Usefulness of serotyping tools in Toxoplasma gondii infections in sheep and pigs

Toxoplasma gondii is the causative agent of toxoplasmosis, one of the most relevant food-borne zoonotic diseases worldwide that causes important economic losses in the livestock sector related to reproductive failure, mainly in sheep and goats. To better understand the epidemiology of toxoplasmosis, predict the clinical consequences and design appropriate control strategies, it is crucial to determine the strain causing the infection. In this sense, serotyping methods offer a cost-effective, rapid, sensitive, and non-invasive alternative to DNA-dependent genotyping techniques, allowing the analysis of subclinical cases. This technique is based on the detection of strain-specific antibodies that react against segments of antigenic proteins presenting polymorphic variations. However, the prediction capability of promising peptides needs to be determined in each animal species, as differences in the strain-specific reactivity patterns have been described. In the present study we sought to evaluate the applicability in sheep and pigs of a panel of peptides previously characterized in mice and humans. To this end, we used 51 sheep serum samples obtained in previous experimental infections (32 type II and 19 type III), 20 sheep samples from naturally infected sheep where the causative strain was genotyped (18 type II and 2 type III), and 40 serum samples from experimentally infected pigs (22 type I and 18 type III). Our ELISA test results show that a combination of GRA3, 5, 6 and 7 peptide homologous pairs can discriminate infections caused by type II and III strains in sheep and pigs. Namely, the best peptide pairs to discriminate infections in these two livestock species were GRA6-I/III-213 vs. GRA6-II-214 and GRA6-II-44 vs. GRA6-III-44, together with GRA3-I/III-38 vs. GRA3-II-38 in sheep and GRA7-II-224 vs. GRA7-III-224 in pigs. Notably, the GRA6-44 peptide pair, which were previously described to be inefficient in mice and humans, showed a high prediction capacity, especially in sheep, while GRA5-38 peptide versions failed to correctly predict the strain type in most sheep and pig samples, underpinning the notion that individual standardization is needed for each animal species. These results lay the grounds for future large-scale serotyping studies in sheep and pigs, as well as its potential extrapolation to other animal species in future studies.