Abstract
Babesiosis is considered an emerging disease because its incidence has significantly increased in the last 30 years, providing evidence of the expanding range of this rare but potentially life-threatening zoonotic disease. Babesia divergens is a causative agent of babesiosis in humans and cattle in Europe. The recently sequenced genome of B. divergens revealed over 3,741 protein coding-genes and the 10.7-Mb high-quality draft become the first reference tool to study the genome structure of B. divergens. Now, by exploiting this sequence data and using new computational tools and assembly strategies, we have significantly improved the quality of the B. divergens genome. The new assembly shows better continuity and has a higher correspondence to B. bovis chromosomes. Moreover, we present a differential expression analysis using RNA sequencing of the two different stages of the asexual lifecycle of B. divergens: the free merozoite capable of invading erythrocytes and the intraerythrocytic parasite stage that remains within the erythrocyte until egress. Comparison of mRNA levels of both stages identified 1,441 differentially expressed genes. From these, around half were upregulated and the other half downregulated in the intraerythrocytic stage. Orthogonal validation by real-time quantitative reverse transcription PCR confirmed the differential expression. A moderately increased expression level of genes, putatively involved in the invasion and egress processes, were revealed in the intraerythrocytic stage compared with the free merozoite. On the basis of these results and in the absence of molecular models of invasion and egress for B. divergens, we have proposed the identified genes as putative molecular players in the invasion and egress processes. Our results contribute to an understanding of key parasitic strategies and pathogenesis and could be a valuable genomic resource to exploit for the design of diagnostic methods, drugs and vaccines to improve the control of babesiosis.
Highlights
Babesiosis is a worldwide emerging infectious disease [1] caused by a protozoan parasite of the genus Babesia which is naturally transmitted by ixodid ticks and infects vertebrate erythrocytes
The size and number of chromosomes are similar to those reported for the B. bovis genome [20] where chromosomes 1, 2, 3 and 4 are estimated to be 1.4, 2.0, 2.8 and 3.2 million base pairs, respectively. These results suggest that the probable ordering for our assembly, where we could assign contigs/scaffolds into chromosomes, is very similar to that observed in B. bovis
The strategy developed shows how genome sequencing and RNA sequencing (RNAseq) can further our understanding of transcriptional patterns of B. divergens blood stages
Summary
Babesiosis is a worldwide emerging infectious disease [1] caused by a protozoan parasite of the genus Babesia which is naturally transmitted by ixodid ticks and infects vertebrate erythrocytes. Parasite infection of natural hosts such as in cattle causes bovine babesiosis, resulting in high economic losses. The severity of human infection has become apparent [2]. The four identified Babesia species confirmed to infect humans to date are B. microti, B. divergens, B. duncani, and B. venatorum [2,3]. B. microti has a worldwide distribution while B. divergens is a causative agent of babesiosis in humans and cattle in Europe [3]. In asplenic, immune-compromised patients, children, elderly people and patient who are experiencing B. divergens infection, the disease can be severe. B. divergens infections are considered medical emergencies and patients require immediate treatment [3]
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