Pediculosis and insect-bite reactions

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Examine wet-combed hair in good light, especially behind ears and nape, and distinguish a live louse from adherent debris. Ask about treatment product, exact application and reinfestation before calling resistance. Avoid stigma: head lice affect clean hair and all social groups.

For bites, map exposed versus covered sites, puncta, crops and household pattern and ask about pets, beds, travel, outdoor and tick exposure. Papules can look red, violet or brown; swelling, central punctum and distribution remain useful across skin tones. Environmental eradication should target a confirmed source rather than indiscriminate insecticide spraying.

Pubic or body lice changes context. Offer confidential sexual-health assessment for pubic lice and safeguarding by age and circumstances. Body lice may signal lack of laundry access and deserves practical support. A tick still attached should be removed promptly with fine-tipped forceps close to skin, followed by symptom safety-netting rather than routine antibiotics for every bite.

Key points

Diagnose head lice only by finding a live moving louse with wet detection combing; nits alone may be empty or non-viable.
Check household members and treat those with live lice on the same day; routine treatment of uninfested contacts is unnecessary.
Use wet combing on days 1, 5, 9 and 13 with a check on day 17, or a licensed physical insecticide exactly as directed.
Many dimeticone products require a second application after seven days; resistance is less relevant to a physical coating mechanism.
Pubic lice warrants treatment of recent sexual contacts and an STI screen; eyelash involvement needs eye-safe specialist advice.
Body lice control depends on laundering or replacing infested clothing and bedding plus access to washing and social support.
Bite reactions are managed with source control, cold compresses, oral non-sedating antihistamine and appropriate topical anti-inflammatory care, not routine antibiotics.
Treat anaphylaxis with intramuscular adrenaline and assess expanding erythema, fever or necrosis separately from a large local allergic reaction.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Head-louse transmission

Pediculus humanus capitis spreads mainly through direct hair-to-hair contact; jumping, flying and dirty hair are myths.

Body and pubic lice

Body lice live chiefly in clothing seams, while pubic lice spread through close bodily contact and sometimes affect coarse hair or eyelashes.

Arthropod saliva

Mosquito, flea, bedbug and other bites inject salivary antigens that provoke immediate or delayed itchy hypersensitivity papules.

Exposure ecology

Pets, travel, accommodation, occupational contact, clothing and household clustering help locate the source without assigning blame.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Blood feeding and eggs
Lice feed repeatedly and cement nits to hair shafts or clothing fibres; empty shells can persist after eradication.
2
Hypersensitivity itch
Immune response to saliva and faeces produces pruritus, excoriation and sometimes delayed symptoms despite active infestation.
3
Papular urticaria
Recurrent arthropod bites trigger grouped itchy papules with central puncta and unusually prolonged lesions, particularly in susceptible children.
4
Barrier injury
Repeated scratching creates erosions, crust, persistent pigment change and an entry portal for staphylococcal or streptococcal infection.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Live head louse

A grey-brown moving insect found by detection comb confirms active head-lice infestation.

Nits without lice

Firm eggshells attached to hair can remain after cure and do not alone justify treatment.

Pubic or eyelash lice

Coarse-hair lice, blue-grey macules or nits on eyelashes require sexual-health or ophthalmic context.

Papular bite crops

Grouped itchy papules with a central punctum on exposed sites support arthropod hypersensitivity.

Anaphylaxis physiologyRed flag

Sudden airway swelling, wheeze, hypoxia, hypotension or collapse after a bite or sting requires intramuscular adrenaline.

Red flags requiring action

Airway, breathing or circulation compromise after a bite, expanding severe infection, fever with travel or tick exposure, eyelash lice with ocular injury, unexplained genital lice in a child or widespread infestation in a vulnerable person requires urgent assessment.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Wet detection combingFirst step
Why
Confirm active head lice before treatment.
Interpretation and limitations
A live louse is diagnostic; dandruff and empty nits are not.
02
Exposure and household map
Why
Identify contacts and likely flea, bedbug, travel or animal sources.
Interpretation and limitations
Distribution and timing guide environmental action; one papule cannot identify an insect species reliably.
03
Bacterial swab
Why
Assess pus or crust after scratching when infection is clinically present.
Interpretation and limitations
Do not swab intact allergic papules or prescribe antibiotics for itch alone.
04
STI testing with consent
Why
Assess co-infection when pubic lice is diagnosed.
Interpretation and limitations
Offer site- and exposure-appropriate testing and partner notification without moral judgement.
05
FBC and targeted vector testing
Why
Investigate anaemia or systemic febrile illness after heavy infestation or relevant travel.
Interpretation and limitations
Tests follow symptoms and epidemiology; routine blood panels are unnecessary for ordinary head lice.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Scabies

Nocturnal itch, burrows in finger webs and genital nodules with close-contact spread favour mites over lice or exposed-site bites.

Eczema or folliculitis

Chronic confluent dermatitis or follicle-centred pustules lacks live lice and the characteristic clustered punctate exposure pattern.

Urticaria

Migrating raised wheals resolve individually within 24 hours without a punctum, unlike persistent excoriated insect-bite papules.

Delusions of infestation

A fixed infestation belief without objective insects requires respectful dermatological examination and avoidance of harmful repeated pesticide exposure.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Confirmed head liceTreat and recheckFirst stepA live louse is found by detection combing.

1Check household members and treat all confirmed cases on the same day.
2Use scheduled wet combing or a licensed dimeticone product, repeating at seven days when its instructions require it.
3Recheck for live lice after the completed regimen and distinguish reinfestation, under-application and nits from failure.

02Pubic or body liceAddress context and contactsLice involve pubic or body sites or clothing seams.

1For pubic lice, arrange treatment, recent partner management and STI screening; obtain specialist advice for eyelashes.
2For body lice, hot-launder or replace clothing and bedding and support access to washing facilities.
3Assess secondary infection, anaemia, vulnerability and safeguarding without stigmatising the patient.

03Insect-bite reactionControl source and inflammationGrouped itchy papules fit bites without anaphylaxis or invasive infection.

1Identify and remove the likely environmental source, including pet or professional pest assessment when indicated.
2Use cold compresses, a non-sedating antihistamine and brief site-appropriate topical corticosteroid for itch.
3Safety-net systemic allergy, fever, spreading pain, target-like rash after tick exposure and persistent unexplained lesions.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

A physical head-lice treatment that coats lice and has no conventional neurotoxic resistance mechanism.

Dimeticone 4% lotion

Apply to dry hair and scalp until fully covered, leave for the product-specified period and repeat after seven days when directed.

Follow the exact licensed product because contact times differ, avoid flames and heat while hair is treated, protect eyes and do not use incomplete leftover quantities.

A generally non-sedating option for symptomatic itch and wheal reduction.

Cetirizine 10 mg tablets

Adults and people aged 12 years or older take 10 mg orally once daily when bite-related itch warrants an antihistamine.

Somnolence can occur; consider driving, alcohol and renal adjustment, and do not substitute it for adrenaline in anaphylaxis.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Secondary infection

Excoriated scalp or bite lesions can subsequently develop bacterial impetigo, spreading cellulitis and tender regional lymphadenopathy.

Anaemia in heavy infestation

Severe chronic lice burden can contribute to iron deficiency, especially in vulnerable children or neglected adults.

Anaphylaxis

Some stings and bites trigger sudden life-threatening airway, respiratory or circulatory compromise requiring immediate intramuscular adrenaline treatment.

Psychological and social harm

Stigma, persistent sleep loss, unnecessary school exclusion and repeated pesticide overuse can exceed the infestation's direct physical burden.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Recheck for live lice after the complete regimen rather than treating persistent nits.
Audit product quantity, coverage, contact time and repeat date before declaring resistance.
Review bite lesions for reduced itch and absence of new crops after source control.
Escalate breathing difficulty, hypotension, fever, spreading infection or tick-associated systemic symptoms.
Document STI, safeguarding and social-support actions sensitively for pubic or body lice.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Live lice define activity

Adherent empty eggs can remain for months and are not proof that treatment failed.

Lice do not jump

Head-to-head contact, not airborne spread or unclean hair, drives most transmission.

Product timing differs

Dimeticone formulations have different contact times, making package-specific instruction essential.

Bite pattern guides source

Crops on exposed sites and household or pet clustering are more informative than lesion colour alone.

11Common pitfallsFrequent interpretation and management errors.

01
Diagnosing head lice from nits alone and repeatedly treating a cleared scalp.
02
Using too little lotion to cover thick or long hair.
03
Treating every household member prophylactically without checking for live lice.
04
Giving antibiotics for sterile bite hypersensitivity without infection.
05
Missing STI or safeguarding assessment when pubic lice occurs in a concerning context.