Abstract
BackgroundThe tick-borne intra-erythrocytic apicomplexan Babesia caballi is one of the etiological agents of equine babesiosis, an economically important disease of equids in most tropical and subtropical areas of the world. Discovering candidate antigens for improved diagnostic tools and vaccines remains needed for controlling equine babesiosis. This study describes the B. caballi sbp4 (Bcsbp4) gene and protein (BcSBP4) and analyzes its antigenicity in infected equids.MethodsBLAST searches of an uncurated B. caballi assembly genome using the B. bovis SBP4 as a query were carried out, followed by PCR amplification and sequencing of a newly identified BcSBP4. Characterization of this novel gene and protein was performed by bioinformatics analysis, western blots, immunofluorescence (IFA) and an in vitro neutralization test using anti SBP4 peptide antibodies. Antigenicity of recombinant BcSBP4 (rBcSBP4) was tested with sera from field animals (n = 18) using an indirect ELISA (iELISA).ResultsBabesia caballi genome searches using B. bovis SBP4 as a query allowed identification of a novel gene termed Bcsbp4. The Bcsbp4 gene encodes for a protein of 30.58 kDa, which is fully conserved among B. caballi isolates from USA and Egypt. Bioinformatics analysis indicates that BcSBP4 contains a signal peptide and lacks additional transmembrane domains. Expression of BcSBP4 in blood stages of B. caballi was confirmed by western blot and IFA using antibodies against synthetic peptides representing putative B-cell epitopes of BcSBP4 predicted by in silico analysis. In vitro neutralization tests using anti-BcSBP4 peptide antibodies showed a marginal, but statistically significant inhibitory effect on the infectivity of B. caballi merozoites in horse red blood cells. Sera from eight B. caballi-infected equids, but none out of ten negative equid control sera, gave a positive signal in an rBcSBP4 based iELISA.ConclusionsThe Bcsbp4 gene is expressed in B. caballi blood stages. The BcSBP4 protein is a potential candidate for developing a novel serological test that could detect B. caballi infection in equids in tropical and subtropical countries worldwide.
Highlights
The tick-borne intra-erythrocytic apicomplexan Babesia caballi is one of the etiological agents of equine babesiosis, an economically important disease of equids in most tropical and subtropical areas of the world
BLAST searches performed with the putative BcSBP4 showed e scores that are very significant among the B. bovis, B. bigemina, and B. ovata spherical body protein 4 (SBP4)
We found that BcSBP4 is more similar to B. bigemina (35% identity), followed by the B. ovata (34% identity) SBP4 proteins, in comparison to other known Babesia SBP4 sequences (Fig. 1)
Summary
The tick-borne intra-erythrocytic apicomplexan Babesia caballi is one of the etiological agents of equine babesiosis, an economically important disease of equids in most tropical and subtropical areas of the world. Mahmoud et al Parasites Vectors (2020) 13:369 of the world and is caused mainly by two intra-erythrocytic haemoprotozoan parasites, Theileria equi and Babesia caballi. These parasites are transmitted by ixodid tick vectors [1, 2]. Both T. equi and B. caballi parasites are responsible for severe hemolytic disease that is characterized by fever, anemia, haemoglobinuria, jaundice, edema, and occasional death of the infected equids [3], which results in great economic losses in the horse industry in endemic areas. Vaccines against B. caballi are currently unavailable, and control of the disease depends largely on accurate diagnostics and treatment with babesiacidal drugs, such as imidocarb. These treatments ameliorate the clinical signs and reduce fatalities [1], efficient control of B. caballi transmission would require sensitive and specific serological and molecular diagnostic methods
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