Babesia are tick borne, as was first shown by Smith and Kilborne (1893). The developmental cycle in the tick is incompletely known. The best accounts are those of Christophers (1907)[216] for B. canis and Koch (1906) for B. bovis, and these accounts are supplementary. The principal stages, so far as known, may be summarized thus:—
(1) The piroplasms taken by the tick in feeding on blood pass into the tick’s stomach. The pyriform parasites, which alone are capable of further development, are set free from the blood corpuscles. In about twelve to eighteen hours they become amœboid, sending out long, stiff, slender, pointed pseudopodia. The nucleus of each parasite divides unequally into two. Similar forms have been obtained in cultures. These stellate forms may be gametes, and according to Koch fuse in pairs.
(2) A spherical stage follows, possibly representing the zygote. This grows, and a uninucleate globular mass results. This form is found in large numbers on the third day, according to the observations of Koch.
(3) A club-shaped organism is next formed. This may represent an oökinete stage. The club-shaped bodies are motile and gregarine-like, and are about four times the size of the blood forms. These club-shaped bodies and subsequent stages were described by Christophers in the development of B. canis in the dog-tick, Rhipicephalus sanguineus.
(4) The club-shaped bodies pass from the gut of the tick into the ovary, and so get into the ova. There they become globular, and later are found in the cells of the developing tick-embryo. The parasites are, then, transmitted hereditarily. Similar globular bodies are found in the tissue cells of the body of tick nymphs which have taken up piroplasms. The globular stage was called the “zygote” by Christophers, but it may correspond to the oöcyst of Plasmodia.
(5) The globular body divides into a number of “sporoblasts,” which become scattered through the tissues of the larval or nymphal tick, as the case may be.
(6) The sporoblasts themselves divide into a large number of sporozoites, which are small uninucleate bodies, somewhat resembling blood piroplasms. The sporozoites collect in the salivary glands of the tick. They are inoculated into the vertebrate when the tick next feeds.
The chief species of Babesia and their pathogenic importance may be listed thus:—
(1) Babesia bovis (Babes) produces infectious hæmoglobinuria of cattle in Europe and North Africa. It is transmitted by Ixodes ricinus. A similar parasite also occurs in deer.