Infection of the intermediate mite host with Wolbachia-depleted Litomosoides sigmodontis microfilariae: impaired L1 to L3 development and subsequent sex-ratio distortion in adult worms

The rodent filaria Litomosoides sigmodontis harbour Wolbachia, endosymbionts essential for worm embryogenesis, larval development and adult survival. To study the effect of tetracycline, which depletes Wolbachia, on the development of microfilariae (L1s, MF) to L3 in the intermediate host Ornithonyssus bacoti, and to observe the development of Wolbachia-depleted L3s in Mongolian gerbils (Meriones unguiculatus); microfilaremic gerbils were treated orally with tetracycline for 6 weeks (primary infected (1°) Tet) or untreated (1° Con). Treatment resulted in a significant reduction of Wolbachia per MF in 1° Tet gerbils. Naïve mites then fed on the 1° Tet and 1° Con gerbils in the week after treatment ended, when MF levels were not significantly different, and used to infect new gerbils (secondary infected (2°) Tet, 2° Con) via natural infection. The infection rate from dissected mites was 9% and 54% (1° Tet and 1° Con, respectively). After 3 months, worms were isolated from 2° gerbils. Significantly fewer female worms developed in 2° Tet gerbils. In contrast, there was no difference in the number of male worms that developed in 2° gerbils, resulting in a male biased sex-ratio. Although 2° Tet male worms had fewer Wolbachia than 2° Con males, development was not impaired. Female worms that developed from Wolbachia-depleted MF had Wolbachia levels equivalent to worms from 2° Con animals. Thus, tetracycline pre-treatment selected for female worms with high numbers of Wolbachia, whereas male worms had median Wolbachia levels significantly lower than 2° Con males. Therefore, female worms require a higher threshold of Wolbachia for their development. The worms analysed were only exposed to tetracycline as MF, ruling out direct effects of tetracycline during larval development in the mites or 2° gerbils, suggesting that the depletion of Wolbachia in MF was the cause of impaired larval development.