Apicomplexan parasite Toxoplasma gondii has three distinct clonal lineages: high, medium and low virulent strains, type I, II and III, respectively. T. gondii avoids the immune response by transforming from fast multiplying tachyzoite to slow multiplying bradyzoite, and establishing a chronic infection. In the present study, we isolated a new strain of T. gondii from cat feces in the Tokachi subprefecture, Hokkaido, Japan and named it as TgCatJpObi1 (Obi1) strain. Genotyping analysis of 12 loci revealed atypical characters close to type II, genotype 4 according to ToxoDB classification. Phenotypically, Obi1 strain shows slow growth rate and the ability of spontaneous cyst formation in both human foreskin fibroblast (HFFs) and mouse peritoneal macrophages in vitro without bradyzoite induction. Parasite virulence was assessed by means of mouse survival upon infection with either Obi1 or ME49 strains. Obi1 strain displayed no mortalities in comparison to type II clonal lineage, ME49 at LD₅₀ to LD₁₀₀ range (1 × 10³–10⁶ tachyzoites). Although virulence of Obi1 strain is significantly lower than that of ME49, nucleotide sequences analyses revealed that genes of virulence factors such as Gra15, Rop5, 16, 17, and 18 in Obi1 strain were 100% identical to those in the type II strain. Thus, characterization of a newly isolated strain, Obi1, is crucial to clarify the development of toxoplasmosis in both humans and animals.

顶复门寄生虫弓形虫具有三个不同的克隆谱系：高、中和低毒株，分别为 I 型、II 型和 III 型。刚地弓形虫通过从快速繁殖的速殖子转变为慢速繁殖的缓殖子并建立慢性感染来避免免疫反应。在本研究中，我们从日本北海道十胜县的猫粪便中分离出一株新的弓形虫，并将其命名为TgCatJpObi1（Obi1）菌株。根据 ToxoDB 分类，对 12 个位点的基因分型分析揭示了接近 II 型、基因型 4 的非典型性状。在表型上，Obi1 菌株在人包皮成纤维细胞 (HFF) 和小鼠腹腔巨噬细胞中均显示出缓慢的生长速度和自发囊肿形成的能力体外无缓殖子诱导。通过感染 Obi1 或 ME49 菌株后的小鼠存活率来评估寄生虫毒力。与 II 型克隆谱系 ME49 相比，Obi1 菌株在 LD ₅₀至 LD ₁₀₀范围内（1 × 10 ³ –10 ⁶速殖子）没有表现出死亡率。虽然Obi1毒株的毒力明显低于ME49，但核苷酸序列分析显示Obi1毒株中Gra15、Rop5、16、17、18等毒力因子基因与II型毒株100%相同。因此，新分离菌株 Obi1 的表征对于阐明人类和动物弓形虫病的发展至关重要。