Acute and persistent infection by a transfected Mo7 strain of Babesia bovis

Role of Rhipicephalus bursa larvae in transstadial transmission and endemicity of Babesia ovis in chronically infected sheep

Babesia ovis, transmitted by Rhipicephalus bursa ticks, is the causative agent of ovine babesiosis, a disease characterized by fever, anemia, hemoglobinuria, and high mortality in sheep. This study investigates whether sheep that survived babesiosis without treatment can serve as a source of infection for B. ovis-free host-seeking R. bursa larvae in a later season. Three donor sheep were experimentally infected with B. ovis, and after six months, persistence of B. ovis was assessed through blood and tick transmission experiments. Blood from donor sheep was intravenously injected into three recipient sheep, while donor sheep were also infested with B. ovis-free R. bursa larvae. Engorged nymphs molted to adults, and new recipient sheep were infested with these ticks. All recipient sheep were monitored for B. ovis for 100 days using microscopic, serological, and molecular approaches. The presence of B. ovis was confirmed in the recipient sheep that received blood, leading to clinical infection in two. However, no B. ovis was detected in recipient sheep infested with ticks. These results suggest that sheep recovering from B. ovis infection do not serve as a source of infection for R. bursa larvae in subsequent seasons.

Advances in Babesia Vaccine Development: An Overview

Babesiosis is a tick-borne zoonotic disease, which is caused by various species of intracellular Babesia parasite. It is a problem not only for the livestock industry but also for global health. Significant global economic losses, in particular in cattle production, have been observed. Since the current preventive measures against babesiosis are insufficient, there is increasing pressure to develop a vaccine. In this review, we survey the achievements and recent advances in the creation of antibabesiosis vaccine. The scope of this review includes the development of a vaccine against B. microti, B. bovis, B. bigemina, B. orientalis and B. divergens. Here, we present different strategies in their progress and evaluation. Scientists worldwide are still trying to find new targets for a vaccine that would not only reduce symptoms among animals but also prevent the further spread of the disease. Molecular candidates for the production of a vaccine against various Babesia spp. are presented. Our study also describes the current prospects of vaccine evolution for successful Babesia parasites elimination.

Comparison of high throughput RNA sequences between Babesia bigemina and Babesia bovis revealed consistent differential gene expression that is required for the Babesia life cycle in the vertebrate and invertebrate hosts

Bovine babesiosis caused by Babesia bigemina and Babesia bovis is an economically important disease that affects cattle worldwide. Both B. bigemina and B. bovis are transovarially transmitted by Rhipicephalus ticks. However, little is known regarding parasite gene expression during infection of the tick vector or mammalian host, which has limited the development of effective control strategies to alleviate the losses to the cattle industry. To understand Babesia gene regulation during tick and mammalian host infection, we performed high throughput RNA-sequencing using samples collected from calves and Rhipicephalus microplus ticks infected with B. bigemina. We evaluated gene expression between B. bigemina blood-stages and kinetes and compared them with previous B. bovis RNA-seq data. The results revealed similar patterns of gene regulation between these two tick-borne transovarially transmitted Babesia parasites. Like B. bovis, the transcription of several B. bigemina genes in kinetes exceeded a 1,000-fold change while a few of these genes had a >20,000-fold increase. To identify genes that may have important roles in B. bigemina and B. bovis transovarial transmission, we searched for genes upregulated in B. bigemina kinetes in the genomic datasets of B. bovis and non-transovarially transmitted parasites, Theileria spp. and Babesia microti. Using this approach, we identify genes that may be potential markers for transovarial transmission by B. bigemina and B. bovis. The findings presented herein demonstrate common Babesia genes linked to infection of the vector or mammalian host and may contribute to elucidating strategies used by the parasite to complete their life cycle.

Immunization against a Conserved Surface Polysaccharide Stimulates Bovine Antibodies with Opsonic Killing Activity but Does Not Protect against Babesia bovis Challenge

Arthropod-borne apicomplexan pathogens remain a great concern and challenge for disease control in animals and humans. In order to prevent Babesia infection, the discovery of antigens that elicit protective immunity is essential to establish approaches to stop disease dissemination. In this study, we determined that poly-N-acetylglucosamine (PNAG) is conserved among tick-borne pathogens including B. bovis, B. bigemina, B. divergens, B. microti, and Babesia WA1. Calves immunized with synthetic ß-(1→6)-linked glucosamine oligosaccharides conjugated to tetanus toxoid (5GlcNH2-TT) developed antibodies with in vitro opsonophagocytic activity against Staphylococcus aureus. Sera from immunized calves reacted to B. bovis. These results suggest strong immune responses against PNAG. However, 5GlcNH2-TT-immunized bovines challenged with B. bovis developed acute babesiosis with the cytoadhesion of infected erythrocytes to brain capillary vessels. While this antigen elicited antibodies that did not prevent disease, we are continuing to explore other antigens that may mitigate these vector-borne diseases for the cattle industry.

Differential expression of calcium-dependent protein kinase 4, tubulin tyrosine ligase, and methyltransferase by xanthurenic acid-induced Babesia bovis sexual stages

Background

Babesia bovis is one of the most significant tick-transmitted pathogens of cattle worldwide. Babesia bovis parasites have a complex lifecycle, including development within the mammalian host and tick vector. Each life stage has developmental forms that differ in morphology and metabolism. Differentiation between these forms is highly regulated in response to changes in the parasite’s environment. Understanding the mechanisms by which Babesia parasites respond to environmental changes and the transmission cycle through the biological vector is critically important for developing bovine babesiosis control strategies.

Results

In this study, we induced B. bovis sexual stages in vitro using xanthurenic acid and documented changes in morphology and gene expression. In vitro induced B. bovis sexual stages displayed distinctive protrusive structures and surface ruffles. We also demonstrated the upregulation of B. bovis calcium-dependent protein kinase 4 (cdpk4), tubulin-tyrosine ligase (ttl), and methyltransferase (mt) genes by in vitro induced sexual stages and during parasite development within tick midguts.

Conclusions

Similar to other apicomplexan parasites, it is likely that B. bovis upregulated genes play a vital role in sexual reproduction and parasite transmission. Herein, we document the upregulation of cdpk4, ttl, and mt genes by both B. bovis in vitro induced sexual stages and parasites developing in the tick vector. Understanding the parasite's biology and identifying target genes essential for sexual reproduction will enable the production of non-transmissible live vaccines to control bovine babesiosis.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04902-3.

Experimental Infection of Calves with Transfected Attenuated Babesia bovis Expressing the Rhipicephalus microplus Bm86 Antigen and eGFP Marker: Preliminary Studies towards a Dual Anti-Tick/Babesia Vaccine

Bovine babesiosis, caused by Babesia bovis and B. bigemina, is a major tick-borne disease of cattle with global economic impact. The disease can be prevented using integrated control measures including attenuated Babesia vaccines, babesicidal drugs, and tick control approaches. Vaccination of cattle with the Rhipicephalus microplus Bm86-based recombinant vaccine reduces the fitness of R. microplus and R. annulatus, but several booster inoculations are required to maintain protection. Herein, we generated a stable transfected strain of B. bovis expressing an enhanced GFP (eGFP) and a chimeric version of Bm86 (B. bovis/Bm86/eGFP). The eGFP was expressed in the parasite cytoplasm, whereas Bm86 was displayed on the surface of merozoites. Three splenectomized calves experimentally infected with B. bovis/Bm86/eGFP showed mild signs of acute disease and developed long-lasting antibody responses to B. bovis and native Bm86. No evidence of sequestration of parasites in the cerebral capillaries was found upon postmortem analysis, confirming attenuation of the strain. This is the first report of transfected B. bovis expressing the tick antigen Bm86 on the merozoite surface that elicits an antibody response to native Bm86. These results represent a proof of concept for a novel live, attenuated, tagged dual-vaccine approach to attempt simultaneous control of babesiosis and tick infestation.

Molecular detection and characterization of Theileria sp. from hedgehogs (Paraechinus aethiopicus) in Saudi Arabia

In this study, we conducted molecular detection and characterization of piroplasms that infect the Ethiopian or desert hedgehogs (Paraechinus aethiopicus) in Saudi Arabia. Blood samples from 112 (68 males and 44 females) desert hedgehogs from Unaizah, Central Saudi Arabia were screened for Theileria/Babesia DNA using the polymerase chain reaction (PCR) employing specific primers amplifying the partial 18S small subunit rRNA gene. Theileria DNA was detected in 51 samples (45·5%), giving a prevalence of 45·5%. Theileria DNA was found in 33 (48·5%) males and 18 (40·9%) females, and there was no significant difference (P > 0·05) in the prevalence between males and females. Similarly, there was no significant difference (P > 0·05) in the prevalence between juveniles (40%) and adults (46·7%). There was a significant difference in the prevalence of Theileria in hedgehogs collected from May to September and the period from October to April (P = 0·003). Four haplotypes of Theileria sp. in hedgehogs were detected and designated as H1-H4. H1 was the predominant haplotype and found in 80·8% of the positive individuals. Partial sequences of the 18S rRNA of Theileria sp. from hedgehogs grouped with Theileria spp. that are benign. This study is the first report of the occurrence of Theileria spp. in Saudi Arabian desert hedgehogs.

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.

Knockout of Babesia bovis rad51 ortholog and its complementation by expression from the BbACc3 artificial chromosome platform

Babesia bovis establishes persistent infections of long duration in cattle, despite the development of effective anti-disease immunity. One mechanism used by the parasite to achieve persistence is rapid antigenic variation of the VESA1 cytoadhesion ligand through segmental gene conversion (SGC), a phenomenon thought to be a form of homologous recombination (HR). To begin investigation of the enzymatic basis for SGC we initially identified and knocked out the Bbrad51 gene encoding the B. bovis Rad51 ortholog. BbRad51 was found to be non-essential for in vitro growth of asexual-stage parasites. However, its loss resulted in hypersensitivity to methylmethane sulfonate (MMS) and an apparent defect in HR. This defect rendered attempts to complement the knockout phenotype by reinsertion of the Bbrad51 gene into the genome unsuccessful. To circumvent this difficulty, we constructed an artificial chromosome, BbACc3, into which the complete Bbrad51 locus was inserted, for expression of BbRad51 under regulation by autologous elements. Maintenance of BbACc3 makes use of centromeric sequences from chromosome 3 and telomeric ends from chromosome 1 of the B. bovis C9.1 line. A selection cassette employing human dihydrofolate reductase enables recovery of transformants by selection with pyrimethamine. We demonstrate that the BbACc3 platform is stably maintained once established, assembles nucleosomes to form native chromatin, and expands in telomere length over time. Significantly, the MMS-sensitivity phenotype observed in the absence of Bbrad51 was successfully complemented at essentially normal levels. We provide cautionary evidence, however, that in HR-competent parasites BbACc3 can recombine with native chromosomes, potentially resulting in crossover. We propose that, under certain circumstances this platform can provide a useful alternative for the genetic manipulation of this group of parasites, particularly when regulated gene expression under the control of autologous elements may be important.

Identification of proteins expressed by Babesia bigemina kinetes

Background

Babesia bigemina is an apicomplexan parasite transovarially transmitted via Rhipicephalus ticks that infect red blood cells and causes bovine babesiosis, a poorly controlled severe acute disease in cattle. New methods of control are urgently needed, including the development of transmission blocking vaccines (TBV). Babesia bigemina reproduces sexually in the gut of adult female R. microplus upon acquisition following a blood meal. Sexual reproduction results in zygotes that infect gut epithelial cells to transform into kinete stage parasites, which invade tick ovaries and infects the egg mass. The subsequent tick generation transmits B. bigemina upon feeding on bovine hosts. An important limitation for developing novel TBV is that the pattern of protein expression in B. bigemina tick stages, such as the kinete stage, remain essentially uncharacterized.

Results

We determined the protein expression profile of three B. bigemina putative tick stage candidates BbiKSP (BBBOND_0206730), CCp2 and CCp3. We found that BbiKSP expression was restricted to B. bigemina kinetes. CCp2 and CCp3, previously shown to be expressed by induced sexual stages, were also expressed by kinetes. Importantly, none of these proteins were expressed by B. bigemina blood stages.

Conclusions

Babesia bigemina kinetes express BbiKSP, CCp2 and CCp3 proteins, therefore, these proteins may play important roles during B. bigemina development within tick hemolymph and may serve as potential candidate targets for the development of TBV.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3531-7) contains supplementary material, which is available to authorized users.

Co-infection with tick-borne disease agents in cattle in Russia

Tick-borne diseases cause significant livestock losses worldwide. In Russia, information concerning single or mixed infections with different Anaplasma, Theileria and Babesia species in cattle is very limited. This study was conducted to determine the level of co-infection with protozoan pathogens (Theileria spp. and Babesia spp.) and rickettsial pathogens (A. marginale and A. phagocytophilum) in cattle in central Russia. Blood samples were examined with real time polymerase chain reaction (RT-PCR) for A. marginale and A. phagocytophilum, and by amplifying the V4 hypervariable region of the 18S rRNA gene, followed by cloning, DNA sequencing, and phylogenetic analyses, for Babesia and Theileria species. In total 67% of examined blood samples were positive for Theileria spp. or A. marginale, and 19% of the animals were co-infected with Theileria spp. and A. marginale. Seasonal variation in prevalence was found for Theileria spp. Phylogenetic analysis based on 18S rRNA gene sequences revealed the presence of five Theileria species: T. annulata, T. orientalis, T. buffeli, T. sergenti, and T. sinensis. No samples were positive for Babesia spp. or A. phagocytophilum. The data obtained for prevalence of bovine theileriosis and anaplasmosis in the central part of Russia underscore the need for improved surveillance and control programs to reduce tick-borne diseases in cattle.

Identification of a putative methyltransferase gene of Babesia bigemina as a novel molecular biomarker uniquely expressed in parasite tick stages

BACKGROUND

Bovine babesiosis is caused by apicomplexan pathogens of the genus Babesia such as B. bigemina and B. bovis. These tick-borne pathogens have a complex life-cycle involving asexual multiplication in vertebrate hosts and sexual reproduction in invertebrate vectors. In the tick midgut, extracellular Babesia parasites transform into gametes that fuse to form zygotes. Understanding the mechanisms that underlie formation of extracellular Babesia tick stages is an important step towards developing control strategies for preventing tick infection and subsequent parasite transmission.

RESULTS

We induced B. bigemina sexual stages in vitro by exposing parasites to Tris 2-carboxyethyl phosphine (TCEP). Subsequently, we identified a novel putative methyltransferase gene (BBBOND_0204030) that is expressed uniquely in all B. bigemina tick stages but not in blood stages. In vitro TCEP-exposed B. bigemina presented diverse morphology including parasites with long projections, round forms and clusters of round forms indicative of sexual stage induction. We confirmed the development of sexual stages by detecting upregulation of previously defined B. bigemina sexual stage marker genes, ccp2 and 3, and their respective protein expression in TCEP-induced B. bigemina cultures. Next, transcription analysis of in vitro TCEP-induced B. bigemina culture based on an in silico derived list of homologs of Plasmodium falciparum gamete-specific genes demonstrated differential expression of the gene BBBOND_0204030 in induced cells. Further examination of ex vivo infected ticks demonstrated that BBBOND_0204030 is transcribed by multiple stages of B. bigemina during parasite development in tick midgut, ovary and hemolymph. Interestingly, ex vivo results confirmed our in vitro observation that blood stages of B. bigemina do not express BBBOND_0204030 and validated the in vitro system of inducing sexual stages.

CONCLUSIONS

Herein we describe the identification of a B. bigemina gene transcribed exclusively by parasites infecting ticks using a novel method of inducing B. bigemina sexual stages in vitro. We propose that this gene can be used as a marker for parasite development within the tick vector. Together, these tools will facilitate our understanding of parasite-tick interactions, the identification of novel vaccine targets and, consequently, the development of additional strategies to control bovine babesiosis.

Stable transformation of Babesia bigemina and Babesia bovis using a single transfection plasmid

Babesia bigemina and Babesia bovis, are the two major causes of bovine babesiosis, a global neglected disease in need of improved methods of control. Here, we describe a shared method for the stable transfection of these two parasites using electroporation and blasticidin/blasticidin deaminase as a selectable marker. Stably transfected B. bigemina and B. bovis were obtained using a common transfection plasmid targeting the enhanced green fluorescent protein-BSD (egfp-bsd) fusion gene into the elongation factor-1α (ef-1α) locus of B. bigemina and B. bovis under the control of the B. bigemina ef-1α promoter. Sequencing, Southern blotting, immunoblotting and immunofluorescence analysis of parasite-infected red blood cells, demonstrated that the egfp-bsd gene was expressed and stably integrated solely into the ef-1α locus of both, B. bigemina and B. bovis. Interestingly, heterologous B. bigemina ef-1α sequences were able to drive integration into the B. bovis genome by homologous recombination, and the stably integrated B. bigemina ef-1α-A promoter is fully functional in B. bovis. Collectively, the data provides a new tool for genetic analysis of these parasites, and suggests that the development of vaccine platform delivery systems based on transfected B. bovis and B. bigemina parasites using homologous and heterologous promoters is feasible.

Molecular detection and characterization of Theileria spp. infecting cattle in Sennar State, Sudan

Tropical theileriosis is a serious animal disease transmitted by tick vectors. The agents of theileriosis are obligate intracellular parasites that cause mild to severe disease in the mammalian host. Tropical theileriosis has been recognized as a burden to the development of the dairy industry in Sudan and causes major economic losses. However, knowledge about the distribution of Theileria spp. in Sudan and the extent of sequence variation within the 18S rRNA gene is currently unknown. The aim of this study was to determine the diversity of Theileria spp. using 18S rRNA-based PCR to detect parasites in cattle followed by cloning and sequencing. We observed an overall prevalence rate of 63% hemoparasite infection in cattle from Sennar state. A subset of samples was used for cloning and sequencing of the 18S rRNA gene. Nineteen of 44 animals were co-infected with more than one species of Theilera. Phylogenetic analysis revealed three Theileria spp. that were predominant in cattle including pathogenic T. annulata and apathogenic T. velifera and T. mutans. The present study provides information regarding the prevalence of theileriosis in Sudan and will help to design strategies to control it. Additionally, more study is needed to determine tick vector competence and degree of coinfection with multiple Theileria spp. in Sudan. This represents the first molecular phylogeny report to identify Theileria spp. in cattle from Sudan.

Geno- and phenotypic characteristics of a transfected Babesia bovis 6-Cys-E knockout clonal line

BACKGROUND

Babesia bovis is an intra-erythrocytic tick-transmitted apicomplexan protozoan parasite. It has a complex lifestyle including asexual replication in the mammalian host and sexual replication occurring in the midgut of host tick vector, typically, Rhipicephalus microplus. Previous evidence showed that certain B. bovis genes, including members of 6-Cys gene family, are differentially expressed during tick and mammalian stages of the parasite's life cycle. Moreover, the 6-Cys E gene is differentially expressed in the T3Bo strain of B. bovis tick stages, and anti 6-Cys E antibodies were shown to be able to inhibit in vitro growth of the phenotypically distinct B. bovis Mo7clonal line.

METHODS

In this study, the 6-Cys E gene of B. bovis T3Bo strain was disrupted by transfection using a plasmid containing 6-Cys gene E 5' and 3' regions to guide homologous recombination, and the egfp-bsd fusion gene under control of a ef-1α promoter, yielding a B. bovis clonal line designated 6-Cys EKO-cln. Full genome sequencing of 6-Cys EKO-cln parasites was performed and in vitro inhibition assays using anti 6-Cys E antibodies.

RESULTS

Full genome sequencing of 6-Cys EKO-cln B. bovis demonstrated single insertion of egfp-bsd gene that disrupts the integrity of 6-Cys gene E. Undistinguishable growth rate of 6-Cys EKO-cln line compared to wild-type 6-Cys E intact T3Bo B. bovis strain in in vitro cultures indicates that expression of gene 6-Cys E is not essential for blood stage replication in this strain. In vitro inhibition assays confirmed the ability of anti-6 Cys E antibodies to inhibit the growth of the wild-type Mo7 and T3Bo B. bovis parasites, but no significant inhibition was found for 6-Cys EKO-cln line parasites.

CONCLUSIONS

Overall, the data suggest that the anti-6 Cys E antibody neutralising effect on the wild type strains is likely due to mechanical hindrance, or cross-reactivity, rather than due to functional requirements of 6-Cys gene E product for survival and development of the erythrocyte stages. Further investigation is underway to determine if the 6-Cys E protein is required for replication and sexual stage development of B. bovis during tick stages.

A novel modified-indirect ELISA based on spherical body protein 4 for detecting antibody during acute and long-term infections with diverse Babesia bovis strains

Background

Cattle persistently infected with Babesia bovis are reservoirs for intra- and inter-herd transmission. Since B. bovis is considered a persistent infection, developing a reliable, high-throughput assay that detects antibody during all stages of the infection could be pivotal for establishing better control protocols.

Methods

A modified indirect enzyme-linked immunosorbent assay (MI-ELISA) was developed using the spherical body protein-4 (SBP4) of B. bovis to detect antibody against diverse strains through all infection stages in cattle. This SBP4 MI-ELISA was evaluated for sensitivity and specificity against field sera from regions with endemic and non-endemic B. bovis. Sera were also evaluated from cattle infected experimentally with various doses and strains during acute and persistent infection with parasitemia defined by nested PCR.

Results

The format variables for SBP4 MI-ELISA were optimized and the cutoff for positive and negative interpretation was determined based on receiver operating characteristic curve analysis using B. bovis positive and negative sera tested in the reference immunofluorescence assay (IFA). The diagnostic specificity of the SBP4 MI-ELISA using IFA-negative sera collected from Texas was 100%, significantly higher than the cELISA (90.4%) based on an epitope in the rhoptry-associated protein-1 (RAP-1 cELISA). The diagnostic sensitivity of the SBP4 MI-ELISA was 98.7% using the IFA-positive sera collected from several areas of Mexico, in contrast to that of the RAP-1 cELISA at 60% using these same sera. In cattle infected with low and high doses of three B. bovis strains, the SBP4 MI-ELISA remained antibody positive for 11 months or more after initial detection at 10 to 13 days post-inoculation. However, the RAP-1 cELISA did not reliably detect antibody after eight months post-inoculation despite the fact that parasitemia was occasionally detectable by PCR. Furthermore, initial antibody detection by RAP-1 cELISA in low-dose infected animals was delayed approximately nine and a half days compared to the SBP4 MI-ELISA.

Conclusions

These results demonstrate excellent diagnostic sensitivity and specificity of the novel SBP4 MI-ELISA for cattle with acute and long-term carrier infections. It is posited that use of this assay in countries that have B. bovis-endemic herds may be pivotal in preventing the spread of this disease to non-endemic herds.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2016-9) contains supplementary material, which is available to authorized users.

Analysis of Stage-Specific Protein Expression during Babesia Bovis Development within Female Rhipicephalus Microplus

Arthropod-borne protozoan pathogens have a complex life cycle that includes asexual reproduction of haploid stages in mammalian hosts and the development of diploid stages in invertebrate hosts. The ability of pathogens to invade, survive, and replicate within distinct cell types is required to maintain their life cycle. In this study, we describe a comparative proteomic analysis of a cattle pathogen, Babesia bovis, during its development within the mammalian and tick hosts with the goal of identifying cell-surface proteins expressed by B. bovis kinetes as potential targets for the development of a transmission blocking vaccine. To determine parasite tick-stage-specific cell-surface proteins, CyDye labeling was performed with B. bovis blood stages from the bovine host and kinetes from the tick vector. Cell-surface kinete-stage-specific proteins were identified using 2D difference in gel electrophoresis and analyzed by mass spectrometry. Ten proteins were identified as kinete-stage-specific, with orthologs found in closely related Apicomplexan pathogens. Transcriptional analysis revealed two genes were highly expressed by kinetes as compared with blood stages. Immunofluorescence using antibodies against the two proteins confirmed kinete-stage-specific expression. The identified cell-surface kinete proteins are potential candidates for the development of a B. bovis transmission blocking vaccine.

Transfected Babesia bovis Expressing a Tick GST as a Live Vector Vaccine

The Rhipicephalus microplus tick is a notorious blood-feeding ectoparasite of livestock, especially cattle, responsible for massive losses in animal production. It is the main vector for transmission of pathogenic bacteria and parasites, including Babesia bovis, an intraerythrocytic apicomplexan protozoan parasite responsible for bovine Babesiosis. This study describes the development and testing of a live B. bovis vaccine expressing the protective tick antigen glutathione-S-transferase from Haemaphysalis longicornis (HlGST). The B. bovis S74-T3B parasites were electroporated with a plasmid containing the bidirectional Ef-1α (elongation factor 1 alpha) promoter of B. bovis controlling expression of two independent genes, the selectable marker GFP-BSD (green fluorescent protein–blasticidin deaminase), and HlGST fused to the MSA-1 (merozoite surface antigen 1) signal peptide from B. bovis. Electroporation followed by blasticidin selection resulted in the emergence of a mixed B. bovis transfected line (termed HlGST) in in vitro cultures, containing parasites with distinct patterns of insertion of both exogenous genes, either in or outside the Ef-1α locus. A B. bovis clonal line termed HlGST-Cln expressing intracellular GFP and HlGST in the surface of merozoites was then derived from the mixed parasite line HlGST using a fluorescent activated cell sorter. Two independent calf immunization trials were performed via intravenous inoculation of the HlGST-Cln and a previously described control consisting of an irrelevant transfected clonal line of B. bovis designated GFP-Cln. The control GFP-Cln line contains a copy of the GFP-BSD gene inserted into the Ef-1α locus of B. bovis in an identical fashion as the HIGST-Cln parasites. All animals inoculated with the HlGST-Cln and GFP-Cln transfected parasites developed mild babesiosis. Tick egg fertility and fully engorged female tick weight was reduced significantly in R. microplus feeding on HlGST-Cln-immunized calves. Collectively, these data show the efficacy of a transfected HlGST-Cln B. bovis parasite to induce detectable anti-glutathione-S-transferase antibodies and a reduction in tick size and fecundity of R. microplus feeding in experimentally inoculated animals.

The cattle tick Rhipicephalus microplus is a hematophagous ectoparasite, responsible for the transmission of lethal parasites such as Babesia sp, limiting cattle production in tropical and subtropical regions of the world. There is an urgent emerging need for improved methods of control for these currently neglected tick and tick borne diseases. It is hypothesized that a dual attenuated-live vector vaccine containing a stably transfected tick antigen elicits protective immune responses against the parasite and the tick vector in vaccinated cattle. Live Babesia vaccines based on attenuated parasites are the only effective method available for preventing acute babesiosis. On the other hand, glutathione-S-transferase from Haemaphysalis longicornis (HlGST) is a known effective antigen against Rhipicephalus microplus, the most common vector for B. bovis. This study describes the development and testing of a transfected, B. bovis vaccine expressing HlGST against the tick R. microplus. A B. bovis clonal line designated HlGST-Cln expressing HlGST and GFP/BSD, and separately a control transfected B. bovis clonal line expressing only GFP/BSD was used to vaccinate calves in two independent experiments. All immunized calves developed mild babesiosis, and only calves immunized with the HlGST-Cln parasite line generated anti-HlGST antibodies. Tick egg fertility and fully engorged female tick weight were reduced significantly in R. microplus feeding on HlGST-Cln-vaccinated calves. Taken together, these data demonstrates the ability of transfected B. bovis to elicit antibodies against a heterologous tick antigen in cattle and to induce partial protection in the vaccinated animals against the cattle tick for the first time, representing a step toward the goal to produce a live vector anti-tick vaccine.

Molecular epidemiology of bovine Babesia spp. and Theileria orientalis parasites in beef cattle from northern and northeastern Thailand

Beef cattle production represents the largest cattle population in Thailand. Their productivity is constrained by tick-borne diseases such as babesiosis and theileriosis. In this study, we determined the prevalence of Babesia bigemina, Babesia bovis and Theileria orientalis using polymerase chain reaction (PCR). The genetic markers that were used for detection of the above parasites were sequenced to determine identities and similarity for Babesia spp. and genetic diversity of T. orientalis. Furthermore the risk factors for the occurrence of the above protozoan parasites in beef cattle from northern and northeastern parts of Thailand were assessed. A total of 329 blood samples were collected from beef cattle in 6 provinces. The study revealed that T. orientalis was the most prevalent (30.1%) parasite in beef cattle followed by B. bigemina (13.1%) and B. bovis (5.5%). Overall, 78.7% of the cattle screened were infected with at least one of the above parasites. Co-infection with Babesia spp. and T. orientalis was 30.1%. B. bigemina and T. orientalis were the most prevalent (15.1%) co-infection although triple infection with the three parasites was observed in 3.0% of the samples. Sequencing analysis revealed that B. bigemina RAP1 gene and B. bovis SBP2 gene were conserved among the parasites from different cattle samples. Phylogenetic analysis showed that the T. orientalis MPSP gene from parasites isolated from cattle in north and northeast Thailand was classified into types 5 and 7 as reported previously. Lack of tick control program was the universal risk factor of the occurrence of Babesia spp. and T. orientalis infection in beef cattle in northern and northeastern Thailand. We therefore recommend training of farmers on appropriate tick control strategies and further research on potential vectors for T. orientalis and elucidate the effect of co-infection with Babesia spp. on the pathogenicity of T. orientalis infection on beef in northern and northeastern Thailand.

Integration of a transfected gene into the genome of Babesia bovis occurs by legitimate homologous recombination mechanisms

This study examines the patterns of gene integration of gfp-bsd upon stable transfection into the T3Bo strain of Babesia bovis using a plasmid designed to integrate homologous sequences of the parasite's two identical ef-1α A and B genes. While the transfected BboTf-149-6 cell line displayed two distinct patterns of gene integration, clonal lines derived from this strain by cell sorting contained only single gfp-bsd insertions. Whole genome sequencing of two selected clonal lines, E9 and C6, indicated two distinct patterns of gfp-bsd insertion occurring by legitimate homologous recombination mechanisms: one into the expected ef-1α orf B, and another into the ef-1α B promoter. The data suggest that expression of the ef-1α orf B is not required for development of B. bovis in cultured erythrocyte stages. Use of legitimate homologous recombination mechanisms in transfected B. bovis supports the future use of transfection methods for developing efficient gene function assignment experiments using gene knockout techniques.

Serological and molecular diagnostic surveys combined with examining hematological profiles suggests increased levels of infection and hematological response of cattle to babesiosis infections compared to native buffaloes in Egypt

BACKGROUND

Babesiosis threatens the development of the cattle and buffaloes industries in Egypt and improved control is needed. The main objectives of this study are surveying the presence of bovine babesiosis in distinct selected bovine and buffalo populations in Egypt using novel molecular and previously validated serological methods, while also comparing the occurrence of hematological alterations among Babesia infected cattle and buffalos.

METHODS

A total of 253 and 81 blood samples from apparently healthy cattle and buffaloes, respectively, were randomly collected from diverse locations in Egypt. All samples were tested for Babesia bovis and B. bigemina infection using blood film examination, competitive ELISA (cELISA) and PCR. Novel semi-nested and nested PCR assays for the detection of B. bovis and B. bigemina respectively, were developed and used to analyze DNA extracted from bovine and buffalo samples. Hematological profiles were studied using a hematological analyzer.

RESULTS

Blood films examination revealed 13.8% and 7.4% Babesia infection rates in cattle and buffaloes, respectively. However, in cattle, the cELISA detected 32.8%, 21.3% and 10.7% infection rates with B. bigemina, B. bovis and mixed infection, respectively. In addition, cELISA identified 22.2%, 22.2% and 6.2% infection rates with B. bigemina, B. bovis and mixed infection, respectively in buffaloes. The semi-nested PCR assay showed that 15% of the tested samples were positive for B. bovis in cattle, but just 3% in buffaloes. Infections with B. bigemina were also found in cattle (32.4%), but not in buffaloes upon nested PCR analysis. Sequencing analysis confirmed the identity of the PCR amplicons and showed that Egyptian genotypes of B. bigemina and B. bovis highly resemble sequences previously deposited in GenBank. Hemograms performed on the sampled animals revealed macrocytic hypochromic anemia associated with reduced platelet counts in infected cattle with babesiosis. In addition, marked increases in total leukocyte and granulocytic counts and decreases in lymphocytic counts were found in infected cattle. In contrast, no such hematological anomalies were found in presumably Babesia-infected buffaloes.

CONCLUSIONS

Frequent occurrence of babesiosis among apparently healthy bovines in Egypt, suggests the need for appropriately designed prevalence studies in this country. Infected bovine, but not buffalo, populations often present hematological disorders compatible with intravascular hemolysis and thrombocytopenia.

Transfection of Babesia bovis by Double Selection with WR99210 and Blasticidin-S and Its Application for Functional Analysis of Thioredoxin Peroxidase-1

Genetic manipulation is an essential technique to analyze gene function; however, limited methods are available for Babesia bovis, a causative pathogen of the globally important cattle disease, bovine babesiosis. To date, two stable transfection systems have been developed for B. bovis, using selectable markers blasticidin-S deaminase (bsd) or human dihydrofolate reductase (hdhfr). In this work, we combine these two selectable markers in a sequential transfection system. Specifically, a parent transgenic B. bovis line which episomally expresses green fluorescent protein (GFP) and human dihydrofolate reductase (hDHFR), was transfected with a plasmid encoding a fusion protein consisting of red fluorescent protein (RFP) and blasticidin-S deaminase (BSD). Selection with WR99210 and blasticidin-S resulted in the emergence of parasites double positive for GFP and RFP. We then applied this method to complement gene function in a parasite line in which thioredoxin peroxidase-1 (Bbtpx-1) gene was knocked out using hDHFR as a selectable marker. A plasmid was constructed harboring both RFP-BSD and Bbtpx-1 expression cassettes, and transfected into a Bbtpx-1 knockout (KO) parasite. Transfectants were independently obtained by two transfection methods, episomal transfection and genome integration. Complementation of Bbtpx-1 resulted in full recovery of resistance to nitrosative stress, via the nitric oxide donor sodium nitroprusside, which was impaired in the Bbtpx-1 KO parasites. In conclusion, we developed a sequential transfection method in B. bovis and subsequently applied this technique in a gene complementation study. This method will enable broader genetic manipulation of Babesia toward enhancing our understanding of the biology of this parasite.

Vaccines against bovine babesiosis: where we are now and possible roads ahead

SUMMARY Bovine babesiosis caused by the tick-transmitted haemoprotozoans Babesia bovis, Babesia bigemina and Babesia divergens commonly results in substantial cattle morbidity and mortality in vast world areas. Although existing live vaccines confer protection, they have considerable disadvantages. Therefore, particularly in countries where large numbers of cattle are at risk, important research is directed towards improved vaccination strategies. Here a comprehensive overview of currently used live vaccines and of the status quo of experimental vaccine trials is presented. In addition, pertinent research fields potentially contributing to the development of novel non-live and/or live vaccines are discussed, including parasite antigens involved in host cell invasion and in pathogen-tick interactions, as well as the protective immunity against infection. The mining of available parasite genomes is continuously enlarging the array of potential vaccine candidates and, additionally, the recent development of a transfection tool for Babesia can significantly contribute to vaccine design. However, the complication and high cost of vaccination trials hinder Babesia vaccine research, and have so far seriously limited the systematic examination of antigen candidates and prevented an in-depth testing of formulations using different immunomodulators and antigen delivery systems.

Targeted surface expression of an exogenous antigen in stably transfected Babesia bovis

Babesia bovis is a tick-borne intraerythocytic protozoan responsible for acute disease in cattle which can be controlled by vaccination with attenuated B. bovis strains. Emerging B. bovis transfection technologies may increase the usefulness of these live vaccines. One use of transfected B. bovis parasites may be as a vaccine delivery platform. Previous transfection methods for B. bovis were limited by single expression sites and intracellular expression of transfected antigens. This study describes a novel transfection system in which two exogenous genes are expressed: one for selection and the other for a selected antigen designed to be delivered to the surface of the parasites. The strategy for duplicating the number of transfected genes was based on the use of the putative bidirectional promoter of the B. bovis 1.4 Kb ef-1α intergenic region. The ability of this region to regulate two independent expression sites was demonstrated using a luciferase assay on transiently transfected B. bovis parasites and then incorporated into a stable transfection plasmid to control independent expression of the selectable marker GFP-BSD and another gene of interest. A chimeric gene was synthetized using sequences from the protective B-cell epitopes of Rhipicephalus microplus tick antigen Bm86 along with sequences from the surface exposed B. bovis major surface antigen-1. This chimeric gene was then cloned into the additional expression site of the transfection plasmid. Transfection of the B. bovis Mo7 strain with this plasmid resulted in stable insertion into the ef-1α locus and simultaneous expression of both exogenous genes. Expression of the Bm86 epitopes on the surface of transfected merozoites was demonstrated using immunofluorescence analyses. The ability to independently express multiple genes by the inclusion of a bidirectional promoter and the achievement of surface expression of foreign epitopes advances the potential of transfected B. bovis as a future vaccine delivery platform.

Bovine babesiosis in the 21st century: advances in biology and functional genomics

Bovine babesiosis caused by the protozoan parasite, Babesia bovis, remains a significant cause of avoidable economic losses to the livestock industry in many countries throughout the world. The molecular mechanisms underlying the pathophysiology of severe disease in susceptible cattle are not well understood and the tools available to study the biology of the parasite, including technologies for genetic manipulation, have only recently been developed. Recent availability of multiple parasite genomes and bioinformatic tools, in combination with the development of new biological reagents, will facilitate our better understanding of the parasite. This will ultimately assist in the identification of novel targets for the development of new therapeutics and vaccines. Here we describe some recent advances in Babesia research and highlight some important challenges for the future.

In vitro culture systems for the study of apicomplexan parasites in farm animals

In vitro culture systems represent powerful tools for the study of apicomplexan parasites such as Cryptosporidium, Eimeria, Sarcocystis, Neospora, Toxoplasma, Besnoitia, Babesia and Theileria, all with high relevance for farm animals. Proliferative stages of these parasites have been cultured in vitro employing a large variety of cell culture and explant approaches. For some, such as Cryptosporidium and Eimeria, the sexual development has been reproduced in cell cultures, while for others, animal experimentation is required to fulfill the life cycle. In vitro cultures have paved the way to exploit the basic biology of these organisms, and had a major impact on the development of tools for diagnostic purposes. With the aid of in vitro cultivation, studies on host-parasite interactions, on factors involved in innate resistance, stage conversion and differentiation, genetics and transfection technology, vaccine candidates and drug effectiveness could be carried out. The use of transgenic parasites has facilitated high-throughput screening of anti-microbial compounds that are active against the proliferative stages. Here, we review the basic features of cell culture-based in vitro systems for apicomplexan parasites that are relevant for farm animals, and discuss their applications with a focus on drug identification and studies of stage differentiation.