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Congenital Toxoplasmosis

Historical Perspective

• In 1908, Nicolle and Manceaux described the parasite in the blood, spleen and liver of a North African rodent–gundi (Ctenodactylus gundi), due to its similar appearance as leishmania they named it Leishmania gondii.
• In 1909, Nicolle and Manceaux renamed the parasite as T. gondii.
• In 1937, Sabin & Olitsky described that Toxoplasma was an obligate intracellular parasite and could be passed onto laboratory animals by intracranial, subcutaneous, intraperitoneal inoculation of brain homogenates (The slurry of tissues and cells which results when cell structure has been mechanically disrupted). They have also suggested that ingestion of Toxoplasma contaminated tissue can result in Toxoplasmosis.
• In 1937 to 1940, Wolf and Cowen have described necrotic and granulomatous lesions on autopsy of a 3 day old infant's brain infected with Toxoplasma. They have also reported that the mothers were asymptomatic but carried antibodies against Toxoplasma and the possibility of congenital transmission was expressed.
• In 1940, Pinkerton and Weinman reported the first fatal case of Toxoplasmosis in an adult.
• In 1948, Sabin and Feldman developed a serological test to identify infected individuals by using antibodies specific to Toxoplasma, called the Sabin Feldman Dye test. The serological test when used in large population studies showed a high proportion of humans and domestic animals carried antibodies against Toxoplasma.
• In 1965, Desmonts described that ingestion of under-cooked and uncooked meat plays a role in the pathogenesis of Toxoplasmosis.
• In 1970, Dubley described the life cycle of the parasite and established that the cats are the definitive hosts and any warm blooded animal can be an intermediate host.

Pathophysiology

Biology of the Parasite

The three infective stages of T. gondii include:

• Tachyzoite: It is the rapidly dividing and invasive form, can invade any vertebrate cell type
• Bradyzoite: These result from the conversion from tachyzoites, they are slowly diving form, and are present in the tissue cysts which can remain in the host throughout the lifetime
• Sporozoite: It is the environmental form present in the oocyts

Mechanism of cell Invasion

• The initial step of invasion is attachment of the tachyzoite to the host cell membrane. A set of protiens help in the adherence and penetration of the host cell membrane, also enhance the growth and virulence of the parasite.
• In the host cell the parasite forms a vacuole where it divides for 6 to 9 cycles after which the parasites are released into the circulation. It is an active process which is dependent on the increase in intracellular calcium stores.

Pathogenesis of Vertical Transmission

• Different modes of transmission of T.gondii to have a primary infection in a healthy mother include:
  • Ingestion of tissue cysts from raw meat and uncooked meat
  • Ingestion of food, fruits, vegetables or water contaminated by oocysts in the cat feces
  • Infection after a solid organ transplant, heart transplant patients are at the highest risk as it can harbor tissue cysts
• Once the patient has a primary infection with tachyzoites in the blood stream during pregnancy a possible transplacental infection can take place
• The tachyzoites colonize in the placenta and can cross the barrier to reach the fetus in 30% of cases leading to the disease
• The frequency of tachyzoites transfer to the fetus is related to the gestational age with lowest risk in the first trimester and highest in the third trimester
• The factors influencing the transfer of tachyzoites to the fetus is not well understood

Differentiating Toxoplasmosis from other Diseases

The most important congenital infections, which can be transmitted vertically from mother to fetus are the TORCH infections. These infections have overlapping features and hence, must be differentiated from Toxoplasmosis  :

Congenital Infection	Cardiac Findings	Skin Findings	Ocular Findings	Hepatosplenomegaly	Hydrocephalus	Microcephaly	Intracranial Calcifications	Hearing deficits
Toxoplasmosis		Petechiae Purpura Maculopapular rash	Chorioretinitis	✔	✔	✔	Diffuse intracranial calcifications
Treponema pallidum		Petechiae Purpura Maculopapular rash	Chorioretinitis Glaucoma	✔
Rubella	Patent ductus arteriosus (PDA) Pulmonary artery stenosis Coarctation of the aorta Myocarditis	Petechiae Purpura	Chorioretinitis Cataracts Glaucoma Microphthalmia	✔	✔	✔		✔
Cytomegalovirus (CMV)	✔	Petechiae Purpura	Chorioretinitis	✔		✔	Periventricular calcifications	✔
Herpes simplex virus (HSV)	Myocarditis	Petechiae Purpura Vesicles	Chorioretinitis	✔	✔	✔		✔
Parvovirus B19	Myocarditis	Petechiae Subcutaneous edema	Chorioretinitis Cataracts	✔

Epidemiology, Demographics

• It is estimated that 25 to 30% of the world's population is infected with Toxoplasma.
• Toxoplasmosis affects 500 to 5000 new borns every year.
• In United States 22.5% of the population above the of 12 is estimated to be infected by Toxoplasma and affects 1.1 million people every year.
• In United States 89% of women in the childbearing age are susceptible to have an acute infection and at risk for transmitting the parasite to the baby if the primary infection occurs during the getational period.
• In countries such as North America, Northern Europe and in Sahelian countries of Africa low seroprevalences of 10% to 30% are observed. In countries of Central and Southern Europe, tropical African countries and Latin America the seroprevalence is around 30 to 50%. This shows the variation within the countries and as well as between the countries.

Race

• The prevalence of Toxoplasmosis is higher in non-Hispanic black population and Mexican Americans than non-Hispanic white population.

Age

• A decreasing trend in prevalence is reported in the population of U.S born persons aged between 12 to 49 years; with 14% between the years 1988 to 1994, and 9% in the years 1999 to 2004. This trend is attributed to the improvement of hygienic conditions, changes in farming systems, the consumption of frozen meat, and the feeding of cats with sterilized food.
• The similar decreasing trend of seroprevalence is been reported in France and Netherlands.

Developing Countries

• In countries with poor hygienic measures and using unfiltered surface water for consumption reported higher seroprevalence rates. In these countries the childhood population is at a higher risk of acquiring the infection, the mean age is reported to be 15 years.

Risk Factors

The main risk factors for acquiring the infection is consuming raw meat and ingestion of food contaminated with Toxoplasma oocysts excreted in cat feces.
The risk factors which predispose the pregnant women for primary infection include:

• Consumption of raw oyesters and clams
• Eating undercooked meat which includes pork and lamb
• Drinking unpasteurized goat’s milk
• Exposure to kitten litter
• Working with meat
• Low socioeconomic status
• Poor Hygiene
• Drinking unfiltered water

Diagnosis

Laboratory Findings

Principles and methods used for the diagnosis of congenital toxoplasmosis:

Principle	Detection	Method	Findings supporting the diagnosis of Toxoplasmosis
Toxoplasma specific humoral responses	IgG, IgM, IgA	Dye test, ELISA, ELISA-like assays,ISAGA, immunofluorescence, agglutination	Positive IgM after 5 days of life and in the absence of blood transfusions Positive IgA after 10 days of life Persistence of Toxoplasma IgG beyond 1 year of age
	IgG, IgM, and IgA to specific Toxoplasma antigen	Western blot	Presence of specific bands only seen in the newborn or bands with higher intensity than maternal ones for IgG and/or IgM and/or IgA in a reference laboratory
Toxoplasma nucleic acid amplification	DNA	PCR	Positive result in any body fluid (e.g: amniotic fluid, cerebrospinal fluid, peripheral blood, urine)
Immunohistochemistry of Toxoplasma specific antigens in tissue	Antigens	Immunoperoxidase	Positive result in any tissue(e.g., brain or other fetal tissue)
Visualization by microscopy	Visual identification of tachyzoites and/or cysts	Stains such as hematoxylin/eosin, Giemsa	Positive identification in a reference laboratory
Isolation of Toxoplasma	Whole live parasite	Inoculation in peritoneal cavity of mice	Detection of live cysts from any body fluid or tissue that has been inoculated in mice in a reference laboratory
Brain imaging	Brain calcifications Hydrocephaly Microcephaly	Ultrasound, CT, brain MRI	Findings can be suggestive but are not diagnostic of congenital Toxoplasmosis since other etiologies may result in similar findings
Retinal exam	Inflammation in choroidal and retinal layers	Ophthalmological exam	Retinochoroidal lesions can be highly suggestive or, at times, diagnostic of congenital Toxoplasmosis

Table adopted from Laboratory Diagnosis of Congenital Toxoplasmosis

References

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