Lesson
Localize the Arthropod Before Treating It
Head lice, body lice, pubic lice, and scabies occupy different habitats. Pruritus reflects hypersensitivity and can lag infestation or persist after organisms are gone.
- Louse versus mite
- Habitat
- Life cycle
- Hypersensitivity
- Secondary infection
Live louse and near-scalp nits
Seams carry organisms and eggs
Sexual-health contact pathway
Burrow, papule, and contact network
Map habitat to disease
Head lice cling to scalp hair. Body lice mainly live in clothing seams. Pubic lice attach to coarse hair. Sarcoptes scabiei mites burrow in the stratum corneum. Pets do not transmit human head lice.
Separate symptoms from viability
First-exposure itch can take weeks to develop, while post-treatment itch can persist. Live lice or mites and new burrows are more informative than itch alone. New papules warrant reassessment, but retained eggs or casings alone do not establish viable post-treatment infestation.
Use life cycle to time therapy
Egg killing differs by drug. Many non-ovicidal products need a repeat timed after hatching and before reproduction. Topical ivermectin is an exception: it impairs survival of newly hatched lice and is generally a single application. Follow the selected product rather than using egg killing alone to decide retreatment. Scabies treatment also coordinates a second exposure when required.
Protect injured skin
Excoriation can cause impetigo or cellulitis. Spreading redness, warmth, pain, pus, fever, lymphangitis, or systemic illness requires evaluation beyond antiparasitic treatment.
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Lesson
Confirm Active Head Lice Without Stigma
Head lice are best diagnosed by a live crawling louse. Nits close to the scalp support recent infestation, but dandruff and distant empty casings are commonly misidentified.
- Live louse
- Nit proximity
- Fine comb
- Nape and ears
- No hygiene blame
Bright light and fine comb
Best evidence of active infestation
Near-scalp attachment supports viability
Direct hair contact drives spread
Find live organisms
Use bright light, section the hair, and comb from scalp outward. Concentrate behind the ears and at the nape. Nymphs and adults move quickly and avoid light.
Interpret nits accurately
Nits are firmly attached. Those within one quarter inch of the scalp may be viable. Distant nits are usually hatched, dead, or empty and do not by themselves prove active infestation.
Reject false biology
Head lice crawl. They do not jump or fly. Direct hair-to-hair contact is the main transmission route, while pets do not acquire or spread human head lice.
Remove stigma
Head lice are not evidence of poor hygiene and do not transmit disease. Use neutral language and protect privacy while organizing examination, treatment, and return to school.
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Lesson
Choose a Head-Lice Product by Biology and Label
Permethrin, pyrethrins, ivermectin, spinosad, and malathion differ in age, egg activity, resistance, retreatment, formulation, and safety.
- Permethrin 1 percent
- Pyrethrins
- Ivermectin 0.5 percent
- Spinosad 0.9 percent
- Malathion 0.5 percent
Repeat needed when eggs survive
Usually one dry-hair application
Repeat only for live lice on day seven
No flame or electrical heat while wet
Use pyrethroids precisely
Permethrin 1 percent is approved from age two months and may need day-nine retreatment. Pyrethrins with piperonyl butoxide begin at age two years, need repeat treatment, and require chrysanthemum or ragweed allergy review.
Place current ivermectin accurately
Ivermectin lotion 0.5 percent is now OTC from age six months. It is usually a single dry-hair application and should not be repeated without clinician advice. Do not confuse it with off-label oral ivermectin. For head lice, CDC describes clinician-prescribed oral doses of 200 or 400 micrograms/kg, repeated in 9 to 10 days. Do not use oral ivermectin for head lice during pregnancy or in children weighing less than 15 kg; do not copy the scabies repeat interval.
Use spinosad by live-louse findings
Spinosad 0.9 percent is labeled from age six months and kills lice and unhatched eggs. Shake and apply to dry scalp and hair for 10 minutes, then rinse with warm water; use enough for complete coverage, up to 120 mL. Repeat only if live crawling lice are seen seven days after the first treatment. Nit combing is optional for the label regimen.
Account for abametapir interactions
Abametapir 0.74% lotion (Xeglyze) is labeled for head lice from age 6 months. Shake, coat dry scalp and hair, leave for 10 minutes, and rinse with warm water. It is a single application; discard unused product. Its label requires avoiding CYP3A4, CYP2B6, or CYP1A2 substrate medicines for 2 weeks afterward; choose a different lice treatment if that cannot be done safely. Do not independently stop necessary medicines. Adult supervision and eye protection are essential; its benzyl alcohol content adds toxicity concerns in young infants and after ingestion.
Understand benzyl alcohol and historical availability
Benzyl alcohol 5% lotion kills lice but not eggs. The FDA Ulesfia label specifies age 6 months or older, complete dry-hair and scalp saturation for 10 minutes, then rinsing and a second application 7 days later. Protect the eyes and monitor irritation. The book identifies the Ulesfia brand as discontinued; a historical approval is not proof of current supply. Confirm availability before making it the treatment plan.
Retire lindane as a routine choice
Current CDC guidance reports lindane unavailable in the United States and warns of serious neurologic toxicity. It is not a routine fallback for treatment failure. Reassess the diagnosis and application, then choose an appropriate alternative rather than repeating a failed insecticide.
Connect malathion chemistry to selectivity and fire safety
Malathion toxicity involves acetylcholinesterase inhibition. Mammalian carboxylesterases detoxify malathion to non-cholinesterase-inhibiting metabolites more effectively than susceptible insects. This contributes to selective insect toxicity; it does not make excessive human exposure safe. Malathion 0.5 percent begins at age six years and is flammable while hair is wet. Avoid smoking, flames, hair dryers, curling devices, and straightening irons until the hair is dry.
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Lesson
Deliver the Full Scalp Dose and Time the Repeat
Correct active ingredient cannot overcome conditioner interference, incomplete saturation, wrong contact time, unsafe rinsing, or a retreatment schedule copied from another product.
- Dry versus prepared hair
- Full saturation
- Contact time
- Sink rinse
- Failure audit
Follow dry or prepared hair label
Volume follows length and density
Do not copy another product
Audit before reapplying
Prepare exactly as labeled
Do not use conditioner or combination shampoo and conditioner before a lice medicine. Confirm whether hair must be dry, how much product is needed, and whether a second bottle is required for long or dense hair.
Use the pyrethrin shampoo as labeled
The reviewed pyrethrins 0.33% plus piperonyl butoxide 4% shampoo is applied to dry hair for 10 minutes, no longer. Add warm water to lather, then rinse thoroughly and comb. Its label requires repeat treatment in 7 to 10 days; CDC specifies 9 to 10 days. Plan the date explicitly. It is labeled from age 2; younger children need clinician advice. Review ragweed or chrysanthemum allergy.
Prepare permethrin cream rinse on damp hair
For the reviewed Nix 1% creme rinse, shampoo without conditioner, rinse, and towel dry until damp rather than wet. Shake and saturate scalp and hair, especially behind the ears and at the nape. Leave for 10 minutes, no longer, and rinse with warm water. The label calls for a second treatment if live lice appear 7 days or more later; CDC identifies day 9 as a useful retreatment time. Do not confuse this scalp product with permethrin 5% scabies cream. The Nix label also directs patients with ragweed allergy to consult a clinician.
Time topical ivermectin from complete coverage
Apply ivermectin 0.5% lotion to dry scalp and hair, from roots through tips. Start the 10-minute timer only after coverage is complete, then rinse with water alone. Wait 24 hours before shampooing. The reviewed tube is single-use: use up to one tube for full coverage and discard the remainder. Nit combing is optional for efficacy. Do not repeat on the same person for the same infestation without clinician advice.
Keep malathion on long enough without heat
Apply malathion 0.5% to dry hair and scalp, allow natural drying, and leave hair uncovered. Shampoo after 8 to 12 hours, rinse, and comb out dead lice and eggs. Repeat only if lice remain 7 to 9 days later. Never speed drying with a hair dryer or use other heat, flame, or smoking around wet treated hair. Wash the product off if irritation develops; recurrent irritation or a burn needs medical advice.
Match shampoo timing to the actual medicine
A blanket instruction to avoid shampoo for 1 to 2 days after every lice treatment is inaccurate. Ivermectin lotion specifies a 24-hour wait, while the reviewed abametapir and benzyl alcohol labels permit shampooing after treatment. Malathion is removed by shampoo at its 8-to-12-hour endpoint. Give the instructions for the selected preparation.
Cover and time accurately
Saturate the scalp and all hair, protect eyes, use the exact leave-on interval, and wash hands. Rinse over a sink with warm rather than hot water when appropriate to limit skin exposure.
Comb with a purpose
Fine-toothed combing can remove lice and nits and support surveillance. It is helpful but not a universal proof of cure, and spinosad does not require nit removal for efficacy.
Audit failure before adding exposure
At eight to twelve hours, slowly moving lice may still be dying. If no dead lice are found or lice remain as active as before, consult before retreating. Review diagnosis, application, conditioner, contacts, reinfestation, and resistance.
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Lesson
Control Contacts and Environment Without Overreaction
Close contacts drive lice transmission. Focused examination, synchronized treatment, limited recent-item cleaning, and school inclusion replace prophylactic pesticide, fumigation, and no-nit policies.
- Contact examination
- Bed sharing
- Two-day item window
- No fumigation
- School inclusion
Coordinate treatment when indicated
Hot laundry and comb soaking
Off-host risk is low
Reject no-nit exclusion
Treat the contact network
Check household members and close contacts. Treat active infestations and people sharing a bed at the same time as directed. Do not apply insecticide prophylactically to every contact without evidence.
Clean what matters
For head lice, wash items used in the prior 2 days in hot water (130 degrees F) and use high-heat drying. Soak combs and brushes at 130 degrees F or hotter for 5 to 10 minutes. Dry-clean or bag nonwashable items for 2 weeks. Vacuum relevant surfaces without extensive household pesticide use.
Stop excessive decontamination
Head lice survive poorly off the host. Spending extensive time and money cleaning the home is unnecessary. Fumigant sprays and fogs add inhalation and skin toxicity and should not be used.
Keep children in school
A child need not be sent home early. After treatment begins at home, the child can return. CDC, AAP, and school nurses oppose no-nit exclusion because distant or dead nits do not transmit like live lice.
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Lesson
Distinguish Pubic and Body Lice Pathways
Pubic lice require a sexual-health pathway, while body lice primarily require clean clothing, bathing, laundry access, and attention to vulnerable living conditions.
- Pubic lice
- Partner management
- STI evaluation
- Body lice
- Clothing seams
Treat all linked exposure
Keep insecticides away from eyes
Inspect clothes and bedding
Make the primary treatment possible
Treat pubic lice as sexual health
CDC recommends permethrin 1% cream rinse or pyrethrins with piperonyl butoxide on affected pubic-hair areas for 10 minutes before washing off. Treat sex partners from the prior month, avoid sex until treatment and decontamination are complete and persistent infestation is excluded, and evaluate for HIV, syphilis, chlamydia and gonorrhea. Reassess after a week if symptoms persist; scalp-product instructions do not automatically establish pubic-site use.
Protect the eyes
Lice on eyebrows or eyelashes require clinician-directed removal or ophthalmic-grade therapy. Ordinary scalp or pubic lice products should not be placed near the eyes.
Find body lice in clothing
Body lice and eggs are usually found in seams rather than living continuously on skin. Inspect clothes and bedding when pruritus occurs in crowded or resource-limited settings.
Treat access as clinical
Regular bathing, clean clothing changes, hot laundering, bedding management, and housing support are primary body-lice interventions. Pediculicide is generally unnecessary when adequate bathing and laundering are maintained. Unlike head lice, body lice can transmit epidemic typhus, Bartonella quintana infection, and epidemic relapsing fever; fever or systemic illness needs clinical evaluation.
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Lesson
Recognize Scabies and Its Contact Window
Classic scabies combines nocturnal pruritus, papules, and burrows in characteristic sites. Young children, older adults, immune compromise, and prior corticosteroids can change the pattern.
- Burrows
- Nocturnal itch
- Characteristic sites
- Dermoscopy
- Two-month contact history
Distribution supports diagnosis
Age changes morphology
Find mite, egg, or scybala
Asymptomatic contacts can transmit
Read distribution and history
Inspect finger webs, wrists, elbows, axillae, waist, buttocks, nipples, genitals, and other involved sites. In infants and young children, include head, neck, palms, and soles as directed.
Confirm when the question needs it
Dermoscopy or skin scraping can identify mites, eggs, or fecal material. Negative testing does not always exclude scabies, but objective confirmation is especially valuable in atypical, institutional, or crusted disease.
Map transmission
Prolonged direct skin contact is the main route. Review household, sexual, caregiving, and institutional contacts during the prior two months. Symptoms can be delayed in a first infestation.
Protect alternatives and complications
Atopic and contact dermatitis, insect bites, drug eruption, folliculitis, and other pruritic conditions can mimic scabies. Pain, pus, fever, severe crusting, or systemic illness needs another pathway.
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Lesson
Treat Classic Scabies as Whole-Body and Whole-Household Care
Permethrin 5 percent remains first-line from age two months. Oral ivermectin is off label, and spinosad has a distinct labeled scabies regimen from age four years.
- Permethrin 5 percent
- Oral ivermectin
- Spinosad
- Complete coverage
- Simultaneous contacts
Comprehensive coverage
Weight and pregnancy limits
Scabies directions differ from lice
Treat the network simultaneously
Apply permethrin comprehensively
Apply 5 percent cream over the prescribed body surface, including folds, under nails, between digits, buttocks, groin, and soles. The reviewed label includes head-to-sole coverage and specifically includes the scalp, temples and forehead in infants. Older adults may also have scalp or hairline involvement. CDC describes neck-down treatment for typical adults; establish the age- and distribution-specific body map with the prescriber and protect the eyes. The label estimates about 30 g for an average adult, but adequate prescribed coverage determines the amount. Reapply to hands if washed during the exposure and remove after eight to fourteen hours.
Choose alternatives for specific constraints
Permethrin 5% may need a second application about a week later. For infants younger than 2 months, clinician-directed compounded sulfur 5% to 10% in petrolatum is an option. Crotamiton 10% is approved for adults but has frequent treatment failures and is not FDA-approved for children. Neither OTC lice products nor household insecticide sprays substitute for a scabies prescription.
Use oral ivermectin accurately
For selected classic scabies, experts use two 200 microgram per kilogram doses with food, seven to fourteen days apart. This is off label, and safety is not established below 15 kg or during pregnancy.
Explain the food instruction discrepancy
The oral ivermectin label for its approved worm indications says to take it fasting. CDC scabies guidance instead recommends taking the off-label scabies doses with food to increase exposure. Confirm and document the indication-specific prescription rather than automatically replacing one instruction with the other.
Check the microgram-to-milligram calculation
For a prescribed scabies dose of 200 micrograms/kg in a 60 kg adult, 60 times 200 = 12,000 micrograms = 12 mg. With 3 mg tablets, that is four tablets per dose. Verify the repeat date separately; a correct first dose does not complete the two-dose regimen. This example calculates an existing prescription, not individual treatment eligibility.
Screen oral ivermectin beyond weight
Review pregnancy, interacting medicines, severe liver disease, and neurologic symptoms. The label reports rare increased INR with warfarin. Significant exposure to Loa loa-endemic West or Central Africa requires pretreatment assessment because heavy infection can cause serious encephalopathy after treatment. Confusion, impaired consciousness, seizures, or severe rash needs urgent assessment.
Keep adverse reactions in their disease context
The book lists fever, joint symptoms and tender enlarged lymph nodes with oral ivermectin. In its label, this cluster appears among inflammatory Mazzotti reactions after microfilarial death in onchocerciasis. Do not teach it as the expected response to treating head lice or scabies. Ivermectin can still cause drug-related gastrointestinal, skin and neurologic adverse effects.
Do not copy spinosad directions between indications
For scabies from age four, shake spinosad and rub it over the body from the neck down through the soles. Patients with a balding scalp also apply it to the scalp, hairline, temples, and forehead. Avoid eye contact. Let the skin dry for ten minutes before dressing and wait at least six hours before showering or bathing. Its ten-minute head-lice contact time is not the scabies regimen.
Treat contacts and recent items
Coordinate simultaneous contact treatment. Wash bedding, clothing, and towels hot and dry hot. Seal nonwashable items at least 72 hours to one week. Avoid direct skin contact until treatment is complete.
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Lesson
Escalate Crusted Scabies and Institutional Transmission
Crusted scabies has a massive mite burden, high contagiousness, and sometimes limited itch. It requires combined therapy, keratin management, precautions, and outbreak coordination.
- High mite burden
- Combined therapy
- Keratolysis
- Contact precautions
- Outbreak tracking
Mite burden can be massive
Severity guides repeated schedules
Reach mites beneath crust
Stop institutional amplification
Recognize the dangerous pattern
Widespread hyperkeratotic crusts, scaling, fissures, nail involvement, institutional exposure, immune compromise, neurologic disease, or inability to scratch should raise concern even when itch is limited.
Combine systemic and topical treatment
CDC describes oral ivermectin 200 micrograms/kg per dose on approximately days 1, 2 and 8; days 1, 2, 8, 9 and 15; or days 1, 2, 8, 9, 15, 22 and 29, according to severity. Combine with full-body permethrin 5%, sometimes daily initially or every 2 to 3 days during the first 1 to 2 weeks. No optimal schedule has consensus; specialist and infection-control teams must specify the complete calendar.
Improve topical penetration
Keratolytic therapy on non-permethrin days may reduce thick crust and improve scabicide penetration. Monitor barrier injury, bacterial infection, hydration, temperature, and medication exposure.
Control the outbreak as a system
Use contact precautions, identify staff and residents with direct exposure, treat contacts in a coordinated window, manage linens and rooms, track completion, and notify infection prevention or public health.
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Close the Loop on Safety, Failure, and Residual Itch
Product concentration, minimum age, body surface, pregnancy, lactation, weight, damaged skin, seizure risk, immune status, and caregiver technique determine safe exposure.
- Age thresholds
- Pregnancy narrative
- Postscabetic itch
- New lesions
- Failure audit
Prevent formulation errors
Use product-specific evidence
Hypersensitivity resolves slowly
Separate activity from recovery
Use exact age and formulation
Permethrin 1 percent for lice and 5 percent for scabies are not interchangeable. Spinosad begins at six months for lice and four years for scabies. Ivermectin lotion begins at six months for lice, while oral ivermectin has distinct off-label limits.
Use narrative reproductive reasoning
For scabies in pregnancy, CDC prefers topical permethrin; oral ivermectin safety is not established. Use product-specific evidence rather than old pregnancy letters. For breastfeeding, LactMed considers topical permethrin acceptable because absorption is low and metabolism is rapid, whereas some US labels retain more cautious nursing language. Reconcile that discrepancy with the clinician instead of routinely stopping breastfeeding. Prevent infant contact with treated skin; a breast-area application needs a feeding and wash-off plan. Do not extrapolate this advice to every scabicide.
Explain residual itch before it happens
Scabies itch can persist for several weeks because hypersensitivity resolves slowly. New burrows, new papules, live mites, or untreated contacts warrant reassessment. CDC also advises reassessment when itching persists more than 2 to 4 weeks after treatment; this does not prove that mites survived.
Audit every failure
Reconfirm organism, product, concentration, amount, coverage, contact time, repeat timing, contact treatment, recent-item control, reinfestation, irritation, and resistance before adding or switching insecticides.
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Module practice
Check the connections.
Each attempt draws 10 questions from the complete 40 question bank.
Each attempt draws a fresh set and rearranges the answer choices.
References
Current clinical foundation.
Core source material was synthesized with the following contemporary guidance. Verify local policy and current guidance before applying clinical information.
- CDC Clinical Care of Head Lice
- CDC Treatment of Head Lice
- CDC Head Lice and School
- CDC Clinical Care of Scabies
- CDC Treatment of Scabies
- DailyMed Spinosad Label
- DailyMed Ivermectin Lotion Label
- DailyMed Xeglyze (Abametapir)
- FDA Ulesfia Label (2020; Historical Product)
- DailyMed Nix 1% Creme Rinse
- DailyMed Malathion 0.5% Lotion
- DailyMed Stromectol: Oral Ivermectin Safety
- CDC STI Guidelines: Ectoparasitic Infections
- DailyMed Pyrethrins/Piperonyl Butoxide Shampoo
- CDC About Body Lice
- DailyMed Permethrin 5% Cream
- FDA Ovide Pharmacology Review: Detoxification
- LactMed: Permethrin During Breastfeeding