Babesia: parasites which develop within erythrocytes

Photo Laboratoire de Biologie Cellulaire et Moléculaire

    The genus Babesia comprises a large group of protozoans which develop within erythrocytes. They are transmitted by tick bites and infect a large range of vertebrates.
The taxonomic classification of these parasites is based on: i) the demonstration of an apical complex, which places the Babesia in the Apicomplexa lineage, ii) the absence of a conoid which positions them in the class Aconoidasida and iii) their pear-shaped morphology (piriform) which implies their membership in the order Piroplasmida.
Siixty-eight species of Babesia are known to date, and at least eighteen are pathogenic in domestic or wild animals (cattle, sheep, goats, horses, pigs, cats, dogs, rodents) or humans. The principal clinical signs of babesiosis are generally fever, hemolytic anemia, and hemoglobinuria. In severe cases, the disease may progress to coma and death.

Babesia microti, a rodent parasite which causes human babesiosis.
Because of its worldwide geographic distribution, the species most frequently implicated in human babesiosis today is Babesia microti. In Europe, however, B. divergens is also responsible for human babesiosis in splenectomized patients, and unidentified species different from B. microti and B. divergens have also been implicated in America, Europe and Asia.

Life cycle of Babesia microti (Photo from the site of the Division of Parasitic Diseases)

The cycle of Babesia microti involves a mammalian host and a tick vector. During a blood meal, a tick infected by B. microti injects sporozoites into its rodent host (1). The sporozoites penetrate the erythrocytes and reproduce asexually by budding, leading to the formation of 2 or 4 merozoites which initiate new endoerythrocyte cycles (2). In the blood, some parasites differentiate into male or female gametocytes (3). The definitive host is a tick of the Ixodides family; once ingested by the appropriate tick (4), the gametes undergo a sporogony cycle which leads to the formation of new sporozoites (5).
Humans enter the cycle whey they are bitten by an infected tick (6). A tick infected by Babesia microti injects sporozoites and an endoerythrocytic cycle is initiated. The multiplication of the blood stages of the parasite is responsible for the clinical manifestations of babesiosis (7). Furthermore, human-to-human transmission via blood transfusion is known, and has been well-documented (8).

Importance of sequencing the genome of Babesia microti
B. microti is a Eukaryote which lends itself well to complete sequencing because of its haploid nature and its relatively small size (5.2 Mb, 3 chromosomes). The genome of B. microti is the smallest genome described to date in the Apicomplexes, which suggests an original genomic structure and a minimal proteome for this parasite. The determination of this minimal genomic organization for parasites with a complex life cycle could lead to a definition, within the Apicomplexes, of the minimal protein information required for an essential function.

Babesia microti infecting human erythrocytes (Photo from the site of CEMERA)

Furthermore, despite recent studies, the precise phylogenetic positioning of B. microti remains uncertain. B. microti seems to belong in the ancestral group of Archaeopiroplasmides, which gave rise to the Theilerides, Babesides and Ungulibabesides groups. For Babesia in the strict sense, only the genome of B. bovis (Ungulibabesides, which exclusively parasitizes cattle) is being sequenced at the moment.
The sequencing of the genome of B. microti will provide the first genomic resource from the family of Piroplasmides which is pathogenic for humans. This sequencing will be of primordial utility for the characterization of antigens implicated in a given function, and could provide the basis for defining a vaccine and/or chemiotherapy against human babesiosis caused by B. microti.