Ectoparasiticides and Special Populations

Permethrin 5% cream is the first-line treatment for common scabies. Permethrin is a synthetic pyrethroid that acts by binding to voltage-gated sodium channels in the mite nervous system and prolonging their open state, causing repetitive neuronal firing, paralysis, and death. The cream is applied from the neck down, left on overnight, then washed off; a repeat application is needed to kill mites that hatched from eggs after the initial treatment to kill mites that hatched from eggs after the initial treatment, as permethrin is not reliably ovicidal. In infants, the application is extended to include the scalp and face, where the scabies mite can infest in this age group. Permethrin has very low dermal absorption and is considered safe in pregnancy and from two months of age.

Oral ivermectin is an effective alternative to permethrin for common scabies and is the preferred agent in institutional outbreak settings, where simultaneous oral treatment of all residents and staff is logistically superior to applying cream to all individuals simultaneously. A repeat dose is given about one week after the first to ensure eradication. Ivermectin is not recommended as first-line in pregnant women or children below 15 kg due to limited safety data.

Crusted scabies occurs in patients with compromised immunity, neurological impairment, or extreme debility. The thick hyperkeratotic crust prevents adequate topical drug penetration, making permethrin alone insufficient. Standard management combines repeated courses of oral ivermectin with topical permethrin plus keratolytic agents to remove the crust and allow drug penetration. Strict contact precautions and simultaneous treatment of all close contacts are essential, as crusted scabies patients are highly contagious and a common source of institutional outbreaks.

Benzyl benzoate 25% lotion is widely used in resource-limited settings as an alternative to permethrin. Its mechanism involves direct neurotoxicity to the mite. It is more irritating to the skin than permethrin and is considered safe from the second trimester of pregnancy.

Pruritus typically persists for two to four weeks after successful scabies treatment, driven by ongoing delayed-type hypersensitivity to mite antigens, feces, and dead mite material remaining in the skin. Failure to counsel patients about this phenomenon leads to unnecessary retreatment. Permethrin resistance in Sarcoptes scabiei has been confirmed molecularly and is associated with mutations in the voltage-gated sodium channel gene analogous to the knockdown resistance (kdr) mutations seen in insect pests.

Permethrin 1% rinse has historically been the first-line treatment for head lice, but knockdown resistance (kdr) — specific point mutations in the voltage-gated sodium channel gene that reduce pyrethroid binding affinity — has reached high prevalence in head louse populations across much of North America, Europe, and Australia. Where kdr prevalence is high, permethrin should not be used as first-line therapy, as treatment failure rates are unacceptably high. Because kdr acts by target site modification, it confers cross-resistance to all pyrethrins and pyrethroids regardless of their specific structure; increasing the dose does not overcome it.

Malathion 0.5% lotion is an organophosphate cholinesterase inhibitor that retains full activity against kdr-resistant lice because its mechanism is entirely independent of sodium channel function. It inhibits acetylcholinesterase in the louse nervous system, causing accumulation of acetylcholine at cholinergic synapses and louse death. Malathion lotion is flammable during application — patients must avoid open flames and hair dryers while the product is on the hair. It is not recommended in young children or in pregnancy due to organophosphate systemic absorption concerns.

Spinosad 0.9% suspension acts by activating nicotinic acetylcholine receptors (nAChR) and glutamate-gated chloride channels in lice, causing neuromuscular hyperexcitation and death. Its distinct mechanism means it is fully active against kdr-resistant lice. Unlike permethrin, spinosad has ovicidal activity, so a single application is usually sufficient without a mandatory second dose. Benzyl alcohol 5% lotion kills lice by a physical mechanism — it asphyxiates lice by blocking their breathing spiracles — and carries no resistance risk whatsoever.

Oral ivermectin is an option for head lice refractory to topical agents, providing a systemic route that bypasses topical application entirely. Body lice infestation is treated primarily by environmental measures — discarding or hot-laundering infested clothing and bedding — because body lice live in clothing seams rather than on skin, and environmental decontamination is the key intervention.

Pyrethrins and pyrethroids bind to voltage-gated sodium channels in the neuronal axon membrane and prolong their open state after depolarization. Normally, voltage-gated sodium channels open briefly during an action potential and then rapidly inactivate; pyrethroid binding prevents inactivation, causing sustained sodium influx, repetitive action potential generation, and ultimately paralysis and death of the target organism. At therapeutic topical doses in humans, systemic toxicity is not a practical concern given low dermal absorption and rapid hepatic metabolism.

Pyrethroids are rapidly metabolized by mammalian hepatic esterases, limiting systemic accumulation at therapeutic topical doses. Cats lack this esterase activity, making permethrin-containing dog products severely toxic if applied to cats.

Knockdown resistance is the primary mechanism of pyrethroid resistance in insect and mite pests. Specific point mutations in the voltage-gated sodium channel gene alter the pyrethroid-binding site, reducing drug binding affinity without significantly impairing the channel's normal function. Because these are target site mutations, they confer cross-resistance to all pyrethrins and pyrethroids — the entire drug class is affected simultaneously. Agents acting through entirely different mechanisms (malathion via acetylcholinesterase inhibition, spinosad via nicotinic acetylcholine receptor and glutamate-gated chloride channel activation, benzyl alcohol via physical asphyxiation) are unaffected by knockdown resistance and retain full activity.

Piperonyl butoxide is an esterase inhibitor combined with natural pyrethrins in many over-the-counter head lice formulations. By preventing metabolic degradation of pyrethrins in the louse, it potentiates pyrethrin activity. However, piperonyl butoxide does not overcome knockdown resistance because knockdown resistance acts at the target site (the sodium channel), not through metabolic degradation. In populations with high knockdown resistance prevalence, pyrethrin-piperonyl butoxide combinations are no more effective than pyrethrins alone.

Malaria in pregnancy causes maternal anemia, placental malaria, low birth weight, preterm delivery, and maternal death — treatment must never be withheld. In the second and third trimesters, artemisinin-based combination therapies are the treatment of choice for uncomplicated falciparum malaria. In the first trimester, quinine plus clindamycin is preferred because safety data on artemisinin-based combination therapies in early pregnancy, while increasingly reassuring, are less complete. For prophylaxis during pregnancy, chloroquine is safe throughout all trimesters for chloroquine-sensitive destinations; mefloquine is used from the second trimester for chloroquine-resistant destinations; atovaquone-proguanil and doxycycline are generally avoided.

Significant helminth infections during pregnancy are associated with maternal anemia, malnutrition, and intrauterine growth restriction, making treatment a net benefit in high-prevalence settings. Single-dose albendazole or mebendazole for soil-transmitted helminthiasis is recommended from the second trimester onward by the World Health Organization (WHO) in endemic regions; the teratogenic risk from a single dose is considered negligible against the established benefit. Both agents are avoided in the first trimester. Praziquantel for schistosomiasis is recommended throughout pregnancy, including the first trimester in high-transmission settings, because untreated schistosomiasis causes significant morbidity and the WHO includes praziquantel in preventive chemotherapy programs for pregnant women.

Permethrin 5% cream is the preferred treatment for scabies in pregnancy given its very low dermal absorption. Benzyl benzoate is an alternative from the second trimester. Lindane (gamma-hexachlorocyclohexane), once used for scabies and lice, is contraindicated in pregnancy due to significant central nervous system toxicity risk from dermal absorption and has been banned or restricted in many countries — it is no longer recommended for any indication in pregnancy or in children. Malathion is avoided in pregnancy as a precaution against organophosphate exposure.

For toxoplasmosis in pregnancy, spiramycin is used when maternal seroconversion is detected to reduce the risk of vertical transmission to the fetus. When fetal infection is confirmed by amniocentesis, pyrimethamine-sulfadiazine-folinic acid is used to treat the infected fetus; pyrimethamine is avoided in the first trimester due to its anti-folate teratogenic mechanism.

Prolonged albendazole therapy for neurocysticercosis or echinococcosis causes transaminase elevation in a significant minority of patients, generally mild to moderate and reversible on dose reduction or discontinuation. Liver function monitoring during treatment cycles is standard practice. Benznidazole for Chagas disease also causes hepatotoxicity as a dose-limiting toxicity. Praziquantel is metabolized extensively by cytochrome P450 3A4 (CYP3A4) and should be used with caution in severe hepatic impairment.

Pyrimethamine causes dose-dependent megaloblastic anemia, leukopenia, and thrombocytopenia by inhibiting dihydrofolate reductase (DHFR) in human bone marrow precursors — making folinic acid co-administration mandatory, as covered in Module 2. Prolonged albendazole and benznidazole can also suppress bone marrow, requiring complete blood count monitoring during treatment. These are the principal agents in the antiparasitic pharmacopeia with significant myelosuppressive potential.

Mefloquine neuropsychiatric toxicity — ranging from vivid dreams and anxiety to psychosis and seizures — is the most clinically important central nervous system adverse effect in the antiparasitic class, covered in Module 1. Ivermectin central nervous system toxicity occurs at therapeutic doses only when the blood-brain barrier is compromised or P-glycoprotein efflux is inhibited. Lindane causes central nervous system toxicity through gamma-aminobutyric acid-A (GABA-A) receptor antagonism, blocking inhibitory chloride channels; seizures have been reported, particularly in children, from dermal absorption — this is the basis for its contraindication in children and pregnancy.

The most clinically significant interactions are: rifampicin reduces praziquantel plasma levels dramatically via CYP3A4 induction and must not be co-administered; corticosteroids reduce praziquantel cerebrospinal fluid penetration; metronidazole inhibits CYP2C9 and potentiates warfarin anticoagulation, requiring international normalized ratio (INR) monitoring; P-glycoprotein inhibitors (ritonavir, verapamil) increase ivermectin central nervous system penetration and should be used with caution when blood-brain barrier integrity is uncertain; mefloquine lowers seizure threshold and should not be combined with other agents that do the same; artemether-lumefantrine prolongs the corrected QT interval and should not be combined with other corrected QT-prolonging agents; pyrimethamine bone marrow toxicity is additive with other myelosuppressants.