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Meixner, Nicola; Sommer, Marie F.; Scuda, Nelly; Matiasek, Kaspar and Müller, Matthias (2020): Comparative aspects of laboratory testing for the detection ofToxoplasma gondiiand its differentiation fromNeospora caninumas the etiologic agent of ovine abortion. In: Journal of Veterinary Diagnostic Investigation, Vol. 32, No. 6: pp. 898-907

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Histologic examination of aborted material is an essential component in the diagnosis of ovine toxoplasmosis. However, the detection ofToxoplasma gondiiin histologic sections, and its differentiation from the closely related protozoanNeospora caninum, is challenging. We developed a chromogenic in situ hybridization (ISH) assay for the identification ofT. gondiiin paraffin-embedded tissue samples. We examined retrospectively the archived placental tissue of 200 sheep abortion submissions for the presence ofT. gondiiby immunohistochemistry (IHC), ISH, and real-time PCR (rtPCR). All placental samples that tested positive forT. gondiiby rtPCR (9 of 200) were also positive by IHC, with inconclusive IHC staining in an additional 7 rtPCR-negative cases. Further testing forN. caninumof all 200 placentas by rtPCR revealed 7Neospora-positive cases.T. gondiiISH was positive in 4 of 9 IHC-positive samples and 1 of the 7N. caninumrtPCR-positive samples. Real-time PCR was used as the reference standard for specificity and sensitivity calculations regarding placenta samples. Specificity of ISH and IHC was 99% and 96-100%, respectively. The sensitivity of ISH (44%) was quite low compared to IHC (100%). The exclusive use of ISH for the detection ofT. gondii, and thus for the diagnosis of ovine toxoplasmosis, was not acceptable. However, combined with rtPCR, both ISH and IHC can be useful detection methods to improve histologic evaluation by visualizing the parasite within tissue sections.

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