Brown Ticks

Brown dog tick Rhipicephalus sanguineus, from here.

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.

Cattle ticks Rhipicephalus microplus on a host, from here.

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.

REFERENCES

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.

The Hard Way to be a Bloodsucker (Taxon of the Week: Ixodidae)

A mature gorged female of Ixodes ricinus, the sheep tick or castor bean tick, ready to lay her eggs. Photo by Jarmo Holopainen.

Ticks are probably the most familiar of all mite groups. Not only do they include by far the largest mite species but they also feed on the blood of vertebrates, a habit guaranteed to bring them to our attention. The ticks themselves would usually be more irritating than dangerous, except on occasions when they attack in large numbers, but many ticks are vectors of some very unpleasant diseases. Ticks are classified into three families of which the largest is the Ixodidae or hard ticks with a little under 700 species (Horak et al., 2002). Hard ticks are distinguished from members of the Argasidae or soft ticks by the presence of a hardened scutum at the front of the dorsum (the third tick family contains a single species, the African Nuttalliella namaqua). In males the scutum can cover almost the entire dorsal surface while the female scutum is restricted to the front part of the body over the legs. Hard ticks also have the capitulum (the 'head') directed forward so that it is easily visible from above while soft ticks have the capitulum pointed downwards (Nicholson et al., 2009).


A female of the cattle tick Rhipicephalus microplus laying her not inconsiderable brood of eggs. Photo from here.


Cattle ticks removed from a single calf. Photo from here.

The hard tick life cycle contains four stages of a single instar each - egg, larva, nymph and mature adult (soft ticks have multiple nymphal instars). Eggs are laid in large clusters of hundreds or thousands - the record number of eggs laid by a single female is 34,000 for a specimen of Amblyomma variegatum (Nicholson et al., 2009). Like other mites, larval ticks have only six legs when they first hatch out; the fourth pair doesn't appear until the nymphal stage. In both the larval and nymphal stages the young ticks will find a suitable host and feed then usually drop off and moult away from the host* (a small number of species don't leave the host before moulting and remain on a single host for their entire life). Some tick species are very choosy about their hosts (the best-known of which being the cattle tick, Rhipicephalus microplus [aka Boophilus microplus]) while others such as the sheep tick Ixodes ricinus are far more catholic. Some species feed on different hosts at different life stages. Even if suitable hosts are few and far between, some ticks can survive for over a year without feeding while they wait for one to turn up (some soft ticks can survive for several years without food). Males of Ixodes do not feed after reaching maturity and usually mate with females before they attach themselves to the final host (though some may mate on the host) while mature males of other ixodid genera do feed and copulation between the sexes takes place on the host (all together now - ewww). After copulation, the attached female gorges herself on her host's blood, swelling up to many times her original size. Once her eggs are mature, she drops off the host, lays her eggs in a suitable sheltered site, and dies (in contrast, female soft ticks can find another host and mate with another male, eventually surviving for several years).

*I have to admit to being surprised when I learnt that ticks didn't just latch onto their final host right away, even though in retrospect it should have been bloody obvious. After all, they would hardly be as much concern as disease vectors if they only ever attacked a single individual.


Back when she was skinny - a female lone star tick Amblyomma americanum sitting on vegetation waiting for a host. Photo by James Gathany.

Phylogenetic analysis supports a basal division in Ixodidae between Ixodes and other genera which is consistent with the differences in life cycles between the two groups (Murrell et al., 2003).

REFERENCES

Horak, I. G., J.-L. Camicas & J. E. Keirans. 2002. The Argasidae, Ixodidae and Nuttalliellidae (Acari: Ixodida): a world list of valid tick names. Experimental and Applied Acarology 28: 27-54.

Murrell, A., N. J. H. Campbell & S. C. Barker. 2003. The value of idiosyncratic markers and changes to conserved tRNA sequences from the mitochondrial genome of hard ticks (Acari: Ixodida: Ixodidae) for phylogenetic inference. Systematic Biology 52 (3): 296-310.

Nicholson, W. L., D. E. Sonenshine, R. S. Lane & G. Uilenberg. 2009. Ticks (Ixodida). In Medical and Veterinary Entomology, 2nd ed. (G. R. Mullen & L. A. Durden, eds) pp. 483-532. Academic Press.