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Background: Cognitive deficits are present in a large majority of Bipolar Disorder (BD) patients and known to be a marker of bad prognosis. Because, these deficits encompass several domains and no specific medical treatment seems to be effective, it is important to better understand the mechanisms underlying cognitive deterioration. As Toxoplasma gondii is known to induce the synthesis of pro-inflammatory cytokines such as IL-6, we will explore here the possible role of T. gondii in the cognitive decline observed in BD. Methods: 42 euthymic BD patients and 36 controls were assessed for episodic verbal memory using the CVLT and for working memory and verbal ability using the WAIS Ill. Patients and controls were also screened for seropositivity to I gondii and evaluated for the levels of IL-fl transcripts. Results: The seropositivity for I gondii was significantly higher in BD patients as compared to controls (p-0.005). The cognitive deterioration index (Dl) was higher in BD patients (p=5.10 ') and correlated to high IL-6 mRNA expression only among those infected by T gondii (rho-0.43, p-0.01). Among deteriorated patients (defined by scores above 0.10 according to Weschler's definition), the IL-6 mRNA expression was twice greater (p=0.01). Limitations: Our results are to be interpreted with caution because of our small sample size and the cross-sectional design. Conclusions: A long-term exposure to inflammation, measured here with IL-6 mRNA expression in T. gonciii infected BD may alter cognitive functioning. IL-6 could thus be a useful predictive marker of cognitive deterioration in BD and may help to design personalized treatment.