Sexuality in piroplasms as revealed by electron microscopy in Babesia microti.

Protozoa of the closely related genera Babesia and Theileria are intraerythrocytic parasites of vertebrates. They have a complex life cycle that includes development in an intermediate vector host, a tick. Whether sexual stages occur in the tick has been a subject of great controversy. The small size of the organism and the complexity of developmental stages in the gut of the tick have prevented a definitive solution of this problem. By means of a simple and straightforward although time-consuming method, it became possible to demonstrate gametes and their sexual fusion in Babesia microti developing in the gut of larvae of the tick Ixodes dammini. Tick larvae fed on hamsters infected with a human strain of B. microti were fixed and processed for electron microscopy. It was found that some of the parasites formed a unique structure shaped like an arrowhead. Because it was suspected that these forms might represent gametes, a search was made for pairs of parasites that were fusing and with each member of the pair emerging from a different erythrocyte. Such a fusing pair could not possibly represent a parasite undergoing division. By study of serial sections such pairs were indeed found. In every case one member of the pair of gametes had an arrowhead structure. This proves sexuality of B. microti and makes highly likely its existence in all members of the genera Babesia and Theileria.