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Neospora caninum is a protozoan parasite that causes abortion and important economic losses in cattle worldwide. There are no treatments or vaccines available; disease control is based on diagnosis and herd management strategies. We developed, validated, and evaluated under field conditions a competitive inhibition ELISA based on the truncated SAG1 protein (tSAG1), expressed in Escherichia coli , and the RafNeo5 monoclonal antibody (ciELISA tSAG1 ). A criterion based on the 3-y sequential serologic analysis of 230 dairy cows by IFAT was used as the gold standard. The assay was validated using 860 serum samples from cows that were consistently positive or negative by IFAT throughout the study period. ciELISA tSAG1 was then used to evaluate the prevalence of neosporosis in 16 beef cow herds (22 samples per herd, 352 total samples). The results were compared with those from IFAT and a commercial cELISA (cELISA VMRD ). The ciELISA tSAG1 cutoff was ≥ 29%I, with a diagnostic sensitivity of 98.7% (95% CI = 96.8-99.7%) and a diagnostic specificity of 97.9% (95% CI = 96.4-99.0%). Concordance among IFAT, cELISA VMRD , and ciELISA tSAG1 was 90.3%. The agreement (κ) between ciELISA tSAG1 and the other 2 tests was ≥ 0.81. The overall prevalence of neosporosis in the 16 beef herds was 30% (range: 5-60%). The ciELISA tSAG1 could be useful for large-scale detection of anti- N. caninum antibodies in cattle and seroepidemiologic investigations, given its appropriate sensitivity and specificity, and the simplicity of production.

The aim of the present study was to characterize the specific immune response in prepubertal female calves inoculated with Neospora caninum. Forty-eight N. caninum-seronegative 6-month-old Angus female calves were randomly allocated into two groups: group A calves were inoculated subcutaneously (sc) with 1 × 10 6 tachyzoites of the low virulence NC-Argentina LP1 isolate in sterile phosphate-buffered saline (PBS); group B calves were mock inoculated sc with sterile PBS. Calves from group A developed a specific immune response characterized by the production of IgG antibodies and the expression of IFN-γ and TNF-α cytokines. Animals did not present any febrile reaction or reactions at the site of inoculation. Although chronic N. caninum infection was developed in 50% of calves of group A after inoculation, according to the presence of antibodies against rNc-SAG4, antigen characteristic of bradyzoites, N. caninum antibodies dropped below the cut-off of ELISA from day 210 post-inoculation onwards. Future trials using the same group of inoculated animals will allow the characterization of the evolution of the immune response during pregnancy and to determine whether the immunization with the local isolate is able to prevent congenital transmission and to protect against heterologous challenges.

We carried out an inter-laboratory trial to compare the serological tests commonly used for the detection of specific Neospora caninum antibodies in cattle in Ibero-American countries. A total of eight laboratories participated from the following countries: Argentina (n = 4), Brazil (n = 1), Peru (n = 1), Mexico (n = 1), and Spain (n = 1). A blind panel of well-characterized cattle sera (n = 143) and sera representative of the target population (n = 351) was tested by seven in-house indirect fluorescent antibody tests (IFATs 1-7) and three enzyme-linked immunosorbent assays (ELISAs 1-3; two in-house and one commercial). Diagnostic performance of the serological tests was calculated and compared according to the following criteria: (1) the "Pre-test information," which uses previous epidemiological and serological data; (2) the "Majority of tests," which classifies a serum as positive or negative according to the results obtained by most tests evaluated. Unexpectedly, six tests showed either sensitivity (Se) or specificity (Sp) values lower than 90%. In contrast, the best tests in terms of Se, Sp, and area under the ROC curve (AUC) values were IFAT 1 and optimized ELISA 1 and ELISA 2. We evaluated a high number of IFATs, which are the most widely used tests in Ibero-America. The significant discordances observed among the tests regardless of the criteria employed hinder control programs and urge the use of a common test or with similar performances to either the optimized IFAT 1 and ELISAs 1 and 2.