Fig. 3

Toxoplasma serology tests in primary infection, reactivation, and in reinfection. a The transmission of Toxoplasma gondii to immunocompetent humans (intermediate host) occurs by ingestion of oocysts, normally via contaminated food or water. Infection can also occur via consumption of undercooked meat or raw meat containing tissue cysts with bradyzoites. The majority of cases (85–90%) are asymptomatic, but around 10–15% of the infected individuals develop systemic symptoms [12,13,14,15]. In either case, chronic (distant) infection can persist for the life of the hosts. If the hosts become immunodeficient or immune-suppressed, bradyzoites reactivate, which causes cerebral or ocular toxoplasmosis. If primary infection occurs during pregnancy, parasites can also infect the fetus by congenital transmission. Typical Toxoplasma serological changes of primary infection in immunocompetent patients are shown in b. The first isotype antibody to appear in the very early phase of the primary infection is IgM, followed by the appearance of IgG, which is required for confirmation of the infection. Toxoplasma IgG avidity test is a critical tool to distinguish recent infection from distant infection in pregnant women, because IgM of Toxoplasma is known to persist in many cases of distant infection [29,30,31,32] (persistent IgM is marked with * in this figure). However, congenital transmission also occurs in distant/latent infected host when the host becomes immunodeficient (a, lower left) or reinfected with different strains of Toxoplasma gondii (a: lower right). In such cases, commercial IgG avidities give high-level results even before the onset of reactivation or reinfection, because the hosts have been distantly/latently infected. Therefore, clinicians cannot interpret IgG avidity test as they do in primary infection. Clinically symptomatic phases are highlighted in yellow. Blue dot line indicates the “distant infection” profile, which consists of three infection profiles: latent infection, reactivation, and reinfection. Different strains of Toxoplasma gondii are described as oocysts-2 or tissue cysts-2. Representative changes of Toxoplasma gondii serology in reactivation and in reinfection are shown in c and d. In d, distinctive serological changes of reinfection from serum of reactivation are written in red letter. IgG, IgM, IgA, Toxoplasma gondii-specific IgG, IgM, IgA; Inf, infection; Pos, positive; Neg, negative; Pos/higher, positive or higher level; v.e, very early phase of infection. Recent infection is highlighted in pink, and distant infection is highlighted in light blue. This figure is modified from Vera Lucia Pereira-Chioccola et al. [15] and O Villard et al. [93]. Ref ^A-K, reference group A–K; Ref. A: [90, 91], Ref. B: [96], Ref. C: [74,75,76,77, 78(p), 82(p), 94, 96]; p, presumed cases; Ref. D: [91], Ref. E: [91], Ref. F: [93], Ref. G: [74, 77, 82, 94], Ref. H: [74, 76, 77], Ref. I: [74, 91, 96], Ref. J: [74, 76, 77], Ref. K: [17, 93], Ref. L: [17]