Primary and secondary ookinetes of Babesia microti in the larval and nymphal stages of the tick Ixodes dammini

Abstract

The ultrastructure of ookinetes (kinetes) of Babesia microti and their relationship to the larval and nymphal stages of the tick vector Ixodes dammini are described. Ookinetes in the larva are elongate, straight, or gently curved, more than 8 μm long, with a clearly defined anterior end, and surrounded by a pellicular complex composed of a plasma membrane and two inner discontinuous membranes. At 13 days following cessation of feeding, the parasite was detected in larval tissues, including salivary glands, fat body, and nephrocytes. A sequence of stages in the asexual reproduction of the parasite is described. In the larva, primary ookinetes dedifferentiate to form a kinetoblast that grows and then redifferentiates yielding secondary ookinetes. Ookinetes reinvade the salivary glands of unfed nymphs beginning immediately after completion of the molt and continuing for a short time thereafter. There they once again dedifferentiate and grow into a vast ramifying sporoblast meshwork, which lies quiescent until feeding stimulates the redifferentiation and formation of infective sporozoites through an asexual process. Babesia microti thus resembles some other species of Babesia in that the parasite can invade somatic tissues other than salivary glands and yet differs from them in the apparent absence of transovarial transmission.