Infectious Disease and Microbiology – Pediculus and Phthirus Species (Lice)

Overview

Pediculus and Phthirus are genera of blood-feeding ectoparasitic insects (lice) that infest humans and cause pediculosis or phthiriasis. The three major human infestations are head lice, body lice, and pubic lice.

The major clinical manifestation is pruritus, which results largely from hypersensitivity to components of louse saliva. Persistent scratching can produce excoriations and predispose to secondary bacterial infection.

Classification

Genera: Pediculus and Phthirus

Important human lice include:

• Pediculus humanus capitis — head louse

• Pediculus humanus corporis — body louse

• Phthirus pubis — pubic or “crab” louse

Organism type: Ectoparasitic insect

Infestation: Pediculosis / phthiriasis

High-Yield Classification

Head lice

→ Pediculus humanus capitis

Body lice

→ Pediculus humanus corporis

Pubic/crab lice

→ Phthirus pubis

Life Cycle

The life cycle consists of three major stages:

Egg

↓

Nymph

↓

Adult

Under favorable temperature conditions, eggs generally hatch within:

7–10 days

The nymphal stage lasts approximately:

7–13 days

The complete egg-to-egg cycle is approximately:

3 weeks

Nits

Louse eggs are commonly called:

Nits

They are attached firmly to:

Hair shafts

in head and pubic lice.

Finding viable lice or appropriate nits during direct examination helps establish the diagnosis.

Epidemiology

Lice occur:

Worldwide

Infestation can affect people in many socioeconomic and hygiene settings.

Crowding and close interpersonal contact facilitate transmission. Body lice, in particular, are strongly associated with situations where clothing cannot be regularly changed or laundered.

Transmission

Transmission varies according to the type of louse.

Head lice

→ Primarily close head-to-head contact

Body lice

→ Associated with infested clothing and bedding

Pubic lice

→ Usually transmitted through close intimate or sexual contact

Pediculus humanus capitis – Head Lice

Pediculus humanus capitis causes:

Pediculosis capitis

The lice primarily infest:

Scalp hair

and may occasionally involve other coarse hair around the head.

Clinical Manifestations of Head Lice

Typical findings include:

• Intense scalp pruritus

• Visible lice

• Nits attached to hair shafts

• Excoriations from scratching

• Secondary bacterial infection in some patients

• Regional lymphadenopathy when secondary infection occurs

The posterior scalp and areas behind the ears are commonly examined carefully for evidence of infestation.

High-Yield Head Lice Pattern

Child

• ●

Intensely itchy scalp

• ●

Nits firmly attached to hair shafts

→ Pediculus humanus capitis

Pediculus humanus corporis – Body Lice

Pediculus humanus corporis causes:

Pediculosis corporis

Unlike head lice, body lice primarily live and deposit their eggs in:

Clothing, especially seams

They move onto the skin mainly to:

Feed on blood

Clinical Manifestations

Body lice may cause:

• Severe itching

• Excoriations

• Papular skin lesions

• Secondary bacterial infection

Chronic heavy infestation may lead to skin thickening and hyperpigmentation.

High-Yield Body Lice Pattern

Severe generalized itching

• ●

Limited access to clean clothing/laundry

• ●

Lice or eggs in clothing seams

→ Pediculus humanus corporis

Body Lice as Disease Vectors

An especially important distinction is that:

Body lice can transmit systemic infectious diseases.

Pediculus humanus corporis can serve as a vector for:

• Rickettsia prowazekii → epidemic typhus

• Bartonella quintana → trench fever

• Borrelia recurrentis → louse-borne relapsing fever

This vector role makes body lice particularly important in infectious disease.

High-Yield Vector Pattern

BODY LOUSE

→ Rickettsia prowazekii

→ Epidemic typhus

→ Bartonella quintana

→ Trench fever

→ Borrelia recurrentis

→ Louse-borne relapsing fever

Phthirus pubis – Pubic Lice

Phthirus pubis is commonly called the:

Pubic louse

or

Crab louse

because of its short, broad, crab-like morphology.

It causes:

Phthiriasis pubis

Distribution

The organism primarily inhabits:

Pubic hair

In extensive infestation, it may also involve:

• Axillary hair

• Chest or other coarse body hair

• Facial hair

• Beard or mustache

• Eyelashes

Eyelash Infestation

Infestation of the eyelashes by Phthirus pubis is called:

Phthiriasis palpebrarum

This may cause:

• Eyelid itching

• Irritation

• Visible lice or nits attached to eyelashes

High-Yield Pubic Lice Pattern

Pubic pruritus

• ●

Visible crab-like lice or nits on pubic hair

• ●

Close intimate/sexual contact

→ Phthirus pubis

Pruritus

The most important symptom produced by both Pediculus and Phthirus is:

ITCHING

Pruritus results largely from a hypersensitivity response to louse feeding and saliva.

Repeated scratching can cause:

Excoriation

↓

Skin barrier disruption

↓

Secondary bacterial infection

↓

Possible regional lymphadenitis

Diagnosis

Diagnosis is primarily made by:

Direct visual examination

The clinician looks for:

• Live adult lice

• Nymphs

• Nits/eggs

Identification of a live louse provides the strongest evidence of active infestation.

Head Lice Diagnosis

Careful examination of the:

Scalp and hair

is performed for lice and nits.

Nits should be distinguished from:

• Dandruff

• Hair casts

• Other debris

Unlike dandruff, nits are generally:

Firmly attached to the hair shaft

Body Lice Diagnosis

For suspected body lice, examination should include:

Clothing

particularly:

Clothing seams

because the lice and their eggs are usually found there rather than continuously living on the skin.

Pubic Lice Diagnosis

Examine:

• Pubic hair

• Other coarse body hair when indicated

• Eyelashes if ocular involvement is suspected

Live lice or nits may be directly visualized.

Treatment of Head Lice

The source lists topical pediculicidal therapy for head lice, including:

1% pyrethrin/pyrethroid-based treatment

Because some treatments do not reliably kill every egg:

Repeat treatment may be required approximately 7–10 days later

to kill newly hatched lice before they mature and reproduce.

Treatment choices should account for local resistance patterns and product-specific instructions.

Important Historical Treatment Note

The source also lists:

Gamma benzene hexachloride (lindane)

for head and pubic lice.

However, lindane is an older treatment with important neurotoxicity concerns and is no longer a preferred routine therapy in many settings. Modern treatment generally favors safer available pediculicides.

Treatment of Pubic Lice

Pubic lice are treated with an appropriate:

Topical pediculicide

Clothing, towels, and bedding used by the affected person should also be appropriately cleaned.

Because pubic lice are commonly transmitted through intimate contact, recent sexual partners may require evaluation and treatment.

Treatment of Body Lice

For body lice, the most important intervention is:

Decontamination of clothing and bedding

Clothing and bedding should be:

Washed and dried using appropriately hot cycles

when possible.

Improving access to:

• Clean clothing

• Regular bathing

• Laundry facilities

is central to eliminating infestation.

Topical pediculicides are not always necessary when clothing and environmental measures can be effectively implemented.

Prevention

Prevention depends on the type of infestation.

Important measures include:

• Prompt identification and treatment

• Avoiding direct contact with active infestation

• Avoiding sharing potentially contaminated clothing or personal items

• Appropriate laundering of clothing and bedding

• Regular examination when exposure is suspected

Prevention of Head Lice

Because head lice primarily spread through close contact:

Avoid direct head-to-head contact with an infested individual

Prompt recognition and treatment can reduce further transmission.

Prevention of Body Lice

The key preventive measures are:

Regular access to clean clothing and bedding

• ●

Appropriate laundering

• ●

Treatment of active infestation

Prevention of Pubic Lice

Because pubic lice are commonly sexually transmitted:

Avoid intimate contact until infestation has been treated

Recent sexual contacts should be considered for evaluation.

Head vs. Body vs. Pubic Lice

Feature

Head Louse

Body Louse

Pubic Louse

Organism

P. h. capitis

P. h. corporis

P. pubis

Main location

Scalp hair

Clothing seams

Pubic/coarse hair

Major symptom

Pruritus

Pruritus

Pubic pruritus

Transmission

Head-to-head

Clothing/bedding

Intimate contact

Important vector?

No major classic role

Yes

No major classic role

Key control

Hair treatment

Launder clothing/bedding

Pediculicide + contact management

Lice vs. Scabies

Lice

→ Visible ectoparasites/nits

→ Hair or clothing involvement

→ Head, body, or pubic distribution

Scabies

→ Sarcoptes scabiei mite

→ Burrows into superficial skin

→ Intense itching, often worse at night

→ Finger webs, wrists, waist, and genital areas are commonly involved

High-Yield Clinical Patterns

Itchy scalp + nits attached to hair

→ Pediculus humanus capitis

Itching + lice in clothing seams

→ Pediculus humanus corporis

Pubic itching + crab-like lice

→ Phthirus pubis

Exam Essentials

Organisms: Pediculus and Phthirus species

Type: Ectoparasitic insects (lice)

Distribution: Worldwide

Life cycle: Egg → nymph → adult

Egg hatching: Approximately 7–10 days

Nymphal stage: Approximately 7–13 days

Egg-to-egg cycle: Approximately 3 weeks

Head lice: Pediculus humanus capitis

Body lice: Pediculus humanus corporis

Pubic/crab lice: Phthirus pubis

Major symptom: Pruritus

Complication: Excoriation with secondary bacterial infection and possible lymphadenitis

Diagnosis: Direct visualization of lice/nymphs/nits

Head lice clue: Nits firmly attached to hair shafts

Body lice clue: Lice and eggs primarily in clothing seams

Pubic lice clue: Pubic/coarse hair involvement and intimate transmission

Important body-louse diseases: Epidemic typhus, trench fever, and louse-borne relapsing fever

Head/pubic lice treatment: Appropriate topical pediculicide, with repeat treatment when indicated

Body lice treatment: Decontamination/laundering of clothing and bedding

Historical treatment: Lindane is no longer a preferred routine option in many settings because of toxicity concerns

Prevention: Prompt treatment, avoidance of direct exposure, and appropriate laundering/environmental control

Key clinical pearl: The three lice are best distinguished by location: P. humanus capitis lives on scalp hair, P. humanus corporis primarily inhabits clothing seams, and Phthirus pubis prefers pubic and other coarse hair. For infectious-disease exams, remember that the body louse is the important vector—it can transmit epidemic typhus (Rickettsia prowazekii), trench fever (Bartonella quintana), and louse-borne relapsing fever (Borrelia recurrentis).