Treatment failure of over-the-counter pediculicide shampoos in Thai schoolgirls: Evidence of insecticide resistance

Abstract

The increasing resistance to chemical pediculicides has raised concerns about the efficacy of head lice treatments, including in Thailand. This study evaluated the efficacy of three commercially available pediculicide shampoos (0.75% permethrin, 0.6% carbaryl, and 0.12% Stemona root crude extract) among infested children in Chonburi Province, Thailand. A pre-test/post-test experimental design was conducted with 135 infested female children assigned to three treatment groups (n = 45 each), with no significant differences in baseline demographic characteristics (level of head lice infestations, hairstyle, and hair length). Each group received two applications (Day 0 and Day 7) of the allocated pediculicide shampoo, following the manufacturer’s instructions. The final proportion of cured individuals was assessed on Day 14 after treatment using a fine-tooth comb. The observed percentages of cured individuals were 42.22% for carbaryl shampoo, 24.44% for Stemona shampoo, and 6.67% for permethrin shampoo. No side effects were reported, although none of the shampoos achieved complete eradication, with particularly poor outcomes in children with heavy infestations. These findings provide clinical evidence of reduced efficacy of over-the-counter pediculicides in Thailand, consistent with emerging resistance trends. More effective alternatives with different mechanisms of action, including ivermectin, abametapir, and dimeticone-based products, should be considered, although their current availability in Thailand remains limited. In the interim, development of locally available herbal formulations, together with school-based screening and simultaneous treatment, represents more feasible strategies. These results highlight the need for updated treatment guidelines, restriction of ineffective products, and regular resistance surveillance to ensure effective control.

Introduction

Head louse (Pediculus humanus capitis) infestation, or pediculosis, is a common condition among schoolchildren. The global prevalence is estimated at 19.0% (95% CI: 18.0–20.0) in both developed and developing countries (Falagas et al. 2008; Hatam-Nahavandi et al. 2020). In Thailand, pediculosis is found mainly in girls and is rarely seen in boys (Ruankham et al. 2016). Infestations and reinfestations are most common at the start of the school year, making this a recurring public health issue. Head-to-head contact, either at school or at home, is the primary route of transmission. Effective management of pediculosis depends on several factors, including treatment efficacy, safety, cost, and accessibility (Leung et al. 2022).

Current treatment options work through different mechanisms. These include neurotoxic chemical pediculicides (such as permethrin, phenothrin, malathion, hexachlorocyclohexane, carbaryl, and ivermectin), occlusive agents (like benzyl alcohol, isopropyl myristate, and dimeticone), and manual removal with a fine-tooth comb (Nolt et al. 2022). Neurotoxic chemical pediculicides remain the first-line treatment in most settings. According to the lice life cycle, which spans about two weeks, two applications are usually required. The first application eliminates motile lice (nymphs and adults), and the second, applied 7–10 days later, targets newly hatched nymphs (Coates et al. 2020). However, the effectiveness of many pediculicides has declined, largely due to resistance (Amanzougaghene et al. 2018; Fox et al. 2020; Mohammadi et al. 2021; Roca-Acevedo et al. 2019; Yoon et al. 2014). Resistance is primarily driven by repeated misuse of pediculicides, leading to sub-lethal exposure. Underdosing occurs when one product pack is divided among several individuals, allowing lice to survive and develop resistance. Persistent chemicals such as permethrin remain on hair shafts and decline slowly in concentration. This process can expose lice to low doses and promote resistance even under correct application (Durand et al. 2012; Gunning et al. 2019). In areas where resistance is suspected, alternative classes of treatments targeting different mechanisms should be considered.

In Thailand, both chemical (permethrin, carbaryl, and benzyl benzoate) and botanical pediculicides are widely available over the counter. Permethrin and carbaryl are among the most used agents. Permethrin, a type I pyrethroid, disrupts sodium channels in lice nerve membranes. It is considered slightly toxic when applied to the skin (dermal LD50 > 2000 mg/kg). The U.S. Food and Drug Administration has approved 1% permethrin as safe for treating lice in children. In Thailand, permethrin is available at 0.75% and 1% concentrations in shampoos and creams. Common side effects include skin irritation, burning, swelling, and eye redness. Carbaryl, a carbamate insecticide, works by inhibiting acetylcholinesterase in insects (Schneider 2000; Sparks and Nauen 2015). Although ingestion can lead to serious symptoms, such as headache, nausea, and muscle weakness, carbaryl has moderate dermal toxicity (LD50 > 2000 mg/kg in animals). Carbaryl has now been discontinued or banned for application to humans in Europe (Boulton 1995). However, it remains available for pediculosis treatment in Thailand. In response to concerns about chemical side effects, botanical products such as Stemona root crude extract (0.12% w/w) shampoo are also available as alternative pediculicides in the country.

Resistance to pyrethroids, including knockdown resistance (kdr) mutations, has been reported in lice populations in Thailand (Brownell et al. 2020). In earlier studies, carbaryl shampoo showed high efficacy, with a 93.0% cured among schoolchildren in Bangkok (Soonwera 2014). However, more recent ex vivo studies suggest that the same concentration may have substantially lower activity in some contexts (Yingklang et al. 2023). These inconsistencies highlight that the real-world clinical effectiveness of many over-the-counter pediculicides in Thailand has not been systematically evaluated. Clinical assessments are essential to provide reliable data on treatment outcomes (Burgess 2022).

This study aimed to evaluate the clinical efficacy of three pediculicide shampoos containing 0.75% permethrin, 0.6% carbaryl, or 0.12% Stemona root crude extract. We used a pre-test/post-test design to assess their effectiveness in treating head lice infestations among primary schoolchildren in Chonburi Province, Thailand. The percentage of cure was evaluated one week after the second treatment, following the manufacturer’s instructions.

Materials and methods

Study design and setting

This study was part of a head lice control project conducted from May to July 2024 in five government primary schools in Mueang District, Chonburi Province, Thailand. A total of 301 girls aged 6–12 years were screened for head lice infestation. Of these, 135 girls were diagnosed with active infestations and enrolled in the intervention phase. They were randomly assigned to one of three treatment groups using simple randomization (computer-generated numbers). Randomization lists were prepared by an independent researcher who was not involved in treatment or outcome assessment. Each group received two applications of the allocated pediculicide shampoo. Lice clearance or cure was assessed on Day 14 after treatment. Common side effects were monitored after each application.

Sample size and sampling

The sample size calculation was based on detecting a minimum 20% difference in cure rates between treatment groups, with 80% power at a 5% significance level (Twisk 2013). The minimum required sample size was 32 infested children per group. However, to increase statistical power and account for potential dropouts, we included all 135 children diagnosed with infestation. A multistage sampling approach was used to select study schools. Chonburi Province was divided into three administrative zones according to the Thai Education Administration. One zone was selected by simple random sampling. Within that zone, Mueang District (with 37 schools) was chosen. Schools were eligible if they were government-run, had more than 50 female students, and provided only primary-level education. This reduced the chance of reinfestation from older students. Based on these criteria, five schools were included.

The eligibility inclusion and exclusion criteria

Children aged 6–12 years who were willing to participate and had not used pediculicide products in the past three months were eligible. Exclusion criteria were active scalp or skin conditions, use of other lice treatments during the study, known allergy to any product component, or refusal to study.

Head lice detection and data collection

At baseline, head lice infestation was defined by the presence of motile lice and/or viable eggs. Detection was performed by trained personnel using a fine-tooth comb. Combing was carried out from scalp to tip in five to six strokes, and all combed lice were collected on a white paper sheet. After examination, the lice were discarded and not returned to the scalp. This procedure was applied uniformly across participants, minimizing potential bias in treatment outcomes. Viable eggs were distinguished from empty nit shells through visual inspection under a magnifying glass, based on their brown coloration, intact operculum, and proximity to the scalp (< 1 cm). Intensity of head lice infestations was classified as severe (more than ten lice after five to six comb strokes or more than ten live lice eggs), moderate (six to nine lice after five to six comb strokes), or mild (one to five lice during five to six comb strokes or one to five live lice eggs) (Kurt et al. 2009). Baseline data included hair type (straight or curly), hair length (short or long), and hygiene practices (who washed the child’s hair). These were collected through staff’s observation and structured interviews with the child, parents, or teachers.

Intervention procedures and outcome measurements

Three commercially available shampoos with different active ingredients were tested (0.6% carbaryl shampoo, 0.12% Stemona shampoo, and 0.75% permethrin shampoo). This study did not use the 1% permethrin formulation because the same active ingredient was already used in shampoo form. The concentrations selected for this study reflect the formulations commonly available over-the-counter in Thailand. These products are routinely used by parents and school health personnel. All shampoos were obtained from local pharmacies in Chonburi Province and used according to their packaging instructions. Before application (Day 0), children were examined for scalp lesions. Those with shoulder-length hair received 15 mL of shampoo; those with shorter hair received 10 mL. Shampoo was applied to wet hair by trained staff, covered with a shower cap for 10 min, then rinsed with tap water. No combing was done post-application. These procedures were performed at their schools. Side effects (including skin irritation, burning, and eye redness) were observed immediately after each treatment by a nurse. Treatments were repeated on Day 7. Before application, the presence of lice was briefly checked using a few combing strokes with the same fine-tooth comb, solely for diagnostic purposes and not as a full comb-out procedure, to avoid influencing treatment outcomes. All lice removed during the examination were discarded and not returned to the scalp. Data were recorded, and the final assessment of lice clearance was conducted one week later (Day 14). Cure was defined as the absence of motile lice (nymphs or adults) on Day 14, with eggs not included in the assessment (Mumcuoglu et al. 2021). Reinfestation was defined as the absence of adult lice on Day 7 and their reappearance on Day 14. Dropout criteria included the use of other lice treatments during the study, severe allergic reactions, or failure to attend follow-up (Fig. 1).

Fig. 1.

Flowchart of participants throughout the study

Data analysis

Data were analyzed using STATA version 10.1 (StataCorp LLC, College Station, TX, USA). Descriptive statistics were used to present baseline characteristics. The percentage of cure was calculated as the proportion of participants with no detectable lice on Day 14. Group differences were analyzed using Chi-square tests and Fisher’s Exact Testing. A p-value of less than 0.05 was considered statistically significant.

Human ethical statement

This study received approval from the human ethics committees at Burapha University (No. HS053/2566; September 2023). The study was conducted in accordance with the Declaration of Helsinki Guidelines. Before the intervention assessment, written informed consent was obtained from the parents or legal guardians of all participating children. Child assent was also obtained according to age: oral assent for children younger than 7 years and written assent for those aged 7–12 years. At the end of this study, children who still had head lice infestations were treated with appropriate pediculicide. The data were analyzed anonymously.

Results

Participant characteristics

All 135 enrolled participants completed the study. At baseline, there were no statistically significant differences among the three treatment groups in terms of head lice intensity, hairstyle, hair length, school grade, hair washing habits, or sleeping arrangements (p > 0.05) (Table 1). All participants adhered to the treatment schedule, and no cases were lost to follow-up or withdrawn from the study.

Table 1.

Baseline demographic characteristics of infested children in each treatment group (n = 45 each group)

Variables	0.6% carbaryl group n (%)	0.12% Stemona root crude extract group n (%)	0.75% permethrin group n (%)	P-value
Intensity of head lice
Mild	25 (55.56)	26 (57.78)	30 (66.67)	0.361b
Moderate	4 (8.89)	3 (6.67)	5 (11.11)
Severe	16 (35.56)	16 (35.56)	10 (22.22)
Sleeping arrangement
With others	32 (71.11)	30 (66.67)	34 (75.56)	0.649a
Alone	13 (28.89)	15 (33.33)	11 (24.44)
Hair washing
By themselves	34 (75.56)	32 (71.11)	38 (84.44)	0.310a
By parents	11 (24.44)	13 (28.89)	7 (15.56)
Hair type
Straight	33 (73.33)	34 (75.56)	33 (73.33)	0.962a
Curly	12 (26.67)	11 (24.44)	12 (26.67)
Hair length
Long hair (shoulder-length)	35 (77.78)	36 (80.00)	31 (68.89)	0.431a
Short hair	10 (22.22)	9 (20.00)	14 (31.11)

^a Based on Chi-square testing

^b Based on Fisher’s Exact Testing

Efficacy outcomes and side effects

Following the first application (Day 0), the percent of cured individuals was 11.11% in the group treated with carbaryl shampoo and 8.89% in the group treated with Stemona shampoo. In contrast, the permethrin shampoo group showed no clinical improvement, with 0% lice clearance on Day 7. By day 14, children treated with carbaryl shampoo showed 42.22% cured, followed by those treated with Stemona shampoo (24.44% cured) and permethrin shampoo (6.67% cured), despite a second treatment on day 7 (Table 2). The difference in cure rates among the three groups was statistically significant (p < 0.05), with the carbaryl shampoo group showing superior efficacy compared to both Stemona and permethrin shampoo treatments (Fig. 2).

Table 2.

Pre-test and post-test of pediculosis treatment in each group (n = 45 each group)

Variables	0.6% carbaryl group		0.12% Stemona root crude extract group		0.75% permethrin group
	Pre-treatment	Post-treatment on 14 days	Pre-treatment	Post-treatment on 14 days	Pre-treatment	Post-treatment on 14 days
Infested children
	45	19/45 (42.22%)	45	11/45 (24.44%)	45	3/45 (6.67%)
Intensity levels
Mild	25	17/25 (68.00)	26	11/26 (42.31)	30	2/30 (6.67%)
Moderate	4	2/4 (50.00)	3	0/3 (0.00)	5	0/5 (0.00)
Severe	16	0/16 (0.00)	16	0/16 (0.00)	10	0/10 (0.00)
Hair type
Straight	33	15/33 (45.45%)	34	10/34 (29.41%)	33	3/33 (9.09%)
Curly	12	4/12 (33.33%)	11	1/11 (9.09%)	12	0/12 (0.00%)
Hair length
Long hair (shoulder-length)	35	12/35 (34.29%)	36	7/36 (19.44%)	31	0/31 (0.00%)
Short hair	10	7/10 (70.00%)	9	4/9 (44.44%)	14	3/14 (21.43%)

Fig. 2.

Comparison of the percentage of cured individuals after treatment with three commercial pediculicide shampoos at the first and second applications (n = 45 infested individuals per group). P-values < 0.05 were considered statistically significant based on the Chi-square testing. Lice clearance or cure was assessed on Day 14 after applications of each product

No complete eradication of motile lice was achieved in any group, and the percent cured was particularly low in children with severe infestations. Treatment response was better among children with mild to moderate infestations and shorter hair length. No side effects, including skin irritation, burning sensation, eye redness, or scalp swelling, were observed in any group after either the first or second application.

Discussion

Current guidelines recommend switching to insecticides with different mechanisms of action when resistance is suspected (Nolt et al. 2022; Sparks and Nauen 2015). However, even with this strategy, complete eradication of head lice remains difficult. This study evaluated the efficacy of commonly used over-the-counter pediculicides (permethrin, carbaryl, and Stemona root extract) among school-aged children in Thailand. None of the treatments achieved complete eradication, underscoring the persistent challenge of managing head lice infestations in this setting.

Permethrin shampoo demonstrated a very low percent cured (6.67%), consistent with previous reports from Thailand, where 1% permethrin formulations also yielded unsatisfactory outcomes (Yingklang et al. 2023). In contrast, higher cure rates following 1% permethrin application for 15 min have been reported in countries such as Iran (Kalari et al. 2019; Kassiri et al. 2021) and Turkey (Tanyuksel et al. 2003). These differences may reflect variations in product formulation, timing of follow-up assessments, or regional patterns of insecticide resistance. Importantly, knockdown resistance (kdr) mutations have been implicated in treatment failure (Durand et al. 2012; Mohammadi et al. 2021) and have also been detected in Thai lice populations (Brownell et al. 2020, 2024), which might have contributed to the poor outcomes.

Carbaryl shampoo achieved a moderate percent cured (42.22%), particularly in children with mild-to-moderate infestations. These findings are consistent with ex vivo studies in Chonburi that reported similarly low efficacy (Yingklang et al. 2023), suggesting the potential emergence of resistance in local head lice populations. Although carbaryl performed better than permethrin, its effectiveness was still markedly lower than the 93% cure rate previously reported in Bangkok (Soonwera 2014). Several factors may account for these differences in outcomes. The earlier study assessed outcomes shortly after the second treatment (Soonwera 2014), which may have reflected short-term clearance rather than sustained eradication. In contrast, our study evaluated the cure one week after the second treatment, a timeframe more consistent with the lice life cycle. Differences in infestation severity, with a higher proportion of heavily infested children, may also have contributed to the lower cure rate in our study. Regional variation in carbamate susceptibility cannot be excluded. These findings highlight the need for standardized follow-up protocols and ongoing resistance monitoring.

Stemona shampoo, a traditional herbal alternative, yielded a 24.44% cured. Despite its perceived safety and popularity, the 0.12% formulation tested here showed limited pediculicidal efficacy, suggesting a need for reformulation or dose optimization. Further research should investigate its pharmacodynamics and active compounds in standardized clinical settings. In this study, no side effects were observed in any group, possibly due to careful application and avoidance of sensitive areas. While reassuring, the relatively small sample size limits conclusions regarding safety, and larger studies are warranted to assess rare side effects.

Although clinical resistance is strongly suspected, molecular or biochemical confirmation was not performed. Pyrethroid resistance in lice is most often associated with kdr mutations, whereas carbaryl resistance in insects is typically mediated by metabolic detoxification. Target-site mutations in the acetylcholinesterase (ace) gene have also been described in other insects (Keïta et al. 2020), but markers for carbaryl resistance in head lice remain poorly defined. Biochemical assays could therefore play a key role in future carbaryl resistance surveillance. Importantly, carbaryl has been withdrawn for human use in Europe due to potential carcinogenicity (Boulton 1995), yet it remains available over the counter in Thailand. Given its modest efficacy and documented safety concerns (Boulton 1995), we recommend that Thai regulatory authorities review the continued over-the-counter availability of carbaryl pediculicides and consider restricting their use.

From a public health perspective, these findings underscore the need to revise strategies for managing head lice among Thai schoolchildren. The low cure rates of commonly used pediculicides, particularly permethrin and carbaryl, raise concerns about widespread resistance and continued reliance on ineffective products. The evidence presented here can inform procurement policies in schools and local health authorities to ensure access to effective, evidence-based treatments. Pediculicides with cure rates exceeding 85% should be prioritized (Mumcuoglu et al. 2021). Consideration should also be given to treatments with alternative mechanisms of action. Physically acting agents, oral medications, ovicidal compounds, and nonchemical approaches have all shown potential. Physically acting agents, such as dimeticone-based products, have shown promise in other settings, while oral medications like ivermectin may provide an option for resistant cases or when topical treatments fail (Mumcuoglu et al. 2021). Abametapir, a metalloproteinase inhibitor that disrupts egg development and hatching, has demonstrated promising efficacy. A single 10 min application resulted in complete inhibition of egg hatching in vitro and achieved over 80% clinical success, with mild side effects (Woods et al. 2022). However, none of these agents are currently approved for pediculosis treatment in Thailand. Their use in Thailand is constrained by regulatory and accessibility barriers, reflecting challenges in drug importation and market availability. These agents should therefore be considered as potential long-term options contingent upon policy approval and improved access. In the meantime, research and development of locally available herbal formulations and safer, accessible pediculicides should be prioritized to provide practical solutions within the Thai context. Preventive strategies at the school level, including annual mass screening and classroom-wide treatment at the beginning of each semester, could help reduce reinfestation. Educational initiatives to promote correct product use and treatment adherence are also critical to improving outcomes and minimizing unnecessary exposure to ineffective chemicals. Although Thailand currently lacks a national pediculosis control policy, these measures could be feasibly implemented within school systems as an initial step toward more comprehensive control.

This study has several limitations. Household members and other children in the community were not assessed or treated, potentially contributing to reinfestation. Genetic or biochemical resistance testing was not performed, limiting mechanistic support. The study population included only girls from a single province, restricting generalizability. Finally, the sample size may have been insufficient to detect rare or delayed side effects. Future research should expand to multiple geographic locations, incorporate molecular or biochemical resistance assays, and evaluate family- or community-based interventions to improve long-term outcomes.

Conclusion

This study demonstrates that commonly used over-the-counter pediculicide shampoos in Thailand (permethrin, carbaryl, and Stemona root crude extract) show limited efficacy in treating head lice infestations among school-aged children, with none achieving complete eradication. These findings highlight an urgent need for more effective treatment strategies. Alternative agents with different mechanisms of action, such as ivermectin, abametapir, or physically acting products, particularly dimeticone-based formulations, should be prioritized. However, although these agents have demonstrated high efficacy in other countries, their availability and regulatory approval in Thailand remain limited. Thus, their practical use would require policy support, regulatory approval, and improved accessibility. In the interim, research into locally available herbal formulations and physically acting products should be encouraged as more feasible options within the Thai context. Public health interventions should also incorporate mass screening and simultaneous treatment at the beginning of each school semester to reduce reinfestation. Policy implications include the need for Thai health authorities to update treatment guidelines, restrict the distribution of ineffective products, and establish regular surveillance of pediculicide resistance to ensure effective control.

Author contributions

Conceptualization: T.T., M.Y., P.K., K.Y.; Data curation: T.T., M.Y., P.K.; Formal analysis: M.Y. P.K.; Funding acquisition: M.Y.; Methodology: T.T., P.K., M.Y., K.Y.; Project administration: M.Y.; Supervision: M.Y.; Validation: T.T., P.K., M.Y., K.Y.; Writing –original draft: T.T., P.K. M.Y.; Writing – review & editing: T.T., P.K., M.Y., K.Y.; All authors have read and agreed to the published version of the manuscript.

Funding

This research project was financially supported by Mahasarakham University (received by T.T.). M.Y. received support from (i) Burapha University, (ii) Thailand Science Research and Innovation (TSRI), and (iii) the National Science, Research and Innovation Fund (NSRF) under the Fundamental Fund (Grant No. 1270/2567). The funding agencies had no involvement in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

This study received approval from the human ethics committees at Burapha University (No. HS053/2566; September 2023). The study was conducted in accordance with the Declaration of Helsinki Guidelines.

Consent for publication

Not applicable.

Clinical trial number

Not applicable.

Competing interests

The authors declare no competing interests.