Differential development and emission of Theileria parva sporozoites from the salivary gland of Rhipicephalus appendiculatus
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The initiation of feeding of infected Rhipicephalus appendiculatus adults induces the rapid development of Theileria parva sporoblasts within the salivary gland acini leading to the production of numerous sporozoites which are inoculated into the mammalian host initiating infection. In this study the pattern of development, host cell specificity and emission of T. parva sporozoites within the salivary glands of heavily infected, 4-day fed adult R. appendiculatus ticks was examined. Infected acini were randomly distributed throughout the salivary gland. Sporozoite development within each gland was not synchronized and wide variation in the rate of parasite development, which correlated with the secretory activity of the individual acinus, was observed in all glands examined. Previous studies had shown that T. parva developed primarily in Type III `e' cells. However, in heavily infected salivary glands sporogony and the emission of mature sporozoites also occurred in `c' cells of Type II acini. Sporozoite emission from infected cells occurred by a process similar to apocrine secretion. The loss of the apical membrane of the infected cell allowed sporozoites free access to the lumen of the acinus and into the collecting ducts of the salivary gland. Sporozoite discharge was gradual since few parasites were found in the acinus valve or in the collecting ducts. Furthermore, the small size of the acinar valve aperature ensures that only small numbers of sporozoites can be released at any one time from an infected acinus.