Toxoplasma gondii and Neospora caninum are closely related apicomplexan parasites of veterinary and public health importance. T. gondii is a zoonotic pathogen for which cats are the definitive host, whereas N. caninum is a major cause of reproductive losses in cattle, with dogs acting as the definitive host. Data on exposure in pet animals in Thailand remain limited. This study investigated seroprevalence and associated risk factors of T. gondii and N. caninum in pet cats and dogs in Pathum Thani Province, an urban area adjacent to Bangkok. Between June 2020 and July 2021, serum samples were collected from 169 owned animals, including 86 cats and 83 dogs, participating in a mobile sterilization program. Antibodies were detected using the indirect fluorescent antibody test (IFAT), and animal characteristics, behaviors, and environmental factors were obtained via owner questionnaires. Serological evidence of exposure to both parasites was detected. Antibodies against T. gondii were detected in 4.73% (8/169) of animals, including 4.65% (4/86) of cats and 4.82% (4/83) of dogs. For N. caninum, the overall seroprevalence was 10.06% (17/169), with a higher prevalence in dogs (15.66%, 13/83) than in cats (4.65%, 4/86). No significant risk factors were identified for T. gondii or N. caninum infection in either cats or dogs (p > 0.05).

Neosporosis, caused by N. caninum, is an emerging protozoan disease responsible for significant economic losses in the global dairy and meat industries, primarily due to abortion in cattle. Dogs serve as both definitive and intermediate hosts and play a key role in the parasite transmission cycle. Currently, effective control strategies remain limited, partly due to insufficient information on infection status. In Vietnam, data on N. caninum infection are scarce and mainly limited to cattle and buffalo. In this study, an indirect enzyme-linked immunosorbent assay (iELISA) based on recombinant NcGRA4 protein was applied and evaluated for the detection of anti-N. caninum antibodies in dogs. A total of 142 shelter dogs from Hanoi, northern Vietnam, were tested to determine seroprevalence. The NcGRA4-based iELISA detected an overall seroprevalence of 28.87% (41/142), whereas the indirect fluorescent antibody test (iFAT) showed a lower prevalence of 14.08% (20/142), indicating substantial exposure to N. caninum among shelter dogs in this region. Using iFAT as the reference method, the NcGRA4-based iELISA demonstrated a sensitivity of 90.00%, a specificity of 81.15%, and an overall accuracy of 82.39%. These findings indicate that the NcGRA4-based iELISA is a suitable screening tool for seroepidemiological surveillance of N. caninum infections in dogs. Univariable and multivariable logistic regression analyses showed no significant associations between N. caninum seropositivity and the investigated variables, including age, sex, breed, and housing conditions. This study also provides the first serological evidence of canine exposure to N. caninum in Vietnam.