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Review
. 2019 Feb;49(2):183-197.
doi: 10.1016/j.ijpara.2018.11.002. Epub 2019 Jan 26.

Unravelling the cellular and molecular pathogenesis of bovine babesiosis: is the sky the limit?

Affiliations
Review

Unravelling the cellular and molecular pathogenesis of bovine babesiosis: is the sky the limit?

Carlos E Suarez et al. Int J Parasitol. 2019 Feb.

Abstract

The global impact of bovine babesiosis caused by the tick-borne apicomplexan parasites Babesia bovis, Babesia bigemina and Babesia divergens is vastly underappreciated. These parasites invade and multiply asexually in bovine red blood cells (RBCs), undergo sexual reproduction in their tick vectors (Rhipicephalus spp. for B. bovis and B. bigemina, and Ixodes ricinus for B. divergens) and have a trans-ovarial mode of transmission. Babesia parasites can cause acute and persistent infections to adult naïve cattle that can occur without evident clinical signs, but infections caused by B. bovis are associated with more severe disease and increased mortality, and are considered to be the most virulent agent of bovine babesiosis. In addition, babesiosis caused by B. divergens has an important zoonotic potential. The disease caused by B. bovis and B. bigemina can be controlled, at least in part, using therapeutic agents or vaccines comprising live-attenuated parasites, but these methods are limited in terms of their safety, ease of deployability and long-term efficacy, and improved control measures are urgently needed. In addition, expansion of tick habitats due to climate change and other rapidly changing environmental factors complicate efficient control of these parasites. While the ability to cause persistent infections facilitates transmission and persistence of the parasite in endemic regions, it also highlights their capacity to evade the host immune responses. Currently, the mechanisms of immune responses used by infected bovines to survive acute and chronic infections remain poorly understood, warranting further research. Similarly, molecular details on the processes leading to sexual reproduction and the development of tick-stage parasites are lacking, and such tick-specific molecules can be targets for control using alternative transmission blocking vaccines. In this review, we identify and examine key phases in the life-cycle of Babesia parasites, including dependence on a tick vector for transmission, sexual reproduction of the parasite in the midgut of the tick, parasite-dependent invasion and egression of bovine RBCs, the role of the spleen in the clearance of infected RBCs (IRBCs), and age-related disease resistance in cattle, as opportunities for developing improved control measures. The availability of integrated novel research approaches including "omics" (such as genomics, transcriptomics, and proteomics), gene modification, cytoadhesion assays, RBC invasion assays and methods for in vitro induction of sexual-stage parasites will accelerate our understanding of parasite vulnerabilities. Further, producing new knowledge on these vulnerabilities, as well as taking full advantage of existing knowledge, by filling important research gaps should result in the development of next-generation vaccines to control acute disease and parasite transmission. Creative and effective use of current and future technical and computational resources are needed, in the face of the numerous challenges imposed by these highly evolved parasites, for improving the control of this disease. Overall, bovine babesiosis is recognised as a global disease that imposes a serious burden on livestock production and human livelihood, but it largely remains a poorly controlled disease in many areas of the world. Recently, important progress has been made in our understanding of the basic biology and host-parasite interactions of Babesia parasites, yet a good deal of basic and translational research is still needed to achieve effective control of this important disease and to improve animal and human health.

Keywords: Babesia; Babesiosis; Parasite vaccines; Pathogenesis; Tick fever.

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Figures

Fig. 1
Fig. 1
Schematic representation of a simplified and partial life cycle of Babesia parasites. (A) Representation of the life-cycle of Babesia parasites in an adult, female tick (e.g. Rhipicephalus microplus) after taking a blood meal from an infected animal. Upon ingestion, the parasite develops sexual forms that fuse to form zygotes. Zygotes mature into kinetes upon invasion of the midgut epithelial cells of the tick which invade the tick hemolymph, where they can invade the ovaries and ultimately infect the larvae of the next generation of ticks. (B) Representation of Babesia infection and asexual reproduction in the bovine host. Sporozoites are introduced from tick saliva into the blood of their bovine host during blood feeding. (I) Sporozoites invade red blood cells (RBCs) and undergo asexual development. (II) Sporozoites mature into trophozoites inside the infected RBC. (III) Trophozoites divide asexually into two daughter merozoites inside infected RBCs. (IV) Merozoites are released into the blood following RBC lysis and then rapidly invade new RBCs. (C) Representation of splenic macrophage-mediated destruction of Babesia-infected RBCs in the blood circulation. Figure generated using BioRender.
Fig. 2
Fig. 2
Pseudo-coloured atomic force microscopy images of the surface of bovine red blood cells (RBCs) infected with late stages of either Babesia bovis or Babesia bigemina. The unique ridge-like features present on the surface of B. bovis-infected RBCs are notably absent from RBCs infected with B. bigemina. The right-hand panels represent a higher magnification view of the surface of the infected RBCs shown in the left-hand panels. The atomic force microscopy appearance of the surface of normal, uninfected bovine RBCs is similar to RBCs infected with B. bigemina (not shown).
Fig. 3
Fig. 3
Schematic representation of protective immune responses in bovines infected with Babesia parasites. (A) Representation of innate immunity in young calves. The innate immunity in young calves is characterised by rapid activation of macrophages, abundant release of Interferon-γ (IFN-γ) and nitric oxide (NO). Young, naïve calves are naturally more resistant to infection and usually survive the challenge upon exposure to Babesia-infected ticks in endemic areas (a process also known as pre-munisation). In contrast, adult animals are more susceptible to Babesia infection and usually develop acute, often fatal, babesiosis. Animals which survive acute infections can develop chronic babesiosis and produce life-long protective immune responses. Further, innate immune responses appear to be more pronounced in young, rather than adult, animals. (B) Representation of adaptive immunity in persistently infected or vaccinated animals. Macrophages and protective neutralising antibodies appear to be essential for control of parasitemia in vaccinated and persistently infected animals. Figure generated using BioRender.

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References

    1. Al-Khedery B., Allred D.R. Antigenic variation in Babesia bovis occurs through segmental gene conversion of the ves multigene family, within a bidirectional locus of active transcription. Mol. Microbiol. 2006;59:402–414. - PubMed
    1. Allred D.R., Al-Khedery B. Antigenic variation as an exploitable weakness of babesial parasites. Vet. Parasitol. 2006;138:50–60. - PubMed
    1. Allred D.R., Hines S.A., Ahrens K.P. Isolate-specific parasite antigens of the Babesia bovis-infected erythrocyte surface. Mol. Biochem. Parasitol. 1993;60:121–132. - PubMed
    1. Alzan H.F., Knowles D.P., Suarez C.E. Comparative bioinformatics analysis of transcription factor genes indicates conservation of key regulatory domains among Babesia bovis, Babesia microti, and Theileria equi. PLoS Negl. Trop. Dis. 2016;10 - PMC - PubMed
    1. Alzan H.F., Lau A.O., Knowles D.P., Herndon D.R., Ueti M.W., Scoles G.A., Kappmeyer L.S., Suarez C.E. Expression of 6-Cys gene superfamily defines Babesia bovis sexual stage development within Rhipicephalus microplus. PLoS ONE. 2016;11 - PMC - PubMed

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