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Viral pathogens

An embryo can be infected with organisms via the placenta. In this case, maternal blood plays an intermediary role (maternal viremia). In addition, the embryo can also be infected via the amniotic cavity as the result of a vaginal infection of the mother (vertical infection).

  • Rubella virus:
    The rubella virus (that causes German measles) is a typical example of a teratogenic pathogen.
    When the mother is infected the virus can pass through the placental barrier thereby infecting the embryo or the fetus. It is thus very important to vaccinate women during childbearing years.
    During the first trimester the danger of anomalies due to infection in the first month amounts to roughly 50%, but decreases in the second month to 25% and in the third month to 15%. Symptoms of this form of embryopathy include cardiac defects, cataracts and deafness. In addition microcephalia, mental deficiency, chorioretinitis, glaucoma, microphtalmia and dental abnormalities are also diagnosed.
    In the 2nd and 3rd trimesters the risk for the appearance of fetal abnormalities are smaller (roughly 10%).

  • Cytomegalovirus:
    An infection with the cytomegalovirus (HHV-5, human herpes virus) is the most frequently occurring one during the fetal period and affects roughly 3% of pregnant women.
    One assumes that during the embryonic period this infection is lethal and leads to a spontaneous miscarriage in the first trimester. Children that are infected in the early part of the fetal period are asymptomatic and are detected thanks to special diagnostic techniques.
    From the 2nd trimester an infection with the virus leads to the following disease pictures: retarded growth, changes in the CNS (microcephalia, cerebral atrophy, hydrocephalia, cerebellary hypoplasia, chorioretinitis, atrophy of the eyes) and hepatosplenomegalia.

  • Herpes simplex:
    As a rule, an infection by the herpes simplex virus (HSV) occurs only in the late phase of the pregnancy. A fetal infection leads to mental deficiency, microcephalia, myocardiopathy, spasticity, retinal dysplasis and characteristic dermal wounds.
    Often the baby gets infected during birth due to a genital herpes infection of the mother.
    Around 50% of the children of infected mothers get infected during the birth process and half of them die from it. Delivery via caesarian section can prevent this.

  • Varicella virus:
    The varicella virus is responsible for congenital abnormalities that appear in the course of the first four months. To these belong scarring, muscle atrophy, hypoplasia of the limbs and fingers, abnormalities of the eyes and the brain (mental deficiency).
    The teratogenic risk has been established only up to the 20th week.

  • HIV (Human Immunodeficiency Virus):
    The HIV is responsible for the acquired immunodeficiency syndrome (AIDS). In the past few years, HIV infection of pregnant women has grown into a huge problem (worldwide 33.4 million people now carry the virus).
    When the mother is seropositive, a third of the children that she gives birth to become infected.
    The infection of the child occurs in utero in 1/3 of the cases. In 2/3 of the cases the infection occurs during the delivery and one supposes that it occurs via the feto-maternal blood exchange shortly or during the delivery or via contact with cervico-vaginal secretions and maternal blood during the passage through her genital apparatus. A caesarian section and an antiviral treatment are recognized measures for reducing the risk of infection. The congenital anomalies that occur due to an in utero infection can be retarded growth, microcephalia and mental deficiency.
Fig. 20 - Vulnerable phases for a few infectious and toxic agents

  1. Embryonic period
  2. Fetal period

Fig. 20

Over the course of the embryonic period (A) the threshold of vulnerability to teratogenic agents in comparison to that during the fetal period (B) are highly increased. The colored bars show the critical phases for various teratogenic agents.