4. Indirect: Infected ticks inject the infective sporozoite stage into the mammalian host and the parasites enter erythrocytes where they multiply by binary fission. After division, merozoites invade other erythrocytes. Ticks become infected by the ingestion of intraerythrocytic parasites, called piroplasms. Sexual development occurs in the tick . B. bigemina is transmitted transtadially (one tick stage to another stage); in addition, it can invade the ovaries of adult female ticks, so can be transmitted transovarially within the tick's developing larval cells.