Fetal infection is one of the greatest fears of toxoplasmosis
and is probably the situation in which veterinarians and physicians
are most often called upon for advice by clients/patients. Unfortunately,
the time for clients/patients to ask questions and learn about
the disease should be before becoming pregnant, but too often
questions arise only after the fact. (Hence, the importance of
on-going client/patient education as part of your practice/professional
activity.) There are about 400-4,000 cases of congenital toxoplasmosis
in the U.S. yearly.
Transmission of T. gondii to the fetus generally only
occurs during an acute infection of the mother, not during latency
(see below regarding HIV mothers). Therefore, risk of congenital
infection of a fetus is restricted to those mothers who are actively
infected during pregnancy, or within 6-8 weeks immediately prior
to conception.
- Confirmation of acute infection during or immediately prior
to pregnancy is based upon detection of anti-Toxoplasma
IgM antibodies.
- However, this is not fool-proof system. A (-) IgM titer has
a 100% predictive value against recent infection, but a (+) titer
has a much lower predictive value for recent infection, because
anti-Toxoplasma IgM titers can remain elevated for 2-6
months.
- Using a combination of IgM assays improves the predictive
value for recent infection to 80%.
- Congenital infection can also be diagnosed by detection of
anti-Toxoplasma IgM, IgA or IgE antibodies in the fetus/neonate
or detection by immunoblot of specific IgG antibodies in the
newborn that are not present in the mother's serum.
- If a woman has IgG Ab to T. gondii, this is evidence
of past infection and she should not be at risk for congenital
transmission to her baby (as long as she is also not HIV [+]
or otherwise immunosuppressed).
- Women with AIDS may be able to reactivate their infection
with subsequent infection of their fetus.
Overall, estimates suggest that 50-70% of women in the U.S.
are at risk (previously uninfected) for infection with T. gondii
during pregnancy.
The chance for congenital transmission across the placenta
and the potential severity of the ensuing damage to the fetus
are related to gestational age at the time of infection:
- The greatest risk for transmission across the placenta follows
infection during the third trimester; however, the severity of
damage is greatest following first trimester infection.
- Hence the need to know how to avoid infection prior to pregnancy!
- Overall, about 1/3 of women who are newly infected during
pregnancy will transmit the organisms to their fetus.
- Antibiotic therapy during pregnancy can reduce this risk
by 60-75%.
The effects on the fetus of congenital infection include:
- spontaneous abortion
- chorioretinitis and blindness (Click here for a photograph of chorioretinitis)
- hydrocephalus and psychomotor deficits(Click here for a photograph of hydrocephalus)
- epilepsy
- mental retardation
- myelitis and paralysis
- Note: most (85-90%) congenitally-infected children will be
asymptomatic at birth, but more than 80% of these children will
subsequently develop disease (especially chorioretinitis) later
in life if they are not diagnosed and treated.
