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Genus Ixodes (Ixodid ticks)Unfed Ixodes ticks are medium in length (3 to 4mm) including mouthparts and the integument has striations. The mouthparts are anterior, and palp articles 2 are longer than articles 1 and 3. The basis capituli has straight lateral margins. Eyes are always absent. A scutum is present in the female and a conscutum in the male. There is no enamel on the scutum or conscutum. Festoons are always absent. The slender legs never have pale rings and pulvilli are always present.
Dorsal view of Ixodes ricinus (Sheep Ticks) in questing posture. Photos: InfluentialPoints.
On the underside (see picture below) spiracular plates are large and posterior to legs 4. They are oval in males, but more or less circular in females. Ventral plates are present in males only. The anal groove forms a loop anterior to the anus. Coxae 4 are of normal size whilst coxae 1 have unequal paired spurs.
Ventral view of engorged female Ixodes hexagonus (Hedgehog tick). Photo: InfluentialPoints
The ixodid genus includes important disease vectors of animals and humans and also some that inject toxins that can cause paralysis. Disease organisms transmitted to man include the bacterium Borrelia burgdorferi, that causes Lyme disease, and tick-borne encephalitis (TBE) virus. In addition the protozoan Babesia and the bacterium Anaplasma cause diseases in livestock. Ixodes hexagonus (Hedgehog Tick)Adult Ixodes hexagonus are brown, but the female ticks are light gray when engorged as below and right. Their palps are shorter than the base of the gnathostoma. The scutum is hexagonal or heart shaped and the palps are not club-shaped. Ixodes hexagonus is best distinguished from Ixodes ricinus (the sheep tick / deer tick) by the shape of the front tarsus. In Ixodes hexagonus the front tarsus has a hump near the end of its tip (just visible in the picture below) whilst in Ixodes ricinus it tapers evenly to the tip.
The hedgehog tick is endophilic and has been described as a nest-dwelling hedgehog specialist. It is also found on foxes, mustelids (including badgers), dogs and cats, but does not usually occur on rodents or birds. It is three-host tick - each stage drops off after feeding and must find another host for the next feed. Ixodes hexagonus is the species most frequently recorded on cats, and as a result may attach to humans. It is widely distributed throughout Europe and north-west Africa and may be common in suburban and urban areas.
Dorsal view of engorged Ixodes hexagonus (Sheep Ticks). Photo: InfluentialPoints. This picture shows a recently engorged hedgehog tick. The palpi are rather 'splayed out' as it has just been extracted from a cat (said cat seemed relieved, the tick less so). Rather little is known about the ecology of Ixodes hexagonus. Arthur (1953) reported that it has spring and autumn peaks of activity on hedgehogs, stoats and weasels. Matuschka et al. (2008) noted that the main hosts were hedgehog and fox. They found that ticks generally detached while it was dark, becoming replete during late evening and early morning hours. This was despite the fact that they fed on nocturnally active hosts which might result in them being dispersed away from the host. However, they tend to detach while host is sleeping which reduces dispersion. Bunnell et al. (2011) looked at factors affecting the parasite loads of individual hedgehogs. Sick hedgehogs tended to have higher loads than healthy hedgehogs apparently because ticks are preferentially attracted to the faecal odour from a sick hedgehog than to that from a healthy hedgehog. Pfäffle et al. (2011) compared the dynamics of what he described as the 'generalist' sheep tick with the 'specialist' hedgehog tick. They found that all hedgehogs examined harboured both species. Adults and nymphs of Ixodes ricinus showed peaks in spring and autumn, while larvae peaked in summer. Seasonal fluctuations of Ixodes hexagonus population density were much weaker compared to Ixodes ricinus and nymphs and larvae showed comparatively little change in population size and no consistent period of peak density. They suggested that density-dependent mechanisms were regulating the population density of the specialist Ixodes hexagonus, but not of the generalist Ixodes ricinus (the latter being a somewhat questionable conclusion...).
Ixodes hexagonus (Hedgehog Tick) feeding on cat. Photo: InfluentialPoints. This picture shows a hedgehog tick feeding on a domestic cat (Felis cattus). The hedgehog tick is a proven vector of Lyme disease, caused by the spirochete Borrelia burgdorferi (Piesman & Gern, 2004). It is generally considered to be less important than Ixodes ricinus as a vector, on the grounds that it is mainly nidiculous - living in the nest of its host, the hedgehog. Nevertheless it can also transmit Babesia microti, tick-borne encephalitis, and has been implicated as the main vector of Theileria annae in dogs in Spain (Camacho et al., 2003). Given the potential for disease transmission, it is important that ticks attaching to dogs and cats should be removed as soon as possible.
Ixodes ricinus (Sheep Tick, Deer Tick)Adult Ixodes ricinus are red-brown, but the female ticks are light gray when engorged. Before feeding, sheep tick males are approximately 2.5-3 mm long and the females 3-4 mm long. When they are engorged, the females can be as long as 1 cm. Their palps are longer than the base of the gnathostoma. Both sexes have setae present on the scutum. The tarsi (terminal leg segment before pulvillus) are moderately long and tapering. A long spur is found on the posterior internal angle of the coxa (basal leg segment) of the first pair of legs; this spur overlaps the coxa of the second pair of legs. The female scutum is almost circular and the female genital aperture is between leg coxae IV.
The sheep tick is another three-host tick where larva, nymph and adult all have different hosts. A full year may separate the active feeding periods of successive instars. Nymphs and larvae favour small mammals especially insectivores. Ixodes ricinus adults favour medium to large mammals including sheep, cattle, deer, dogs and humans. It is found in woodlands, heathlands and rough pasture throughout Europe. In north Africa it restricted mainly to the cooler and more humid areas that are associated with the Atlas mountains and occurs in Tunisia, Algeria and Morocco. The pictures below show two adult sheep ticks at rest in the questing posture, a female (first image) and a male (second image).
Male and female Ixodes ricinus (Sheep Ticks) An unfed tick adopts one of two characteristic attitudes when at rest (Lees, 1948). The 'questing' posture shown above is assumed when the tick is on the alert. The forelegs are extended rigidly to the front. Sometimes they are held out immobile, sometimes they are waved actively. The forelegs, which bear Haller's organ and other sensilla have, indeed, often been compared with the antennae of insects. A hungry tick spends some time in questing even in the absence of sensory stimulation, but when the tick is entirely undisturbed an attitude of rest with the forelegs folded (see below) is more usually adopted. This posture may be maintained for weeks or months on end. A hungry tick in repose quests immediately in response to certain forms of stimulation.
Ixodes ricinus (Sheep Tick) in repose posture. Photo: InfluentialPoints. Ixodes ricinus has been intensively studied, and much is known about many aspects of its biology and ecology. Free-living ticks only survive at places where the microclimatic relative humidity does not fall below about 80% for extended periods of time, and the litter layer in woodland plays an important role in this respect. The reason for the biseasonal pattern of tick abundance has been much researched (for example Walker 2001). For a long time it was ascribed to the existence of two cohorts, a spring and an autumn population, with some exchange between the two. However, Randolph et al. (2002) have shown that the observed pattern of abundance can be better explained by recruitment of a single cohort of each stage of ticks each year in the autumn. Ticks either feed immediately in early autumn and then enter diapause, or do not feed until the following spring - both groups moult to the next stage in the following autumn.
Larval Ixodes ricinus (sheep tick) biting human, and skin reaction. Photo: InfluentialPoints. The first image above shows a larval Ixodes ricinus. Note it only has three pairs of legs - the foremost appendages are the palps. Such larvae are commonly termed pepper ticks because of their extremely small size (about 0.8 mm long), although their size belies the reaction that a pepper tick bite can provoke - note the localised reddening of the skin in the second picture. Fortunately there is rather little chance of getting infected with a tick disease if bitten by pepper ticks as the infection rate (resulting from transovarial transmission or interrupted feeds) is very low.
Nymph of Ixodes ricinus (sheep tick) on vegetation. Photo: InfluentialPoints. The first image above shows a nymphal Ixodes ricinus Note that, whilst still very small (about 1.4 mm long), the nymph like the adult has four pairs of legs. The infection rate of nymphal ticks with disease agents such as Borrelia is much higher than that of the larvae, because all nymphs have taken a meal from a potential reservoir host such as a shrew or rodent. Bites by nymphal ticks are responsible for most transmission of Lyme disease. Ixodes ricinus is an important disease vector in Europe. Humans can be infected with Lyme disease (caused by Borrelia burgdorferi), Q fever (caused by Coxiella burnetti), the viral infection 'louping ill' and (most dangerously) tick-borne encephalitis. Cattle can be infected with redwater fever (Babesia spp.), Lyme disease, sheep tick pyraemia (Staphylococcus aureus, cattle tick-borne fever, anaplasmosis (both Anaplasma species) and various other diseases.
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