Validation of an Attenuated Live Vaccine Against Babesiosis in Native Cattle in an Endemic Area

A Culture-Adapted Strain of Babesia bovis Has Reduced Subpopulation Complexity and Is Unable to Complete Its Natural Life Cycle in Ticks

Babesia bovis natural field strains are composed of several geno-phenotypically distinct subpopulations. This feature, together with possible epigenetic modifications, may facilitate adaptation to variable environmental conditions. In this study we compare geno-phenotypical features among long-term (more than 12 years) (LTCP) and short-term cultured B. bovis parasites (STCP) derived from the B. bovis S74-T3Bo strain. LTCPs intraerythrocytic forms are smaller in size than STCPs and have faster in vitro growth rate. In contrast to its parental strain, the LTCP lack expression of the sexual stage specific 6cysA and 6cysB proteins and are unable to develop sexual forms upon in vitro sexual stage induction. Consistently, in contrast to its parental strain, LTCPs have reduced virulence and are not transmissible to cattle by vector competent Rhipicephalus microplus (R. microplus). Similar to previous comparisons among attenuated and virulent B. bovis strains, the LTCP line has decreased genomic diversity compared to the STCP line. Thus, LTCP may contribute to our understanding of adaptive mechanisms used by the parasites in response to environmental changes, protective immunity, virulence, and transmission by ticks. In addition, LTCPs may be considered as candidates for a non-tick transmissible vaccine against bovine babesiosis.

Comparative Study of Indirect Fluorescent Antibody, ELISA, and Immunochromatography Tests for Serological Diagnosis of Bovine Babesiosis Caused by Babesia bovis

Simple Summary

Currently serological diagnosis of bovine babesiosis is based on the detection of Babesia-specific antibodies (immunoglobulin-G). Antibody detection is commonly used in seroepidemiological studies or in the assessment of antibabesial antibody titers after cattle vaccination. The indirect fluorescent antibody test (IFAT) and enzyme-linked immunosorbent assay (ELISA) are the most widely used diagnostic tests, although there their implementation has some drawbacks, principally due to the requirements for trained personnel, specific materials, and special laboratory equipment. This study compared a newly designed rapid immunochromatography test (ICT), which has been reported recently and used for Babesia bovis-specific antibody detection with promising results, with an in-house ELISA for the serological diagnosis of cattle exposed to B. bovis (Babesia bovis) in Mexico. Higher sensitivity and specificity values were found by ICT, proving its effectiveness over ELISA. ICT also had better concordance than ELISA when IFAT was used as the “gold standard”. The rapid ICT was shown to have diagnostic utility for the detection of antibodies against B. bovis and could be used as a field test in Mexico due to its practicality, as it does not need laboratory equipment for implementation and interpretation of results.

Abstract

The indirect fluorescent antibody test (IFAT) is the most frequently used test to conduct seroepidemiological studies so far, and it is regarded as the "gold standard" test for the serological diagnosis of bovine babesiosis. The aim of the present study was to compare the enzyme-linked immunosorbent assay (ELISA) and the rapid immunochromatography test (ICT) for use in the serological diagnosis of cattle exposed to B. bovis in Mexico. The evaluation of test performance was carried out with 30 positive and 30 negative reference sera. A total of 72 bovine sera samples collected from cattle in a region with endemic bovine babesiosis were analyzed by ELISA and ICT, and the results were compared with those of IFAT. Kappa value (k) was also calculated to determine the agreement between tests. The sensitivity and specificity of ELISA for detecting antibodies against B. bovis were 87% (26/30) and 80% (24/30), respectively. The sensitivity and specificity of ICT for detecting antibodies against B. bovis were 90% (27/30) and 83.3% (25/30), respectively. The overall concordance determined for ELISA and ICT was 94.4% (68/72) and 98.6% (71/72), respectively, when the results were compared with those of IFAT. ICT was more sensitive and specific in this comparative study, showing good strength of agreement (k = 0.79) with respect to IFAT. ICT combines a strip-based assay system that is fast, practical, and sensitive for detection of antibodies to B. bovis, which suggests that it could be applied in the field without requiring any laboratory equipment for its use and interpretation of test results.

Bovine Babesiosis in Turkey: Impact, Current Gaps, and Opportunities for Intervention

Bovine babesiosis is a global tick-borne disease that causes important cattle losses and has potential zoonotic implications. The impact of bovine babesiosis in Turkey remains poorly characterized, but several Babesia spp., including B. bovis, B. bigemina, and B. divergens, among others and competent tick vectors, except Rhipicephalus microplus, have been recently identified in the country. Bovine babesiosis has been reported in all provinces but is more prevalent in central and highly humid areas in low and medium altitude regions of the country housing approximately 70% of the cattle population. Current control measures include acaricides and babesicidal drugs, but not live vaccines. Despite the perceived relevant impact of bovine babesiosis in Turkey, basic research programs focused on developing in vitro cultures of parasites, point-of-care diagnostic methods, vaccine development, “omics” analysis, and gene manipulation techniques of local Babesia strains are scarce. Additionally, no effective and coordinated control efforts managed by a central animal health authority have been established to date. Development of state-of-the-art research programs in bovine babesiosis to address current gaps in knowledge and implementation of long-term plans to control the disease will surely result in important economic, nutritional, and public health benefits for the country and the region.

Comparative analysis of gene expression between Babesia bovis blood stages and kinetes allowed by improved genome annotation

Throughout their life cycle, Babesia parasites alternate between a mammalian host, where they cause babesiosis, and the tick vector. Transition between hosts results in distinct environmental signals that influence patterns of gene expression, consistent with the morphological and functional changes operating in the parasites during their life stages. In addition, comparing differential patterns of gene expression among mammalian and tick parasite stages can provide clues for developing improved methods of control. Hereby, we upgraded the genome assembly of Babesia bovis, a bovine hemoparasite, closing a 139 kbp gap, and used RNA-Seq datasets derived from mammalian blood and tick kinete stages to update the genome annotation. Of the originally annotated genes, 1,254 required structural changes, and 326 new genes were identified, leading to a different predicted proteome compared to the original annotation. Next, the RNA-Seq data was used to identify B. bovis genes that were differentially expressed in the vertebrate and arthropod hosts. In blood stages, 28% of the genes were upregulated up to 300 fold, whereas 26% of the genes in kinetes, a tick stage, were upregulated up to >19,000 fold. We thus discovered differentially expressed genes that may play key biological roles and serve as suitable targets for the development of vaccines to control bovine babesiosis.

Kinetic characterization of a novel cysteine peptidase from the protozoan Babesia bovis, a potential target for drug design

C1A cysteine peptidases have been shown to play an important role during apicomplexan invasion and egress of host red blood cells (RBCs) and therefore have been exploited as targets for drug development, in which peptidase specificity is deterministic. Babesia bovis genome is currently available and from the 17 putative cysteine peptidases annotated four belong to the C1A subfamily. In this study, we describe the biochemical characterization of a C1A cysteine peptidase, named here BbCp (B. bovis cysteine peptidase) and evaluate its possible participation in the parasite asexual cycle in host RBCs. The recombinant protein was obtained in bacterial inclusion bodies and after a refolding process, presented typical kinetic features of the cysteine peptidase family, enhanced activity in the presence of a reducing agent, optimum pH between 6.5 and 7.0 and was inhibited by cystatins from R. microplus. Moreover, rBbCp substrate specificity evaluation using a peptide phage display library showed a preference for Val > Leu > Phe. Finally, antibodies anti-rBbCp were able to interfere with B. bovis growth in vitro, which highlights the BbCp as a potential target for drug design.

An Overview of Current Knowledge on in vitro Babesia Cultivation for Production of Live Attenuated Vaccines for Bovine Babesiosis in Mexico

The instrumentation of the in vitro culture system has allowed researchers to learn more about the metabolic and growth behavior of Babesia spp. The various applications for in vitro cultivation of Babesia include obtaining attenuated strains for vaccination or pre-munition, the selection of pure lines with different degrees of virulence, studies on biological cloning, ultrastructure, antigen production for diagnostics, drug sensitivity assessments, and different aspects of parasite biology. Although there are different types of vaccines that have been tested against bovine babesiosis, so far, the only procedure that has offered favorable results in terms of protection and safety has been the use of live attenuated vaccines. In countries, such as Australia, Argentina, Brazil, Uruguay and Israel, this type of vaccine has been produced and used. The alternative to live vaccines other than splenectomized calf-derived biological material, has been the in vitro cultivation of Babesia bovis and B. bigemina. The development of in vitro culture of Babesia spp. strains in a defined medium has been the basis for the initiation of a source of parasites and exoantigens for a variety of studies on the biochemistry and immunology of babesiosis. The use of live immunogens from attenuated strains derived from in vitro culture is highlighted, which has been proposed as an alternative to control bovine babesiosis. In several studies performed in Mexico, this type of immunogen applied to susceptible cattle has shown the induction of protection against the experimental heterologous strain challenge with both, Babesia-infected blood and animal exposure to confrontations on tick vector-infested farms. The combination of transfection technologies and the in vitro culture system as integrated methodologies would eventually give rise to the generation of genetically modified live vaccines. However, a greater challenge faced now by researchers is the large-scale cultivation of Babesia parasites for mass production and vaccine distribution.

In silico and phylogenetic analyses of partial BbRAP-1, BbCP2, BbSBP-4 and BbβTUB gene sequences of Babesia bovis isolates from cattle in South Africa

Background

Bovine babesiosis is one of the most economically important tick-borne diseases threatening the livestock industry globally including South Africa. This disease is induced by members of Babesia bovis species. Antigenic variations among geographical strains of B. bovis, and these heterogeneities are cited as the mechanism by which parasites evade from host immune system and they hamper the successful development of a single vaccine that could confer absolute protection. Given the economic importance of livestock industry in South Africa, the extent of genetic diversity among field isolates of B. bovis merits extensive investigation.

In this study, we genetically characterized partial genes of B. bovis and studied the phylogenetic relationship among B. bovis isolates of South African origin. The genes, which were PCR-amplified from bovine samples collected from different locations across South Africa, coded for rhoptry-associated protein 1 (BbRAP-1), cysteine peptidase 2 (BbCP2), spherical body protein 4 (BbSBP-4) and β-tubulin (BbβTUB). Phylogenies were inferred from newly determined sequences using the neighbour-joining approach.

Results

Nested PCR assays with gene-specific primers indicated that, of the 54 bovine samples tested, 59.3% (32/54; 95% CI = 46.0–71.3%), 27.8% (15/54; 95% CI = 17.6–40.9%), 37.0% (20/54; 95% CI = 25.4–50.4%) and 29.6% (16/54; 95% CI = 19.1–42.8%) possessed BbRAP-1, BbCP2, BbSBP-4 and BbβTUB fragments, respectively. Sequencing of PCR-generated fragments revealed that nucleotide sequences of each of the four genes were highly conserved among the B. bovis isolates examined. Phylogenetic analyses of BbCP2, BbSBP-4 and BbβTUB sequences indicated a close phylogenetic relatedness among South African-derived sequences and those of global B. bovis strains.

Conclusion

The data reported in this study indicated that there is a high conservation among the genes of B. bovis isolates from cattle in South Africa. These findings give an indication that immunologically important proteins encoded by these genes could potentially be considered for exploitation as viable candidates for inclusion in recombinant subunit vaccines.

Recovery of bovine Babesia spp. after long-term cryostorage and comparison of bovine donor erythrocytes and serum

Cultured Babesia bovis and Babesia bigemina were recovered from liquid nitrogen storage nearly 30 years after they were cryopreserved. Four cattle were compared as donors of erythrocytes and serum for microaerophilous stationary phase (MASP) cultures for recovery of B. bigemina. Erythrocytes and serum from only one (#913) of the four animals supported growth of B. bigemina. Two B. bigemina (frozen in 1986 and 1987) and two B. bovis (both frozen in 1986) cryostocks were recovered from liquid nitrogen storage and all four recovered and thrived in #913 erythrocytes and serum. In the third passage after recovery, B. bovis cultures were cryopreserved. Six months later they were successfully recovered using #913 erythrocytes and serum. This study shows that B. bovis and B. bigemina stored nearly 30 years in liquid nitrogen can be successfully recovered in the MASP system. This study also confirms previous observations that selection of a suitable bovine donor of erythrocytes and serum is critical to the success of the culture.

Co-immunization of cattle with a vaccine against babesiosis and Lactobacillus casei increases specific IgG1 levels to Babesia bovis and B. bigemina

The effect of Lactobacillus casei administered along with a live attenuated vaccine vs. bovine babesiosis (VAC) on induction of IgG1 and IgG2 antibodies to Babesia bovis and Babesia bigemina was assessed by the indirect fluorescent antibody test (IFAT) in bovines of an endemic babesiosis area before (day 0) and after vaccination (days 15 and 30). We previously reported that L. casei increases the efficiency of VAC under controlled conditions and under extreme conditions in the field; however, the levels of IgG1 and IgG2 antibodies to B. bovis and B. bigemina are not known in vaccinated animals. Twenty-one dairy cows were allocated into three groups (seven animals per group): unvaccinated, vaccinated with VAC and vaccinated simultaneously with VAC and L. casei (VAC-LC). All animals were kept in a babesiosis endemic area at Tlalixcoyan, Veracruz. At days 15 and 30 after vaccination, the average levels of IgG1 to B. bovis and to B. bigemina were significantly higher in VAC-LC group than levels observed in VAC and control groups (P<0.01). Levels of IgG2 were similar in VAC and VAC-LC groups but higher than in the control group (P<0.01). When tested in in vitro cultures of B. bovis, sera from VAC-LC group significantly inhibited parasite growth as compared with the sera of the other two groups. It is suggested that the efficiency improvement of VAC, in part, is due to the L. casei boost of IgG1 over IgG2 antibodies to B. bovis and B. bigemina when the bacteria is co-inoculated with this vaccine.

Genetic conservation of potentially immunogenic proteins among Brazilian isolates of Babesia bovis

Bovine babesiosis caused by Babesia bovis remains an important constraint for the development of cattle industries worldwide. Effective control can be achieved by vaccination with live attenuated phenotypes of the parasite. However, these vaccines have a number of drawbacks, which justifies the search for better, safer vaccines. In recent years, a number of parasite proteins with immunogenic potential have been discovered. However, there is little information on the genetic conservation of these proteins among different parasite isolates, which hinders their assessment as immunogens. The aim of the present study was to evaluate the conservation of the genes ama-1, acs-1, rap-1, trap, p0 and msa2c among five Brazilian isolates of B. bovis. Through polymerase chain reaction, genetic sequencing and bioinformatics analysis of the genes, a high degree of conservation (98-100%) was found among Brazilian isolates of B. bovis and the T2Bo isolate. Thus, these genes are worth considering as viable candidates to be included in a recombinant cocktail vaccine for cattle babesiosis caused by B. bovis.