The influence of heat on the development of Babesia microti in unfed nymphs of Ixodes dammini

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Abstract
Several colonies of tick larvae, each colony deriving from the eggs of a separate female, were fed on hamsters infected with Babesia microti isolated from a human infection. The nymphs were used at 13, 22, and 30 weeks post-larval-feeding (plf). At each time a batch of unfed nymphs was placed at 37.degree. C for 3-7 days. Nymphs from each of the 22- and 30-week plf groups either remained at 21.degree. C or were fed on an uninfected hamster for 64 h. In the nymphs kept at 21.degree. C, Babesia remained in the sporoblast meshwork stage. After ticks had been feeding for 64 h, sporozoites developed in both the 22- and 30-week plf groups. In unfed nymphs exposed to 37.degree. C for 3-7 days, sporozoites were already present in the two younger groups after 4 days, 1-2 days earlier and in better condition than those described previously. They contained all essential organelles and were similar in appearance to sporozoites from the fed nymphs, although they were fewer in number. The oldest group of nymphs contained only sporoblast meshwork. The present findings demonstrated that in unfed nymphs exposed to hyperthermia, the genetic makeup of the colony plays a decisive role in the success of sporozoite development. In both our present and previous experiments, sensitivity to hyperthermia was lost in older nymphs, and was lower in the very young, suggesting that at certain stages in the life cycle, age supersedes the genetic influence in sensitivity to hyperthermia.

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