Lice

Morphology and internal structure

The body of lice is flattened dorsoventrally, and wings are completely reduced. The coloration depends on the color of the cuticle and the presence of ingested blood in the gut. Sizes range from 1 to 4.5 mm depending on the species and sex.

The body is clearly divided into three parts:

• Head. Connected to the thorax by a thin neck. It bears simple eyes and 5-segmented antennae. The mouthparts are of the piercing-sucking type; at rest, they are retracted into a special sheath inside the head. During blood-sucking, the secretion of the salivary glands enters the wound, causing severe burning and an allergic reaction.
• Thorax. Bears one spiracle on each side. Three pairs of 5-segmented legs are located on the ventral side. Each leg ends in a powerful claw ideally adapted for firmly grasping a hair.
• Abdomen. Oval-shaped with scalloped (notched) margins. The structure of the posterior end of the abdomen depends on the sex. In males, it is rounded, and the copulatory apparatus is located on the dorsal side. In females, the last segment is forked, the genital opening is on the penultimate (9th) segment, and sickle-shaped appendages—gonapophyses—are located beneath it.

Species of lice parasitizing humans

Humans are parasitized by members of the family Pediculidae, which are obligate monophages:

1. Head louse (Pediculus humanus capitis). Inhabits the scalp hair. Sizes: males 2–3.5 mm, females 3–4.5 mm. Characterized by deep notches along the margins of the abdomen. Causes pediculosis and transmits the causative agents of louse-borne epidemic typhus and trench fever.
2. Body louse (Pediculus humanus humanus). Localized in the folds of undergarments and clothing, crawling onto the skin only to feed. Unlike the head louse, it has thinner and longer antennae, and the notches on the abdomen are less deep. It is a vector of epidemic and relapsing typhus, as well as trench fever.
3. Pubic louse / crab louse (Phtirus pubis). Inhabits areas of the body with sparse hair cover: the pubic region, armpits, beard, eyebrows, and eyelashes. The body is shorter and wider, and significantly smaller in size (males about 1 mm, females 1.5 mm). Transmitted through close contact (including sexual contact) or shared underwear. It is not a vector of infectious diseases, but causes severe itching (phthiriasis).

Life cycle and ecology

Lice are permanent parasites that feed on blood at all stages of development after hatching from the egg. They tolerate starvation extremely poorly; daily blood-sucking is required for their survival.

Development occurs with incomplete metamorphosis:

1. Egg (nit). Pointed, with a cap. The female attaches it to a hair or clothing fiber using a drop of transparent secretion from the glue glands.
2. Larva. Hatches in 5–8 days and immediately begins feeding on blood.
3. Nymph I and Nymph II.
4. Imago (mature adult). Forms within 8–11 days after three molts.

The process is highly dependent on temperature. The optimum is 28–30 °C. At 25 °C, development is suppressed, and at temperatures below 15 °C, females stop laying eggs.

Mechanisms of vector-borne disease transmission

Lice transmit infections, but the mechanisms of human infection are fundamentally different from mosquito or tick bites.

Epidemic typhus (causative agent — Rickettsia prowazekii): Rickettsiae enter the louse's gut with the patient's blood, multiply in the epithelium, and are excreted in huge quantities with feces during blood-sucking. The pathogen does not penetrate the insect's salivary glands, so the bite itself is not infectious. A person becomes infected via contamination by rubbing itchy feces into scratch marks and micro-abrasions on the skin.

Relapsing fever (causative agent — Borrelia recurrentis): Obermeier's spirochetes enter the insect's gut and then penetrate the body cavity (hemolymph), where they actively multiply. Human infection occurs only when the louse is crushed (breaching the integrity of its body). Infected hemolymph gets onto the skin, and the pathogen enters the bloodstream through abrasions.

Clinical presentation and prevention

Pediculosis is accompanied by prominent symptoms: severe itching, skin coarsening, pigmentation (melanoderma), and excoriations (scratch marks). Damage to the skin frequently leads to secondary bacterial infection with the development of severe dermatitis and pyoderma.

Prevention and treatment include regular screenings in organized groups, sanitation of hair and clothing, as well as disinfection of premises. Insecticides (in shampoos and soaps) and oral chemotherapeutic agents (e.g., phenylbutazone) that make the blood toxic to lice are used to destroy the parasites.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed structure of the piercing-sucking mouthparts of insects
• Life cycle of rickettsiae in the epithelium of the louse gut
• Specific laboratory diagnostic methods for pediculosis
• Mechanism of action of systemic insecticides (using phenylbutazone as an example)
• Sanitary and epidemiological protocols upon detecting pediculosis in a hospital setting

Other topics in the "Medical Arachnoentomology" section

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