Experimental congenital toxoplasmosis. III. Toxoplasmosis in the offspring of mice infected by the vaginal route. Incidence and manifestations of the disease.

A study has been made of congenital toxoplasmosis in the offspring of mice infected with Toxoplasma by the vaginal route during pregnancy. Some of the young mice were retarded in postnatal development, and some became ill or died in the 2nd to 4th weeks of life while the majority remained symptom-free in spite of the presence of toxoplasmic lesions of varying degrees of severity. Congenital toxoplasmosis developed only in offspring whose mothers had been infected on the 7th to 9th day of pregnancy. Infection of the offspring without active toxoplasmosis in the mother was not observed. The highest incidence of congenital infection (57.6 per cent) was obtained by giving 2 vaginal instillations of Toxoplasma-infected mouse brain on the 8th and 9th days of pregnancy. Mice infected before the 7th day developed placental toxoplasmosis but rarely delivered viable young. When the mother was infected after the 9th day, the offspring were normal. When congenital toxoplasmosis occurred in a litter, a majority or all of the individual offspring were usually infected. Although pathologic changes were not present in the suckling mice at birth, and did not appear before the 9th postnatal day, reasons are stated for excluding the possibility of postnatal contact or milk-borne infection. It cannot be assumed from the experimental disease that the vagina is a portal of entry of Toxoplasma in human congenital toxoplasmosis. Any route of infection leading to a maternal parasitemia during pregnancy might result in toxoplasmosis of the placenta and transmission of the disease to the offspring before birth. Unlike the restricted time interval effective in the mouse, there is a long period during the later months of pregnancy in the human being in which transplacental passage of the infection may occur. When transmission to the fetus takes place shortly before parturition, evidence of disease in the human infant, as in the mouse, may not become manifest until several weeks postpartum, and the prenatal origin of the infection may not be apparent. When the fetus becomes infected well before parturition, symptoms of congenital toxoplasmosis may be present at birth. The asymptomatic character of the infection in many of the young mice would appear to have a counterpart in certain instances of human congenital toxoplasmosis.