BACKGROUND: Horizontal transmission of Toxoplasma gondii occurs primarily via ingestion of environmental oocysts or consumption of undercooked/raw meat containing cyst-stage bradyzoites. The relative importance of these two transmission routes remains unclear. Oocyst infection can be distinguished from bradyzoite infection by identification of IgG antibodies against T. gondii-embryogenesis-related protein (TgERP). These antibodies are, however, thought to persist for only 6-8 months in human sera, limiting the use of TgERP serology to only those patients recently exposed to T. gondii. Yet recent serological survey data indicate a more sustained persistence of anti-TgERP antibodies. Elucidating the duration of anti-TgERP IgG will help to determine whether TgERP serology has epidemiological utility for quantifying the relative importance of different routes of T. gondii transmission. METHODS: We developed a sero-catalytic mathematical model to capture the change in seroprevalence of non-stage-specific IgG and anti-TgERP IgG antibodies with human age. The model was fitted to published datasets collected in an endemic region of Brazil to estimate the duration of anti-TgERP IgG antibodies, accounting for variable age-force of infection profiles and uncertainty in the diagnostic performance of TgERP serology. RESULTS: We found that anti-TgERP IgG persists for substantially longer than previously recognised, with estimates ranging from 8.3 to 41.1 years. The Brazilian datasets were consistent with oocysts being the predominant transmission route in these settings. CONCLUSIONS: The longer than previously recognised duration of anti-TgERP antibodies indicates that anti-TgERP serology could be a useful tool for delineating T. gondii transmission routes in human populations. TgERP serology may therefore be an important epidemiological tool for informing the design of tailored, setting-specific public health information campaigns and interventions.