Acute Toxoplasma infection among family members of patients with acute lymphadenopathic toxoplasmosis.

Studies were performed to determine how frequently acute infections with Toxoplasma gondii occur among family members of patients with acute acquired lymphadenopathic toxoplasmosis. In five of the nine families studied, more than one family member had serologic evidence of recent acute infection with T gondii. In three of the families, the immunoglobulin M and immunoglobulin G antibody titers to T gondii among infected family members were similar irrespective of whether lymphadenopathy was present. Lymphadenopathy developed in one family member in each of two families (families VIII and IX) three and eight months after lymphadenopathy had developed in the index case (proband). These results suggest that common-source outbreaks occur with surprising frequency among family members of patients with lymphadenopathic toxoplasmosis and that the humoral response is similar with different manifestations of the infection.