impaginato piccolo - Società Italiana di Parassitologia (SoIPa)

Parassitologia 50: 51-53, 2008
Toxoplasmosis in pregnancy: evaluation of diagnostic
methods
V. Meroni, F. Genco
Infectious Diseases Department University of Pavia, Foundation IRCCS Policlinico San Matteo Pavia
Introduction
Abstract. Toxoplasmosis in pregnancy is usually subclinic or associated with non specific symptoms.
Diagnosis and timing of infection are usually based on serological tests. In this short review we tried to summarize
the serological patterns we can encounter and to discuss the interpretation of test results
Toxoplasma gondii is an obligate intracellular protozoan
that infects almost a third of the world’s population.
Primary infection with T. gondii in pregnant
women may result in congenital toxoplamosis via
transplacental transmission. The extent of damage
depends mainly on when the mother gets infected, with
consequences being more severe during the early phases
of gestation. However, transmission is more frequent
during late pregnancy (Dunn et al., 1999).
As parasitaemia lasts only few days and infection is
often asymptomatic, the diagnosis relies mainly on
serology (Montoya and Liesenfeld, 2004).
Diagnosis of toxoplasmosis in pregnancy has two goals:
to evaluate the immune status of the woman, and in
case of acute infection, to date the time of infection. It
is therefore advisable to perform serological tests
before or at the beginning of pregnancy.
Serological Tests
Key words: toxoplasmosis in pregnancy, serological tests, prenatal diagnosis
Given the kinetics of antibody production, the screening
tests for toxoplasmosis are based on measurement
of specific anti-Toxoplasma IgG and IgM antibodies
with automated tests, which usually have a good sensitivity
and specificity.
Anti–Toxoplasma IgG antibodies are produced
throughout life after infection. Detection in a single
sample at any titre with any test is a marker of previous
infection.
IgM antibodies are detected in recently acquired infections
but may persist for more than a year. Furthermore
false positive, aspecific reaction may be recorded in
IgG negative patients.
Four different serological patterns may be seen
(Montoya and Liesenfeld, 2004; Sensini 2006).
Correspondence: Valeria Meroni
Infectious Diseases Department University of Pavia,
Foundation IRCCS Policlinico San Matteo Pavia,
e-mail: v.meroni@smatteo.pv.it
1-IgG negative IgM negative
No immunity. Hygienic alimentary prophylaxis and
monthly follow up (if possible until one month after
delivery) in order to avoid seroconversion are advised.
Indeed there is some evidence that health education
may reduce the risk of seroconversion and consequently
of congenital toxoplasmosis. (Gollub et al., 2008).
In Italy, the screening test in pregnancy is not mandatory
but National Health System pays for a preconceptional
test and for the follow-up of negative women
(DPR245 10/09/98) so most of pregnant women
undergo the monthly controls.
2-IgG positive IgM negative.
Previous immunity, if the tests have been done before
or at the beginning of pregnancy.
No further sampling including further pregnancies is
required.
In the third trimester, a negative IgM test cannot
exclude an infection in the first trimester; in these cases
it is necessary to perform other tests such as IgG avidity
and further testing one month later to evaluate serological
stability.
3-IgG negative IgM positive.
Early seroconversion or false positive result (Gussetti
et al., 1990).
In any seroconversion, IgG must must be produced, so
it is mandatory to repeat a weekly sampling to detect
IgG. If the patient has been treated, however, IgG production
might be delayed and decreased. If this is the
case, it is preferable to employ different tests (IgG-IgM
Immunoblot, cellular immunity tests ) to obtain a diagnosis
as early as possible.
If a seroconversion has been proved, by using additional
tests ,the clinician should prescribe the correct therapy.
This also allows to advice the woman for prenatal
diagnosis.
If seroconversion is not confirmed ,treatment can be
safely discontinued..
Immunoblot for IgG and IgM is a very specific test
when purified antigens (not commercially available)
are employed.
The presence of 3 bands for IgG and two bands for IgM
against 30-40 kD proteins can confirm seroconversion
earlier than any other test (Sharma et al., 1983).