Molecular and in vitro characterization of a Babesia divergens-like agent from eastern cottontail rabbits (Sylvilagus floridanus) on Nantucket Island

Abstract

A Babesia sp. isolated from eastern cottontail rabbits (Sylvilagus floridanus) is morphologically similar and genetically identical, based on SSU rRNA gene comparisons, to two agents responsible for human babesiosis in North America and is closely related to the European parasite, Babesia divergens. The ribosomal RNA (rRNA) internal transcribed spacer regions (ITS1 and ITS2) and the 5.8S rRNA genes of Babesia isolates were sequenced and analyzed. The rRNA ITS region sequences of three isolates, one each from Kentucky, Massachusetts and Great Britain, considered Babesia divergens-like organisms, were compared to two Babesia microti isolates, two Babesia odocoilei isolates and a well defined Babesia divergens isolate. The two B. divergenslike isolates from North America shared identical rRNA ITS1-5.8S-ITS2 region sequences, and the clones of these isolates clustered into one clade in three phylogenetic analyses, suggesting that these isolates are conspecific. In vitro comparison of host erythrocyte specificity between the rabbit Babesia sp. and B. divergens was employed to discriminate between the two organisms and to determine the usefulness of in vitro techniques for Babesia sp. characterization. In vitro growth of the rabbit Babesia sp. was supported in human and cottontail rabbit erythrocytes, but not in bovine cells. Babesia divergens in vitro growth was supported in human and bovine erythrocytes, but not in cottontail rabbit cells. Morphological characteristics and size differences also distinguished the two parasites from one another. The erythrocyte specificity and parasite size differences reported in this study agree with previous in vivo results and validate the use of in vitro methods for characterization of Babesia species.