Ixodes ricinus

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Vector of the causative agents of Lyme borreliosis and tickborne encephalitis

■ Ixodes ricinus, (common sheep tick, castor bean tick) is the most frequent hard tick species in central Europe. The medical significance of this species is due to its role as vector of the causative agents of Lyme borreliosis, tickborne encephalitis (European tickborne encephalitis, "early summer meningoencephalitis," ESME), and other pathogens. Ticks that have attached to the skin should be mechanically removed as soon as possible to reduce the risk of infection. ■

Morphology. Male: about 2-3 mm long with a highly chitinized scutum covering the entire dorsal surface. Female: 3-4 mm, up to 12 mm when fully engorged after a blood meal; the scutum covers only the anterior portion of the body (Fig. 11.1a). Adults and nymphs (the latter about 1 mm long) have four pairs of legs, the smaller larvae (about 0.5 mm long) only three pairs. Ticks possess characteristic piercing mouthparts.

— Arthropod Parasites of Man

— Arthropod Parasites of Man

Phlebotomus Papatasi
Fig.11.1 a Ixodes ricinus, female engorged with blood; b Sarcoptes scabiei, female; c body louse; d crab louse; e sandfly (Phlebotomus papatasi) feeding on human skin; f dog flea (Ctenocephalides canis). (Image e: H. M. Seitz, Institut für Medizinische Parasitologie, Bonn.)

Biology. The various stages of I. ricinus are dependent on blood meals from vertebrates throughout their developmental cycle. Having selected a suitable location on a host, a female tick inserts her piercing mouthparts into the skin within about 10 minutes. Using clawlike organs at the tip of stylettelike mouthparts, the chelicerae, the tick cuts a wound into which the unpaired, barbed, pinecone-shaped hypostome is then inserted to anchor the parasite in the skin. While sucking blood, ticks secrete large amounts of saliva, containing cytolytic, anticoagulative, and other types of substances. They ingest blood, tissue fluid and digested tissue components. The weight of the female increases considerably during a blood meal. When completely engorged, the tick resembles a ricinus seed. The epidemiologically important factor is the possible ingestion of pathogens with the blood meal, which can, at a following blood meal in the tick's next developmental stage, be inoculated into another vertebrate host (horizontal transmission). Female ticks even transmit certain pathogens by the transovarial route to the next generation of ticks (vertical transmission).

Table 11.2 summarizes the life cycle of I. ricinus. The overall development period may be interrupted by periods of inactivity and starvation (maximum starvation capacity 13-37 months, depending on the stage) and can therefore take from one to three years.

Epidemiology. I. ricinus occurs widely in Europe, both in lowland and mountainous regions up to 800-1000 m above sea level, occasionally even higher. The habitats preferred by this species include coniferous, deciduous, and mixed forests with plentiful underbrush and a dense green belt. The different

Table 11.2 Life Cycle of Ixodes ricinus

Developmental stages: Egg ! Larva ! Nymph ! Imago

Host groups commonly - Rodents, Birds, Domestic and used for blood feeding: birds, mammals2, wild ruminants,

(humans)1 humans dogs, cats, horses, and other animal species2, humans

Duration of bloodsucking,

- 2-6 3-7

5-14

in days:

Tick habitat when not

Humid soil, low vegetation, areas

of woodland with

attached to a host:

underbush, meadows with high g

rass, gardens etc.

1 Occasionally. 2 Many different host species; in Europe about 35.

1 Occasionally. 2 Many different host species; in Europe about 35.

stages of ticks inhabit grass, ferns, and branches in this low vegetation either quite close to the ground (mainly larvae and nymphs) or somewhat higher (up to about 80-100 cm, mainly adults) in questing for suitable hosts. When hosts approach, the ticks either let themselves drop onto them or cling to the skin on contact. I. ricinus becomes active at 7-10 °C. Maximum tick activity is registered in the periods May-June and August-October.

The great epidemiological significance of I. ricinus in central Europe is predominantly due to its function as vector of the causative agents of Lyme bor-reliosis (Borrelia spp., p. 324f.) and the European tickborne encephalitis (TBE) (virus of TBE, p. 443f.). In northern and eastern Europe the TBE virus is transmitted by Ixodes persulcatus; Ixodes scapularis (syn. I. dammini) is the vector of Borrelia burgdorferi in the USA.

Diagnosis. Identification of I. ricinus is either done macroscopically or with the help of a magnifying glass. Differential species identification requires the skills of a specialist. Skin reactions, in particular the erythema chronicum migrans resulting from a borreliosis infection, often provide indirect evidence of an earlier tick bite.

Tick bite prevention. Tick habitats with dense undergrowth, ferns, and high grasses should be avoided as far as possible. If this is unavoidable, proper clothing must be worn: shoes, long socks, long trousers (tuck legs of trousers into socks), long sleeves that fit closely around the wrists. Additional protection is provided by spraying the clothes with acaricides, especially pyrethroids, which have a certain repellent effect (e.g., flumethrin). The effect of repellents applied to the skin (see malaria) is in most cases insufficient to protect against ticks.

After staying in a tick habitat persons should search their entire body for ticks and remove any found attached to the skin as quickly as possible by mechanical means (do not apply oil or other substances to attached ticks!). Any bites should be watched during the following four weeks for signs of reddening (erythema), swelling, and inflammation. A "migrating," i.e., spreading rash (erythema migrans) is indicative for a Borrelia infection. On the other hand, this sign is not observed in all infected persons.

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