Ixodid ticks

External structure and morphology

Ixodid ticks are relatively large—unfed specimens reach 2–13 mm. The tick's body is conventionally divided into two main parts:

1. Idiosoma (body). Typically oval in shape, unsegmented, and bears the walking legs. In unfed specimens, it is flattened, but swells significantly after feeding. Festoons—characteristic folds of the integument—may be present along the body margins.
2. Gnathosoma (capitulum). The mouthpart complex. In ixodid ticks, it is located terminally, i.e., at the anterior end of the body (unlike argasid ticks, in which it is ventral).

A critical diagnostic feature is the chitinous shield (Scutum) on the dorsal surface. It accounts for marked sexual dimorphism:

• In males, a dense chitinous shield covers the entire dorsal surface.
• In females, it occupies only the anterior third (behind the base of the capitulum). The rest of the integument is capable of extensive stretching during blood ingestion.

In most species, a pair of eyes is present on the lateral margins of the dorsal shield, but in representatives of the genus Ixodes, visual organs are completely absent.

Feeding features and saliva properties

These arthropods are highly specialized obligate hematophages. Feeding begins with the insertion of the capitulum into the host's skin. Ticks are able to securely anchor and feed for prolonged periods due to the unique properties of their saliva, which provides:

• Anesthesia at the bite site (which is why the attack often goes unnoticed).
• Local lysis — dissolution of surrounding tissues.
• Increased permeability of blood vessel walls.
• Anticoagulant effect (prevention of blood clotting).
• Formation of a dense "cement-like" sheath around the capitulum, which firmly holds the parasite in the skin.

Life cycle

The development of ixodid ticks proceeds via incomplete metamorphosis and includes four stages: Egg $\rightarrow$ Larva $\rightarrow$ Nymph $\rightarrow$ Adult tick (imago).

Each stage has its own morphological features:

• Larvae: very small (0.2–0.7 mm) and feed on small animals, although they can also attack humans. They have 3 pairs of walking legs, and the reproductive system is undeveloped. Spiracles (stigmata) are absent—larvae respire through the entire body surface.
• Nymphs: larger (1.2–1.7 mm), also feed on small animals and humans. They develop a 4th pair of legs and spiracles, which open ventrally posterior to the 4th pair of legs. The reproductive system remains undeveloped.
• Imago: adult females (3–4 mm) and males (2–3 mm). They attack large animals and humans. They have 4 pairs of legs, spiracles, and a fully developed reproductive system with pronounced sexual dimorphism (the genital pore is located on the ventral side).

Epidemiological significance

Ixodid ticks are vectors for a wide range of pathogens. Those of greatest medical importance belong to the genera Dermacentor and Ixodes.

Major infections:

• Viral: spring-summer (taiga) encephalitis, Omsk and Crimean hemorrhagic fevers, louping ill.
• Bacterial: tularemia, brucellosis, plague, pseudotuberculosis, salmonellosis.
• Rickettsial: tick-borne typhus, Mediterranean spotted fever, Q fever, Rocky Mountain spotted fever.
• Spirochetal: tick-borne borreliosis (Lyme disease).

Key representatives:

1. Taiga tick (Ixodes persulcatus) and castor bean tick (sheep tick) (Ixodes ricinus) are specific vectors of tick-borne encephalitis, Lyme disease, and tularemia.
2. Pasture ticks (Dermacentor pictus, Dermacentor marginatus) are specific vectors of Omsk hemorrhagic fever, tularemia, tick-borne encephalitis virus, and typhus.

Differential diagnosis

To identify the arthropod species, it is important to know the differences between the family Ixodidae (Ixodidae) and Argasidae (Argasidae):

Class and exam questions

What else to study on this topic

• Features of the genus Ixodes: absence of eyes and role in the transmission of borreliosis
• Characteristics of pasture ticks of the genus Dermacentor
• Comparative characteristics of ixodid and argasid ticks
• Structure and functions of the mouthpart complex (gnathosoma)
• Tick life cycle: from egg to imago

Other topics in the "Medical Arachnoentomology" section

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