The Small Piroplasms B Where Do They Belong?

Source:  Fred M. Borgstede Institute for Animal Science and Health, Lelystad, The Netherlands

The systematic position of many of the so-called 'small'  babesias has always been doubtful and has led to many controversies and ongoing debate over the years.  The classification of parasites into the genera Babesia  or Theileria  is primarily based on differences in their development in the vertebrate host and in the tick vector.  In vertebrates Babesia develop exclusively within erythrocytes, whereas Theileria have a pre-erythrocytic development in lymphocytes before erythrocytes are invaded.  In the tick, Babesia kinetes enter the salivary glands and several other organs, including ovaries.  Babesia parasites can thus being transmitted transovarially from one generation of ticks to the next.  This does not occur in Theileria  species. 

Molecular-genetic techniques provide another tool to demonstrate differences and similarities 
between these organisms.  Molecular sequence evidence, for example, indicates that some species formerly assigned to the genus Babesia should be reassigned to different genera.  Babesia equi has recently been reclassified as Theileria equi (Mehlhorn & Schein 1998) as a result of 18S rDNA sequence analysis (Allsopp, Cavalier-Smith, De Waal, and Allsopp, 1994) which confirmed previous observations of pre-erythrocytic stages in lymphocytes ((Schein, Rehbein, Voigt, and Zweygarth, 1981; Moltmann, Mehlhorn, Schein, Rehbein, Voigt, and Zweygarth, 1983; Moltmann et al., 1983).  Development in lymphocytes has also been reported for Babesia microti (Mehlhorn, Raether, Schein, Weber, and Uphoff, 1986) and 18S gene sequence evidence suggests that this organism should be removed from the genus Babesia.  Doubt has also been raised on the assignment of Babesia rodhani to this genus. (Ellis, Hefford, Baverstock, Dalrymple, and Johnson, 1992; Allsopp et al.,  1994; Mackenstedt, Luton, Baverstock, and Johnson,  1994). 

In a recent phylogenetic analysis employing three different algorithms (neighbour-joining, maximum parsimony and maximum likelihood), two monophyletic groups, one consisting of 3 B. microti isolates, Babesia rodhani, and a newly described small piroplasm of dogs and another consisting of the Babesia sensu stricto, were always demonstrated (Zahler, Rinder, and Gothe,  2000).  Neighbour-joining (distance-matrix) and parsimony methods additionally demonstrated the monophyly of a group containing all the Theileria spp. as well as Cytauxzoon felis and B. gibsoni USA. However, the maximum-likelihood analysis of this group was paraphyletic, with B. gibsoni USA and C. felis being ancestral to the remaining Theileriae and to the classic Babesiae.  Thus it would appear that a third phylogenetic group exists which, as the authors suggest, might support the existence of a third family besides the Theileriidae and the Babesiidae. 

Another study (Holman, Madeley, Craig, Allsopp, Allsopp, Petrini, Waghela, and Wagner, 2000) compared small Babesia isolated from an elk (Cervus elaphus canadensis) and a caribou (Rangifer tarandus caribou) to Babesia odocoilei from white-tailed deer (Odocoileus virginianus) and found that the SSU rDNA sequences of these Babesia were identical.  A phylogenetic tree constructed from SSU rRNA gene sequences showed that B. odocoilei was most closely related to B. divergens, both of which branched together with the Babesia sensu stricto. 

Genotyping different isolates of canine small babesiae, previously regarded as Babesia gibsoni on the basis of their similar morphology, has revealed an unexpected variability.  B. gibsoni has strong superficial similarities to T. equi, since the forms most frequently seen in erythrocytes are more or less round with occasional Maltese cross forms seen in some isolates, mainly from America.  These forms appear to be absent in parasites described from Asia.  18S rRNA gene sequences from 4 isolates of B. gibsoni, originating from Japan, Malaysia and Sri Lanka showed almost identical genotypes but differed widely from an isolate from California, USA.  The American isolate segregates together with T. equi while the Asian isolates showed a close relationship with the true Babesia, B. divergens, B. odocoilei, B. caballi and B. bigemina. (Zahler, Rinder, Zweygarth, Fukata, Maede, Schein, and Gothe,  2000) These authors concluded that the Asian parasites clearly belong to the genus Babesia while the American parasite should be attributed to the genus Theileria.  The saga continues, however, when another small 'Babesia', found in the blood of a dog from Spain, was compared with other B. gibsoni isolates.  The Spanish isolate clustered separately from the Asian and USA B. gibsoni isolates and was most closely positioned to B. microti and B.  rodhani. (Zahler, Rinder, Schein, and Gothe,  2000) Although the authors suggest the name Theileria annae for this parasite, it is premature in my opinion as they have not studied the life cycle in vivo nor have they shown that it can be transmitted from dog to dog or even attempted to identify the tick vector(s).  

It is also of interesting to note that phylogenetic analysis of 18S rRNA gene sequences amplified from blood of a baboon chronically infected with Entopolypoides macaci demonstrated this parasite to be most closely related to B. microti (Bronsdon, Homer, Magera, Harrison, Andrews, Bielitzki, Emerson, Persing, and Fritsche,  1999) giving support to Levine's speculation that the two genera are synonymous. 

(Levine, 1988). 

One cannot but wonder where the avian Babesia spp. would fit.  It is time that someone starts sequencing these as well! 

References

  Allsopp, M.T., Cavalier-Smith, T., De Waal, D.T. & Allsopp, B.A. 1994 . Phylogeny and evolution of the piroplasms. Parasitology, 108: 147-152. 

·  Bronsdon, M.A., Homer, M.J., Magera, J.M., Harrison, C., Andrews, R.G., Bielitzki, J.T., Emerson, C.L., Persing, D.H. & Fritsche, T.R.  1999. Detection of enzootic babesiosis in baboons (Papio cynocephalus) and phylogenetic evidence supporting synonymy of the genera Entopolypoides and Babesia. J. Clin. Microbiol., 37: 1548-1553. 

·  Ellis, J., Hefford, C., Baverstock, P.R., Dalrymple, B.P. & Johnson, A.M.,  1992.  Ribosomal DNA sequence comparison of Babesia and Theileria. Mol. Biochem. Parasitol., 54: 87-95. 

·  Holman, P.J., Madeley, J., Craig, T.M., Allsopp, B.A., Allsopp, M.T., Petrini, K.R., Waghela, S.D. & Wagner, G.G. 2000. Antigenic, phenotypic and molecular characterization confirms Babesia odocoilei isolated from three cervids. J. Wildl. Dis., 36: 518-530. 

·  Levine, N.D. 1988. The Protozoan Phylum Apicomplexa. Vol 2: p. 154. 

·  Mackenstedt, U., Luton, K., Baverstock, P.R. & Johnson, A.M.,  1994. Phylogenetic relationships of Babesia divergens as determined from comparison of small subunit ribosomal RNA gene sequences. Mol. Biochem. Parasitol., 68: 161-165. 

·  Mehlhorn, H., Raether, W., Schein, E., Weber, M. & Uphoff, M.  1986. [Light and electron microscopy studies of the developmental cycle and effect of pentamidine on the morphology of intra-erythrocyte stages of Babesia microti]. Dtsch. Tierärztl. Wochenschr., 93: 400-405. 

·  Mehlhorn, H. & Schein, E.  1998. Redescription of Babesia equi Laveran, 1901 as Theileria equi Mehlhorn, Schein 1998. Parasitol. Res., 84: 467-75. 

·  Moltmann, U.G., Mehlhorn, H., Schein, E., Rehbein, G., Voigt, W.P. & Zweygarth, E. 1983. Fine structure of Babesia equi Laveran, 1901 within lymphocytes and erythrocytes of horses: an in vivo and in vitro study.  J. Parasitol., 69: 111-20. 

·  Schein, E., Rehbein, G., Voigt, W.P. & Zweygarth, E. 1981. Babesia equi (Laveran 1901) 1. Development in horses and in lymphocyte culture. Tropenmed. Parasitol., 32: 223-7. 

·  Zahler, M., Rinder, H. & Gothe, R. 2000. Genotypic status of Babesia microti within the piroplasms. Parasitol. Res., 86: 642-6. 

·  Zahler, M., Rinder, H., Schein, E. & Gothe, R.  2000. Detection of a new pathogenic Babesia microti-like species in dogs. Vet. Parasitol., 89: 241-8. 

·  Zahler, M., Rinder, H., Zweygarth, E., Fukata, T., Maede, Y., Schein, E. & Gothe, R. 2000. 'Babesia gibsoni' of dogs from North America and Asia belong to different species. Parasitology, 120: 365-9. 

(This interesting summary of recent developments was contributed by Theo de Waal B TW)