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In Boophilus decoloratus infected by transovarian passage with B. bigemina, primary schizogony occurred as a continuous repetitive process in all 3 stages of the tick's life cycle spent on the host. The primary schizonts and the large merozoites (= vermicules) produced by them were observed in the gut epithelium, haemocytes, muscles, ad peritracheal cells. Secondary schizogony which led to the formation of small merozoites (= infective forms) occurred mainly in the salivary glands, but was also observed in the cortex of the synganglion. Mature small merozoites were observed in nymphal and adult ticks only. An infective stabilate was prepared from nymphae collected on Day 14 and Day 15 post larval infestation. The infections resulting from intravenous injection of the stabilate had a prepatent period of 8 days.

The development of Babesia caballi in the salivary glands of Hyalomma truncatum was studied at the electron microscopic level. Kinetes were first observed in the salivary glands of ticks on Day 2 of tick feeding and on each subsequent day of feeding until engorgement on Day 8. Sporogony appeared to involve the formation of cytomeres. After continued nuclear division, sporozoites formed when individual rounded nuclei were incorporated into portions of cytoplasm. Sporozoites were first observed on Day 4 of tick feeding and contained typical Babesia spp. organelles with a polar ring and up to 4 rhoptries, spherical bodies, a nucleus, mitochondria, endoplasmic reticulum and micronemes. The infection rate in the ticks was approximately 80%.

The development of small pyriform merozoites of B. bovis in the granule-secreting cells of the salivary glands of B. microplus larvae, studied with a light microscope, showed a close resemblance to that of B. argentina described by Riek (1966) in the same tick vector. Dvelopment took place through a process of schizogony and resulted in the formation of many merozoites. A study of the ultrastructure of developing merozoites in the schizont revealed the following: a poorly defined outer membrane; a granular osmiophilic inner membrane; anterior and posterior polar rings; rhoptries; micronemes; microtubules; a nucleus; spherical bodies of varying size. The schizonts were membrane-bound but no parasitophorous vacuoles were seen.

The electron microscope was used to study the structure of merozoites, merozoites in the process of transformation to trophozoites, trophozoites, and the method of multiplication of B. bigemina. The merozoites were piriform in shape and surrounded by 3 peripheral membranes of which the 2 inner ones often appeared as a single thick osmiophilic structure (inner membrane). Anterior and posterior polar rings, microtubules, micronemes, rhoptries and mitochondria with and without tubular cristae were discernible. A single large unidentified spherical body was present in most of the mature merozoites. After penetration of an erythrocyte, merozoites developed into trophozoites through a transformation process which involved the loss of the inner membrane of the pellicle, rhoptries, most of the micronemes and the spherical body. The trophozoites were surrounded by a single membrane, were pleomorphic in shape and contained large inclusions of host cell cytoplasm, but no cytostomes or food vacuoles could be identified. Reproduction took place through a process resembling schizogony resulting in the production of 2 merozoites, the cytoplasmic constituents of the original trophozoite (mother cell) being virtually entirely incorporated into the daughter cells in the process. None of the parasites were contained in parasitophorous vacuoles.

Electron microscopic studies on merozoites of Babesia bovis in epithelial cells of the gut of Boophilus microplus revealed that the pellicle apparently consists of 3 membranes, and an osmiophilic layer intimately associated with microtubules. Micropores in the pellicle were often associated with micronemes. An unidentified tubular structure extended from the anterior polar ring to the nuclear region where it appeared to be associated with the nuclear envelope. A Golgi complex, typical protozoan mitochondria, food vacuoles and rhoptries could not be identified.

The development of Babesia occultans in the salivary glands of adult Hyalomma marginatum rufipes was studied with the electron microscope. Sporogony involved a process of multiple fission in which sporozoites formed from the periphery of a polymorphous sporont. Different stages of development were found concurrently in individual acini as well as within individual acinar cells. Mature sporozoites were found on Day 3 post-tick attachment and measures 3.0-3.5 X 1.5 micron. The apical complex consisted of a polar ring and 4-6 rhoptries. Micronemes were concentrated anteriorly and 1 or 2 spherical bodies were identified in each sporozoite. The general pattern of development was similar to that described in several other Babesia spp. but distinct morphologic differences were noted.

Splenectomized vaccine donor calves undergoing primary reactions to Babesia bovis infections may develop cerebral babesiosis which leads to death if not treated in time. A brain biopsy was performed on an artificially-infected animal showing nervous symptoms and the tissue was immediately processed for electron microscopic examination. Virtually every erythrocyte in the brain capillaries sectioned was infected with B. bovis. Intra-erythrocytic merozoites, trophozoites and dividing trophozoites were indentified. Important features of the piriform merozoites included a reduced apical complex consisting of the anterior polar ring, microtubules, rhoptries and micronemes. Unidentified membrane-bound bodies, mostly spherical in shape, were observed anterior to the nucleus. The trophozoites showed very little structural differentiation and no food vacuoles or micropores could be detected. Each trophozoite produced 2 identical merozoites and the parent cell became totally incorporated in the daughter merozoites in the multiplication process. Projections were seen radiating from the surface of infected erythrocytes which appeared to adhere to other surfaces on contact. This probably resulted in the sludging of infected erythrocytes in the capillaries. The latter observations coincide with those described for Babesia argentina.