From the perspective of a parasite: What factors determine success for a tick?

Background/Question/Methods

Seldom does the study of parasite-host relationships consider the perspective of a parasite living in the landscape of a host’s body. Yet for parasites that spend significant time on another animal, such as black-legged ticks (Ixodes scapularis), the conditions on their host may strongly influence their success. While there are many factors that influence the distribution of questing ticks in an environment, such as humidity and host density, simply finding a host does not guarantee reward. After landing on a host, ticks face a novel, hostile environment with risks and rewards that must be navigated successfully. For this study, the environment in question is on the white-footed mouse, Peromyscus leucopus. What affects the success of ticks on a mouse host? We exposed 34 P. leucopus to known numbers of I. scapularislarvae and monitored tick activity every 12 hours. We then used path analysis to evaluate and test whether fixed factors such as host sex, number of ticks applied, and minutes of restraint from grooming directly or indirectly affect three measures of tick success: the proportion of ticks that landed on the host, the proportion of successfully engorged ticks, and length of time ticks took to detach from the host.

Results/Conclusions

Our model was supported, yielding a Chi-square value of 6.468 (df = 4, p = 0.167). With the three fixed factors, we can explain 30-50% of the variation in our measures of tick success: proportion of ticks on host (R² = 0.31), proportion of engorged ticks (R² = 0.53), and the tick drop period duration (R² = 0.37). Longer restraint from grooming immediately following infestation positively affected both the proportion of ticks that landed on the mouse (p = 0.002) and the length of time that a mouse had ticks on it (p = 0.029). In turn, longer periods of tick detachment were associated with higher proportions of successfully engorged ticks (p = 0.005). Intrinsic qualities such as host sex did not significantly affect tick success. Life on a host is a balance of risks and rewards, as a tick attempts to find a feeding site and avoid removal. Our results suggest that ectoparasites face the same influences of predation, density dependence, and even foraging strategy as other traditionally-studied organisms.