Lyme disease transmission by severely impaired ticks

Abstract

It has been demonstrated that impairing protein synthesis using drugs targeted against tRNA amino acid synthetases presents a promising strategy for the treatment of a wide variety of parasitic diseases, including malaria and toxoplasmosis. This is the first study evaluating tRNA synthetases as potential drug targets in ticks. RNAi knock-down of all tested tRNA synthetases had a strong deleterious phenotype on Ixodes ricinus feeding. Our data indicate that tRNA synthetases represent attractive, anti-tick targets warranting the design of selective inhibitors. Further, we tested whether these severely impaired ticks were capable of transmitting Borrelia afzelii spirochaetes. Interestingly, biologically handicapped I. ricinus nymphs transmitted B. afzelii in a manner quantitatively sufficient to develop a systemic infection in mice. These data suggest that initial blood-feeding, despite the incapability of ticks to fully feed and salivate, is sufficient for activating B. afzelii from a dormant to an infectious mode, enabling transmission and dissemination in host tissues.

Keywords: Borrelia; Lyme disease; borreliosis; tRNA synthetase; tick; transmission.

Figure 1.

RNAi effect on tick feeding capacity. Silencing of the Asparaginyl-tRNA synthetase, Valyl-tRNA synthetase, Leucyl-tRNA synthetase and Lysyl-tRNA synthetase and their impact on engorgement weight in I. ricinus (a) adult females and (b) nymphs. Photographic images of five individual representative ticks after 8 days (adults) and 3 days (nymphs) of feeding are shown on top. Bottom graphs show weights of all ticks after 8 days (adults) and 3 days (nymphs) of feeding. Bars indicate standard errors of means. Success rate indicates the percentage of how many ticks managed to fully engorge.

Figure 2.

Effect of tRNA synthetase silencing on salivary gland morphology. Microscopic images of representative salivary glands dissected from silenced I. ricinus nymphs fed for 3 days. Scale bars represent 100 µm. The graph represents the size of 10 individual acini in each experimental group. The area of acini was measured using the ImageJ program. Bars indicate standard errors of means. ****, p < 0,0001.

Figure 3.

Quantitative evaluation of Borrelia spirochaete loads in murine tissues after transmission by ‘zombie’ ticks. Each data point represents the number of B. afzelii spirochaetes per 10⁶ murine genomes in the individually analysed ear, bladder and heart biopsy specimens. Bars indicate standard errors of means. Differences in spirochaete load between mice infested with asn-, val-, leu-, lys- and gfp-dsRNA were not statistically significant.