In an earlier post on this site, I gave a brief overview of the hard ticks, those lovable suckers of blood and (often) vectors of disease. Today, I'll take one particular subgroup of the hard ticks to look at: the genus Rhipicephalus.
Rhipicephalus species are generally referred to as 'brown ticks' as, for the most part, they lack any prominent spots or other markings. Rhipicephalus species are found worldwide, though the highest diversity is in Africa, home to about three-quarters of the known species (Walker et al. 2000). They are mostly parasites of mammals, but individual species may be found on a range of host species. A few species are economically significant as vectors of such pathogenic organisms as rickettsias and various Sporozoa, notably the brown dog tick R. sanguineus and the cattle tick R. microplus. The former species has been estimated to cause about US$168 million of losses per year in Africa, while the latter costs Australia about US$100 million a year (Murrell & Barker 2003). A brief drive north of Perth is enough for me to see the impact of the cattle tick on Australian agriculture: as one passes the southernmost limit of the tick's range, there is a noticeable shift between the Europe-derived cattle breeds (such as shorthorns and Herefords) kept in the south of the country, and the tick-resistant India-derived breeds (such as Brahmans) kept in the north.
Distinguishing features of Rhipicephalus from other tick genera include the presence of adanal shields in the males, and a short hypostome and palps. Until recently, R. microplus and four other species were separated into their own genus, Boophilus, but a number of analyses, particularly molecular ones, have indicated that Boophilus is nested within Rhipicephalus (e.g. Beati & Keirans 2001) and the genera were synonymised by Murrell & Barker (2003). Members of the now-subgenus Boophilus differ from the remaining Rhipicephalus species in lacking festoons, a series of crimped grooves running around the posterior body margin (visible in the photo at the top of this post). They are also one-host parasites (that is, they remain on a single host through their lifespan and do not leave the host when moulting) while most other Rhipicephalus (such as R. sanguineus) are three-host ticks (they leave their host when moulting and then find a new host). However, close relatives of Boophilus in the subgenus Digineus are two-host ticks, only changing host when moulting from nymph to adult (Murrell & Barker 2003). However, it is worth noting that none of the analyses that led to the subsuming of Boophilus within Rhipicephalus included any representatives of the genus Margaropus, similar to Rhipicephalus but distinguished by possessing broad heavily-segmented legs. A close relationship between Boophilus and Margaropus was indicated by the morphological analysis of Klompen et al. (1997). If Boophilus is nested within Rhipicephalus, it seems quite possible that Margaropus is as well.
Beati, L., & J. E. Keirans. 2001. Analysis of the systematic relationships among ticks of the genera Rhipicephalus and Boophilus (Acari: Ixodidae) based on mitochondrial 12S ribosomal DNA gene sequences and morphological characters. Journal of Parasitology 87 (1): 32-48.
Klompen, J. S. H., J. H. Oliver Jr, J. E. Keirans & P. J. Homsher. 1997. A re-evaluation of relationships in the Metastriata (Acari: Parasitiformes: Ixodidae). Systematic Parasitology 38: 1-24.
Murrell, A., & S. C. Barker. 2003. Synonymy of Boophilus Curtice, 1891 with Rhipicephalus Koch, 1844 (Acari: Ixodidae). Systematic Parasitology 56: 169-172.
Walker, J. B., J. E. Keirans & I. G. Horak. 2000. The Genus Rhipicephalus (Acari, Ixodidae): a guide to the brown ticks of the world. Cambridge University Press.