The Complexity of Piroplasms Life Cycles

Abstract

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.

Keywords: Babesia; Theileria; developmental cycle; gamogony; merogony; piroplasms; sporogony.

Figure 1

Lifecycle of piroplasms. (A) Lifecycle of species of Theileria sensu stricto lineage and Theileria equi includes intra-leucocytic schizogony prior to intra-erythrocytic merogony; schizogony is often accompanied by neoplastic transformation of host leucocytes. In contrast to Babesia species, the gametes of Theileria parasites form two morphologically distinguishable cell types, micro- and macro-gametes. Kinetes, which are produced in tick midgut cells, migrate directly to tick salivary glands where sporogony takes place. (B) Lifecycle of Babesia sensu stricto species comprise exclusively intra-erythrocytic asexual multiplication. Fertilization, which takes place in the tick midgut, is provoked by fusion of two morphologically indistinguishable gametes. The primary kinetes released from tick midgut cells invade various tick tissues, where secondary kinetes are produced. These then invade the tick salivary glands and undergo sporogony. All species of the lineage Babesia sensu stricto exploits transovarial transmission, a unique strategy of parasite invasion into ovarian cells by primary kinetes which results in Babesia-infected tick eggs and subsequent larvae. (C) The life cycle of Babesia microti group, the basal lineage of piroplasms, differs from species of the Babesia sensu stricto lineage by the lack of transovarial transmission.