Trends in Parasitology
ReviewWill new ticks invade North America? How to identify future invaders
Section snippets
What is a tick invasion?
Ticks are a clade within Acari (mites) of obligately blood-feeding parasites. With over 700 species, the hard ticks or Ixodidae are present on all continents and occupy a broad array of ecological niches [1]. Tick infestation can cause debilitating or fatal paralysis or exsanguination, and ticks also transmit a large array of pathogens to their hosts. An invasive species is an organism that expands beyond its previous range into a new region with negative impacts on the health of humans,
Case studies in tick invasions in North America
Ticks that have already invaded North America (Table 1) represent a diversity of genera, life history characteristics, and potential health impacts. The most noteworthy recent example of long-range introduction followed by rapid expansion in North America is the Asian longhorned tick, Haemaphysalis longicornis. Prior to detection in the USA in August 2017 on a heavily parasitized sheep, the longhorned tick was found only in Asia and Australasia [13]. Although archival analysis suggests that
Host tropism
Host availability is the primary resource that limits invading ticks, and host mobility is the primary mechanism by which ticks move to new locations. With the exception of the cattle fever ticks, all of the invading ticks described here are three-host ticks – feeding on a separate host at every life stage. The cattle fever ticks attach to their host as larvae and then continue their life cycle on the same host until engorged females have mated and dropped off to lay eggs in the environment [50
A history of invasion predicts future invasion
Previous and ongoing invasion events in other locations may provide valuable clues that a tick is likely to be invasive in a new location. The parthenogenetic form of the longhorned tick was first detected in Australia (where H. longicornis is called the bush tick) in 1897, but was absent from Western Australia until 1983, at which point expansion was rapid, with rapidly increasing population size [83,84]. The brown dog tick has proven to be invasive around the world, although there may be a
Identification of other potentially invasive ticks
Identification of commonalities, listed in Box 1, among invasive ticks enables the development of a tool that may be used to identify ticks with invasion potential, and may inform surveillance and targeted modeling efforts for tick species that are novel to some area. For example, the Mediterranean Hyalomma tick, Hyalomma marginatum, matches a number of the criteria in Box 1, including wide host tropism and preference for both large wildlife and domestic livestock hosts, and has been projected
Concluding remarks
Some next steps are critical for addressing the inevitable threat of new vector distribution in a world with rapidly changing drivers and limitations to invasion, with several essential research questions outlined in this review (see Outstanding questions). One such step is to identify and assess all potential invading ticks and to prioritize their risks for invasion based on transport and climate change. Once potential invaders are identified, a number of additional tools are essential for
Acknowledgments
The authors thank the members of the Foley Laboratory who provided indispensable suggestions and support to improve this manuscript. We acknowledge funding from the Pacific Southwest Regional Center of Excellence for Vector-Borne Diseases funded by the U.S. Centers for Disease Control and Prevention (Cooperative Agreement 1U01CK000516).
Declaration of interests
The authors declare no competing interests.
Glossary
- Invasion
- an event in which a species moves into a previously unoccupied location.
- Parthenogenetic
- asexual reproduction that does not require fertilization of an egg.
- Peridomestic
- surviving around human residences and in the built environment.
- Propagule pressure
- the number of individuals introduced to, and released into, an area.
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