Genetic diversity and antigenicity variation of Babesia bovis merozoite surface antigen-1 (MSA-1) in Thailand

Genetic Diversity of Merozoite Surface Antigens in Global Babesia bovis Populations

Cattle can be severely infected with the tick-borne protozoa Babesia bovis, giving rise to serious economic losses. Invasion of the host's RBCs by the parasite merozoite/sporozoites depends largely on the MSA (merozoite surface antigens) gene family, which comprises various fragments, e.g., MSA-1, MSA-2a1, MSA-2a2, MSA-2b and MSA-2c, highlighting the importance of these antigens as vaccine candidates. However, experimental trials documented the failure of some developed MSA-based vaccines to fully protect animals from B. bovis infection. One reason for this failure may be related to the genetic structure of the parasite. In the present study, all MSA-sequenced B. bovis isolates on the GenBank were collected and subjected to various analyses to evaluate their genetic diversity and population structure. The analyses were conducted on 199 MSA-1, 24 MSA-2a1, 193 MSA-2b and 148 MSA-2c isolates from geographically diverse regions. All these fragments displayed high nucleotide and haplotype diversities, but the MSA-1 was the most hypervariable and had the lowest inter- and intra-population gene flow values. This fragment also displayed a strong positive selection when testing its isolates for the natural selection, which suggests the potential occurrence of more genetic variations. On the contrary, the MSA-2c was the most conserved in comparison to the other fragments, and displayed the highest inter- and intra-population gene flow values, which was evidenced by a significantly negative selection and negative neutrality indices (Fu's Fs and Tajima's D). The majority of the MSA-2c tested isolates had two conserved amino acid repeats, and earlier reports have found these repeats to be highly immunogenic, which underlines the importance of this fragment in developing vaccines against B. bovis. Results of the MSA-2a1 analyses were also promising, but many more MSA-2a1 sequenced isolates are required to validating this assumption. The genetic analyses conducted for the MSA-2b fragment displayed borderline values when compared to the other fragments.

Molecular characterization and genetic diversity of Babesia bovis and Babesia bigemina of cattle in Thailand

Babesia bovis and B. bigemina are the most common tick-borne parasites that cause bovine babesiosis which effects livestock production, leading to economic losses in tropical and subtropical areas of the world. The aims of this study were to determine the molecular detection, genetic diversity and antigenicity prediction of B. bovis based on spherical body protein 2 (sbp-2) gene and B. bigemina based on rhoptry-associated protein 1a (rap-1a) gene in cattle in Thailand. By PCR assay, the molecular detection of B. bovis and B. bigemina infection revealed levels of 2.58% (4/155) and 5.80% (9/155), respectively. The phylograms showed that B. bovis sbp-2 and B. bigemina rap-1a sequences displayed 5 and 3 clades with similarity ranging between 85.53 to 100% and 98.28 to 100%, respectively, when compared within Thailand strain. Diversity analysis of sbp-2 and rap-1a sequences showed 18 and 4 haplotypes, respectively. The entropy analysis illustrated 104 and 7 polymorphic sites of sbp-2 and rap-1a nucleic acid sequences, respectively, while those of sbp-2 and rap-1a amino acid sequences showed 46 and 4 high entropy peaks, respectively. Motifs analysis exhibited the distribution and conservation among sbp-2 and rap-1a sequences. The continuous and discontinuous B-cell epitopes have also been evaluated in this work. Therefore, our findings may be used to ameliorate the understanding inputs of molecular phylogeny, genetic diversity and antigenicity of B. bovis and B. bigemina Thailand stains.

Erythrocyte Adhesion of Merozoite Surface Antigen 2c1 Expressed During Extracellular Stages of Babesia orientalis

Babesia orientalis, a major infectious agent of water buffalo hemolytic babesiosis, is transmitted by Rhipicephalus haemaphysaloides. However, no effective vaccine is available. Essential antigens that are involved in parasite invasion of host red blood cells (RBCs) are potential vaccine candidates. Therefore, the identification and the conduction of functional studies of essential antigens are highly desirable. Here, we evaluated the function of B. orientalis merozoite surface antigen 2c1 (BoMSA-2c1), which belongs to the variable merozoite surface antigen (VMSA) family in B. orientalis. We developed a polyclonal antiserum against the purified recombinant (r)BoMSA-2c1 protein. Immunofluorescence staining results showed that BoMSA-2c1 was expressed only on extracellular merozoites, whereas the antigen was undetectable in intracellular parasites. RBC binding assays suggested that BoMSA-2c1 specifically bound to buffalo erythrocytes. Cytoadherence assays using a eukaryotic expression system in vitro further verified the binding and inhibitory ability of BoMSA-2c1. We found that BoMSA-2c1 with a GPI domain was expressed on the surface of HEK293T cells that bound to water buffalo RBCs, and that the anti-rBoMSA2c1 antibody inhibited this binding. These results indicated that BoMSA-2c1 was involved in mediating initial binding to host erythrocytes of B. orientalis. Identification of the occurrence of binding early in the invasion process may facilitate understanding of the growth characteristics, and may help in formulating strategies for the prevention and control of this parasite.

Genetic Diversity of Babesia bovis MSA-1, MSA-2b and MSA-2c in China

The apicomplexan parasite Babesia bovis is a tick-borne intracellular hemoprotozoan parasite that is widespread across China. Genetic diversity is an important strategy used by parasites to escape the immune responses of their hosts. In our present study, 575 blood samples, collected from cattle in 10 provinces, were initially screened using a nested PCR (polymerase chain reaction) for detection of B. bovis infection. To perform genetic diversity analyses, positive samples were further amplified to obtain sequences of three B. bovis merozoite surface antigen genes (MSA-1, MSA-2b, MSA-2c). The results of the nested PCR approach showed that an average of 8.9% (51/575) of cattle were positive for B. bovis infection. Phylogenetic analyses of the predicted amino acid sequences revealed that unique antigen variants were formed only by Chinese isolates. Our findings provide vital information for understanding the genetic diversity of B. bovis in China.

Use of molecular markers can help to understand the genetic diversity of Babesia bovis

Cattle babesiosis is a tick-borne disease responsible for significant losses for the livestock industries in tropical areas of the world. These piroplasms are under constant control of the host immune system, which lead to a strong selective pressure for arising more virulent or attenuated phenotypes. Aiming to better understand the most critical genetic modifications in Babesia bovis genome, related to virulence, an in silico analysis was performed using DNA sequences from GenBank. Fourteen genes (sbp-2, sbp-4, trap, msa-1, msa-2b, msa-2c, Bv80 (or Bb-1), 18S rRNA, acs-1, ama-1, β-tub, cp-2, p0, rap-1a) related to parasite infection and immunogenicity and ITS region were selected for alignment and comparison of several isolates of Babesia bovis from different geographic regions around the world. Among the 15 genes selected for the study of diversity, only 7 genes (sbp-2, sbp-4, trap, msa-1, msa-2b, msa-2c, Bv80) and the ITS region presented sufficient genetic variation for the studies of phylogeny. Despite this genetic diversity observed into groups, there was not sufficient information available to associate molecular markers with virulence of isolates. However, some genetic groups no were correlated with geographic region what could indicate some typical evolutionary characteristics in the relation between parasite-host. Further studies using these genes in herds presenting diverse clinical conditions are required. The better understanding of evolutionary mechanisms of the parasite may contribute to improve prophylactic and therapeutic measures. In this way, we suggest that genes used in our study are potential markers of virulence and attenuation and have to be analyzed with the use of sequences from animals that present clinical signs of babesiosis and asymptomatic carriers.

Diagnostic Tools for the Identification of Babesia sp. in Persistently Infected Cattle

Bovine babesiosis is a tick-borne disease of cattle caused by the protozoan parasites of the genus Babesia. Babesia bovis, Babesia bigemina and Babesia divergens are considered by International health authorities (OIE) as the principal species of Babesia that cause bovine babesiosis. Animals that recover from a babesial primo infection may remain as persistent carriers with no clinical signs of disease and can be the source of infection for ticks that are able to acquire Babesia parasites from infected cattle and to transmit Babesia parasites to susceptible cattle. Several procedures that have been developed for parasite detection and diagnosis of this infectious carrier state constitute the basis for this review: A brief description of the direct microscopic detection of Babesia-infected erytrocytes; PCR-based diagnostic assays, which are very sensitive particularly in detecting Babesia in carrier cattle; in-vitro culture methods, used to demonstrate presence of carrier infections of Babesia sp.; animal inoculation, particularly for B. divergens isolation are discussed. Alternatively, persistently infected animals can be tested for specific antibabesial antibodies by using indirect serological assays. Serological procedures are not necessarily consistent in identifying persistently infected animals and have the disadvantage of presenting with cross reactions between antibodies to Babesia sp.

Variable and Variant Protein Multigene Families in Babesia bovis Persistence

Cattle infected with Babesia bovis face a bifurcated fate: Either die of the severe acute infection, or survive and carry for many years a highly persistent but generally asymptomatic infection. In this review, the author describes known and potential contributions of three variable or highly variant multigene-encoded families of proteins to persistence in the bovine host, and the mechanisms by which variability arises among these families. Ramifications arising from this variability are discussed.

Genetic diversity of Babesia bovis in beef cattle in a large wetland in Brazil

Babesia bovis is the etiological agent of bovine babesiosis, a disease transmitted by Rhipicephalus microplus, which affects cattle herds in tropical and subtropical regions of the world, causing significant economic losses due to decreasing meat and milk yield. This study used molecular techniques to determine the occurrence and genetic diversity of B. bovis, based on the genes encoding the spherical body protein (sbp-2) and the merozoite surface antigens (MSAs) genes, in a herd of 400 Nellore (Bos indicus) sampled from beef cattle farms in the Pantanal region, state of Mato Grosso do Sul, Midwestern Brazil. The results of the nested PCR assays based on the sbp-2 gene indicated that 18 (4.5%) calves were positive for B. bovis; out of them, while 77.7% (14/18) were positive for the B. bovis msa-2b fragment, 66.6% (12/18) were positive for the msa-2c fragment. The phylogenetic analysis based on the maximum likelihood method using 14 sequences from msa-2b clones and 13 sequences from msa-2c clones indicated that the sequences detected in this study are clearly distributed in different cladograms. These findings corroborated the diversity analysis of the same sequences, which revealed the presence of 14 and 11 haplotypes of the msa-2b and msa-2c genes, respectively. Furthermore, the entropy analyses of amino acid sequences revealed 78 and 44 high entropy peaks with values ranging from 0.25 to 1.53 and from 0.27 to 1.09 for MSA-2B and MSA-2C, respectively. Therefore, the results indicate a low molecular occurrence of B. bovis in beef cattle sampled in the Brazilian Pantanal. Despite this, a high degree of genetic diversity was found in the analyzed B. bovis population, with possibly different haplotypes coexisting in the same animal and/or in the same studied herd.

Serological and molecular surveys of Babesia bovis and Babesia bigemina among native cattle and cattle imported from Thailand in Hue, Vietnam

Serum and DNA from blood samples collected from Vietnamese yellow cattle (n=101) and cattle imported from Thailand (n=54) at a Vietnamese slaughter house were screened for Babesia bovis and Babesia bigemina infections by enzyme-linked immunosorbent assay (ELISA) and PCR. The positive rates determined by ELISA (B. bovis and B. bigemina) or PCR (B. bigemina) in the Vietnamese cattle were significantly higher than those found in Thai cattle. Some PCR-positive Vietnamese animals were ELISA-negative, whereas all PCR-positive Thai cattle were ELISA-positive, suggesting that the animals were infected in Thailand. Importing Babesia-infected cattle may lead to the introduction of new parasite strains, possibly compromising the development of anti-Babesia immune control strategies in Vietnam.

Low levels of genetic diversity associated with evidence of negative selection on the Babesia bovis apical membrane antigen 1 from parasite populations in Thailand

Babesia bovis, a parasite infecting cattle and buffalo, continues to spread throughout the developing world. The babesial vaccine was developed to be a sustainable alternative treatment to control the parasite. However, genetic diversity is a major obstacle for designing and developing a safe and effective vaccine. The apical membrane antigen 1 (AMA-1) is considered to be a potential vaccine candidate antigen among immunogenic genes of B. bovis. To gain a more comprehensive understanding of B. bovis AMA-1 (BbAMA-1), three B. bovis DNA samples were randomly selected to characterize in order to explore genetic diversity and natural selection and to predict the antigen epitopes. The sequence analysis revealed that BbAMA-1 has a low level of polymorphism and is highly conserved (95.46-99.94%) among Thai and global isolates. The majority of the polymorphic sites were observed in domains I and III. Conversely, domain II contained no polymorphic sites. We report the first evidence of strong negative or purifying selection across the full length of the gene, especially in domain I, by demonstrating a significant excess of the average number of synonymous (dS) over the non-synonymous (dN) substitutions. Finally, we also predict the linear and conformational B-cell epitope. The predicted B-cell epitopes appeared to be involved with the amino acid changes. Collectively, the results suggest that the conserved BbAMA-1 may be used to detect regional differences in the B. bovis parasite. Importantly, the limitation of BbAMA-1 diversity under strong negative selection indicates strong functional constraints on this gene. Thus, the gene could be a valuable target vaccine candidate antigen.

A longitudinal study of Babesia and Theileria infections in cattle in Sri Lanka

Throughout the world, infections with the Babesia and Theileria parasites often result in economically significant clinical disease in cattle. We conducted a longitudinal survey of Babesia and Theileria infections in cattle from the Polonnaruwa (n=75; dry zone) and Nuwara Eliya (n=161; wet zone) districts of Sri Lanka. DNA from blood samples collected in June, September, and December 2014 and March 2015 was screened for Babesia bovis, Babesia bigemina, Theileria annulata and Theileria orientalis using specific polymerase chain reactions (PCRs). Additionally, serum samples collected from the animals were screened using enzyme-linked immunosorbent assays (ELISAs) to detect B. bovis- and B. bigemina-specific antibodies. All of the animals surveyed in Polonnaruwa and 150 (93.2%) of the animals surveyed in Nuwara Eliya were PCR-positive for Babesia and/or Theileria at least once during the study period. A greater percentage of the cattle in Polonnaruwa were positive for T. annulata and T. orientalis than B. bovis or B. bigemina at all time points. T. orientalis was the most common infection in Nuwara Eliya. Additionally, more cattle were seropositive for B. bigemina than B. bovis in both districts. Although significant variations were sometimes observed in the rates of animals that were positive for B. bigemina, T. annulata, and T. orientalis at the different sampling time points, the rates of new infections with these parasites (by PCR or ELISA) on second, third, and fourth time points among the parasite-negative samples at the first, second, and third time points, respectively, did not differ between the sampling in either district-suggesting that the parasite species infected cattle at a constant rate in these locations. However, in Polonnaruwa, the rates of new infection with T. annulata were higher than the rates of new infection with T. orientalis. The rates were also higher than those in Nuwara Eliya. In Nuwara Eliya, the rates of new infection with T. orientalis were higher than the rates of new infection with T. annulata. The rates were also higher than those in T. orientalis in Polonnaruwa. These differences might be due to variations in the density and activity of the specific tick vectors within and between the districts. Our findings suggest the need for year-round control measures against bovine Babesia and Theileria infection in Sri Lanka. Further studies to determine the densities of the vector tick species in the different geographical areas of the country are warranted.