impaginato piccolo - Società Italiana di Parassitologia (SoIPa)

52
V. Meroni, F. Genco - Toxoplasmosis diagnosis in pregnancy
Once therapy has been started it is very difficult to
measure antibodies production. Tests to evaluate cellular
immunity can then be employed in reference laboratories.
A positive Stimulation Index, the expression of CD25
marker on lymphocytes, γ interferon production after
stimulation with purified Toxoplasma antigen, all confirm
the infection and are not affected by therapy
administration. (Fatoohi et al., 2003; Ciardelli et al.,
2008).
4-IgG positive IgM negative
Recent infection. IgM can persist for many months but
we need to date the time of infection.
If the infection was acquired before conception the
mother must be reassured.
If the infection occurred during pregnancy the woman
must be treated (spiramycin or pyrimethamine plus sulfadiazine
according with the trimester ) and advised for
prenatal diagnosis. Immunoenzymatic tests for titration
of specific IgA are not recommended as these antibodies
may persist for many months after the acute phase
and sometimes they are not produced after therapy.
These antibodies may be diagnostic in newborns sera
(like IgM they do not cross the placenta) and during
reactivation in immunocompromised patients.
IgG Avidity test
This test measures the affinity of specific IgG antibodies
for toxoplasmic antigens. (Hedman, et al., 1989).
The avidity value increases with time and the time
required for its maturation depends on the method
employed. This test, is very useful to better define the
timing of infection in IgG and IgM positive patients,
provided some conditions are met:
1-only sera with a sufficient amount of IgG can be analyzed
for avidity
2- the patient must not be treated at the time of testing
(any therapy modifies the kinetics of antibodies and of
avidity too) (Sensini et al., 1996; Petersen et al., 2005)
3- the test must be done in the first trimester of pregnancy.
The Avidity test can be fully or partially automated or
manual, but whatever their features, a high avidity
index excludes an acute infection in the previous 4
months. No information can be inferred from a low
avidity index: in some cases a low avidity may persist
for many months.
If acute infection is suspected, diagnosis cannot rely
upon only one test but must consider also the different
antibody kinetics and clinical evaluation.
Prenatal diagnosis
If an acute infection is diagnosed, different approaches
should be taken depending on whether the onset was
during the first part of pregnancy (before 22w -24w) or
afterwards. For late infection the patient should be
treated with pyrimetamine plus sulfadiazine and folinic
acid until 15 days before the delivery. This time period
was chosen based on the possibility of therapeutical
interruption of pregnancy and varies across countries.
In all other cases of acute infection the woman is counselled
for prenatal diagnosis.
Prenatal diagnosis consists of a PCR performed on
amniotic fluid drawn not earlier than the 18 th week of
gestation and 4-6 weeks after the presumable seroconversion.
Previous studies showed a good specificity and positive
predictive value for PCR but sensitivity resulted quite
low ( 64 %) . Many infected newborns, mainly with
maternal infection in the first trimester had a negative
PCR on amniotic fluid. (Thalib et al., 2005).
At present , a larger amount of amniotic fluid (10 ml ),
a new sequence as target (gene bank accession number
AF146527 ) that is repeated 200 -300 times instead of
B1gene (Genebank accession number AF 179871)
repeated only 20 times in toxoplasmic genome and the
use of real time PCR improved the performance of the
test. (Cassaing et al., 2006) .
In our hands a nested PCR with the high repetitive gene
target AF14 performed on 140 amniotic fluids showed
a sensitivity and specificity of 100 %, evaluated as
neonatal outcome independently from the period of
maternal infection. (Meroni, unpublished)
Conclusions
One hundred years from the discovery of T. gondii,
much progress has been made in the diagnosis of toxoplasmosis
in pregnancy, and new tests are now available
that give us more precise information on the
infection. By using all the means in our possession the
right diagnosis is possible in many cases. However,
many unsolved questions such as the efficacy of therapy,
its effect on the immune response, the strain virulence
and the host/parasite relationship await further
investigation.
References
Cassaing, S., M. H. Bessieres, et al. (2006). Comparison between
two amplification sets for molecular diagnosis of toxoplasmosis
by real-time PCR. J Clin Microbiol 44: 720-724.
Ciardelli, L., V. Meroni, et al. (2008). Early and accurate diagnosis
of congenital toxoplasmosis. Pediatr Infect Dis J 27: 125-129.
Dunn, D., M. Wallon, et al. (1999). Mother-to-child transmission of
toxoplasmosis: risk estimates for clinical counselling. Lancet
353: 1829-1833.
Fatoohi, A. F., G. J. Cozon, et al. (2003). Cellular immunity to
Toxoplasma gondii in congenitally infected newborns and
immunocompetent infected hosts. Eur J Clin Microbiol Infect
Dis 22: 181-184.
Gollub, E. L., V. Leroy, et al. (2008). Effectiveness of health education
on Toxoplasma-related knowledge, behaviour, and risk of
seroconversion in pregnancy. Eur J Obstet Gynecol Reprod Biol
136: 137-145.
Gussetti, N., R. D’Elia, et al. (1990). Natural immunoglobulin M
antibodies against Toxoplasma gondii during pregnancy. Am J
Obstet Gynecol 162: 1359-1360.
Hedman, K., M. Lappalainen, et al. (1989). Recent primary