Chapter 38 — Antiparasitic Drugs — Module 4 — Ectoparasiticides and Special Populations
1. A 78-year-old nursing-home resident is diagnosed with scabies, and over the next week three more residents and two staff members develop the characteristic itchy rash. The facility has 60 residents and rotating staff. The infection-control team must treat cases and contacts simultaneously to halt the outbreak. Which approach is the most practical for this institutional outbreak?
A) Apply permethrin 5% cream to the index resident only and observe the others for 2 weeks before any treatment
B) Quarantine affected residents and provide no pharmacologic treatment, relying on isolation alone
C) Treat all residents and staff on the same day with oral ivermectin 200 micrograms per kilogram, because a single oral dose given facility-wide is logistically far easier to coordinate than applying cream correctly to every individual at once
D) Treat only staff members, since they move between rooms, and leave residents untreated
E) Give a single dose of oral albendazole to all residents and staff
ANSWER: C
Rationale:
In an institutional scabies outbreak, the logistical challenge is treating every case and contact at the same time to prevent reseeding. Oral ivermectin 200 micrograms per kilogram given facility-wide on a single day is far easier to coordinate than ensuring correct neck-down cream application for dozens of residents and rotating staff simultaneously, which is why simultaneous oral ivermectin is preferred for outbreak control in nursing homes and similar settings.
Option A: Option A is incorrect: treating only the index case and watching the others allows ongoing transmission; cases and contacts must be treated together.
Option B: Option B is incorrect: isolation without scabicidal treatment does not eradicate the mite and will not stop the outbreak.
Option D: Option D is incorrect: treating staff but leaving residents untreated leaves the largest reservoir of infestation in place and will not control the outbreak.
Option E: Option E is incorrect: albendazole treats helminth infections, not the scabies mite, so it is ineffective for a scabies outbreak.
2. A 54-year-old man with advanced HIV (human immunodeficiency virus) infection is admitted with thick, scaly, hyperkeratotic plaques on his hands, feet, and scalp. Skin scrapings reveal innumerable mites. He was treated 2 weeks ago with a single application of permethrin 5% cream without improvement. What is the most appropriate management?
A) Combination therapy with repeated oral ivermectin plus topical permethrin plus a keratolytic agent (such as salicylic acid) to remove the crust, together with contact precautions and treatment of close contacts
B) A second single application of permethrin 5% cream alone, identical to the first
C) Oral metronidazole for 7 days as monotherapy
D) A topical corticosteroid alone to soften the plaques, with no scabicide
E) Reassurance only, as the plaques will resolve spontaneously without treatment
ANSWER: A
Rationale:
This is crusted (Norwegian) scabies, occurring here because of advanced HIV-related immunocompromise, with an enormous mite burden under thick hyperkeratotic crust. Single-agent topical therapy fails because the crust blocks penetration, which is exactly what happened with his first permethrin application. Correct management combines repeated oral ivermectin (reaching mites systemically), topical permethrin, and a keratolytic to dissolve the crust and allow penetration, plus contact precautions and simultaneous treatment of close contacts because these patients are highly contagious.
Option B: Option B is incorrect: repeating a single permethrin application will fail again, because the crust prevents the cream from reaching the mites.
Option C: Option C is incorrect: metronidazole has no activity against the scabies mite and is not a treatment for crusted scabies.
Option D: Option D is incorrect: a corticosteroid is not scabicidal and could worsen the infestation by further suppressing local immunity.
Option E: Option E is incorrect: crusted scabies does not self-resolve; untreated, it is severe, highly contagious, and a source of outbreaks.
3. An 8-year-old girl has had head lice for a month. Her parents have correctly applied permethrin 1% rinse twice, 9 days apart, with good technique, but live lice are still present. The family lives in a region where the knockdown resistance (kdr) sodium-channel mutation is documented in more than half of head lice. What is the best next step?
A) Apply permethrin 1% rinse a third time at double the usual concentration
B) Switch to a different over-the-counter pyrethrin-piperonyl butoxide shampoo
C) Add an oral antihistamine as the definitive treatment for the infestation
D) Switch to an agent with a different mechanism of action, such as spinosad or malathion, which remain effective against kdr-resistant lice because they do not act on the sodium channel
E) Stop all treatment and conclude that the infestation cannot be cleared
ANSWER: D
Rationale:
Two correctly applied permethrin courses have failed in a high-kdr setting, which strongly implicates target-site pyrethroid resistance. Because kdr alters the sodium-channel binding site and confers cross-resistance to all pyrethroids regardless of dose, the rational move is to switch to an agent with a different mechanism. Spinosad (nicotinic acetylcholine receptor and chloride-channel activation) and malathion (acetylcholinesterase inhibition) act off the sodium channel and retain activity against kdr-resistant lice.
Option A: Option A is incorrect: a higher permethrin concentration cannot overcome a target-site mutation, because the binding site itself is altered; cross-resistance is dose-independent.
Option B: Option B is incorrect: pyrethrin-piperonyl butoxide shampoos are pyrethroid-based and share kdr cross-resistance, and are in fact less effective than permethrin against resistant lice.
Option C: Option C is incorrect: an antihistamine may relieve itch but does not kill lice and is not a treatment for the infestation.
Option E: Option E is incorrect: the infestation is readily curable with a different-mechanism agent, so abandoning treatment is unjustified.
4. A 29-year-old woman who is 20 weeks pregnant develops the intensely itchy rash and burrows of scabies. She asks which treatment is safe for her pregnancy. Which agent is preferred?
A) Lindane lotion applied to the whole body
B) Permethrin 5% cream, the preferred scabicide in pregnancy, because its very low dermal absorption (under 2 percent) means little drug reaches the systemic circulation or the fetus
C) Oral ivermectin as the routine first choice for scabies in any pregnant patient
D) Diethylcarbamazine, because it is specifically indicated for scabies in pregnancy
E) No treatment until after delivery, since scabies should never be treated in pregnancy
ANSWER: B
Rationale:
Permethrin 5% cream is the preferred scabicide in pregnancy. Its dermal absorption is below 2 percent, so very little drug reaches the maternal circulation or the fetus, and it has a long record of safe use. Scabies should be treated during pregnancy, and permethrin is the agent of choice for this patient.
Option A: Option A is incorrect: lindane is contraindicated in pregnancy because of significant central nervous system absorption and neurotoxicity.
Option C: Option C is incorrect: ivermectin has limited pregnancy safety data and is generally avoided for elective use in pregnancy, reserved for life-threatening indications; it is not the routine first choice.
Option D: Option D is incorrect: diethylcarbamazine (DEC) treats lymphatic filariasis and is contraindicated in pregnancy; it is not a scabies treatment.
Option E: Option E is incorrect: scabies can and should be treated in pregnancy with a safe agent such as permethrin; withholding treatment is inappropriate.
5. A 41-year-old man being treated for cerebral toxoplasmosis with pyrimethamine and sulfadiazine returns after 10 days with fatigue and easy bruising. Laboratory studies show a falling white-cell count, anemia, and a low platelet count. On review, he was never started on a recommended supportive medication. What is the most likely explanation and the correct action?
A) The marrow suppression is from sulfadiazine alone, and pyrimethamine never causes this problem
B) The findings are unrelated to his therapy and require no change to the regimen
C) Pyrimethamine has caused iron-deficiency anemia, so the correct action is iron supplementation
D) The cytopenias are an expected harmless effect that needs no intervention
E) Pyrimethamine inhibits dihydrofolate reductase in human bone marrow precursors, causing megaloblastic anemia, leukopenia, and thrombocytopenia; folinic acid (leucovorin) was omitted, so it should be started to rescue the marrow, with complete blood count monitoring
ANSWER: E
Rationale:
Pyrimethamine inhibits dihydrofolate reductase (DHFR) in human bone marrow precursor cells, producing dose-dependent megaloblastic anemia, leukopenia, and thrombocytopenia. Folinic acid (leucovorin) supplies a usable form of folate that rescues human marrow without blunting the antiparasitic effect, and it is mandatory during pyrimethamine therapy, with periodic complete blood count (CBC) monitoring. This patient's cytopenias reflect the omission of folinic acid, which should now be started.
Option A: Option A is incorrect: pyrimethamine is itself a potent cause of marrow suppression through DHFR inhibition; the toxicity is not attributable to sulfadiazine alone.
Option B: Option B is incorrect: the cytopenias are a recognized, mechanism-based effect of pyrimethamine, so the regimen does require intervention (adding folinic acid).
Option C: Option C is incorrect: the anemia is megaloblastic (folate-related), not iron-deficiency anemia, so iron supplementation does not address the mechanism.
Option D: Option D is incorrect: progressive pancytopenia is not a harmless effect; it requires folinic acid rescue and monitoring to prevent serious marrow toxicity.
6. A 35-year-old woman is planning travel to a malaria-endemic region and asks about weekly mefloquine for prophylaxis. Her history includes major depression with a prior hospitalization and one adult seizure. What is the most appropriate response?
A) Avoid mefloquine and select a different antimalarial for prophylaxis, because mefloquine carries a boxed neuropsychiatric warning and is contraindicated in patients with a history of psychiatric illness or a seizure disorder
B) Prescribe mefloquine, because a psychiatric history makes it safer and more effective
C) Prescribe mefloquine at a higher dose to overcome any psychiatric interference
D) Tell her no malaria prophylaxis is needed at all
E) Prescribe lindane as malaria prophylaxis instead
ANSWER: A
Rationale:
Mefloquine carries a boxed warning for neuropsychiatric adverse effects (anxiety, vivid nightmares, dizziness, insomnia, and, rarely, psychosis, seizures, and encephalopathy) and lowers the seizure threshold. It is contraindicated in patients with a history of psychiatric illness, a seizure disorder, or cardiac conduction abnormalities. Given this woman's depression and prior seizure, mefloquine should be avoided and a different antimalarial chosen for prophylaxis.
Option B: Option B is incorrect: a psychiatric history is a contraindication to mefloquine, not a reason it is safer; the neuropsychiatric risk is precisely the concern.
Option C: Option C is incorrect: increasing the dose increases neuropsychiatric risk rather than mitigating it, and does not address the contraindication.
Option D: Option D is incorrect: travel to an endemic region warrants effective prophylaxis; the issue is choosing a safer agent, not omitting prophylaxis.
Option E: Option E is incorrect: lindane is a topical ectoparasiticide, not an antimalarial, and has no role in malaria prophylaxis.
7. A 47-year-old man is being treated with praziquantel for neurocysticercosis. He is newly diagnosed with active tuberculosis, and the team plans to start rifampicin. The pharmacist flags a concern. What is the interaction, and what should be done?
A) Rifampicin raises praziquantel levels to toxic ranges, so praziquantel must be stopped immediately
B) There is no interaction, because praziquantel is not metabolized by the liver
C) Rifampicin is a potent inducer of CYP3A4 and accelerates praziquantel's metabolism, lowering praziquantel plasma levels by about 85 percent and risking treatment failure, so the two should not be co-administered and the regimen must be re-planned
D) Rifampicin and praziquantel bind in the gut, so taking them 2 hours apart resolves the problem
E) Rifampicin increases praziquantel absorption, improving its efficacy, so no change is needed
ANSWER: C
Rationale:
Rifampicin is a potent inducer of cytochrome P450 3A4 (CYP3A4), the enzyme that metabolizes praziquantel. Induction speeds praziquantel breakdown and lowers its plasma levels by roughly 85 percent, which can cause treatment failure of the neurocysticercosis. Because the reduction is so large, the two should not be co-administered; the team must re-plan (for example, sequencing therapy or selecting alternative management) rather than give them together.
Option A: Option A is incorrect: enzyme induction lowers, not raises, praziquantel levels, so the risk is treatment failure rather than toxicity from elevated levels.
Option B: Option B is incorrect: praziquantel is extensively metabolized by hepatic CYP3A4, which is exactly why a CYP3A4 inducer affects it.
Option D: Option D is incorrect: the interaction is metabolic (enzyme induction), not gut binding, so spacing the doses does not prevent the loss of efficacy.
Option E: Option E is incorrect: rifampicin reduces praziquantel exposure through faster metabolism; it does not improve efficacy, and the combination should be avoided.
8. A 14-month-old infant weighing 10 kg is diagnosed with common scabies. A colleague suggests oral ivermectin to simplify treatment. What is the most appropriate management for this child?
A) Oral ivermectin at the standard weight-based dose, since ivermectin is preferred at any age
B) Topical permethrin 5% cream, which is the preferred agent for scabies in a child below 15 kg, because oral ivermectin is generally reserved for children of at least 15 kg given the more permeable blood-brain barrier and limited safety data in smaller infants
C) Lindane lotion applied to the whole body
D) Oral mefloquine
E) No treatment until the child reaches 15 kg
ANSWER: B
Rationale:
For scabies in a child below 15 kg, topical permethrin 5% cream is the preferred agent. Oral ivermectin is generally reserved for children weighing at least 15 kg (about 2 years of age), because smaller infants have a more permeable blood-brain barrier and there are limited safety data, raising concern about central nervous system toxicity. Permethrin's low systemic absorption makes it the safer effective choice for this 10-kg infant (with application extended to the face and scalp in infants).
Option A: Option A is incorrect: ivermectin is not recommended below 15 kg outside life-threatening situations, so it is not appropriate for this 10-kg infant.
Option C: Option C is incorrect: lindane is contraindicated in all children because of central nervous system neurotoxicity from dermal absorption.
Option D: Option D is incorrect: mefloquine is an antimalarial with no role in treating scabies.
Option E: Option E is incorrect: scabies in an infant should be treated promptly with a safe agent such as topical permethrin; waiting until 15 kg is inappropriate.
9. A 58-year-old man living in a crowded shelter presents with intense itching and excoriations on the trunk. Examination of his clothing reveals lice and eggs concentrated in the seams, while few are found on the skin. The clinician is mindful that body lice can transmit serious infections in such settings. What is the primary intervention?
A) Lifelong suppressive oral ivermectin
B) Daily topical corticosteroid to the skin for 2 weeks as the main treatment
C) A single oral dose of albendazole
D) Environmental decontamination, meaning hot-laundering or discarding the infested clothing and bedding, because the body louse lives and lays eggs in clothing seams rather than on the skin
E) Surgical excision of affected skin
ANSWER: D
Rationale:
The body louse spends most of its life in the seams of clothing and comes onto the skin only to feed, which is why the lice and eggs are concentrated in the seams here. The primary intervention is environmental: hot-laundering or discarding the infested clothing and bedding eliminates the louse population at its source. Topical permethrin may be used as an adjunct, but environmental decontamination is the key measure. This is also important because body lice transmit serious infections (epidemic typhus, trench fever, louse-borne relapsing fever) in crowded conditions.
Option A: Option A is incorrect: body lice are eradicated by treating the clothing and environment, not by lifelong drug suppression.
Option B: Option B is incorrect: a topical corticosteroid reduces inflammation and itch but does not kill lice or clear them from clothing.
Option C: Option C is incorrect: albendazole treats helminth infections, not louse infestation.
Option E: Option E is incorrect: body lice infestation is not a surgical condition; no skin excision is involved.
10. A 26-year-old man has been treated for the acute blood-stage of Plasmodium vivax malaria and now needs primaquine to achieve radical cure (to clear the dormant liver forms and prevent relapse). What must be done before primaquine is given?
A) Obtain a bone density scan, because primaquine weakens bone
B) Begin lifelong proton pump inhibitor therapy to protect the stomach
C) Check a fasting lipid panel, because primaquine raises cholesterol
D) Give primaquine immediately with no testing, as no screening is required
E) Test glucose-6-phosphate dehydrogenase (G6PD) status first, because primaquine can trigger severe hemolysis (destruction of red blood cells) in patients who are G6PD-deficient
ANSWER: E
Rationale:
Primaquine imposes an oxidative load on red blood cells and can cause acute hemolytic anemia in patients deficient in glucose-6-phosphate dehydrogenase (G6PD), the enzyme that protects red cells from oxidative stress. For this reason, G6PD testing is mandatory before primaquine is given to any patient, so that deficient individuals can be identified and the hemolysis risk managed before radical-cure therapy proceeds.
Option A: Option A is incorrect: primaquine does not cause bone loss, so a bone density scan is not the required screen.
Option B: Option B is incorrect: a proton pump inhibitor protects the gastric mucosa but does nothing to prevent primaquine-induced hemolysis.
Option C: Option C is incorrect: primaquine's characteristic toxicity is hemolysis in G6PD deficiency, not a lipid effect, so a lipid panel is not the relevant test.
Option D: Option D is incorrect: testing is required; giving primaquine without checking G6PD status risks severe hemolysis in a deficient patient.
11. A worried pet owner calls because she applied a concentrated canine "spot-on" permethrin flea product to her cat, which is now tremoring, twitching, and hypersalivating. She asks why the same product her dog tolerates well is harming her cat. What is the explanation?
A) Cats metabolize permethrin very slowly (they have limited glucuronidation capacity), so permethrin accumulates to neurotoxic levels and causes severe, sometimes fatal neurotoxicity; concentrated canine permethrin products must never be applied to cats
B) Cats are simply allergic to the inactive dye in the product, and the tremor is an allergic reaction unrelated to permethrin
C) Permethrin is harmless to all mammals, so the cat's signs must be due to an unrelated illness
D) The cat absorbed too little permethrin, and the signs reflect underdosing rather than toxicity
E) Permethrin is toxic to dogs but completely safe in cats, so the product was simply mislabeled
ANSWER: A
Rationale:
Cats are uniquely vulnerable to permethrin because their hepatic metabolism of the compound is markedly slow, owing to limited glucuronidation capacity. Instead of being cleared rapidly as in dogs and humans, permethrin accumulates to neurotoxic levels, producing tremors, twitching, hypersalivation, and seizures that can be fatal. This is why concentrated canine permethrin spot-on products must never be applied to cats. It is the same rapid-metabolism principle that protects other mammals, failing specifically in the cat.
Option B: Option B is incorrect: the signs are pharmacologic permethrin neurotoxicity, not an allergic reaction to a dye.
Option C: Option C is incorrect: permethrin is not harmless to all mammals; cats are distinctly sensitive, and the neurologic signs are classic permethrin toxicity.
Option D: Option D is incorrect: the tremors and hypersalivation reflect toxic accumulation, not underdosing.
Option E: Option E is incorrect: the toxicity is reversed from this claim; permethrin is well tolerated by dogs and dangerous to cats, and the problem is feline sensitivity, not mislabeling.
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