Ixodes scapularis: All Bite and No Bark

         The hard-bodied tick Ixodes scapularis has an impressive arsenal of morphological traits that make it a highly adapted ectoparasite. As mentioned previously, in 1904 George Nuttall described the ability of Ixodes to climb vertical glass surfaces fully engorged from a blood meal. The powerful climbing legs of the tick terminate in strong pointed claws, making them more than capable of ascending to an ideal feeding spot on their vertebrate host.

            But once I. scapularis has positioned itself to feed, the truly impressive biology of this organism becomes apparent. A diagram of tick mouthparts taken from Service’s Medical Entomology for Students is shown below.

 Photo credit: Mike Service

Absent a “true head” structure, the capitulum (‘false head’) is slightly visible from above, with the majority of the visible structure consisting of the holdfast structures (Service, 2008). Palps are club-shaped, and covered with several spikes for anchoring into flesh. Both the hypostome (harpoon-like structure) and chelicerae (grasping mandibles) are injected through host tissue during a feed. The salivary glands secrete an adhesive substance that fuses the mouthparts in place. In 2006, Patricia Nuttall and Christian Paesen filed a patent claim for the ixodid cement polymer coded by a specific sequence in the I. scapularis genome. Research is currently being done on the glue and its derivatives as a surgical adhesive. 

Apart from adhesive function, tick saliva also functions as an anticoagulant. According to Narasimhan et al., ixodid ticks usually take 4 to 10 days to complete a feed. During this time, the anticoagulant agents found in the saliva modulate host immune response, facilitating the transmission of pathogens. Two genes, Salp14 and over 30 paralogs (similar genes) are found in adult and nymph Ixodes. This abundance of variety among anticoagulants has proven particularly troublesome to efforts to produce effective vaccines.

            A clever solution to this problem, reported by Narasimhan et al. in 2003, is the implementation of post-transcriptional gene silencing by RNA interference (RNAi). Single-stranded binds to and destroys specific mRNA molecules before protein production can complete. Because Salp14 paralogs share 80-90% of nucleotide sequences, RNA interference effectively circumvents the tick’s genetic arsenal of variable anticoagulants. 

By introducing these foreign sequences of DNA, the tick’s ability to produce Salp14 family anticoagulants is greatly reduced. The paper reported a 50-70% decline in engorgement (post-feeding) weights in the ticks. While infection rate of Borellia burgdorferi (the agent that causes Lyme disease) was not measured in the study, and the authors do not anticipate RNAi-based treatments fully preventing pathogenic transmission, they do predict reduced transmission as a result of 50%+ reduction in engorgement (Narasimhan et al., 2009).

            Much work is still required in developing vaccines against pathogenic infection through tick vectors such as Ixodes scapularis. Still, RNAi techniques may prove essential to effectively combatting these diseases. Until then, we'll just have to appreciate Ixodes salivary compounds for what they are: some pretty spiffy spit.

Works Cited

Service, Mike. Medical Entomology for Students. 4th ed. Cambridge: Cambridge University Press, 2008. Digital.

Narasimha, S., Montgomery, R., DePonte, K., Tschudi, C., Marcantonio, N., Anderson, J., Sauer, J., Cappello, M., Kantor, F., Fikrig, E. 2003. Disruption of Ixodes scapularis anticoagulation by using RNA interference. Proc. Natl. Acad. Sciences. Vol. 101(5): 1141-1146.