Passively-acquired anti toxoplasma antibody and pseudo- seroconversion

 In a number of clinical scenarios (e.g. HIV/AIDS, organ transplantation, pregnancy), toxoplasma infection can result in severe or life-threatening complications and, where seroconversion is observed, significant clinical intervention may be required.

• In organ transplantation in particular, but also in a range of clinical complications in HIV/AIDS and pregnancy, blood transfusions or immunoglobulins may be required. Since the prevalence of toxoplasma infection in the blood donor population in the United Kingdom is likely to be 20% or more, there is a significant chance that anti-toxoplasma antibodies may be acquired as a result.
• In apparent sero-conversion in pregnancy such passively-acquired antibody can be rapidly identified in most cases due to the absence of IgM. This is important since an uninfected woman may be wrongly considered immune and hence may not receive the advice aimed at reducing the risk of infection given to women who are susceptible to toxoplasma infection during pregnancy.
• The transplacental transfer of antibody complicates the investigation of congenital infection in the neonate.

In the immunosuppressed/immunodeficient, where apparent seroconversion occurs absence of IgM is less reliable in identifying passively acquired antibody. In such cases either IgG avidity testing (passively acquired IgG should have an avidity consistent with a convalescent immune response) or repeat serology several months later (to confirm clearance of passively-acquired antibody) should be considered.