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Containing group: Hematozoa

Introduction

Order Piroplasmorida Wenyon, 1926

Piroplasms are species-rich haemosporidians, owing their name to pear-shaped (pyriform) intracellular stages formed in the host erythrocytes. Some piroplasmids cause severe diseases in ruminants, horses, pigs, dogs, cats, wild mammals and, exceptionally, in humans (Babesia microti, B. divergens, and the newly described B. duncani). They were found also in birds and reptiles. These diseases lead to fevers and often kill the host. The symptoms are usually hyperthermia, anemia and hemoglobinuria. Causative agents were discovered in 1888 by Victor Babeş. Vectors of these blood-parasitizing stages are the ixodid ticks, the involvement of which was also detected rather early by Smith and Kilborne in 1893. The complete and rather complicated life cycles, however, were described in the last 30 years.

The East Coast fever and tropical theileriosis in cattle and water buffaloes are caused by Theileria parva and Theileria annulata, respectively. Several Babesia species are responsible for babesiosis of cattle, horses, dogs, and rarely also humans. Poor growth, low milk production, and mortality of infected animals resulted in several efforts to control piroplasmoses. Before implementation of successful eradication programs focused on vectors, the direct and indirect annual loss caused by the piroplasmosis was estimated at more than $100 millions in the U.S. only. While under control in developed countries, these diseases still cause serious economical loss in tropical and subtropical countries. Since the eradication of tick vectors is not realistic in most tropical countries, there is a demand for effective control of piroplasmoses by alternative approaches. Vaccines using live Theileria parasites, soluble antigen from Babesia species or vaccines composed of subunits are being developed or have even reached the stage of clinical trials, but have yet to be tested on a large scale. Recombinant forms of T. parva and T. annulata sporozoite surface antigens induce protection against parasite challenge in cattle. In the future, vaccines might be improved by inclusion of tick peptides in multivalent vaccines.

Characteristics

Members of the Piroplasmasida represent a moderately consistent group of vertebrate blood cell parasites, which can be either piriform, or pear-shaped, round, amoeboid, or rod-shaped in morphology. Unique morphological features of piroplasmids are the absence of a conoid and the reduction of the apical complex to the polar ring. Extreme reduction is characteristic for the family Theileriidae, which lacks subpellicular microtubules, the inner membrane complex, as well as the micronemes. Blood protozoans of the genera Babesia and Theileria represent worldwide distributed parasites of both domestic animals and wildlife with zoonotic potential. So far, over 30 species of piroplasms have been described from the common domestic animals.

After entering the host cell, piroplasms escape from the parasitophorous vacuole and, with few exceptions (T. buffeli and T. separata), digest host hemoglobin without producing any pigment or other visible residues. For those reasons, piroplasms are generally called non-pigment forming haemoparasites. The heteroxenous life cycle is composed of merogony taking place in a wide range of mammals (to a lesser extent in birds and reptiles), with gamogony and sporogony occurring in the gut and salivary glands of invertebrates, respectively. So far, only hard ticks (Ixodidae) were identified as vectors, although for the majority of species, vectors are yet to be found.

Piroplasms are generally considered as highly specific parasites on the level of both intermediate (vertebrate) and definitive (tick serving as a vector) host. High specificity on the level of vectors has been repeatedly documented; however, the level of host specificity in their vertebrate hosts may be lower than described.

Discussion of Phylogenetic Relationships

Previous authors such as Babeş described the formation of parasites inside the red blood cells as coccal (Babesia) or bacillus-like (Theileria) bacteria. Lately it was confirmed that these pathogens are true protozoans, although their exact systematic position remained unclear for along time. Present systematics of piroplasms is based almost entirely upon criteria derived from taxonomic interpretation of a combination of morphology of the intraerythrocytic stages, host specificity, vector, mode of pathogenesis and pathogenicity, cross-immunity, frequency of schizonts and piroplasms, and spatial distribution.

The phylogenetic relationships within the order Piroplasmorida Wenyon, 1926 and the systematic position of the whole group and even the names of the genera and species of these piroplasms have been subject to constant revision and ongoing debate, and opinions have remained divergent to date. Historically, several genera had been described as belonging to the piroplasms: Theileria and Rangelia belonging to the family Theileriidae, which is characterized by preerythrocytic stages in lymphocytes, and Babesia, Nicollia, Nuttallia, Smithia, Rossiela, and Gonderia belonging to the family Babesiida and differing morphologically in the aspect of their intraerythrocytic form. Additionally, five more genera have been distinguished: Microbabesia, Babesiella, Achromaticus, Cytotauxzoon, and Piroplasma. The system has been recently reduced to the family Theileriidae du Toit 1918 comprising only the genus Theileria Bettencourt, Franca and Borges, 1907, consisting of 40 species. The family Babesiida Poche, 1913 contains the genus Babesia Starcovici, 1893 containing more than 110 species, and additionally the monotypic genera Echinozoon Garnham, 1951 and Entopolypoides Mayer, 1933. A third and forth family Anthemosomatidae Levine, 1981 (monotypic) and Haemohormidiidae Levine, 1984 with two genera have also been erected, but the life cycle and vectors of their members remain unknown.

The species belonging to the two well-accepted genera, Theileria and Babesia, seem to be significantly separated from each other by several biological features. However, the intraerythocytic stages of some species within these genera may have similar morphology. In vertebrate hosts Theileria species have a preerythrocytic development within lymphatic cells, while such schizogony is lacking in Babesia species. In ticks various stages of piroplasms enter not only the salivary glands, but also several other organs of their vectors and specifically in the genus Babesia even the eggs. Parasites are thus transmitted not only by tick saliva (alimentary infection) to vertebrate hosts but also trans-stadially (from larva to nymph) and trans-ovarially (from egg to larva); the latter path of transmission is absent in Theileria species. Molecular evidence obtained in recent studies, however, indicates that phylogenetic position and assignment of at least some species (e.g. Babesia microti, B. rodhaini, B. gibsoni and etc.) are problematic and these parasites differ from both Theileria and Babesia.

The validity of the genus Cytauxzoon remains unclear.  It contains the exoerythrocytic stages in its life cycle, which distinguish it from the genus Babesia. Cytauxzoon is transmitted by Dermacentor ticks. It occurs naturally in felines and causes a uniformly fatal disease in domestic cats. Two species are known, Cytauxzoon felis and C. manul. Its life cycle in the vertebrate host includes an intra-erythrocytic phase and a tissue phase consisting of large schizonts that develop in macrophages or monocytes, thus this piroplasm belongs to the family Theileriidae. However, its phylogenetic position is closely related to the above-mentioned species (e.g. Babesia gibsoni).

Other Names for Piroplasmorida Wenyon 1926

• Piroplasmasina

Information on the Internet

micro*scope: Piroplasmorida