Head Lice Life Cycle & Characteristics

Head Lice 

Life Cycle and Characteristics 

Clinical Presentation 

Infectious Agent 

Reservoir 

Occurrence/Susceptibility 

Transmission 

Identification of live louse. 

Identification of nits on the hair shaft. 3 

Itching, scratching caused by an allergic reaction to the bites. 4,7 

Allergic reaction begins 1–2 weeks after infestation (1 to 5 lice). This is a 

reaction to the anti-coagulant products that the louse injects locally as it takes 

its blood meal. Others may be asymptomatic. 2 

In long standing cases a roseo-like exanthema on the body may accompany 

lice infestation. 4,7,2 

May be excoriation and crusting with secondary bacterial infection and regional 

lymphadenopathy. 4,7 

Tickling feeling of something moving in the hair. 

Head lice are insects from the arthropod family and are of the order of the 

Anoplura. The adult lice are obligate blood sucking ectoparasites that require 

warmth and a source of human blood to survive. Although infestation generally 

occurs close to the scalp, behind the ears and the back of the neck, it has been 

reported to occur anywhere on the scalp. 4,9,11,12 

Humans 4,11,13 

Worldwide. Head lice are more common in children and the literature indicates 

the prevalence is higher in females. Caucasians are more frequently infested than 

other ethnic groups irrespective of social economic class. Any person may 

become louse infected under suitable conditions of exposure. 13,5,2 

Primary mode of transmission is direct person to person contact. 

Infestations are more commonly spread within families than within schools. 

because of close personal contact and shared brushes and combs. 16 

Secondary mode of transmission is fomites or indirect through personal 

belongings, i.e. hats, bedding, combs, and hair accessories. 

Head lice crawl but do not fly or jump. 5,9,13 

Although lice can crawl relatively quickly, they rarely travel far from a preferred 

habitat, such as the warmth of the scalp. 17 

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Incubation Period 

Life Cycle of Head Louse 

1. Eggs or Nits 

2. Nymph 

3. Adult Lice 

Physical Characteristics 

Egg to egg cycle averages 3 weeks 

Life cycle composed of 3 stages (see diagram): 

- Eggs 

- Nymph 9 

- Adult 

Egg to nymph stage 6 – 12 days 

Nymph to adult 7 – 14 days 9 

Adult 7 – 10 days 

Most suitable temperature for incubation 32°C 

Most suitable humidity 70% 

Eggs do not hatch at < 20° C or >38° C 3,9,14 

Description: 

Eggs are laid in their shells (the nits). 

Shape: oval, teardrop or flask shaped and have a lid (operculum) that covers 

the free end. 4,11 

Colour: documented as grayish-white, pearly or silvery white and even 

yellow-white; transparent, glistening or opalescent in nature. 

Size: 0.3 mm – 0.8 mm, looks like grains of sand . 9,11 

Easier to detect than adult lice. 

After hatching, empty white to dull yellow and almost clear colored nit casing 

or shells are left behind. They remain tightly adhered to hair shaft. 3,9,11 

Often confused with dandruff or hair spray droplets, which easily come off of hair. 

Viable nits are always found close to the scalp (1 – 4 mm). 

Human hair grows slowly at an average rate of approximately 0.37 mm/ day. 

Nits found several millimetres from the scalp are now empty egg cases. 

Duration of infestation can be estimated by the distance of nits from the scalp. 4,9,11 

Egg is tightly glued to the base of the hair shaft, nearest the scalp with a 

special cement bond called chitin. 

Eggs are usually deposited 1 – 4 mm from the scalp. 

Eggs are difficult to dislodge. 

Eggs hatch within 6 – 12 days. 

2 – 12% of eggs do not hatch. 

Only eggs deposited by inseminated female lice will hatch. 

May persist on hair for months, even after successful treatment, 

if not removed. 3,5,6,7,9,11,13 

Looks like adult louse, but smaller and immature. 

Size: 1 mm in length, size of pinhead. 

Matures in 3 stages. 

They grow by molting or shedding their skin 3 times before reaching adulthood. 

Nymphs mature after three molts and become adults (able to reproduce) about 

7 – 14 days after hatching. 5,11 

Nymphs tend to remain on the head where they hatch. 3,8,9 

Wingless, elongated body with 3 segments (head, thorax and abdomen). 

Short antennae with 4 segments. 

Small head with anterior piercing mouthparts that are used to pierce the scalp 

and feed on blood. 

(continued on next page) 

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Feeding habits: 

Period of Communicability 

Diagnosis 

Active or Recent Infestation 

Previous or Old Infestation 

Helpful Hints for Diagnosis 

Detection 

Method 1 

6 short and stubby legs with single tarsal segments, with powerful hook-like 

claws at the end of each leg. These claws are used to grasp the hair shaft. 

Dorsoventrally flattened. 

Size: 2 – 4 mm in length – approximate size of a match head or sesame seed 

(females usually larger than males). 

Do not leave head until nearly fully grown. 

Mating only occurs once fully grown. 

Colour: have been documented as grey, tan, brown, red or black. 

Colour changes to rust coloured after a blood meal. 

Elusive and well camouflaged. 

Darker in color in dark hair. They tend to reflect the colour of their surrounding. 

The louse injects a local anaesthetic through their saliva into the scalp to 

prevent their host from feeling any pain and an anticoagulant to prevent the 

blood from clotting, thus making it easier to feed. 

They blood feed several times each day. 

They move rapidly and are difficult to spot. 

Lice are not able to burrow into the scalp. 

Usually found close to the scalp to maintain its body temperature. 

Adult female lays 3 – 10 eggs per day. 

Adult female may deposit 100 – 300 nits in her lifetime. 3,4,5,11,13 

As long as lice or viable eggs remain alive on the infested head. 

Nits will not hatch off the head as they are extremely temperature and 

humidity sensitive. 

Lice that fall off the head and are without a blood meal have been noted to 

die within 20 – 48 hours. 

Lice are capable of surviving 24 hours immersed in H2O. 5,11 

Live nymphs or adults. 

Viable nits 1 – 4 mm from scalp 1,11 Highly Suggestive of Active Infestation: 

Numerous nits within 6 mm (1/4”) from scalp. 11 

Nits only; more than 6 mm (1/4”) to 12 mm (1/2”) from scalp. 

Nearly always hatched, when located at this distance. 1,5 

1. Duration of infestation can be estimated by the distance of the nits from the 

scalp: human hair, on average, grows at a rate of 0.37 mm / day. 

2. Infested heads can carry anywhere from 8 – 24 lice at any given time. They 

have a high natural mortality rate; heads may have hundreds of viable, dead 

or hatched eggs. 

Dampen hair (to decrease static electricity that can cause lice to be repelled 

from the comb). 

Bend head over a plain sheet of paper. 

Comb hair with a fine toothed comb (teeth about 0.2 mm apart). 

Observe for lice to drop. 

Part hair and look for moving lice. 

Magnifying glass may be useful. 

Use illumination, bright light – they scurry to hide making it easier to spot. 1,13 

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Method 2 

Method 3 

Bibliography 

Exclusion from School/Day 

care 

January, 2008 

White hair conditioner may be applied to the dry hair, covering root to tip. 

Immediately after application, a head lice comb is used and the combings wiped 

onto tissue paper and examined for lice or eggs. 17 However this conditioner 

should be thoroughly removed if lice treatment is necessary. 

Tap head with piece of transparent adhesive tape. If there are lice, they will 

stick to the tape. It then becomes a convenient coverslip for a microscopic slide. 

1 Adams RM (1996). You Ought to Have your Head Examined: Pediculosis Capitis 

Redux. The Pediatric Infectious Disease Journal. Vol. 15, No. 11, p. 1056-1057 

2 Diseases and Conditions. Lice Treatment and Shakedown. Health News. New 

England Journal of Medicine. 1995 

3 Eichenfield LF, Colon-Fontanez F (1998). Treatment of Head Lice. Concise 

Reviews of Pediatric Infectious Diseases. p. 421-422 

4 Elston DM (1999). What’s Eating You? Pediculus humanus (Head Louse and 

Body Louse). CUTIS. Vol. 63, May 1999 p. 259 

5 Fact Sheet: Head Lice Infestation(revised 2005). Centers for Disease Control and 

Prevention. Available: www.cdc.gov/ncidod/dpd/parasites/lice/factsht_head_lice.htm 

6 Headlice. Caring for Kids Index. Internet 

7 Head Lice Alert. Health News. February 1996, University of Toronto, Faculty of 

Medicine. Vol. 14, No. 1, p. 7 

8 Ibarra J (1995). A Non-Drug Approach to Treating Head Lice. Nurse Prescriber/ 

Community Nurse. Vol. 1, No. 8, p. 25 

9 Ibarra J, Hall DMB (1996). Head Lice in Schoolchildren. Archives of Disease in 

Childhood. Vol. 75, p. 471 

10 Nguyen VX, Robert P (1997). Treatment of Head Lice. The New England Journal 

of Medicine. March 6, 1997, p. 734 

11 Parasites and Health: Head Lice (2005). Causal Agent, Clinical Features. 

Available: www.dpd.cdc.gov/dpdx/HTML/HeadLice.htm 

12 Pigott KG (1997). Lice and Scabies. Lippincott’s Primary Care Practice. Vol. 1, 

No. 1, p. 93-96 

13 Pollack RJ. (1999). Head Lice: Information and Frequently Asked Questions. 

Department of Immunology and Infectious Diseases, Harvard School of Public 

Health. Available: www.hsph.harvard.edu/headlice.html 

14 Sauer GC, Hall JC (1996). Dermatologic Parasitology. Manual of Skin Diseases. 

7th Edition p. 239 

15 Hansen, Ronald C. Overview: The State of Head Lice Management and Control. 

The American Journal of Managed Care, September 2004, pS260 

16 Frankowski, Barbara. American Academy of Pediatrics Guidelines for the 

Prevention and Treatment of Head Lice Infestation. The American Journal of 

Managed Care, vol.10,no.9,sup. P S270 

17 Counahan,M et al. Head Lice Prevalence in Primary Schools in Victoria, 

Australia. J.Paediatr.Child Health(2004) 40, p 617 

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Head Lice Life Cycle & Characteristics

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