Skin Parasites

Sarcoptes scabiei var hominis is a mite that burrows into the stratum corneum, where the female tunnels and lays eggs. Around 206.6 million people are infested at any moment, with roughly 622.5 million new cases and 5.3 million DALYs a year ^{1Reference 1Li J et al. · 2024The disability-adjusted life years (DALYs), prevalence and incidence of scabies, 1990-2021: A systematic analysis from the Global Burden of Disease Study 2021}. The WHO added scabies to its list of neglected tropical diseases in 2017.

The symptom is intense itch, worst at night, and it is an allergic response to the mite and its products rather than the burrowing itself — which is why it takes four to six weeks to appear on first infestation but hours on reinfestation, and why the itch persists for weeks after successful treatment. Typical sites are finger webs, wrists, axillae, waistband, and genitals; in infants and the elderly it spreads more widely.

Crusted (Norwegian) scabies is the severe form seen in immunosuppression and neurological impairment, with thousands to millions of mites, heavy crusting, and extreme contagiousness.

Treatment is topical 5% permethrin cream or oral ivermectin at 200 µg/kg, with household contacts treated simultaneously and bedding laundered. Scabies is also a major precursor of impetigo and, downstream of that, of post-streptococcal glomerulonephritis and rheumatic heart disease in endemic populations — which is a large part of why it is taken seriously as a public-health target.

Pediculus humanus capitis is the most common childhood ectoparasitosis worldwide. Lice cannot jump or fly; transmission is by direct head-to-head contact. The itch is again allergic, so a first infestation is often silent for weeks.

Conventional treatment is losing ground. Permethrin resistance through kdr sodium-channel mutations is now widespread ^{2Reference 2Batır MB et al. · 2024First Report of the Gene Mutations Associated with Permethrin Resistance in Head Lice (Pediculus humanus capitis De Geer, 1767) from Primary School Children in Istanbul (Türkiye) and Nagarkot (Nepal)}, and cure rates for 1% permethrin in resistant populations have fallen far enough that trial comparators sometimes perform at 40% or less. Dimeticone — a silicone that works physically by suffocation rather than neurotoxically — has largely taken over, along with malathion, spinosad and ivermectin.

Body lice (P. h. corporis) live in clothing rather than on skin and are associated with homelessness and crowding; they are the vector for epidemic typhus and trench fever, which makes them a different kind of problem. Pubic lice (Pthirus pubis) are sexually transmitted.

Demodex folliculorum and D. brevis are mites that live in hair follicles and sebaceous glands, and carriage is normal — a Chilean biopsy series found skin prevalence around 13.5%, rising with age ^{3Reference 3Gutiérrez B et al. · 2021Demodex skin prevalence by biopsy, Chile — 13.5%}. They become clinically relevant when density rises, most importantly as a cause of chronic blepharitis: itchy, crusted, irritated lid margins with cylindrical dandruff at the lash base, plus dry-eye symptoms ^{4Reference 4Liu et al. · 2010Pathogenic role of Demodex mites in blepharitis}.

Treatment changed recently. Tea tree oil lid scrubs were the standard approach for two decades; lotilaner 0.25% ophthalmic solution (Xdemvy) is now FDA-approved and outperforms it in meta-analysis ^{5Reference 5Talha M et al. · 2024Meta-analysisLotilaner 0.25% ophthalmic solution for Demodex blepharitis — meta-analysis; mite-eradication RR 3.55}. Tea tree oil has real evidence behind it but is no longer first-line.

Tunga penetrans — the sand flea, chigoe, or jigger. The fertilised female burrows into skin, usually on the feet, and swells to about a thousand times her original volume while feeding, producing a painful nodule with a black central punctum. It is a WHO neglected tropical disease and a disease of extreme poverty: pooled prevalence in sub-Saharan Africa runs around 33–38%, with local peaks of 60–72% ^{6Reference 6Abrha et al. · 2021Systematic reviewTreatment and prevention of tungiasis — a systematic review}.

Treatment is dimeticone (NYDA) or careful mechanical extraction. Botanicals do best at prevention.

Infestation of tissue by fly larvae — Cordylobia in Africa, Dermatobia in the Americas, Cochliomyia in wound myiasis. Sporadic, affecting travellers and the rural tropical poor. Management is larval removal, occlusion with petroleum jelly to force the larva to surface, or surgical extraction, with ivermectin as an adjunct. Myiasis has essentially no botanical human evidence — the entire literature is in vitro work on surrogate blowflies.

The ectoparasite evidence is unusually clinically grounded. Unlike malaria and Leishmania research, which is dominated by IC50 screens on cultured parasites, most of the notable ectoparasite botanicals have human trials.

The bias is commercial. A handful of well-marketed commercial products dominate the literature — often through single-group or industry-affiliated trials — while unbranded traditional remedies sit at 1970s uncontrolled case series or a single in vitro report. And the positive results belong to those specific formulations. Zanzarin, Licener, the EO/LP solution and the coconut-anise spray are products, not plants; generic coconut oil and generic tea tree oil have not been shown to do what they did.

Head Lice

Coconut and anise spray beat permethrin in a randomised parallel-group trial of 100 children: applied twice nine days apart, it cured 41/50 (82%) against 21/50 (42%) for 0.43% permethrin lotion (p < 0.0001) ^{7Reference 7Burgess et al. · 2010Clinical trialClinical trial showing superiority of a coconut and anise spray over permethrin 0.43% lotion for head louse infestation}. A separate open trial of a coconut/anise/ylang remedy in 119 patients matched a permethrin–malathion product almost exactly (92.3% vs 92.2%) ^{8Reference 8Mumcuoglu et al. · 2002AnimalThe in vivo pediculicidal efficacy of a natural remedy}. Anethole from anise provides the neurotoxic action; the coconut fatty acids coat and suffocate.

The 42% cure in the permethrin arm is a drug failing in a resistant population, not a drug performing normally. The alcohol vehicle also caused irritant reactions on excoriated skin in 33 participants.

Eucalyptus plus Leptospermum petersonii (EO/LP) gave 83% cure against 36% for a pyrethrins/piperonyl-butoxide mousse in a multicentre assessor-blind RCT, and was 100% pediculicidal after a single application ^{9Reference 9Greive et al. · 2018RCTThe efficacy of Australian essential oils for the treatment of head lice: a randomised controlled trial}. Pyrethrin resistance is common, so the relative advantage is partly the comparator’s failure. There is no head-to-head cure comparison against dimeticone.

Neem seed extract shampoo gave 60/60 (100%) cured after a single 10-minute application in a randomised investigator-blinded trial of 119 children, against 54/57 (94.7%) for dimeticone ^{10Reference 10Abdel-Ghaffar et al. · 2017Length of treatment with a neem seed extract shampoo needed to eliminate head lice}; an earlier study found a three-minute submersion killed all motile stages ^{11Reference 11Abdel-Ghaffar et al. · 2012Efficacy of a single treatment of head lice with a neem seed extract}.

The investigators attribute this to a physical mechanism — blocking spiracles — rather than a neurotoxic one, which means it competes on the same terms as dimeticone rather than offering a distinct botanical action. Both trials come from the same product-affiliated group.

Quassia tincture has a historical record: in an uncontrolled series of 454 head-lice patients in 1978, two applications a week apart left only three with hatched lice at follow-up, attributed to reinfestation, with no side effects ^{12Reference 12Jensen et al. · 1978Pediculosis capitis treated with quassia tincture}. No control group, no randomisation, and nothing modern. There is also a species ambiguity between Quassia amara and Picrasma excelsa.

Clove oil and eugenol are ovicidal and adulticidal against head lice in vitro ^{13Reference 13Yang et al. · 2003Ovicidal and adulticidal effects of Eugenia caryophyllata bud and leaf oil compounds on Pediculus capitis14Reference 14Bagavan et al. · 2011Contact and fumigant toxicity of Syzygium aromaticum flower-bud extract and its compounds to head lice} with no human cure trial. Eugenol is a dermal sensitiser.

Scabies

Neem has human scabies data. A randomised controlled trial in 61 patients found 10% neem leaf-extract lotion produced no significant difference from 5% permethrin in lesion count, with earlier improvement at day 7 in the neem arm ^{15Reference 15Widasmara et al. · 2025RCTRandomised comparison of 10% neem leaf-extract lotion, 10% brotowali stem-extract lotion and 5% permethrin for scabies}. Earlier, an uncontrolled 1992 pilot of a neem-plus-turmeric paste in 814 people reported 97% cure in 3–15 days ^{16Reference 16Charles et al. · 1992The use and efficacy of Azadirachta indica ADR (“Neem”) and Curcuma longa (“Turmeric”) in scabies}.

That is equivalence in one small trial, plus one large uncontrolled pilot. No blinded superiority trial exists, and the turmeric contribution was never isolated. The mechanistic work is in rabbit-mite surrogates, identifying an octadecanoic-acid tetrahydrofuran diester as acaricidal via SOD, POD and Ca²⁺-ATPase suppression ^{17Reference 17Du et al. · 2014Acaricidal activity of an octadecanoic acid tetrahydrofuran diester from neem oil against Sarcoptes scabiei var cuniculi}. Topical only — oral neem oil causes a Reye-like encephalopathy in infants.

5% TTO and its constituent terpinen-4-ol kill S. scabiei rapidly in vitro, including mites from crusted-scabies patients that tolerate permethrin and ivermectin ^{18Reference 18Walton et al. · 2004Acaricidal activity of Melaleuca alternifolia (tea tree) oil against Sarcoptes scabiei var hominis}, and one ex vivo assay found 5% TTO comparable to 5% permethrin for killing ^{19Reference 19Yürekli et al. · 2022Ex vivo comparison of tea tree oil and permethrin against Sarcoptes scabiei}. The human cure-rate trial, however, exists only as a published protocol with no results ^{20Reference 20Thomas et al. · 2018RCTA tea tree oil-based gel for scabies in Aboriginal children — randomised controlled trial protocol (no results published)}. Tea tree oil is not a proven scabies cure, and permethrin at ~90% cure remains the benchmark.

Manuka (Leptospermum scoparium) β-triketones — leptospermone, isoleptospermone and flavesone — are miticidal and ovicidal in vitro, apparently through inhibition of HPPD in the tyrosine-catabolism pathway ^{21Reference 21Nammunige NA et al. · 2025In vitroManuka oil β-triketones (leptospermone, isoleptospermone, flavesone) are miticidal and ovicidal against Sarcoptes scabiei via HPPD inhibition — in vitro}. A novel target; strictly preclinical, and isolated-constituent rather than whole-oil.

Tungiasis — Prevention

A coconut-oil-based repellent (Zanzarin, with jojoba and aloe) prevents sand-flea infestation. Applied twice daily to the feet it reduced newly embedded sand fleas by 92% and lesions by 87% in a Brazilian crossover cohort ^{22Reference 22Feldmeier et al. · 2006A plant-based repellent protects against Tunga penetrans infestation and sand flea disease}; a randomised controlled field study in Madagascar drove the median attack rate to zero within two weeks, significantly better than shoes or control ^{23Reference 23Thielecke M et al. · 2014RCTTreatment of Tungiasis with dimeticone: a proof-of-principle study in rural Kenya — randomised controlled trialView study →}; and intermittent application maintained the effect, with embedded fleas reaching zero in 98% during the intensive phase ^{24Reference 24Buckendahl et al. · 2011Control of tungiasis through intermittent application of a plant-based repellent}. A Lancet Infectious Diseases systematic review of eight RCTs in 808 patients named coconut-oil lotion the most promising prevention and dimeticone the most promising treatment ^{6Reference 6Abrha et al. · 2021Systematic reviewTreatment and prevention of tungiasis — a systematic review}.

It has three limits. It is prevention only — it does nothing for fleas already embedded. It depends entirely on twice-daily adherence. And the trials come from a single research group, with Zanzarin being a specific coconut/jojoba/aloe formulation rather than plain coconut oil.

Demodex Blepharitis

Tea tree oil lid scrubs — 50% weekly in office plus 5% daily at home — drove Demodex counts to zero in 7 of 9 patients within four weeks, where baby shampoo never reached zero ^{25Reference 25Gao et al. · 2005In vitroIn vitro and in vivo killing of ocular Demodex by tea tree oil}.

Screening all 13 TTO constituents identified terpinen-4-ol as the active, killing mites at around 1% and more potently than whole tea tree oil ^{26Reference 26Tighe et al. · 2013Terpinen-4-ol is the most active ingredient of tea tree oil to kill Demodex mites}.

Several RCTs followed: a double-blind trial of 3% TTO gels in 49 patients reduced Demodex presence from 54.2% to 20.6% with improved symptom scores ^{27Reference 27Ergun et al. · 2019RCTDouble-blind randomised trial of two 3% tea tree oil gels in chronic blepharitis}; a trial against intense pulsed light gave 75% eradication for 5% TTO versus 100% for IPL ^{28Reference 28Zhang et al. · 2019RCTIntense pulsed light versus 5% tea tree oil for ocular Demodex eradication — randomised trial}; a TTO-plus-chamomile swab matched baby shampoo on mite counts with better symptom scores ^{29Reference 29Mergen et al. · 2021RCTTea tree oil and chamomile swabs versus baby shampoo in Demodex blepharitis — double-blind RCT}.

Tea tree oil was once standard and is now second-line. FDA-approved lotilaner 0.25% is superior in meta-analysis (mite-eradication RR 3.55 versus control) ^{5Reference 5Talha M et al. · 2024Meta-analysisLotilaner 0.25% ophthalmic solution for Demodex blepharitis — meta-analysis; mite-eradication RR 3.55}, the TTO trials are small and heterogeneous, and TTO at scabicidal strength irritates the ocular surface — the 50% scrubs are poorly tolerated, which is the direct trade-off against potency.

Gaps

Body and pubic lice have no species-specific botanical human trial. The head-lice results are assumed to transfer and have never been tested — and body lice live in clothing, which changes the intervention entirely.

Myiasis has no human botanical evidence. Tea tree oil repels blowfly oviposition and kills eggs and first-instar larvae in a Lucilia cuprina surrogate model ^{30Reference 30Callander et al. · 2011Insecticidal and repellent effects of tea tree oil against Lucilia cuprina}, and betel leaf and Cassia alata extracts are larvicidal against Chrysomya ^{31Reference 31Kumarasinghe et al. · 2002Larvicidal effects of betel-leaf (Piper betle) and Cassia alata extracts against Chrysomya megacephala} — but the flies causing human myiasis were not tested. In that work, adding TTO to a larvicide reduced the larvicide’s kill, because larvae avoided the treated area. That is a reason for care, not a treatment.

Lippia multiflora essential oil is pediculocidal and scabicidal in a single in vitro report ^{32Reference 32Oladimeji et al. · 2000Pediculocidal and scabicidal properties of Lippia multiflora essential oil} — an African ethnobotanical lead with no dose-response or human data.