Toxoplasma gondii, Neospora caninum, and Sarcocystis spp. in brain samples of dogs with and without neurological signs in Southern Brazil (2008-2023)

Encephalitis can be associated with protozoa of the phylum Apicomplexa. Sarcocystis spp., Toxoplasma gondii, and Neospora caninum share morphological similarities that complicate differentiation in histopathology, leading to inaccurate diagnoses without complementary techniques. Clinical manifestations often associate with host immunosuppression secondary to other diseases. This study examined brain samples from dogs undergoing necropsy in Rio Grande do Sul, Brazil, from 2008 to 2023, using two groups: dogs with neurological signs caused by protozoa (G1) and dogs without neurological signs (G2). G1 aimed to verify if protozoa identified histopathologically matched molecular techniques. G2 observed the frequency of protozoa detection using PCR in dogs without neurological signs. A total of 230 samples were analyzed: 11 paraffin-embedded in G1 and 219 frozen in G2. DNA extraction used commercial kits, and all samples underwent PCR for the 18S rRNA gene. G1 samples also underwent Restriction Fragment Length Polymorphism (RFLP) analysis. Protozoal DNA was detected in six samples (6/11) from G1 with varying molecular identifications. In G2, 82 samples (82/219; 37.45%) were PCR-positive. These findings highlight the importance of molecular techniques for accurate etiological confirmation and diagnosis, extending epidemiological knowledge and emphasizing Toxoplasma gondii, Neospora. caninum, and Sarcocystis spp. as differential diagnoses for central nervous system diseases in dogs.