Which of the following is indicative of a recent infection with Toxoplasma gondii?

• A. Low Avidity Anti-Toxoplasma IgG
• B. Anti-Toxoplasma IgM
• C. High Avidity Anti-Toxoplasma IgG
• D. Anti-Toxoplasma IgE

Answer: (A)

The timing of a Toxoplasma gondii infection is best inferred by IgG avidity. When a person is newly infected, the IgG antibodies produced have low binding strength to the parasite—low avidity. As the immune response matures over weeks to months, these IgG antibodies undergo affinity maturation and their avidity increases. Therefore, a low avidity anti-Toxoplasma IgG result points to a recent infection, while high avidity IgG suggests the infection occurred longer ago.

IgM can appear early and may persist for months, but it isn’t a perfectly reliable sole indicator of acute infection, since it can linger or reappear in some cases. Anti-Toxoplasma IgE is not a standard marker used to determine infection timing. Hence, low avidity IgG is the best indicator among these options for a recent infection.