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Tick males and females are obligatory blood-feeders and must eat blood in each of their three life stages to survive. Of the approximately 900 tick species identified worldwide, only 10 to 20 species transmit human pathogens. Still, these few species carry a greater variety of infectious agents than any other group of blood-feeding arthropods. Ticks are on the rise, and their population in the U.S. has more than doubled over the last 14 years. My research aims to understand better the neuronal mechanisms that drive tick behavior, to support efforts to help restrict the spread of ticks and tick-borne diseases. However, our current knowledge of tick neurophysiology and their sensory abilities is limited, and we have no good understanding of what makes a tick tick.
Many sensory abilities have been proposed to be used by ticks to locate and select their hosts. These include the detection of body odor, carbon dioxide (CO2), moisture, heat, vibrations, and visual contrast, most of which have been attributed to a single multisensory organ – the tick Haller’s organ. This organ is located at the first tarsal segment of the tick forelegs, but rigorous studies of its sensory abilities are lacking. For example, it is widely accepted that ticks are attracted to breath-like CO2 concentrations (4%) and body heat. Yet, quantitative and qualitative studies that describe CO2 responses and heat-sensing for the most economically important tick species are absent. Furthermore, anatomical structures and receptors involved in tick CO2 detection and heat-sensing, although suspected to lie within the Haller's organ, have yet to be identified. It is also unknown whether CO2 detection potentiates the integration of other sensory host-emitted cues such as body heat.
My study focuses on Ixodes scapularis, the primary vector for Lyme disease in the U.S. and one of the most economically important tick species. I tested I. scapularis’ ability to sense CO2 and heat-stimuli in a behavioral assay. My results show that I. scapularis responses to CO2 are context-dependent in that the state of the animal determines the behavior elicited at elevated CO2 levels. I also show that CO2 is indeed exclusively detected by the foreleg Haller’s organ. However, my data indicate that I. scapularis do not show a solid attraction to heat-stimuli. This was independent of whether heat stimuli were presented alone or in combination with CO2, suggesting that tick host-seeking strategies differ from other blood-feeding animals, such as mosquitos and parasitic nematodes, for example.
