Fig. 1. SEM-micrograph of Phthirus pubis (× 20)
Order of Insects
The order Phthiraptera is subdivided into two suborders, Anoplura (bloodsucking lice) and Mallophaga (feeding on skin, keratinous substances of feathers and hairs, and dermal secretion fluids). Both groups show the following common features:
Members of the Mallophaga (Fig. 2) are furthermore characterized by a head which is broader than the thorax and by visible chewing mouth parts, whereas the bloodsucking mouth parts of members of the Anoplura are hidden inside their short and stumpy proboscis (Fig. 3).
Of the about 500 known species of the suborder Anoplura, only three species are ectoparasites of human: Pediculus humanus capitis (head louse), living in head hair, P. humanus corporis (body or clothing louse), occupying the clothes and visiting the body only to feed, and Phthirus pubis (crab louse), developing mainly in the hairs of the genital region, but regularly also colonizing the other coarse hairs of the head and body, e.g. the eyelashes (Fig. 1). These lice are highly host-specific, obligate parasites, spending their entire life cycle on the host and only infesting humans and monkeys. Animal lice only occasionally attack humans. Only P. humanus transmits rickettsial or bacterial diseases.
In human lice (Fig. 2), after a temperature-dependent embyonic development of about 8 days, the first instar nymphs (1 mm long) hatch and within 7–10 days after two additional nymphal stages, the adults. (The term larvae is used by scientists in Central Europe, but the term nymph by other scientists.) The whole developmental cycle (egg to egg) lasts about 2–3 weeks. Bacterial and fungal symbionts, which are restricted to special organs (mycetomes) near the gut or ovaries and transmitted transovarially to the eggs, are necessary for larval development and adult reproduction.
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Lice occur worldwide, clothing lice regularly in poor regions where people possess only one set of clothes. In the USA, white people are more frequently infested with head lice than Afro-Caribbean people, probably because of the better adaptation of the claws of the lice to hairs which are round or oval in cross-section, respectively.
The relatively small, narrow head of lice has very short antennae (5 segments), and eyes are strongly reduced to two big ommatidia. The mouth parts are hidden inside the head. The labrum ensheaths the four long, thin stylets made from the two maxillae, the labium and the hypopharynx, the latter containing the salivary channel, whereas blood is ingested through the tube formed by the maxillae. The thoracic segments are fused, and the short legs bear strong claws, that are optimally adapted to the diameter of the hairs of the host and cling onto hairs or fibres. The cuticle is very tough. The Pediculus species are slender insects of about 2–4 mm length, much longer than wide. Males and females can be distinguished by the body and claw sizes, patterns on the thorax, the shape of the abdomen and the sclerotized penis-like genitalia of males. Phthirus pubis is about twice as long as wide and about 1.5–2 mm long. The eggs (so called “nits”) are about 0.8–1 mm long and about 0.3 mm wide and glued onto hairs or cloth fibres. After eclosion of the nymphs, the eggs appear white and remain glued. Eggs of the genera Pediculus and Phthirus can be distinguished by the shape and the appearance of the pores on the operculum, those of Ph. capitis show similar pores as P. humanus corporis, but are more intensively glued to hairs. The pores (aeropyls) are needed for the oxygen supply of the embryo.
Crossbreeding of the two Pediculus species is possible (therefore regarded as subspecies by different authors), but both can be distinguished by their tibial lengths, habitat preference, i.e. cloth or hair, and temperature preference, 28–29°C for head lice and 31–33°C for clothing lice. The individual colour is genetically determined, darker clothing lice occurring more often in association with inuits and other dark-skinned humans.
Only P. h. corporis colonies breed in the laboratory after adaptation to feeding on rabbits. Other species are fed on volunteers.
Adults copulate shortly after emergence or at a later point in time. About 24 hours after mating, oviposition begins. Females of P. h. corporis, P. h. capitis and Ph. pubis live about 5, 3 and 4 weeks and lay about 300, 90 or 30 eggs, respectively. P. h. capitis prefers to deposit eggs singly onto hairs in the neck and behind the ears, P. h. corporis in clusters on the fibres of clothes, e.g. on the seams, and Phthirus pubis lays several eggs on a single hair.
Lice are transmitted by interhost contact and/or by shared use of combs, hats, clothes, etc. Usually less than 10 lice per person occur, but more than 20,000 P. humanus or several hundred P. capitis have been collected from one person.
Lice are attracted to the host by warmth and odours. They are permanent ectoparasites, capillary feeders who suck blood about every 2–3 hours. The ingested blood is stored and digested in a capacious anterior midgut, followed by a narrow posterior midgut and the hindgut. The saliva causes itching and the resulting scratches secondary bacterial infections. However, louse feces usually induces the first irritations.
Only P. h. corporis can transmit classic epidemic typhus, trench fever and louse-borne relapsing fever, but experimental transmission of the pathogens is possible using Phthirus pubis.
Classical epidemic typhus is caused by Rickettsia prowazekii and transmitted only among humans by pathogens present in the deposited feces (Fig. 3). These pathogens invade through skin lesions or are inhalated. The pathogens are infective in the faeces for up to three months. The disease is prevalent in Europe, Africa and South America, but incidence is declining.
Trench or five-day fever, occurring in Europe, is caused by Rochalimaea quintana, showing a mode of transmission similar to that of typhus.
Louse-borne relapsing fever is caused by Borrelia recurrentis and transmitted by crushing infected lice between the fingers or teeth. Thereby, bacteria present in the haemolymph or intestinal tract can invade skin lesions or the mucous epithelia. This disease occurs in Europe, Africa, South America and Asia, but not in Australia.
If the lice suck blood within the first ten days of illness, R. prowazekii is transmitted and multiplies in the lumen of the gut, but also in the cells of the intestinal wall. R. prowazekii is pathogenic to lice due to the destruction of gut cells. In the other bacterial infections no pathogenic effects are reported.
Borrelia recurrentis invades the haemocoel of the insect about four days after ingestion, slowly multiplying there.
By regular macroscopic control of hairs for the white eggs, the head- and crab lice, and by use of a fine-toothed comb, the head lice can be detected. In cases of itching of the skin of head and genital regions, a careful control for nits or lice should be performed. In heavy infestations with P. h. corporis, skin is darkened and hardened (morbus errora), and in Ph. pubis infestations bluish spots (maculae caeruleae) develop, since these lice prefer to bite repeatedly in the same places. In the latter infestations, black spots of louse faeces also occur in the underwear.
Information beginning with the parents of children in kindergarten and school can strongly reduce the infestations. However, it should be pointed out that usually more than one member of a household is infested.
All lice can be killed using insecticides either as powder (clothing lice) or in lotions or shampoos (head- and crab lice). Malathion, carbaryl and pyrethroids are used. Lotions left for some hours on the hair are more effective than shampoos. During the last years head lice have occurred more often, especially on young children, mainly due to the attitude of some parents who do not wish to use insecticide lotions or shampoos. All stages of clothing lice are killed within less than 30 min at 50°C, and within 1 min at 90–100°C; using a tumble drier for 15 min and >60°C is also sufficient. Exposition to -20°C has not been investigated, but 24 hours should be sufficient for killing. Since lice show a relatively weak starvation capacity, Ph. capitis and P. h. corporis will die after 3 and 4 days at 23°C, respectively; for the latter a storage of clothes for 17 days in a polythene bag is recommended. No vaccination is available (insectizides).
In several countries lice are resistant to DDT, carbaryl, lindane, malathion and since recent years to permethrin (see Vol. 2).
