toxoplasmosis pdf - د.احمد

Toxoplasmosis

Toxoplasma gondii:
Toxoplasma gondii  (T.gondii)  is  an  obligate  intracellular  parasite  found  in
many species throughout the world causing a variety of clinical syndromes
in human and animals.
T.gondii  was  discovered  in  1908  in  a  desert  rodent  called  Ctenodactylus
gondii.
The  discovery  of  the  life  cycle  of  the  parasite  has  been  completed  only  in
1970 when domestic cats were recognized as the definitive hosts. In 1980s
T.gondii  was  regarded  as  important  opportunistic  infection  in  immune
compromised subjects.

Stages of the parasite:
The organism exists in three life forms:
Oocysts

• It is resistant form.
• Formed as a result of gametogony (sexual cycle) in the cats’ intestine

(feline intestine).

• Released with the cat feces for periods of 1-3 weeks after cat

infection.

• Remains for 1 year in warm moist soil
• Sporulation occurs outside the cat within 1 to 5 days of excretion

depending upon oxygenation and temperature. They do not sporulate
below 4C° or over 37C°.

Morphology:
Unsporulated oocysts are subspherical to spherical and are 10 by 12 µm in
diameter.
Under  light  microscopy  the  non  sporulated  oocyst  wall  consists  of  two
colorless layers.
Sporulated  oocysts  are  subspherical  to  ellipsoidal  and  are  11  by  13  µm  in
diameter.  Each  sporulated  oocyst  contains  two  ellipsoidal  sporocysts  .
Sporocysts measure 6 by 8 µm; each sporocyst contains four sporozoites

Toxoplasmosis

Tachyzoites:

• Intracellular microorganisms
• Presents in both cats (definitive host) and in human, mammals and

birds (Intermediate hosts).

• They are rapidly dividing forms observed in the acute phase of the

infection and in the reactivated infection.

• Their detection is a hallmark of active disease.

Morphology:
Crescent shape
2-4µm x4-6 µm in dimensions
Oval shape nucleus located at the blunt end of the parasite
They are stained with Giemsa stain, the cytoplasm appears blue and the
nucleus red.

A-‐ Unsporulated oocyst

B-‐ sporulated oocyct with 2 sporocysts , 4 sporozoites
are clear in one sporocyst

Toxoplasmosis

Toxoplasma gondii. Tachyzoites from the peritoneal exudates of infected
mice stained with Giemsa (× 100).

Bradyzoites:

•  Similar to tachyzoites in morphology
•  But slower multiplication
•  Found in tissue cysts
•  Usually localized in muscles, brain and eyes
•  They  can  live  in  the  tissues  of  the  host  for  long  time  and  when

ingested in the undercookedor raw meat can transmit the infection.

• Tissue cysts are round, oval, 100-200 µm in diameter containing 100s

of bradyzoites.

Toxoplasmosis

Life cycle:

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.  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.  Oocysts  transform  into  tachyzoites
shortly  after  ingestion  by  mammals  (intermediate  host).  These  tachyzoites
localize in tissues and develop into tissue cyst bradyzoites.
Cats  become  infected  after  consuming  intermediate  hosts  harboring  tissue
cysts  containing  bradyzoites.  Cats  may  also  become  infected  directly  by
ingestion of sporulated oocysts.

Toxoplasmosis

Summary of modes of Toxoplasma transmission to humans:
  Ingestion  of  undercooked  infected  meat  (chicken,  mutton,  beef)
containing Toxoplasma cysts
  Ingestion  of  the  oocyst  from  contaminated  hands  or  food
(vegetables)
  Accidental skin penetration and inoculation of tachyzoites
  Allograft  organ  transplantation:  heart,  kidney,  liver,  or  bone
marrow (cyst or tachyzoite)
  Congenital transplacental transmission (tachyzoite)
  Consumption of raw milk (oocyst)
  Inhalation of oocysts
  Transcojunctival transmission( tachyzoite)
  Water supply contamination (oocyst)
The tachyzoite occasionally may penetrate mucosal surfaces such
as the conjunctiva.

Geographic Distribution:

Serologic  prevalence  data  indicates  that  toxoplasmosis  is  one  of  the  most
common human infections throughout the world.
A high prevalence of infection in France has been related to a preference for
eating raw or undercooked meat, while a high prevalence in Central America
has been related to the frequency of stray cats in a climate favoring survival
of oocysts and soil exposure.
Studies  showed  that  prevalence  of  Toxoplasma  gondii  infection  among
young  women  in  Mosul  was  around  49.85%  while  prevalence  of  infection
among pregnant ladies with history of abortion or dead fetus in Duhok was
59%.
Pathogenesis:

  T.gondii is  an  intracellular  coccidian  parasite  that  disseminates  via
blood and lymph to reach the different organs and tissues.
The rate of the organ or tissue to get parasite infection depends on the
vascular  supply,  immunity  (local  and  general)  and  the  regenerative
ability of the host cells.

Toxoplasmosis

  The  acute  phase  infection  is  characterized  by  wide  spread  of
tachyzoites  that  invade  all  nucleated  cells,  and  after  5-6  parasitic
division  the  host  cells  will  be  lysed  releasing  the  tachyzoites  that
infect the surrounding cells and tissues leading to cellular death, tissue
necrosis  and  intense  mononuclear  inflammatory  response  in  the
infected tissues.
After that, the infection goes into chronic dormant stage in which the
parasitic  stage  is  bradyzoites  enclosed  in  cysts  that  formed  after  8
days from infection, located in body tissues as brain, skeletal muscles,
heart,  eyes  and  other  organs.  Reactivation  of  infection  occurs  when
immunity declines as in AIDS patients.

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
and/or  a  flu-like  illness.   The  clinical  course  is  usually  benign  and
self-limited; symptoms usually resolve within a few days to weeks

• Immunodeficient patients often have central nervous system (CNS)

disease but may have retinochoroiditis, pneumonitis, or other systemic
disease.

• Congenital toxoplasmosis results from an acute primary infection

acquired  by  the  mother  during  pregnancy.  Congenital  infection
develops  in  30%  to  50%  of  infants  born  to  mothers  with  acquired
toxoplasmosis  during  pregnancy.  The  incidence  of  congenital
infection varies with the trimester during which the mother becomes
infected.  The  lowest  incidence  occurs  in  the  first,  and  the  highest
incidence  is  in  the  third  trimester.  However  severe  disease  occurs  at
early  stages  of  the  pregnancy  with  high  rate  of  abortion.  Many  and
most of cases in late pregnancy are either mild or asymptomatic.

• Ocular toxoplasmosis: it is severe destructive intra ocular

inflammation, which is either due to rupture of cysts or due to allergic
reaction to the parasite. It may occur in the immunocompetent

Toxoplasmosis

patients, but more common in patients with HIV or other
immunocompromised individuals.

• Mental abnormalities due to T. gondii infection: About 40-70% of

asymptomatic newborns develop neurocognitive or neuro motor
abnormalities; and some of them are diagnosed as schizophrenia.

Immune response against T.gondii

T.gondii induces strong humoral and cellular immune response, which
will limit the extension of the infection saving life of the host.
Humoral immunity

• GIT mucosa releases secretory IgA Abs, which prevents parasite

adherence to the mucosal surface.

• IgG and IgM antibodies are important for effective opsonosation

and phagocytosis of tachyzoites.

• Antibodies can also inhibit cellular invasion by the tachyzoites
• Antibodies activate complement cascade, which kills the

tachyzoites.

• Humoral immunity including IgG and IgM antibodies has a

diagnostic importance

Cellular immunity

Cellular immune response is important to survive the acute phase via
the NK cells and macrophages and faces the chronic phases of the
infection and preventing the reactivation of the disease through both
CD8 and CD4 T lymphocytes immune response.

Laboratory Diagnosis
The diagnosis of toxoplasmosis may be documented by:
• Microscopic observation of parasites in patient specimens, such as

bronchoalveolar lavage material from immunocompromised patients,
or lymph node biopsy.

• Isolation of parasites from blood or other body fluids, by intraperitoneal

inoculation into mice or tissue culture. The mice should be tested for
the presence of Toxoplasma organisms in the peritoneal fluid 6 to 10

Toxoplasmosis

days post inoculation; if no organisms are found, serology can be
performed on the animals 4 to 6 weeks post inoculation.

• Detection of parasite genetic material by PCR, especially in detecting

congenital infections in utero.

•

Serologic testing is the routine method of diagnosis (antigen or

antibody detection):

Sabin Feldman dye test: (The gold standard test of T.gondii

diagnosis)

It is a serological method to diagnose toxoplasma gondii. This test is
rarely used these days because it is expensive and needs specialized
labs.

Antibody screening for diagnosis of toxoplasmosis:

  The  detection  of  Toxoplasma-specific  antibodies  is  the  primary
diagnostic  method  to  determine  infection  with  Toxoplasma.  The
enzyme immune assay, as ELISA tests for IgG and IgM antibodies are
the tests most commonly used today.

  Persons  should  be  initially  tested  for  the  presence  of  Toxoplasma-
specific IgG antibodies to determine their immune status. A positive IgG
titer indicates infection with the organism at some time.
  If more precise knowledge of the time of infection is necessary, then an
IgG positive person should have an IgM test performed. A negative IgM
test  essentially  excludes  recent  infection,  but  a  positive  IgM  test  is
difficult  to  interpret  because  Toxoplasma-specific  IgM  antibodies  may
be  detected  by  enzyme  immune  assays  for  as  long  as  18  months  after
acute acquired infection.

If the patient is pregnant, and IgG/IgM positive, IgG avidity test should
be performed. Serological tests for detection of other antibodies are also
available as detection of IgE and IgA Abs.

Treatment
Medicines used for treatment of toxoplasmosis:
Pyrimethamine (Daraprim) first choice in non-pregnant patient
Spiramycin is first choice treatment during pregnancy

Toxoplasmosis

Prevention
Preventive measures to minimize human contact with the infectious form of
the parasite are important in controlling both the acquired and the congenital
form of the disease.

1. Cooking meat to temperatures in excess of 66°C destroys the tissue

cyst and prevents transmission.

2. Freezing meat to -20°C for 24 hours or more kills the cyst form of the

parasite.

3. Avoiding areas contaminated with cat feces, such as litter pans in

homes, and sandboxes and soil around houses where cats are present,
is an important preventive measure.

4. Hands should be washed with soap and water after handling raw or

undercooked meat or cats.

5. Pregnant women who are identified to be negative serologically for

toxoplasmosis should be advised not to come into contact with cats
and to get rid of cats during their pregnancy.

6. Laboratory workers should be advised to wear gloves when handling

infected needles and when working with viable Toxoplasma
organisms.

7. Blood transfusion constitutes another mode of transmission of T.

gondii.  Persons  who  are  Toxoplasma  antibody-positive  should  be
excluded  as  donors  of  organs  or  blood  transfusions  to  Toxoplasma
antibody-negative recipients.

8. Screening tests for toxoplasmosis are recommended to identify the

unimmunized woman of childbearing age and warn her against these
modes of transmission of toxoplasmosis.