[Congenital toxoplasmosis. Transitory negative serology]

Toxoplasmosis serology may become temporarily negative in children with congenital toxoplasmosis, leading to a risk of misdiagnosis and inadequate surveillance. The purpose of our work was to better understand the time course of toxoplasmosis serology which has not been studied specifically and to propose practical recommendations.

We conducted a prospective study in 217 children born with congenital toxoplasmosis between January 1988 and December 1997. Clinical, ophthalmological and serology data were collected every three months during their first year of life then every six months until three years of age and every year thereafter for all patients. Negative serology was defined as the absence of IgG at indirect immunofluorescence and ELISA (enzyme linked immunosorbent assay) and by the absence of IgM at ISAGA (immunosorbent agglutination assay).

During the mean follow-up of 66 +/- 33 months (range 12-126 months), 33 children (15%) presented a period where the toxoplasmosis serology (ELISA and indirect immunofluorescence) was negative for a transient period reaching a mean 5 months. The dye test was performed in 25 of these children and was negative in 6 (24%). Among the negative conversions observed at routine testing, 73% occurred in children taking pyrimethamine/sulfadoxin therapy and the others occurred a mean 11.7 months after interruption of treatment. There was a positive association between maternal treatment and transient seronegativity in the cases where the maternal contamination had occurred during the first 2 trimesters of pregnancy. The serology became positive again in 30 of the 33 children (91%) and in 22 children there was a rebound. At last follow-up, the 3 other children still had negative serology (mean duration 35 months, range 3-62 months).

Transient negative toxoplasmosis serology is a frequent phenomenon in children with congenital toxoplasmosis. Although the underlying pathophysiological mechanism remains unknown, it is crucial to avoid questioning the initial diagnosis of congenital toxoplasmosis and to continue regular routine monitoring.