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Keep an eye out for toxoplasmosis
Timing determines severity
by Stephanie Goyette and Brian J.
Ward, MD
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CASE
PRESENTATION
Mrs. M. is 31 years old. She's pregnant
for the first time and comes to your clinic for her routine
prenatal exam. She mentions that she adopted a stray kitten
that appeared at her door 4 weeks ago. A concerned friend
warned her that this might be dangerous for the baby. Her
pregnancy has progressed normally up to this point, but because
of the risk for toxoplasmosis, she's seeking serologic testing
and medical advice.
Physical examination
- 24 weeks' gestation
- normal, afebrile, no lymphadenopathy
Investigations
- Toxoplasma gondii serology:
immunoglobulin IgM+, IgG-
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Mrs. M. is diagnosed with acute toxoplasmosis. Her doctor provides
counselling for her and her husband. She's prescribed spiramycin
to prevent further transmission to the fetus. An amniocentesis is
scheduled to determine whether or not the baby has been infected.
Making the case
Toxoplasmosis is caused by the protozoan Toxoplasma gondii.
It's one of the world's most common parasitic infections. On average,
one-third of Canadians carry this bug, and in some regions of Europe
and South America, prevalence rates can reach as high as 90%. Fortunately,
in healthy adults, the parasite is rapidly controlled by the host
immune system, and most infected individuals experience only subclinical
or mild disease. The protozoan forms tissue cysts throughout the
body, however, which can remain latent for years.
Fetus at risk
The parasite can be transmitted to the developing fetus when infection
is acquired during pregnancy. In utero complications include hydrocephalus,
intracranial calcification, retinochoroiditis and psychomotor retardation.
Stillbirth or spontaneous abortion may also result.
It's worth mentioning that there's an inverse relationship between
the rate and timing of transmission to the fetus and the severity
of disease manifestations. For instance, transmission across the
placenta is rare if the mother is infected shortly before or in
the few weeks following conception, but the fetus has a much greater
risk of developing serious complications. Conversely, the parasite
has a higher chance of crossing the placenta when acquired later
in pregnancy, but complications are much less common. Infants infected
during the third trimester are typically born without any signs
or symptoms. These children are at high risk, though, for developing
complications later in life and should be treated and monitored
closely. Women who are already infected with T. gondii before
they become pregnant rarely, if ever, transmit the parasite to the
fetus.
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