Toxoplasma gondii Infection: From Environmental Transmission and Host–parasite Interactions to Clinical Disease, Diagnosis, and Translational Opportunities

Abstract

Toxoplasma gondii is an obligate intracellular apicomplexan that links environmental contamination, food-animal production, wildlife ecology, maternal-fetal health, and opportunistic disease. The outcome of infection is determined by interactions among parasite developmental stage and genotype, host immune competence, tissue context, route and magnitude of exposure, and the parasite’s ability to reprogramme host-cell biology. This review integrates environmental transmission with molecular pathogenesis, immunity, persistence, clinical disease, diagnosis, and translational opportunities. Felids shed environmentally resistant oocysts that contaminate soil, water, and food, whereas tissue cysts maintain foodborne transmission. After invasion, tachyzoites form a non-fusogenic parasitophorous vacuole and inject rhoptry and dense-granule effectors that modulate host transcription, interferon-mediated restriction, cell death, migration, and nutrient acquisition. While IL-12/IFN-γ-driven cellular responses define protective immunity, differences between murine and human cell-autonomous immunity limit direct extrapolation from experimental models. Maintenance of chronic infection is biologically active rather than inert and involves persistence and reactivation through the BFD1-BFD2 differentiation circuit (Waldman et al., 2020; Licon et al., 2023), tissue-cyst architecture (Blader et al., 2015; Sullivan & Jeffers, 2012; Weiss & Kim, 2013; Eberhard et al., 2025), metabolic remodelling (Wang et al., 2026), and ongoing immune surveillance (Matta et al., 2021; Eberhard et al., 2025). Clinically, disease ranges from asymptomatic infection to congenital disease, ocular toxoplasmosis, encephalitis, and disseminated disease in immunocompromised hosts. Serology remains the main diagnostic tool, but determining when infection occurred is difficult because IgM may persist and assays vary. Molecular assays provide direct evidence of parasite DNA, but their performance is highly dependent on specimen type, parasite load, extraction procedure, and target. Current therapies effectively suppress replicating tachyzoites but do not reliably clear tissue cysts, which remains a major translational bottleneck in chronic-stage biology. Standardised diagnostics, human-relevant experimental systems, stage-specific therapeutics, multistage vaccines, integrated parasite-host omics, and One Health surveillance linking human cases with livestock, felids, wildlife, food, water, and environmental contamination will be required for future advances.