Ixodes scapularis - The Blacklegged Tick

Ixodes scapularis

The blacklegged tick Ixodes scapularis, commonly called the deer tick for its habit of parasitizing the white-tailed deer, is an ectoparasite (external parasite) of many birds and mammals – including humans. I. scapularis has received widespread scientific attention for its competence as a vector of many human diseases, especially as climate change potentially widens its range through North America (Ogden et al., 2008).

A member of the genus Ixodes (hard-bodied ticks), I. scapularis and its 700 sister species have almost global distribution, with the largest concentrations of I. scapularis found in the northeastern United States. Concentrations of hard-bodied ticks can generally be found in areas experiencing high humidity (favorable for metamorphosis) and seasonal temperature drops for signaling nymph and adult stages in the lifecycle. Population size is directly dependent on availability of potential hosts (Nuttal, 1905). While large mammals such as deer are generally required to sustain large populations of I. scapularis, smaller vectors, particularly white-footed mice (Peromyscus leucopusare), are suitable hosts for larval and nymph stages (Anderson et al., 1987). Anderson et al. (1987) report unsubstantial reduction in the presence of larval stages one year following an island-localized 70% reduction white-tailed deer population. This is an excellent, if frustrating, example of the robust nature of I. scapularis.

Morphologically, I. scapularis can vary significantly depending on its lifecycle stage and how recently it has taken a blood meal. Larvae are often no larger than the period at the end of this sentence, but adult stages can reach roughly 3 mm in length and are visible unaided. Adults lack eyes and festoons (marginal structures, often resembling loops), and are generally dark brown to black, though females may exhibit dimorphism in the form of orange or red coloration posterior to the scutum (a thickened, often pointed dorsal plate). Male ticks may not feed at all in their final lifecycle stage, and may attach to a host only to reproduce. Adult females, however, may feed for several days on a single host, often becoming engorged and swelling to several times their normal size before dropping off to lay eggs. In his report to the Epidemiological Society of London, Dr. George Nuttall described Ixodes as well adapted to a parasitic lifestyle, with ample denticles (tooth-like serrations) and mandibles as holdfasts and the ability to climb vertical glass surfaces, even fully engorged (Nuttal, 1904). Ticks can generally be found in leaf litter, grass, or shrubbery, where they wait to be picked up by a suitable host brushing by.

Zoonotic diseases (those which can be passed from non-human to human hosts) may be acquired during a larval, nymphal, or adult feed, and may remain virulent throughout the lifespan of the individual (Nuttall, 1904). Additionally, infected adults may lay eggs that develop into infected larvae, though larvae may not necessarily convey infection during their initial larval feeds (de Vignes et al., 2001).  In North America, I. scapularis is the primary vector of several pathogens, including Babesia microti (causes babesiosis, a malaria-like sickness), Ehrlichia phagocytophila (a rickettsial bacteria), and perhaps the most commonly recognized Borrelia burgdorferi (Lyme disease) (Benjamin et al., 2002). In addition to disease transmission to humans, these ticks can maintain steady numbers on livestock, and can cause tick paralysis in dogs. 

Although I. scapularis has been formally known to science for over a century, scientific interest has increased significantly since its role as a disease vector has become understood. Studies regarding potential control methods, including fungal biological controls (Deuteromycetes), forest clearing, and intermediate host limitation have been undertaken (Benjamin et al., 2002; Anderson et al., 1987). As of yet, no control methods have been shown to effectively combat this capable and potentially dangerous parasite.

Works Cited

Anderson, J., Johnson, R., Magnarelli, L., Hyde, F., Myers, J., 1987. Prevalence of Borrelia burgdorferi and Babesia microti in Mice on Islands Inhabited by White-Tailed Deer. Applied and Environmental Microbiology. 53(4): 892-894.

Benjamin, M., Zhioua, E., Ostfeld, R. 2002. Laboratory and Field Evaluation of the Entomopathogenic Fungus Metarhizium anisoplia (Deuteromycetes) for Controlling Questing Adult Ixodes scapularis (Acari: Ixodidae). J. Med. Entomol. 39(5): 723-728.

des Vignes, F., Piesman, J., Heffernen, R., Schulze, T., Stafford III, K., Fish, D. 2001. Effect of Tick Removal on Transmission of Borrelia burgdorferi and Ehrlichia phagocytophila by Ixodes scapularis Nymphs. Journal of Infectious Diseases.

Kierans, J.,Hutcheson, H., Durden, L., Klompen, J., Ixodes (Ixodes) scapularis: Redescription of all active stages, distribution, hosts, geographical variation, and medical and veterinary importance. 1996. J. Med. Entomol. 33(3): 297-318.

Nuttall, George H. F. (1904). "Ticks and tick-transmitted diseases". Transactions of the Epidemiological Society of London 24: 12–26.

Ogden, N., Bigras-Poutlin, M., Hanicová, K., Maarouf, A., O’Callaghan, C., Kurtenbach, K. Projected effects of climate change on tick phenology and fitness of pathogens transmitted by the North American tick Ixodes scapularis. 2008. Journal of Theoretical Biology 25(3): 621-632.

Ogden, N., Trudel, L., Artsob, H., Barker, G., Beauchamp, D., Charron, B., Drebot, M., Galloway, T., O’Handley, R., Thompson, R., Lindsay, L. 2006. Ixodes scapularis Ticks Collected by Passive Surveillance in Canada: Analysis of Geographic Distribution and Infection with Lyme Borreliosis Agent Borrelia burgdorferi. J. Med. Entomol. 43(3): 600-609.