Babesia bigemina: Sporozoite Isolation fromBoophilus microplusNymphs and Initial Immunomolecular Characterization

The Complexity of Piroplasms Life Cycles

Although apicomplexan parasites of the group Piroplasmida represent commonly identified global risks to both animals and humans, detailed knowledge of their life cycles is surprisingly limited. Such a discrepancy results from incomplete literature reports, nomenclature disunity and recently, from large numbers of newly described species. This review intends to collate and summarize current knowledge with respect to piroplasm phylogeny. Moreover, it provides a comprehensive view of developmental events of Babesia, Theileria, and Cytauxzoon representative species, focusing on uniform consensus of three consecutive phases: (i) schizogony and merogony, asexual multiplication in blood cells of the vertebrate host; (ii) gamogony, sexual reproduction inside the tick midgut, later followed by invasion of kinetes into the tick internal tissues; and (iii) sporogony, asexual proliferation in tick salivary glands resulting in the formation of sporozoites. However, many fundamental differences in this general consensus occur and this review identifies variables that should be analyzed prior to further development of specific anti-piroplasm strategies, including the attractive targeting of life cycle stages of Babesia or Theileria tick vectors.

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

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.

Tick capillary feeding for the study of proteins involved in tick-pathogen interactions as potential antigens for the control of tick infestation and pathogen infection

Background

Ticks represent a significant health risk to animals and humans due to the variety of pathogens they can transmit during feeding. The traditional use of chemicals to control ticks has serious drawbacks, including the selection of acaricide-resistant ticks and environmental contamination with chemical residues. Vaccination with the tick midgut antigen BM86 was shown to be a good alternative for cattle tick control. However, results vary considerably between tick species and geographic location. Therefore, new antigens are required for the development of vaccines controlling both tick infestations and pathogen infection/transmission. Tick proteins involved in tick-pathogen interactions may provide good candidate protective antigens for these vaccines, but appropriate screening procedures are needed to select the best candidates.

Methods

In this study, we selected proteins involved in tick-Anaplasma (Subolesin and SILK) and tick-Babesia (TROSPA) interactions and used in vitro capillary feeding to characterize their potential as antigens for the control of cattle tick infestations and infection with Anaplasma marginale and Babesia bigemina. Purified rabbit polyclonal antibodies were generated against recombinant SUB, SILK and TROSPA and added to uninfected or infected bovine blood to capillary-feed female Rhipicephalus (Boophilus) microplus ticks. Tick weight, oviposition and pathogen DNA levels were determined in treated and control ticks.

Results

The specificity of purified rabbit polyclonal antibodies against tick recombinant proteins was confirmed by Western blot and against native proteins in tick cell lines and tick tissues using immunofluorescence. Capillary-fed ticks ingested antibodies added to the blood meal and the effect of these antibodies on tick weight and oviposition was shown. However, no effect was observed on pathogen DNA levels.

Conclusions

These results highlighted the advantages and some of the disadvantages of in vitro tick capillary feeding for the characterization of candidate tick protective antigens. While an effect on tick weight and oviposition was observed, the effect on pathogen levels was not evident probably due to high tick-to-tick variations among other factors. Nevertheless, these results together with previous results of RNA interference functional studies suggest that these proteins are good candidate vaccine antigens for the control of R. microplus infestations and infection with A. marginale and B. bigemina.

Vaccination with proteins involved in tick-pathogen interactions reduces vector infestations and pathogen infection

Tick-borne pathogens cause diseases that greatly impact animal health and production worldwide. The ultimate goal of tick vaccines is to protect against tick-borne diseases through the control of vector infestations and reducing pathogen infection and transmission. Tick genetic traits are involved in vector-pathogen interactions and some of these molecules such as Subolesin (SUB) have been shown to protect against vector infestations and pathogen infection. Based on these premises, herein we characterized the efficacy of cattle vaccination with tick proteins involved in vector-pathogen interactions, TROSPA, SILK, and Q38 for the control of cattle tick, Rhipicephalus (Boophilus) microplus infestations and infection with Anaplasma marginale and Babesia bigemina. SUB and adjuvant/saline placebo were used as positive and negative controls, respectively. The results showed that vaccination with Q38, SILK and SUB reduced tick infestations and oviposition with vaccine efficacies of 75% (Q38), 62% (SILK) and 60% (SUB) with respect to ticks fed on placebo control cattle. Vaccination with TROSPA did not have a significant effect on any of the tick parameters analyzed. The results also showed that vaccination with Q38, TROSPA and SUB reduced B. bigemina DNA levels in ticks while vaccination with SILK and SUB resulted in lower A. marginale DNA levels when compared to ticks fed on placebo control cattle. The positive correlation between antigen-specific antibody titers and reduction of tick infestations and pathogen infection strongly suggested that the effect of the vaccine was the result of the antibody response in vaccinated cattle. Vaccination and co-infection with A. marginale and B. bigemina also affected the expression of genes encoding for vaccine antigens in ticks fed on cattle. These results showed that vaccines using tick proteins involved in vector-pathogen interactions could be used for the dual control of tick infestations and pathogen infection.

Expression analysis of heat shock protein 20 and rhoptry-associated protein 1a in sexual stages and kinetes of Babesia bigemina

Heat shock protein 20 (HSP-20) and rhoptry-associated protein 1a (RAP-1a) are two proteins considered as candidates to be included in vaccines or diagnostics methods for the control of bovine babesiosis. It has been hypothesized that both genes have a basic function in the cellular physiology of erythrocyte-infecting stages; it is not known if they have a functional role in tick stages. The objective of this work was to analyze whether hsp-20 and rap-1a are expressed in sexual stages and kinetes of Babesia bigemina. Purified RNA from sexual stages and kinetes was analyzed by reverse transcriptase (RT)-PCR with specific primers for hsp-20 or rap-1a. Expression analysis was carried out using an indirect immunofluorescence test with specific antibodies against HSP-20 and RAP-1a. Results obtained by RT-PCR showed amplicons for hsp-20 and rap-1a in sexual stages and kinetes. Positive signals were also detected when sexual stages and kinetes were analyzed with specific antibodies for HSP-20 and RAP-1a. The results obtained here confirm the hypothesis that the genes hsp-20 and rap-1a from B. bigemina are expressed in sexual stages and kinetes and stress the importance of these proteins in the cellular physiology of tick stages.