Scabies, caused by infestation with the mite Sarcoptes scabiei var. hominis, is one of the most common skin diseases globally, estimated to affect over 200 million people at any one time. It is transmitted by direct prolonged skin-to-skin contact and in institutional settings by fomite contact with infested bedding or clothing. The disease exists in two clinically distinct forms that require different treatment strategies: common scabies and crusted (Norwegian) scabies, a hyperinfestation state occurring in immunocompromised or neurologically impaired individuals in which the mite burden can reach millions rather than the 10 to 15 mites found in typical infestation.2
Permethrin. Permethrin 5% cream is the first-line treatment for common scabies in adults and children above 2 months of age in most countries. Permethrin is a synthetic pyrethroid (discussed in detail in Section 3) that acts by prolonging the opening of voltage-gated sodium (Na⁺) channels in the mite nervous system, causing repetitive neuronal firing, paralysis, and death. The cream is applied from the neck down (including all skin folds, under fingernails, and between toes), left for 8 to 14 hours, then washed off; the full application is repeated in 1 week to kill mites that hatched from eggs after the initial treatment, since permethrin is not reliably ovicidal. In infants below 2 months of age and in some guidelines for infants up to 2 years, application is extended to include the face, scalp, and neck (areas not treated in adults), because the scabies mite can infest these areas in infants. Cure rates with two properly applied permethrin 5% cream treatments exceed 90 percent in immunocompetent individuals.1
Oral Ivermectin. Oral ivermectin is an effective alternative to permethrin for common scabies and is the preferred agent in several clinical contexts: when topical application is impractical (institutionalized patients, outbreak settings), when compliance with topical treatment is poor, or when permethrin has failed. The standard dose is 200 micrograms per kilogram (mcg/kg) as a single oral dose, repeated in 1 to 2 weeks. Two-dose ivermectin achieves cure rates comparable to two applications of permethrin 5% cream in head-to-head randomized controlled trials. For institutional scabies outbreaks in nursing homes or psychiatric facilities, simultaneous oral ivermectin treatment of all residents and staff is logistically superior to applying cream to all individuals simultaneously; both contacts and cases are treated on the same day. Ivermectin is not recommended as first-line therapy in pregnant women or in children below 15 kg, though its risk-benefit ratio is reassessed in outbreak settings. Co-administration with food increases ivermectin absorption approximately 2.5-fold and improves efficacy.1
Crusted Scabies. Crusted (Norwegian) scabies presents with hyperkeratotic plaques, particularly on the hands, feet, elbows, and scalp, containing enormous numbers of mites. It occurs in patients with compromised immunity (human immunodeficiency virus (HIV) infection, organ transplant recipients, hematological malignancy), neurological impairment (Down syndrome, dementia, spinal cord injury with loss of sensation), or extreme debility (malnutrition, older age). Topical permethrin alone is inadequate because the thick hyperkeratotic crust prevents adequate drug penetration. The standard management combines repeated courses of oral ivermectin (200 mcg/kg on days 1, 2, 8, 9, and 15; more frequent dosing may be required for very severe disease) with daily or twice-daily application of permethrin 5% cream and the addition of keratolytic agents (such as 5 to 10% salicylic acid) to remove the crust and allow drug penetration. Strict contact precautions, isolation, and simultaneous treatment of all close contacts are essential, as crusted scabies patients are extremely contagious and the source of institutional outbreaks. Environmental decontamination of bedding, clothing, and furniture is required; the mite survives off the host for up to 72 hours in crusted scabies (compared with 24 to 36 hours in common scabies).1
Benzyl Benzoate. Benzyl benzoate 25% lotion is widely used in resource-limited settings as an alternative to permethrin, primarily because of its lower cost and wider availability in sub-Saharan Africa and parts of Asia. Its mechanism involves direct neurotoxicity to the mite through disruption of octopamine receptor signaling (an invertebrate-specific receptor not present in mammals), causing hyperexcitation and death of the mite. It is applied to the whole body from the neck down for 24 hours, washed off, and reapplied after 24 hours; the standard WHO regimen is two 24-hour applications on consecutive days, repeated after one week. Benzyl benzoate is more irritating to the skin than permethrin, particularly on the face and genitalia, and causes a burning sensation on application that may reduce compliance. It is diluted to 12.5% for children. Benzyl benzoate is considered safe in the second and third trimesters of pregnancy. Crotamiton 10% cream is a less effective alternative sometimes used in infants where other agents are not available; its scabicidal mechanism is poorly characterized and cure rates are substantially lower than permethrin.3
Post-Treatment Itch and Treatment Failure. Pruritus typically persists for 2 to 4 weeks after successful scabies treatment, driven by ongoing delayed-type hypersensitivity (DTH) reactions to mite antigens, feces, and dead mite material remaining in the skin after eradication. Failure to counsel patients about this phenomenon leads to unnecessary retreatment. Oral antihistamines and topical corticosteroids are used to manage post-scabicidal pruritus. True treatment failure, defined as persistent viable mites or new lesions after a complete two-dose treatment course, should prompt consideration of: permethrin resistance (emerging in some regions), inadequate application technique, reinfection from an untreated contact, or underlying crusted scabies. Permethrin resistance in S. scabiei has been confirmed molecularly and is associated with mutations in the voltage-gated sodium channel similar to those causing knockdown resistance (kdr) in insecticide-resistant insects.56
Permethrin 5% cream: apply neck-down 8–14 hrs; repeat at 1 week (eggs hatch after initial treatment); infants: include face, scalp, neck. Ivermectin 200 mcg/kg ×2 doses (1–2 weeks apart): preferred for institutional outbreaks, poor compliance, failed topical therapy. Crusted scabies: ivermectin + permethrin + keratolytics mandatory; topical alone fails. Post-treatment itch: normal for 2–4 weeks after cure — do not retreat unless mites confirmed viable. Permethrin resistance: confirmed; associated with sodium channel kdr mutations.
Pediculosis refers to infestation by lice of three species: Pediculus humanus capitis (head lice), Pediculus humanus corporis (body lice), and Phthirus pubis (pubic or crab lice). Head lice infestation is endemic in school-age children worldwide and does not reflect poor hygiene. Body lice are vectors of serious infectious diseases (epidemic typhus caused by Rickettsia prowazekii, trench fever by Bartonella quintana, and louse-borne relapsing fever by Borrelia recurrentis) in conditions of crowding and poverty. Treatment of pediculosis has been substantially complicated by widespread resistance to permethrin and other pyrethroids, requiring systematic knowledge of alternative agents.
Permethrin for Pediculosis. Permethrin 1% creme rinse (for head lice) acts by the same sodium channel mechanism as the 5% scabies formulation, causing paralysis and death of lice. It is applied to shampooed and towel-dried hair, left for 10 minutes, then rinsed; a second application is recommended at day 9 to kill lice that hatched from nits (eggs) after the first treatment, as permethrin is incompletely ovicidal. Permethrin 1% has historically been highly effective, but resistance has become a significant clinical problem in many countries. The prevalence of the kdr (knockdown resistance) mutation, a specific point mutation (T929I or L932F) in the voltage-gated sodium channel gene of P. humanus capitis that reduces permethrin binding, has reached 50 to 99 percent in head lice populations in parts of the United States, Europe, Australia, and Israel. In these high-resistance settings, permethrin should not be used as first-line therapy, and treatment failure should prompt switching to an alternative agent rather than repeating permethrin.45
Malathion. Malathion 0.5% lotion is an organophosphate cholinesterase inhibitor applied to dry hair for 8 to 12 hours before washing off. It acts by inhibiting acetylcholinesterase (AChE) in the louse nervous system, causing accumulation of acetylcholine (ACh) at cholinergic synapses and producing overstimulation of nicotinic and muscarinic receptors, leading to louse death. Malathion retains ovicidal and pediculicidal activity against kdr-resistant lice strains because its mechanism is entirely independent of sodium channel function. It is effective even against louse strains fully resistant to permethrin and is a first-line alternative in resistance-prevalent settings. Adverse effects include scalp irritation; because malathion has a residual effect on hair of approximately 6 weeks due to binding to hair shafts, reapplication for recurrence is not required within this period. The lotion is flammable during application; patients should avoid open flames, cigarettes, and hair dryers while the product is on the hair and during the drying period. Malathion is not recommended in children below 6 years or in pregnant women due to organophosphate systemic absorption concerns, though actual systemic exposure is low with correct use.4
Spinosad and Benzyl Alcohol. Spinosad 0.9% topical suspension and benzyl alcohol 5% lotion are newer agents approved specifically for head lice. Spinosad is derived from the soil bacterium Saccharopolyspora spinosa and acts by binding to insect nicotinic acetylcholine receptors (nAChR) and glutamate-gated chloride channels, causing neuromuscular hyperexcitation and paralysis of lice. Its distinct mechanism of action means it retains full activity against kdr-resistant lice. Spinosad 0.9% suspension is applied to dry hair for 10 minutes then rinsed; a single treatment is sufficient in most cases (unlike permethrin, which requires a mandatory second application), because spinosad has ovicidal as well as pediculicidal activity. Benzyl alcohol 5% lotion acts by a physical rather than neurotoxic mechanism: it asphyxiates lice by blocking their breathing spiracles, preventing air exchange; it has no known resistance risk. Both agents are approved for children 6 months and above (spinosad) or 6 months and above (benzyl alcohol) and are options when pyrethroids have failed.4
Oral Ivermectin for Pediculosis. Oral ivermectin 400 mcg/kg given as two doses 7 to 10 days apart is an effective alternative for head lice refractory to topical agents, particularly when compliance with topical application is an issue or when kdr resistance renders topical pyrethroids ineffective. It provides a systemic route of action that bypasses topical application altogether, making it useful for scalp conditions (psoriasis, eczema, extensive crusting) that impair topical drug penetration. For pubic lice, permethrin 1% cream or oral ivermectin are the preferred agents; close contacts and sexual partners must be treated simultaneously and bedding and clothing laundered in hot water. Body lice infestation is treated by environmental measures (discarding or hot-laundering infested clothing and bedding) as the primary intervention; topical permethrin applied to clothing and the body is used as adjunct treatment, but environmental decontamination is the key because body lice live in clothing seams rather than on the body itself.4
| Agent | Mechanism | kdr Resistance | Application | Notes |
|---|---|---|---|---|
| Permethrin 1% | Na⁺ channel prolonged opening | Affected (T929I/L932F) | 10 min; repeat day 9 | Widely resistant; avoid in high-prevalence kdr settings |
| Malathion 0.5% | AChE inhibition | Not affected | 8–12 hrs; single application | Flammable; avoid in children <6 yrs; no flames during use |
| Spinosad 0.9% | nAChR + GluCl activation | Not affected | 10 min; single application usually sufficient | Ovicidal; approved ≥6 months; no mandatory 2nd dose |
| Benzyl alcohol 5% | Spiracle asphyxiation (physical) | No resistance risk | 10 min; repeat in 9 days | Not neurotoxic; approved ≥6 months |
| Ivermectin oral | GluCl channel opening | Not affected | 400 mcg/kg ×2 doses (7–10 days apart) | Systemic; useful for kdr-resistant lice or failed topical treatment |
kdr resistance (T929I/L932F sodium channel mutation) is prevalent in >50% of head lice in the US, Europe, Australia: permethrin first-line should be reconsidered in these settings. Malathion and spinosad retain full activity against kdr-resistant lice. Malathion: flammable on hair — warn patients about flames and hair dryers during application. Body lice: environmental decontamination of clothing is the primary treatment; lice live in clothing seams, not on skin. Pubic lice: treat sexual partners simultaneously. Manual nit removal with fine-tooth comb is adjunctive to all topical treatments but not sufficient as sole therapy.
Pyrethrins are naturally occurring insecticidal compounds derived from the flowers of Chrysanthemum cinerariaefolium, while pyrethroids are structurally modified synthetic analogs designed for greater stability, potency, and bioavailability. Together they constitute the most widely used class of insecticides globally, with applications in public health, agriculture, veterinary medicine, and the treatment of ectoparasitic infestations. Understanding their mechanism and resistance biology is essential for rational selection among available ectoparasiticides.
Mechanism of Action. Pyrethrins and pyrethroids act by binding to voltage-gated sodium (Na⁺) channels in the neuronal axon membrane and prolonging their open state after depolarization. Normally, voltage-gated Na⁺ channels open briefly (1 to 2 milliseconds) during an action potential and then rapidly inactivate; pyrethrin binding prevents inactivation, causing prolonged Na⁺ influx, repetitive action potential generation, and ultimately hyperexcitation followed by paralysis and death of the target organism. The channel returns to a closed state much more slowly in the presence of these compounds (tail current prolonged by milliseconds to seconds depending on the compound and temperature). Pyrethroids are subdivided into Type I compounds (no alpha-cyano group; examples include permethrin and bifenthrin) that cause repetitive firing and tremors, and Type II compounds (with an alpha-cyano substituent; examples include cypermethrin, deltamethrin, and lambda-cyhalothrin) that cause persistent depolarization, choreoathetosis, and salivation. The selectivity for invertebrate over mammalian sodium channels is the primary basis for their safety margin in mammals: pyrethroid-bound invertebrate channels recover far more slowly at cool temperatures, and mammals maintain higher body temperatures that accelerate channel recovery. Additionally, pyrethroids are rapidly metabolized by mammalian esterases and CYP450 (cytochrome P450) enzymes, limiting systemic toxicity at therapeutic doses.6
Knockdown Resistance. Knockdown resistance (kdr) is the primary mechanism of pyrethroid resistance in insect and mite pests, including head lice, body lice, and Sarcoptes scabiei. The kdr mechanism involves specific point mutations in domain II (particularly segments 4 and 5) of the voltage-gated sodium channel gene, most commonly at amino acid positions 929 (T929I) and 932 (L932F) in lice, and analogous positions in other arthropods. These mutations alter the binding site of pyrethroids on the sodium channel, reducing binding affinity without significantly impairing the channel's normal electrophysiological function. Super-kdr mutations involve additional changes at position 918 that confer even higher levels of resistance. Because kdr acts by target site modification, it confers cross-resistance to all pyrethrins and pyrethroids regardless of their specific structure, and cannot be overcome by increasing the dose. Agents with entirely different mechanisms of action (malathion for AChE inhibition, spinosad for nAChR/GluCl activation, benzyl alcohol for physical asphyxiation) are unaffected by kdr and retain full activity.677
Spectrum and Clinical Applications. In medical ectoparasiticide use, permethrin is the prototypical pyrethroid. As described in Sections 1 and 2, permethrin 5% cream is first-line for scabies and permethrin 1% creme rinse for head lice (where kdr resistance permits). Beyond these primary indications, pyrethrins combined with piperonyl butoxide (piperonyl butoxide inhibits esterase-mediated metabolic degradation of pyrethrins, dramatically potentiating their activity by preventing premature inactivation) are available as over-the-counter shampoo formulations for head lice (for example, Rid and A-200 brand products). Pyrethrin-piperonyl butoxide shampoos are less effective than permethrin against kdr-resistant lice. Permethrin-treated clothing, bednets, and tents are widely used in military and outdoor recreational settings for protection against mosquitoes, ticks, and other arthropod vectors; permethrin-impregnated bednets are a cornerstone of malaria vector control programs in sub-Saharan Africa.8
Toxicology and Safety. At therapeutic topical doses, pyrethrins and pyrethroids produce minimal systemic toxicity in adults because of poor dermal absorption (permethrin skin absorption is below 2 percent) and rapid hepatic metabolism. The primary adverse effects are skin irritation, contact dermatitis, and, on mucosal surfaces, burning and stinging. Sensory nerve endings in the skin are more sensitive to pyrethroid stimulation than motor neurons, producing a transient paresthesia (tingling, burning) that is pharmacological rather than toxic and resolves without treatment. In the context of massive occupational or accidental oral exposure, pyrethroids cause the neurological effects of their class: Type I compounds produce fine tremors, incoordination, hypersensitivity to stimuli; Type II compounds produce choreoathetosis, hypersalivation, and seizures. Pyrethroids do not cause organophosphate-type cholinergic crisis. Very young infants and cats are substantially more sensitive to pyrethroid toxicity than adults due to slower hepatic metabolism; permethrin is highly toxic to cats (applying dog permethrin products to cats causes severe neurotoxicity) and should never be used in this species.1112
Mechanism: prolong Na⁺ channel open state → repetitive firing → paralysis. Type I (permethrin): tremors. Type II (cypermethrin, deltamethrin): choreoathetosis + salivation. kdr resistance (T929I/L932F): target site mutation; cross-resistance to ALL pyrethroids; use malathion, spinosad, or benzyl alcohol instead. Piperonyl butoxide in OTC formulations: esterase inhibitor that potentiates pyrethrin activity; does not overcome kdr resistance. Permethrin and cats: severely toxic due to slow feline hepatic metabolism; never apply to cats. Dermal absorption <2%: minimal systemic toxicity at therapeutic topical doses in adults.
Prescribing antiparasitic therapy during pregnancy requires careful balancing of the risk posed by untreated parasitic infection against the potential teratogenic, embryotoxic, or fetotoxic risk of the antiparasitic agent. In many situations, the risk of the untreated infection (anemia, malnutrition, preterm birth, intrauterine growth restriction, congenital transmission) substantially outweighs the pharmacological risk, particularly when the drug in question has well-established safety data in pregnancy. The following guidance summarizes current evidence and World Health Organization (WHO) recommendations.
Malaria in Pregnancy. Malaria in pregnancy carries the highest burden of all parasitic infections in pregnant women globally, causing maternal anemia, placental malaria, low birth weight, preterm delivery, and maternal death. Treatment must not be withheld due to pregnancy. Artemisinin-based combination therapies (ACTs) are the treatment of choice for uncomplicated malaria in the second and third trimesters; artemether-lumefantrine is the most widely used combination and has a well-documented safety profile in late pregnancy. In the first trimester, quinine plus clindamycin is the preferred regimen because artemisinin-based combination therapy (ACT) safety data in the first trimester, while increasingly reassuring, remain less complete; quinine is used despite its recognized adverse effects (cinchonism: tinnitus, high-frequency hearing loss, headache, nausea) because the risk of untreated falciparum malaria in the first trimester (miscarriage, severe maternal illness) is far greater. For malaria chemoprophylaxis during pregnancy, weekly chloroquine (for chloroquine-sensitive Plasmodium species) is safe throughout pregnancy; mefloquine is safe from the second trimester; atovaquone-proguanil has limited safety data in pregnancy and is generally avoided.8
Intestinal Helminthiasis and Schistosomiasis in Pregnancy. Single-dose albendazole or mebendazole for soil-transmitted helminthiasis is recommended from the second trimester onward by WHO in high-prevalence settings; the teratogenic risk from single-dose administration is considered negligible compared with the burden of helminth-related anemia and malnutrition in pregnancy. Both agents are avoided in the first trimester. Praziquantel for schistosomiasis is recommended by WHO throughout pregnancy, including the first trimester in high-transmission settings, because untreated schistosomiasis causes significant morbidity. DEC (diethylcarbamazine) is contraindicated in pregnancy due to fetal harm in animal studies. Ivermectin use during pregnancy has limited human safety data; it is generally avoided for elective indications but may be used for life-threatening strongyloidiasis hyperinfection where the benefit clearly outweighs risk.9
Toxoplasmosis in Pregnancy. Primary Toxoplasma gondii infection in pregnancy poses a risk of congenital toxoplasmosis, with fetal infection risk and severity both depending on gestational age at maternal infection. Spiramycin is used to reduce the risk of vertical transmission when maternal seroconversion is detected before fetal infection is confirmed (spiramycin concentrates in placental tissue but does not effectively cross the placenta to treat established fetal infection). When fetal infection is confirmed by amniocentesis (positive polymerase chain reaction (PCR) for T. gondii), pyrimethamine-sulfadiazine-folinic acid combination is used to treat the infected fetus; pyrimethamine is avoided in the first trimester due to teratogenic risk (anti-folate activity). Fetal surveillance by ultrasound for signs of cerebral calcification, hydrocephalus, or intrauterine growth restriction is conducted throughout the remainder of the pregnancy.11
Ectoparasiticides in Pregnancy. Permethrin 5% cream is considered safe in pregnancy (it is the preferred treatment for scabies in pregnant women) given its very low dermal absorption and extensive safety data. Benzyl benzoate is an alternative for scabies from the second trimester. Lindane (gamma-hexachlorocyclohexane), once used for both scabies and lice, is contraindicated in pregnancy due to significant dermal absorption and central nervous system (CNS) toxicity risk; it is no longer recommended as first-line therapy for any indication in adults or children and has been banned in many countries. Malathion is avoided in pregnancy as a precaution against organophosphate exposure, though actual dermal absorption is low with correct topical application. For head lice in pregnancy, permethrin 1% remains the preferred agent; mechanical removal with a fine-tooth comb is a safe adjunctive or primary approach in women who prefer to avoid all chemical treatments.3
| Drug | Indication | First Trimester | Second/Third Trimester | Notes |
|---|---|---|---|---|
| Artemether-lumefantrine | Malaria (uncomplicated) | Limited data; quinine + clindamycin preferred | Recommended (ACT) | Do not withhold for malaria treatment in pregnancy |
| Quinine | Malaria (1st trimester) | Preferred with clindamycin | Use if ACT contraindicated | Cinchonism; uterotonic at high doses |
| Albendazole/mebendazole | Soil-transmitted helminths | Avoid | WHO-recommended single dose | Benefit exceeds risk in high-prevalence settings |
| Praziquantel | Schistosomiasis | WHO recommends in high-transmission | Safe; recommended | Untreated schistosomiasis high morbidity |
| Permethrin 5% | Scabies | Safe (low dermal absorption) | Preferred | Preferred ectoparasiticide in pregnancy |
| DEC | Lymphatic filariasis | Contraindicated | Contraindicated | Fetal harm in animal studies |
| Ivermectin | Strongyloidiasis/scabies | Avoid unless life-threatening | Limited data; avoid elective use | Use for hyperinfection if benefit outweighs risk |
| Lindane | Scabies/lice | Contraindicated | Contraindicated | Significant CNS absorption; banned many countries |
Never withhold malaria treatment in pregnancy: maternal and fetal risk from untreated malaria far exceeds drug risk. Schistosomiasis: praziquantel recommended throughout pregnancy including first trimester in high-transmission settings. Helminths: single-dose albendazole or mebendazole safe from second trimester (WHO MDA). DEC: absolutely contraindicated in pregnancy. Lindane: contraindicated; no longer recommended anywhere. Permethrin 5%: preferred scabies agent in pregnancy. Spiramycin: reduces vertical transmission of toxoplasmosis; does not treat established fetal infection.
Pediatric antiparasitic dosing differs from adult dosing in important ways: many agents are dosed by weight in kilograms, minimum age or weight restrictions exist for several drugs, and younger children have higher rates of certain infections (soil-transmitted helminthiasis, head lice, pinworm) while being more vulnerable to specific drug toxicities. Mass drug administration programs for school-age children constitute one of the largest pediatric pharmacological interventions globally, making antiparasitic prescribing in children a public health as well as individual clinical issue.
Benzimidazoles in Children. Albendazole 400 mg single dose is safe and effective for soil-transmitted helminthiasis in children above 1 year of age (children 12 to 24 months receive 200 mg). Mebendazole 500 mg single dose is an equivalent alternative. For children below 12 months, WHO recommends treatment only in special circumstances and under medical supervision. For extended-course albendazole used for neurocysticercosis (NCC) or echinococcosis in children, dosing is weight-based at 7.5 to 15 mg/kg/day in two divided doses (maximum 800 mg/day), with the same liver function test (LFT) and complete blood count (CBC) monitoring requirements as in adults. Both benzimidazoles are teratogenic in animal models; they are avoided in children under 12 months for routine mass drug administration and the benefit-risk assessment is individualized for younger infants with serious helminthiasis requiring treatment.10
Ivermectin in Children. Ivermectin is approved for use in children weighing 15 kg or more (approximately 2 years of age or older) for strongyloidiasis and onchocerciasis at 200 mcg/kg. Below 15 kg, limited pharmacokinetic data exist; higher blood-brain barrier (BBB) permeability in young infants raises concern about central nervous system (CNS) toxicity. For scabies in children below 15 kg, topical permethrin 5% cream remains the preferred agent. The exception is life-threatening strongyloidiasis hyperinfection in a child below 15 kg, where ivermectin use is justified by the severity of the condition; dosing in this context should be managed by a specialist experienced in tropical medicine. WHO has conditionally expanded ivermectin mass drug administration (MDA) eligibility to children above 5 kg for community programs in some settings, with ongoing safety monitoring.11
Antimalarials in Children. Artemisinin-based combination therapies (ACTs) are the first-line treatment for uncomplicated Plasmodium falciparum malaria in children of all ages. Artemether-lumefantrine is dosed by weight in four weight bands (5 to 14 kg, 15 to 24 kg, 25 to 34 kg, above 34 kg) and requires co-administration with food or milk for adequate lumefantrine absorption. Dihydroartemisinin-piperaquine is an alternative artemisinin-based combination therapy (ACT) with once-daily dosing that improves compliance in young children. Quinine is used for severe malaria (intravenous (IV) in hospital settings) and for first-trimester malaria; it is dosed at 10 mg/kg every 8 hours. Chloroquine retains its use for Plasmodium vivax and Plasmodium ovale malaria in children in regions where chloroquine sensitivity is preserved. Primaquine for P. vivax/P. ovale radical cure requires glucose-6-phosphate dehydrogenase (G6PD) testing before use in all patients (including children) to avoid hemolytic anemia in G6PD-deficient individuals.8
Ectoparasiticides in Young Children. Permethrin 5% cream for scabies is approved from 2 months of age; in infants below 2 months, treatment should be managed by a pediatric specialist. For infants, the cream is applied to the entire body surface including scalp and face, not just neck-down as in older children and adults. Benzyl alcohol 5% lotion for head lice is approved from 6 months of age. Spinosad 0.9% for head lice is approved from 6 months of age. Malathion 0.5% lotion is not recommended below 6 years. Lindane is contraindicated in all children due to CNS neurotoxicity risk from dermal absorption, with particular risk in infants and young children whose skin is more permeable and whose CNS is in active development. Crotamiton 10% cream may be used in infants when no other option is available but has substantially lower efficacy than permethrin. For pinworm (enterobiasis) in children, mebendazole 100 mg or albendazole 400 mg (above 2 years) as a single oral dose, repeated in 2 weeks, is standard; all household members should be treated simultaneously regardless of symptoms.10
Albendazole 400 mg: safe from 12 months (200 mg for 12–24 months); weight-based extended dosing for NCC/echinococcosis. Ivermectin: approved 15 kg/2 years and above; below 15 kg use permethrin for scabies; specialist guidance for hyperinfection in young children. ACTs (artemether-lumefantrine): weight-band dosing; take with food. G6PD testing mandatory before primaquine in all patients. Permethrin 5%: approved from 2 months; extend to scalp and face in infants. Lindane: contraindicated in all children. Malathion: not for under 6 years. Pinworm: treat all household contacts simultaneously.
Antiparasitic agents collectively span a broad range of toxicity profiles, from agents with virtually no systemic toxicity at standard doses (permethrin, single-dose benzimidazoles) to agents with potentially life-threatening toxicities requiring active monitoring (prolonged albendazole, mefloquine, halofantrine). This section integrates organ-system toxicity and drug interaction information across the antiparasitic classes covered in this module and the preceding modules of Chapter 38, providing a pharmacovigilance framework for clinical practice.
Hepatotoxicity. Prolonged albendazole therapy (for neurocysticercosis (NCC), echinococcosis, or visceral larva migrans) causes transaminase elevation in 10 to 17 percent of patients; most elevations are mild to moderate and reversible upon dose reduction or treatment discontinuation. Liver function test (LFT) monitoring at 2-week intervals during each 28-day treatment cycle is standard practice. Benznidazole (for Chagas disease) causes hepatotoxicity as one of its dose-limiting toxicities; complete blood count (CBC) and LFT monitoring at weeks 2, 4, and 8 of the 60-day course is required. Praziquantel causes transient aminotransferase elevations in some patients, particularly those with Schistosoma-related hepatic fibrosis; it is metabolized extensively by cytochrome P450 3A4 (CYP3A4) and should be used with caution in Child-Pugh class C hepatic impairment. The antimonials (meglumine antimoniate, sodium stibogluconate) for leishmaniasis carry significant hepatotoxicity risk and require regular LFT monitoring throughout the treatment course.12
Neurotoxicity. The most clinically important antiparasitic neurotoxicity is mefloquine-associated neuropsychiatric adverse effects (anxiety, nightmares, dizziness, insomnia, and, rarely, psychosis, seizures, and encephalopathy), which occur in approximately 1 in 200 to 1 in 1,700 patients at prophylactic doses and more frequently at treatment doses. Mefloquine is contraindicated in patients with a history of psychiatric illness, seizure disorder, or cardiac conduction abnormalities. A black box warning requires disclosure of neuropsychiatric risks before prescribing. Ivermectin central nervous system (CNS) toxicity occurs at normal doses only when the blood-brain barrier (BBB) is compromised or P-glycoprotein (P-gp) efflux is inhibited; clinical scenarios include human immunodeficiency virus (HIV) encephalopathy, high-dose P-gp inhibitor co-administration (ritonavir, verapamil), and rare multidrug resistance 1 (MDR1) loss-of-function variants. Quinine and quinidine cause dose-dependent QTc (corrected QT interval) prolongation and, at toxic doses, cinchonism (tinnitus, visual disturbance, altered mental status). Lindane causes CNS toxicity through gamma-aminobutyric acid-A (GABA-A) receptor antagonism (blocking inhibitory chloride channels); seizures have been reported in children following skin absorption.1112
Bone Marrow Suppression. Pyrimethamine (used for toxoplasmosis) causes dose-dependent megaloblastic anemia, leukopenia, and thrombocytopenia by inhibiting dihydrofolate reductase (DHFR) in human bone marrow precursors; folinic acid (leucovorin) co-administration at 10 to 25 mg/day is mandatory to prevent this toxicity. CBC monitoring weekly during acute toxoplasmosis treatment is recommended. Benznidazole causes leukopenia requiring CBC monitoring. Prolonged albendazole causes bone marrow suppression (leukopenia, thrombocytopenia) in a minority of patients, detected by CBC every 2 weeks. Flucytosine (used in combination antifungal regimens rather than antiparasitic regimens but often encountered in similar patient populations) and co-trimoxazole at high doses can compound bone marrow suppression in immunocompromised patients taking multiple myelosuppressive agents simultaneously.11
Key Drug Interactions Across the Antiparasitic Classes. The most clinically significant interactions are: (1) rifampicin reduces praziquantel plasma levels by 85 percent through CYP3A4 induction (avoid co-administration); (2) ritonavir and other potent CYP3A4 and P-glycoprotein (P-gp) inhibitors increase ivermectin CNS penetration (use with caution); (3) warfarin anticoagulation is enhanced by metronidazole (CYP2C9 inhibition) requiring INR (international normalized ratio) monitoring; (4) mefloquine lowers seizure threshold and should not be combined with other agents that lower seizure threshold; (5) halofantrine (an older antimalarial rarely used now) causes fatal QTc prolongation and torsades de pointes, particularly when taken after mefloquine; (6) artemether-lumefantrine prolongs QTc and should not be combined with other QTc-prolonging agents; (7) corticosteroids reduce praziquantel cerebrospinal fluid (CSF) penetration by approximately 50 percent and increase benznidazole plasma levels. A systematic review of each patient's current medications for CYP3A4 interactions, QTc-prolonging agents, and bone marrow suppression additive effects is required before starting antiparasitic therapy in complex cases.12
Albendazole prolonged: LFTs every 2 weeks; CBC every 2 weeks. Benznidazole: CBC at weeks 2, 4, 8. Praziquantel: CYP3A4 substrate; rifampicin reduces levels 85% — never co-administer; corticosteroids reduce CSF levels 50%. Pyrimethamine: folinic acid mandatory; weekly CBC during acute treatment. Ivermectin: avoid with CYP3A4/P-gp inhibitors (ritonavir, verapamil) if BBB integrity uncertain. Mefloquine: black box psychiatric warning; contraindicated with history of psychiatric illness or seizures; avoid halofantrine after mefloquine (fatal QTc). Metronidazole + warfarin: monitor INR. Lindane: contraindicated in all children, pregnancy; neurotoxic via GABA-A block; banned in many countries.
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