Ultrastructure of sporogony in Babesia equi in salivary glands of adult female Boophilus microplus ticks.

The development of Babesia equi was studied in the salivary glands of adult female ticks, Boophilus microplus, using a transmission electron microscope (TEM). Engorged nymphs were obtained from splenectomized foals experimentally infected with B. equi and fed in the adult phase for 5 days on rabbits. Sporogony in B. equi involves the development of sporoblasts and sporozoites, which form from finger-like projections on the surface and through radial budding. Mature sporozoites (2.0 x 1.1 microns), typically pyriform, showed a polar ring, rhoptries, micronemes, nuclei, and mitochondria, and a high concentration of free ribosomes were observed from the 2nd day of the ticks, feeding on the rabbits. In general, sporogony of B. equi in the salivary glands of B. microplus showed similarities to the development of this parasite in species of Hyalomma, although with some significant differences in the sporozoite's dimensions. The results of this study indicate that B. equi is capable of multiplying in the salivary glands of adult female B. microplus, forming sporozoites with specialized organelles characteristic of the invasive form, and suggest that B. microplus can act as a natural vector of B. equi in endemic areas where there is no other probable source of infection or where it is the only tick species present on horses.