Hyperthermia and sporogony of <i>Babesia microti</i> in the tick <i>Ixodes dammini</i>: an ultrastructural study

Abstract

An ultrastructural study of Babesia microti sporozoites in fed and unfed heated nymphs of Ixodes dammini was performed to explain differences in the efficiency of sporozoite production. Three colonies of tick larvae, each deriving from eggs of a single female, were fed on hamsters infected with Babesia microti isolated from a human infection. They were kept at 19 °C in conditions of saturated humidity through the larval and nymphal stages. At the time of the experiment, the ages of nymphs from colonies 1, 2, and 3 were 2–2½, 4–4½, and 6½–7 months postmolt, respectively. Nymphs from all colonies were exposed to each of the following conditions: (i) no feeding at 19 °C for 4–7 days; (ii) feeding on an uninfected hamster for 62–64 h at room temperature; (iii) no feeding at 37 °C for 4–7 days. In unfed nymphs at 19 °C, Babesia remained in the sporoblast stage in all three colonies. In nymphs fed for 62–64 h, sporozoites developed in all three colonies. In unfed nymphs exposed for 4–7 days to a temperature of 37 °C, sporozoites formed only in the younger colonies (1 and 2). Only a part of some sporoblasts developed sporozoites and only a few sporozoites had all characteristic structures. Some of them lacked major specialized organelles and were apparently damaged or dead, particularly in colony 1. It appears from this study that colonies differ in their reaction to hyperthermia. In the oldest colony (6½–7 months postmolt), no sporozoites were formed after 4–7 days of exposure to 37 °C, indicating that sensitivity to hyperthermia is lost in older nymphs.