Isolation of <i>Toxoplasma gondii</i> from the placenta of northern fur seals (<i>Callorhinus ursinus</i>) and potential transplacental transmission of the parasite

Toxoplasma gondii infects almost all warm-blooded animals, including marine mammals. Toxoplasmosis has been reported in wild and captive marine mammals in North America; however, no viable T. gondii strains have been isolated from northern fur seals. In this study, reproduction and T. gondii infection status were investigated in 10 northern fur seals (Callorhinus ursinus), from tissues collected from 2012 to 2024 in China. Toxoplasma gondii infections were determined by the modified agglutination test (MAT), PCR, immunohistochemical (IHC) staining, and isolation of the parasite by bioassay in mice. MAT was performed using placenta or tissue exudates to detect anti-T. gondii IgG antibodies. Four of the 10 seals had anti-T. gondii antibodies; Toxoplasma gondii DNA was detected by PCR in placenta tissues of two of these four animals, and T. gondii antibody positive reactions were observed in four seals by IHC. A viable T. gondii strain, TgFurSealCHn1, was isolated from placenta of one seal by bioassay in mice. In all, five seals had signs of T. gondii infection, and three of them had fetal stillbirth. One stillborn fetus had T. gondii nucleic acid detected by PCR, indicating potential vertical transmission of the parasite. Multilocus genetic typing of the TgFurSealCHn1 isolate revealed ToxoDB #5 genotype, which had demonstrated avirulence in Swiss Webster outbred mice, and the ROP18/ROP5 type was 2/2. ToxoDB #5 is the dominant genotype of wild terrestrial and marine mammals in North America. This is the first report of a viable T. gondii strain isolated from northern fur seal placenta.