Pediculidae

There are two common species or one species with two subspecies in this family. These lice occur on New World monkeys, gibbons, great apes, which are infested by Pediculus schaeffi, and humans. Generally, the head and body louse of humans are considered as distinct subspecies: Pediculus humanus humanus and Pediculus humanus capitis. It is often impossible to distinguish a single specimen as either head louse or body louse, but populations of these two lice can be separated on a few morphological features. These two forms interbreed readily in the laboratory, but may not in natural habitats. However, there is evidence that subspecies rank may not be appropriate. Studies of a fragment of the cytochrome oxidase I (COI) gene of mitochondrial DNA in lice collected from nine countries indicate that the head and body lice belong to the same species (conspecific). The COI sequences from the head and body lice studied did not come from reciprocally monophyletic lineages. Instead, they shared three of the 10 haplotypes found, which is evidence for conspecificity. Head lice and body lice are considered here as one species, Pediculus humanus, with two distinguishable subspecies.

Body louse, Pediculus humanus humanus (Fig. 14.2a, b)

The adult female is 2.4-3.6 mm long and the male is 2.3-3.0 mm long; the body is light to dark gray. The head

Linognathus Setosus
Figure 14.1 Phthiraptera. (a) Linognathus setosus female (dorsal/ ventral view); (b) L. setosus male (dorsal/ventral view); (c) Polyplax serrata female (dorsal/ventral view); (d) P. serrata male (dorsal/ventral view).
Dorsal View Cockroach
Figure 14.2 Phthiraptera. (a) Pediculus humanus humanus female (dorsal/ventral view); (b) P. humanus humanus male (dorsal/ventral view).

is short, and constricted to create a short neck. Eyes are well developed; the antennae are five-segmented, and antennal segment 3 is usually longer than wide. The abdomen has lateral lobes, and there are sclerotized plates on lobes 3-8. Legs are subequal in shape and size, and with a claw. Nymphs resemble adults. All stages stay on clothing and in continuous contact with the body of the host, but make contactwith the body while feeding. In severe infestations, some lice may remain on the host when clothing is removed. Eggs are deposited in clusters of three or four in the seams or other crevices of clothing, only rarely are they attached to coarse body hairs. Females deposit 3-11 eggs per day; hatching occurs in about 8 days at 30 ° C; eggs do not hatch above 38 °Corbelow23 °C. Fecundity is about 110 eggs; 91-94% hatch. Nymphs feed immediately after hatching, and frequently during the day and night throughout their development. Development is complete in 8-12 days; unfed lice die within 85 h at 23 °C and 45 h at30 °C. Lice and eggs will die if clothing remains unworn for 17 days. Fecal material in spiral threads is extruded as the louse feeds, and the feces dry quickly in the air.

Location of lice in clothing is specific for life stage and sex of the adult. Females are more often found along the seams of clothing, while males are generally found over the surface of clothing. Adult lice move farther than nymphs, and the proportion of adults is higher in clothing farther from the skin. The body louse prefers wool clothing, but it survives in other fabrics. In warm climates where wool is not worn, body lice are few, regardless of personal hygiene. This louse feeds on skin close to clothing, and usually where the skin is soft or folded, as in the joints. While the mouthparts are inserted and the louse is feeding, it usually retains its hold on the adjoining fabric. Dissemination is through infested clothing and less by physical contact. Body lice leave the host when body temperature drops or increases, such as with a fever. Adults can travel a distance of about 23 cm in 1 min. The temperature of the human body is the optimum condition for this insect, and a rise of 4-5 °C is fatal to the louse within a few hours. Severe infestations of thousands oflice have been reported, butheavy infestations are not common. Most infected persons carry about a dozen lice. Body lice infestations often occur during times of war and civil unrest, when there is poor sanitation and crowding. Louse-borne typhus, Rickettsia prowazekii, and relapsing fever, Borrelia recurrentis, are diseases transmitted by body lice.

Head louse, Pediculus humanus capitis The adult female is 2.4-3.3 mm long and the male is 2.1-2.6 mm long. The body is gray to translucent, but usually resembles the hair color of the host. The head is short, constricted at the base and with a short neck. Antennal segment 3 is as long as it is wide. Eyes are distinct and set behind the base of the antennae; antennae are five-segmented. Legs are subequal in shape and size, and have a well-formed claw. The abdomen has lateral sclerites; the male abdomen is pointed at the tip, in the females the abdomen ends in two triangular projections. Nymphs resemble the adults, and range from 0.90 mm long for first stage to 2.70 mm long for third stage. Adults and nymphs live and feed on the body of the host; they are usually found on the neck and head, particularly behind the ears and on the back of the neck. Eggs are glued to hairs on the head and neck.

Eggs are about 0.8 mm long and yellowish white. Females attach eggs singly, about 1 mm from the base of a host hair. Hatching occurs in 7-10 days at 29-32 °C. The percentage of eggs hatching but not duration of egg stage is influenced by humidity, with the highest rate at 75% relative humidity. Females lay about seven eggs in 24 h, and a total of about 55 eggs in a lifetime. The maximum time for eggs to survive unhatched is 3-4 weeks. Scalp hair grows about 0.4 mm per day, and as it grows, the egg or nit is moved progressively farther and farther from the scalp. By the time the egg hatches, the empty chorion will be about 6 mm from the scalp. Immature development is completed in 8-9 days. Adults mate frequently throughout life. Males and females live about 10 days; adults and nymphs survive about 55 h at 23 °C and about 24 h at 30 °C away from the host without feeding. Dissemination is by physical contact, and usually by the exchange of clothing with stray hairs with eggs or lice attached, or by close and prolonged physical contact. Infestations occur under a range of sanitary conditions, particularly among schoolchildren. Obtaining lice in a school environment is common in developed and developing countries around the world, and is often wrongly associated with neglect or unclean conditions at home.

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