Head Louse (Pediculus capitis)
Structure, Appearance and Characteristics
Body length approx. 2.0-3.7mm.
Females are slightly larger than males.
Antennae have no more than 5 segments.
Head is narrower than thorax.
Thoracic segments are fused and cannot be moved independently from the abdomen.
Gradual / Incomplete metamorphosis (egg – nymph – adult). All stages of life cycle are spent on the host. Female lays about 5-6 eggs per day, normally during periods of host inactivity (80-100 eggs in its lifetime). Eggs (nits) are attached directly to the hairs of the host with a bead of gluey liquid, close to the scalp. Temperature and relative humidity are critical to incubation. Egg mortality is high below 24C or above 37C. Eggs hatch in 7-9 days (at 29-34C). 3 nymphal stages occur in approx. 8-9 days. Adults survive about 22-23 days.
Infests hair on the head of mammals.
Nymphs and adults feed on blood from the host.
Parasites of humans. Can cause irritation, scratching and in some cases infection of bite wounds. Large amounts of injected saliva can cause drowsiness and irritability, hence the carrier feels 'lousy'. Spread by personal contact and items such as combs, brushes, hats, pillow cases etc. Head lice are not noted as being disease vectors as opposed to body lice which do spread diseases such as typhus. Adults and nymphs are both considered pests.
Strengths include short life cycle, ease of transmission to other hosts, firmly placed eggs which are very hard to dislodge. Their weaknesses include having a single food source (blood), identifiable means of transmission to new hosts, confined area of feeding (scalp), and spending entire life on host. Control can be achieved through hygiene, isolating infected hosts. Transmission can be stopped by not sharing items such as hats, brushes, combs, pillow cases etc with infected hosts. Approved chemical treatments should be used on infected persons in the form of rinses that reach the scalp where adults live. Eggs must be physically combed out or else time must be allowed for nymphs to hatch and be chemically treated.