysts are usually only shed for 1–3 weeks,
large numbers may be shed. Oocysts take 1–5 days to
sporulate in the environment and become infective.
Intermediate hosts in nature (including birds and rodents)
become infected after ingesting soil, water or plant
material contaminated with oocysts 2.

15. Oocysts transform into tachyzoites shortly after
ingestion. These tachyzoites localize in neural and
muscle tissue and develop into tissue cyst bradyzoites 3.

16. Cats become infected after consuming intermediate
hosts harboring tissue cysts 4.
Cats may also become infected directly by ingestion
of sporulated oocysts. Animals bred for human
consumption and wild game may also become
infected with tissue cysts after ingestion of
sporulated oocysts in the environment 5.

17. Humans can become infected by any of several
routes:
Eating undercooked meat of animals harboring
tissue cysts 6.
Consuming food or water contaminated with cat
feces or by contaminated environmental samples
(such as fecal-contaminated soil or changing the
litter box of a pet cat) 7.
Blood transfusion or organ transplantation 8.
Transplacentally from mother to fetus 9.

18. In the human host, the parasites form tissue cysts, most
commonly in skeletal muscle, myocardium, brain, and
eyes; these cysts may remain throughout the life of the
host. Diagnosis is usually achieved by serology, although
tissue cysts may be observed in stained biopsy specimens

19. Diagnosis of congenital infections can be achieved by
detecting T. gondii DNA in amniotic fluid using
molecular methods such as PCR

20. Epidemiology
Toxoplasma gondii infection in humans is widespread
throughout the world. Approximately half a billion humans
have antibodies to T gondii. The incidence of infection in
humans and animals may vary in different parts of a country.
The cause for these variations is not yet known: environmental
conditions, cultural habits, and animal species are among
factors that may determine the degree of natural spread of
Toxoplasma gondii.

21. Only a small proportion (less than 0.1 percent) of people acquire
infection congenitally.

22. Infective stages
There are three infectious stages of T. gondii: the
tachyzoites (in groups or clones), the bradyzoites (in
tissue cysts), and the sporozoites (in oocysts).

23. Pathogenesis
Most cases of toxoplasmosis in humans are probably acquired
by the ingestion of either tissue cysts in infected meat or oocysts
in food contaminated with cat feces. Bradyzoites from the tissue
cysts or sporozoites released from oocysts penetrate the
intestinal epithelial cells and multiply in the intestine.
Toxoplasma gondii may spread both locally to mesenteric
lymph nodes and to distant organs by invading the lymphatics
and blood. Necrosis in intestinal and mesenteric lymph nodes
may occur before other organs become severely damaged.

24. Focal areas of necrosis may develop in many organs. The
clinical picture is determined by the extent of injury to these
organs, especially to vital and vulnerable organs such as the
eye, heart, and adrenals. Toxoplasma gondii does not produce a
toxin; necrosis is caused by intracellular multiplication of
tachyzoites.
Opportunistic toxoplasmosis in AIDS patients usually
represents reactivation of chronic infection. The predominant
lesion of Toxoplasmosis encephalitis in these patients is
necrosis, which often results in multiple abscesses, some as
large as a tennis ball

25. Host Defenses
The host may die from toxoplasmosis but much more often
recovers and acquires immunity. Inflammation usually follows
necrosis. By about the third week after infection, Toxoplasma
gondii tachyzoites begin to disappear from visceral tissues and
may localize as tissue cysts in neural and muscular tissues.
Toxoplasma tachyzoites may persist longer in the spinal cord
and brain because immune responses are less effective in these
organs. Chronic infections may be reactivated locally (for
example, in the eye).

26. Reactivation possibly results from the rupture of a tissue
cyst. Probably tissue cysts rupture periodically during
the life of the host, and the bradyzoites released are
normally destroyed by the host's immune responses. This
reaction may cause local necrosis accompanied by
inflammation. Hypersensitivity is said to play a major
role in such reactions; however, in immunocompetent
hosts the infection usually subsides, with no local
renewed multiplication of Toxoplasma.

27. In immunosuppressed patients, rupture of a tissue
cyst may result in renewed multiplication of
bradyzoites into tachyzoites, and the host may die
from toxoplasmosis. The cause of cyst rupture is not
known.

28. Clinical Presentation
Acquired infection with Toxoplasma in immunocompetent
persons is generally an asymptomatic infection. However,
10% to 20% of patients with acute infection may develop
cervical lymphadenopathy( swollen lymph nodes in the
neck) and/or a flu-like illness. The clinical course is
usually benign(not harmful in effect) and self-limited;
symptoms usually resolve within a few weeks to
months. In rare cases ocular infection with visual loss can
occur.

29. Immunodeficient patients often have central nervous
system (CNS) disease but may have retinochoroiditis,
pneumonitis, or other systemic disease. In patients with
AIDS, toxoplasmic encephalitis is the most common
cause of intracerebral mass lesions and is thought to
usually be caused by reactivation of chronic
infection. Toxoplasmosis in patients being treated with
immunosuppressive drugs may be due to either newly
acquired or reactivated latent infection.

30. Congenital toxoplasmosis results from an acute
primary infection acquired by the mother during
pregnancy. The incidence and severity of congenital
toxoplasmosis vary with the trimester during which
infection was acquired. Because treatment of the
mother may reduce the incidence of congenital
infection and reduce sequelae (after effects of a
disease) in the infant, prompt and accurate diagnosis
is important.

31. Many infants with subclinical infection at birth will
subsequently develop signs or symptoms of congenital
toxoplasmosis.( read the signs of congenital
toxoplasmosis) Ocular Toxoplasma infection, an
important cause of retinochoroiditis in the United States,
can be the result of congenital infection, or infection after
birth. In congenital infection, patients are often
asymptomatic until the second or third decade of life,
when lesions develop in the eye.

32. Diagnosis
Diagnosis of toxoplasmosis can be aided by serologic or
histocytologic examination. Clinical signs of
toxoplasmosis are nonspecific and cannot be depended on
for a definite diagnosis; toxoplasmosis clinically mimics
several other infectious diseases.

33. Many serologic tests have been used to detect antibodies
to T gondii. The most reliable of these is the Sabin-
Feldman dye test. Live virulent tachyzoites of T gondii
are used as antigen and are exposed to dilutions of the
test serum and to a complement accessory factor
resembling complement that is obtained from
Toxoplasma-antibody free-human serum. This test is
sensitive and so far is the most specific test for
toxoplasmosis. Its main disadvantages are its high cost
and the human hazard of using live organisms.

34. The indirect fluorescent antibody test (IFAT) overcomes
some of the disadvantages of the dye test. In IFAT, killed
tachyzoites of Toxoplasma, which are available commercially,
are used as antigen. Titers obtained by IFAT are similar to those
from the dye test. Disadvantages of the IFAT are that a
microscope with UV light is needed, fluorescent anti-species
globulin is required for each species to be tested, and false-
positive titers may occur in hosts with anti-nuclear antibodies.
The suitability of IFAT in animal diagnostic work is therefore
limited, but it has proved useful in diagnosing acquired human
toxoplasmosis.

35. Other serologic tests including the indirect
hemagglutination test, the latex agglutination
test, modified agglutination test, and the enzyme-
linked immunoabsorbent assay (ELISA), offer
some advantages. For example, agglutination tests
are easy to perform.

36. Soluble antigens used for indirect hemagglutination tests
are now commercially available in several countries,
including the United States. Although this test is easy to
perform, it usually does not detect antibodies during the
acute phase of toxoplasmosis. In the modified
agglutination test, whole killed tachyzoites are used as
antigen, and the test serum is treated with 2-
mercaptoethanol to eliminate nonspecific agglutinins.
The ELISA test using soluble antigens appears to be
specific and may become the standard test in the future.

37. A single positive serum sample proves only that the host
has been infected at some time in the past. Serologic
evidence for an acute acquired infection is obtained when
antibody titers rise by a factor of 4 to 16 in serum taken 2
to 4 weeks after the initial serum collection, or when
specific IgM antibody is detected. The finding of
antibody in even undiluted serum is useful in the
diagnosis of ocular toxoplasmosis because patients with
this disorder usually have low T gondii antibody titers.

38. Diagnosis can be made by finding T gondii in host
tissue removed by biopsy or at necropsy. This
procedure is particularly useful in
immunosuppressed patients or patients with AIDS,
in whom antibody synthesis may be delayed and
low.

39. Treatment
Sulfonamides and pyrimethamine (Daraprim) are two
drugs widely used to treat toxoplasmosis in humans.