Recommendation to standardize pediculicidal and ovicidal testing for head lice (Anoplura: Pediculidae)

High efficacy of a dimeticone-based pediculicide following a brief application: in vitro assays and randomized controlled investigator-blinded clinical trial

Background

Increasing resistance of head lice against neurotoxic agents and safety concerns have led to the search for treatment alternatives. Dimeticones with a physical mode of action are safe, and bear a reduced risk for the development of resistance.

Methods

We performed in vitro bioassays to assess pediculicidal and ovicidal activities of a new dimeticone-based product, and a randomized controlled clinical trial to assess efficacy, following 10 min application. Of 153 individuals screened, 100 participants with active head louse infestations were randomly assigned to treatment with either a dimeticone-based test product, or a 0.5% permethrin-based reference product (50 participants per group). Participants received two topical applications of either the test (10 min) or reference products (45 min) at days 0 and 7 or 8. Outcome measures included the efficacies of treatment and their safety, as well as global and local tolerability at baseline, and days 1, 7, and 10.

Results

After 10 min exposure, all lice treated with the dimeticone test product were classified as non-viable in the in vitro assay. Ovicidal activity after treatment of eggs with the dimeticone test product was 96.8%. In the clinical trial, 96 patients completed all study visits. In the full analysis set (FAS) population, on day 1 after one application, 98% of patients were cured in the test group, as compared to 84% cured in the reference group. All participants in both groups were free of head lice on day 10, following two applications (100% cure rate). In total, 42 adverse events (AEs) in 23 patients of both treatment groups were recorded, with the majority of AEs classified as mild.

Conclusions

We have shown a high level of pediculicidal and ovicidal activity, and clinical efficacy and safety, of a brief application of a new dimeticone-based product. The short application time and reduced risk for the development of resistance are key drivers for improved patients’ compliance.

Trial registration

EU Clinical Trials Register EudraCT 2016–004635-20. Registered 14 November 2016.

Prevalence and Treatment of Pediculus Humanus Capitis with 1% Permethrin and 0.4% d-Phenothrin in Turkey

Pediculosis humanus capitis (head lice) is an important public health problem among school children. In our study, 20,612 schoolchildren (10,367 boys, 10,245 girls) were examined for Pediculus humanus capitis in 36 elementary schools between December 1996 and February 1998 in Ankara, Turkey. The prevalence of pediculosis capitis infestation was found to be 3.4% (701 / 20, 612). Of these, 382 students were treated with application of 1% permethrin cream rinse, and 184 students with 0.4% d-phenothrin shampoo. On day 14 of the controlled trial, the success rates were 93.7 % in the 1% permethrin cream rinse group and 75.5 % in the 0.4% d-phenothrin shampoo group. The 1% permethrin cream rinse was also significantly more active in pediculicidal efficacy when compared to the 0.4% d-phenothrin shampoo (p<0.001). As a result, these findings demonstrate that pediculosis capitis still remains a widespread health problem.

The Medicinal Use of Essential Oils and Their Components for Treating Lice and Mite Infestations

Recent studies have demonstrated that essential oils, and in particular, pennyroyal, tea tree and anise, have potent insecticidal and acaricidal (mite-killing) activity. The individual components of essential oils are now being investigated in order to give a rational basis to discover which essential oils may prove to be the most effective all-round agents for killing headlice and their eggs, and treating scabies, and for eliminating house dust mites, a major cause of asthma.

Safety, Efficacy, and Physicochemical Characterization of Tinospora crispa Ointment: A Community-Based Formulation against Pediculus humanus capitis

The high prevalence of pediculosis capitis, commonly known as head lice (Pediculus humanus capitis) infestation, has led to the preparation of a community-based pediculicidal ointment, which is made of common household items and the extract of Tinospora crispa stem. The present study aimed to evaluate the safety, efficacy, and physicochemical characteristics of the T. crispa pediculicidal ointment. The physicochemical properties of the ointment were characterized, and safety was determined using acute dermal irritation test (OECD 404), while the efficacy was assessed using an in vitro pediculicidal assay. Furthermore, the chemical compounds present in T. crispa were identified using liquid-liquid extraction followed by ultra-performance liquid chromatography quadruple time-of-flight mass spectrometric (UPLC-qTOF/MS) analysis. The community-based ointment formulation was light yellow in color, homogeneous, smooth, with distinct aromatic odor and pH of 6.92±0.09. It has spreadability value of 15.04±0.98 g·cm/sec and has thixotropic behavior. It was also found to be non-irritant, with a primary irritation index value of 0.15. Moreover, it was comparable to the pediculicidal activity of the positive control Kwell^®, a commercially available 1% permethrin shampoo (P>0.05), and was significantly different to the activity of the negative control ointment, a mixture of palm oil and candle wax (P<0.05). These findings suggested that the community-based T. crispa pediculicidal ointment is safe and effective, having acceptable physicochemical characteristics. Its activity can be attributed to the presence of compounds moupinamide and physalin I.

Identification of repellent odorants to the body louse, Pediculus humanus corporis, in clove essential oil

The control of body lice is an important issue for human health and welfare because lice act as vectors of disease such as typhus, relapsing fever, and trench fever. Body lice exhibit avoidance behavior to some essential oils, including clove essential oil. Therefore, odorants containing clove essential oil components may potentially be useful in the development of repellents to body lice. However, such odorants that induce avoidance behavior in body lice have not yet been identified from clove essential oil. Here, we established an analysis method to evaluate the avoidance behavior of body lice to specific odorants. The behavioral analysis of the body lice in response to clove essential oil and its constituents revealed that eugenol, a major component of clove essential oil, has strong repellent effect on body lice, whereas the other components failed to induce obvious avoidance behavior. A comparison of the repellent effects of eugenol with those of other structurally related odorants revealed possible moieties that are important for the avoidance effects to body lice. The repellent effect of eugenol to body lice was enhanced by combining it with the other major component of clove essential oil, β-caryophyllene. We conclude that a synthetic blend of eugenol and β-caryophyllene is the most effective repellent to body lice. This finding will be valuable as the potential use of eugenol as body lice repellent.

Ex vivo effectiveness of French over-the-counter products against head lice (Pediculus humanus capitis De Geer, 1778)

Head lice infestation is still a public health problem worldwide, with an intracountry and intercountry prevalence variation of 0.7 to 59%. There is a large variety of over-the-counter anti-louse products, but their efficacy is not always well assessed. Our objective was to test the pediculicidal and ovicidal efficacy of 21 over-the-counter head louse products, available in France during the period of 2008 to 2012. We tested children living in Tours City in central France and visiting preschools, primary schools, kindergarten, camps, and child care facilities, as well as children in their family houses, and were examined for the presence of lice. The products were collected from randomly selected pharmacies by covert investigators and then tested in the laboratory on an ex vivo sample of head lice and their eggs, collected from the hair of infested children. Living lice and unharmed eggs were collected from the scalps of 3-12 years old. The laboratory conditions for ex vivo testing mimicked the manufacturers' instructions for exposure time and application method. In 21 runs, 3919 living lice and 4321 undamaged living eggs were collected from the scalp of over 400 children. The 21 products were classified in three groups: 6 products in a group of potentially 100% pediculicidal activity and potentially 100% ovicidal activity, 8 products in a group of potentially 100% pediculicidal activity but insufficient ovicidal activity (including 2 products with claims of single application treatment), and 7 products in a group of insufficient pediculicidal activity and ovicidal activity. The pharmaceutical market for head lice products in France is swamped with poorly tested and ineffective products. Rigorous efficacy testing preregistration and periodic screening and testing of effectiveness in the post-registration period should be endorsed by the health authorities.

Lice update: new solutions to an old problem

An estimated 6 to 12 million children are affected by lice annually in the United States.(1) Knowledge of the various treatment options for this diagnosis is essential. This paper aims to provide an overview of the myriad therapeutic options available for lice infestations in children. U.S. Food and Drug Administration-approved drugs, off-label drug therapies, over-the counter-products, and herbal remedies are all discussed in detail. Clinicians may use this information to teach patients and families about the latest available care for the eradication of head lice in pediatric patients.

The use of videodermatoscopy to monitor treatment of scabies and pediculosis

A great pitfall for scabies and pediculosis therapeutic studies to date is that primary and secondary study outcomes are indirectly assessed (presence or absence of live parasites, including eggs, determined by gross clinical inspection) and data time points are non-standardized (highly variable) relative to the time of therapeutic application. Videodermatoscopy (VD) is a non-invasive technique that allows a rapid and magnified in vivo observation of the skin up to × 1000 with the visualization of morphologic features invisible to the naked eye. Utilization of VD in establishing highly definitive and precise quantitative data products used in the treatment of scabies and pediculosis provides enormous advantages in the quest to establish reproducible quantitative methodology for efficacy studies in these parasitoses. VD enhances the monitoring of clinical response to treatment in scabies and pediculosis and allows determination of the optimal timing of drug application both in vivo and ex vivo. This may be particularly important in minimizing risk of overtreatment, reducing the potential for side effects, and enhancing patient compliance.

Contact and fumigant toxicity of hexane flower bud extract of Syzygium aromaticum and its compounds against Pediculus humanus capitis (Phthiraptera: Pediculidae)

The head lice, Pediculus humanus capitis De Geer is an obligate ectoparasite of humans that causes pediculosis capitis, a nuisance for millions of people worldwide, with high prevalence in children. P. humanus capitis has been treated by methods that include the physical remotion of lice, various domestic treatments, and conventional insecticides. None of these methods render complete protection, and there is clear evidence for the evolution of resistance and cross-resistance to conventional insecticides. Non-toxic alternative options are hence needed for head lice treatment and/or prevention, and natural products from plants are good candidates for safer control agents that may provide good anti-lice activity. The plant extracts are good and safe alternatives due to their low toxicity to mammals and easy biodegradability. The present study carried out the pediculocidal activity using the hexane flower bud extract of Syzygium aromaticum (Myrtaceae) against P. humanus capitis examined by direct contact and fumigant toxicity (closed- and open-container methods) bioassay. The chemical composition of S. aromaticum flower bud hexane extract was analyzed by gas chromatography-mass spectrometry. The major chemical constituent (58.79%) of flower bud hexane extract S. aromaticum was identified as chavibetol (5-allyl-2-methoxyphenol) by comparison of mass spectral data and retention times. The hexane extract of S. aromaticum was subjected to gas chromatography analysis, and totally 47 compounds were detected, of which chavibetol was predominantly present. The other major constituents present in the hexane extract were eugenol acetate (phenol,2-methoxy-4-(2-propenyl)-,acetate (15.09%), caryophyllene-(I1) (2,6,10,10-tetramethyl bicyclo [7.2.0] undeca-1,6-diene (13.75%), caryophyllene oxide (3.04%), 2,6,6,9-tetramethyl-1,4,8-cycloundecatriene (1.67%), and copaene (1.33%). The filter paper contact bioassay study showed pronounced pediculicidal activity in the flower bud hexane extract of S. aromaticum. The toxic effect was determined for every five in an 80-min treatment. The result showed percent mortality of 40, 82, and 100 at 5, 10, and 20 min, and the median lethal time (LT(50)) value was 5.83 (0.5 mg/cm(2)); 28, 82, and 100 at 5, 10, and 30 min. (LT(50) = 6.54; 0.25 mg/cm(2)); and 13, 22, 42, 80, and 100 at 5, 10, 20, 40, and 80 min (LT(50) = 18.68; 0.125 mg/cm(2)), respectively. The vapor phase toxicity was tested at 0.25 mg/cm(2). There was a significant difference in pediculicidal activity of S. aromaticum extract against P. humanus capitis between closed- and open-container methods. Adult mortalities were determined for every five in 60 min (closed method) and for every ten in 180 min (open method). The closed method showed the percent mortality was 45, 88, and 100 at 5, 10, and 15 min (LT(50) = 5.39), respectively. In the open-container method, the percent mortality was observed 5, 20, 47, 84, and 100 at 10, 20, 60, 120, and 180 min (LT(50) = 47.91), respectively. The mortality was more effective in the closed containers than in open ones, indicating that the effect of hexane extract was largely a result of action in the vapor phase exhibited fumigant toxicity. Studies of anti-lice activity of extract provide the basis for preliminary conclusions of structure activity relationships; although no clear patterns can yet be drawn. We here attempt to provide a concise compilation of the available information on anti-lice activity of plant extracts and plant-derived compounds.

In vitro screening of anti-lice activity of Pongamia pinnata leaves

Growing patterns of pediculocidal drug resistance towards head louse laid the foundation for research in exploring novel anti-lice agents from medicinal plants. In the present study, various extracts of Pongamia pinnata leaves were tested against the head louse Pediculus humanus capitis. A filter paper diffusion method was conducted for determining the potential pediculocidal and ovicidal activity of chloroform, petroleum ether, methanol, and water extracts of P. pinnata leaves. The findings revealed that petroleum ether extracts possess excellent anti-lice activity with values ranging between 50.3% and 100% where as chloroform and methanol extracts showed moderate pediculocidal effects. The chloroform and methanol extracts were also successful in inhibiting nymph emergence and the petroleum ether extract was the most effective with a complete inhibition of emergence. Water extract was devoid of both pediculocidal and ovicidal activities. All the results were well comparable with benzoyl benzoate (25% w/v). These results showed the prospect of using P. pinnata leave extracts against P. humanus capitis in difficult situations of emergence of resistance to synthetic anti-lice agents.

In vitro efficacy of over-the-counter botanical pediculicides against the head louse Pediculus humanus var capitis based on a stringent standard for mortality assessment

Infestation of the head louse Pediculus humanus var capitis DeGeer (Phthiraptera: Pediculidae) is an important public health problem in Australia, with up to a third of children infested in some primary schools. Insecticide resistance and inadequate attention to the application instructions of topical pediculicides are common reasons for treatment failure. This study evaluated six popular Australian over-the-counter products against head lice, primarily comprised of different botanical extracts, and compared them with permethrin 1% (Quellada) and a non-treatment control in order to assess their in vitro efficacy. We also assessed commonly used criteria for evaluating pediculicide efficacy in vitro. All tested products failed to demonstrate high levels of efficacy with the exception of Tea Tree Gel((R)), which outperformed 1% permethrin. Permethrin had a high level of efficacy, but using stringent criteria 18% of lice were not dead at 3 h, indicating some resistance to Quellada. Commonly used less stringent criteria were shown to overestimate mortality of head lice as a result of the protective phenomenon of stasis or sham death observed in exposed lice that may recover after some time. Using two different levels of stringency resulted in different rankings of efficacy for most products, with the exception of the first ranked product, Tea Tree Gel. Rankings of efficacy also varied over time, even within the different assessment criteria. Government regulatory agencies should require standard in vitro tests using stringent mortality criteria, with an observation period of >or= 6 h, to determine the efficacy of new pediculicides, and only products that cause a minimum mortality rate (e.g. 80%) in head lice collected from the target population should be licensed for sale.

High in vitro efficacy of Nyda L, a pediculicide containing dimeticone

BACKGROUND

The therapy of pediculosis remains a common problem in clinical practice. As resistance to commonly used chemical pediculicides is constantly increasing, there is a need for new effective compounds.

STUDY DESIGN

The efficacy of Nyda L, a new pediculicide containing a high concentration of dimeticone, was assessed in an in vitro trial and compared with three reference products (Hedrin containing 4% dimeticone, Lyclear containing 1% permethrin, and Prioderm containing 1% malathion) and a negative control group. Head lice were collected from heavily infested patients in a resource-poor community in Northeast Brazil, where no resistance against pediculicides has been reported thus far. In each of the five groups, 50 adult fully vital lice were tested. We used a dip test (immersing head lice in the undiluted products for 3 min, washing off products after 20 min). Using predefined criteria for mortality, the lice were monitored at different points in time, for a period of 24 h.

RESULTS

Nyda L and Prioderm killed all head lice already after 5 min. The efficacy of Nyda L was better than of Lyclear. Lice treated with Hedrin resurrected after several hours and did not show a significantly higher mortality compared with the control group after 24 h.

CONCLUSION

Nyda L can be regarded as a very efficacious pediculicidal compound, killing all lice in vitro within 5 min.

Effectiveness of lotions based on essential oils from aromatic plants against permethrin resistant Pediculus humanus capitis

In Argentina, field populations of the head louse Pediculus humanus capitis De Geer (Phthiraptera: Pediculidae) have developed resistance to permethrin and other pyrethroids. Thus, the aim of this work was the development of a lotion containing essential oils from plants and an alcoholic coadjuvant to improve biological effect. Ethanol + isopropanol (1 + 1 in volume) 50% in water and ethanol 96% were taken as bases for preparation of experimental lotions containing essential oils from plants. We found that experimental lotions containing lavender, peppermint and eucalyptus oils in a 5% composition and the combination of eucalyptus and peppermint in a total concentration of 10%, dissolved in 50% ethanol + isopropanol (1 + 1) in water, showed the best knockdown effect. On the other side, lotion containing peppermint oil and eucalyptus oil (1 + 1) 10%, dissolved in ethanol 96%, showed to be as effective as the best commercial lotion now available in Argentina. Furthermore, addition of 1-dodecanol in all cases increased the effectiveness of all the experimental lotions. This difference is significantly important for 1-dodecanol concentration of 10%, reaching a toxic activity compared to the best commercial lotion available in the market.

In vitro pediculicidal and ovicidal activity of an extract and oil from fruits of Melia azedarach L

BACKGROUND

Head louse infestation is difficult to control because of increasing lice resistance to synthetic pediculicidal drugs.

OBJECTIVE

To test the activity of extract and oil obtained from fruits of Melia azedarach L. against the head louse Pediculus humanus capitis.

METHODS

A filter paper diffusion bioassay was carried out in order to determine the pediculicidal and ovicidal activity of extract and oil from M azedarach L. fruits.

RESULTS

Both vegetable products, tested either individually or in combinations, showed high levels of mortality on adult lice, with values ranging between 62.9% and 96.5%. The highest mortality rate was obtained with a combination of 20% ripe fruit extract with 10% ripe fruit oil. A formulation made with both extract and oil at 10% plus the addition of emulsifier and preserving agents showed 92.3% pediculicidal activity. The products were also successful in delaying or inhibiting nymph emergence, with the formulation being the most effective, with a complete inhibition of emergence.

LIMITATIONS

Because adult lice are sensitive to starvation and therefore control mortalities are often higher than 20% in tests with field specimens, the results may not reflect the direct effect of the extract.

CONCLUSIONS

These results demonstrate the possibility of using Melia products for controlling head lice, which are difficult to control because of their resistance to the currently used anti-louse agents.

Control of head lice: past, present and future

Pediculosis capitis is common among children. Rapid and accurate detection is an underlying principle of effective control and the use of a fine-tooth comb in wet, conditioned hair is most sensitive. However, none of the available treatments are fully effective. Of the current pediculicides available over the counter in the UK, those based on malathion are more effective than pyrethroids. Carbaryl is likely to be the most effective insecticide but is only available on prescription. Wet combing with conditioner has proven to be as effective as some over-the-counter lotions for unsupervised use in the home. There are several promising candidates for new insecticidal actives that are likely to be highly effective for a new generation of pediculicides, as well as a variety of unconventional novel treatments.

Lethality of essential oil constituents towards the human louse, Pediculus humanus, and its eggs

Essential oils have been widely used in traditional medicine for the eradication of lice, including head lice, but due to the variability of their constitution the effects may not be reproducible. In an attempt to assess the contribution of their component monoterpenoids, a range of common individual compounds were tested in in vitro toxicity model against both human lice (Pediculus humanus, an accepted model of head lice lethality) and their eggs, at different concentrations. No detailed study into the relative potencies of their constituent terpenoids has so far been published. Adult lice were observed for lack of response to stimuli over 3 h and the LT(50) calculated, and the percentage of eggs failing to hatch was used to generate ovicidal activity data. A ranking was compiled for adult lice and partially for eggs, enabling structure-activity relationships to be assessed for lethality to both, and showed that, for activity in both life-cycle stages, different structural criteria were required. (+)-Terpinen-4-ol was the most effective compound against adult lice, followed by other mono-oxygenated monocyclic compounds, whereas nerolidol was particularly lethal to eggs, but ineffective against adult lice.

Safety and efficacy of pediculicides for head lice

Head lice infestation is a common and growing problem, primarily affecting school-aged children. There are growing numbers of treatment failures due to the emergence of treatment-resistant lice to the popular over-the-counter products that have been used for the past several decades. Resistance has also decreased the efficacy of lindane, a prescription pediculicide that has been commonly used for several generations. Malathion, recently reintroduced in the US as a prescription pediculicide, has been associated with some treatment resistance depending upon its formulation. Other insecticidal treatments, such as ivermectin, will have to be developed further, given the limited options presently available for the treatment of head lice. Given the number of anecdotal and market-driven reported studies on head lice, assessment of topical lice therapies requires standardised in vitro testing. Based on concerns about safety and decreasing efficacy due to resistance, a reassessment of the general topic of pediculicides for head lice is warranted.

Head lice: Scientific assessment of the nit sheath with clinical ramifications and therapeutic options

Head lice, like many insects, produce a protective coating for their newly laid eggs that is essential to the survival of the species. Knowledge of the composition of the sheath, which is the glue by which the egg is attached to human hair, and the nit laying process could lead to production of agents that could be used to attack louse infestations by interfering with the normally protected environment of nymph development within the egg. The physical removal of nits has become an important part of treatment of head louse infestations given the "no-nit" policy in schools. Biochemical analysis has revealed that the nit sheath of the head louse is composed of 4 bands of protein, possibly cross-linked to aliphatic components with a tertiary structure of beta sheeting. Nature has protected the louse by making the nit sheath similar in composition to the hair; thereby, agents designed to unravel the nit sheath may also damage human hair. Possible targets to destroy the nit sheath include proteases, denaturants, beta sheet breaker proteins, and small protein inhibitors of sheath formation. Better understanding of insect glues may allow us to develop compounds so that the liquid secretions of the collateral glands of the female louse, which becomes the nit sheath, do not solidify by oxidation when placed with the louse egg onto human hair. Knowledge of insect behavior, such as oviposition, may also suggest methods for repelling female lice from laying eggs onto hair. Alternatively, agents that coat the nits and restrict the oxygen transfer to the developing larvae may prove beneficial.

Relationship of treatment-resistant head lice to the safety and efficacy of pediculicides

Head lice infestation is a common and growing problem that primarily affects school-aged children. Most cases of head lice are diagnosed and treated by nonphysicians. Misdiagnosis may lead to treatment when no lice are present. Treatment failure may lead to repeated use of and improperly applied pediculicides, potentially resulting in overexposure to pesticides. These treatment failures are primarily due to the emergence of treatment-resistant lice. In regions where resistant lice are common, patients may self-treat numerous times with over-the-counter pediculicides before seeking treatment from a physician. Resistance has decreased the efficacy of lindane, a prescription pediculicide that has been used for decades. In addition, the Food and Drug Administration recently warned of potentially serious adverse effects associated with lindane and recommended strict controls for its use. Malathion, recently reintroduced in the United States as a prescription pediculicide, has not been associated with treatment resistance or notable adverse effects, although it is flammable due to its alcohol base. Because of concerns about decreasing efficacy due to resistance and safety concerns about over-the-counter products and some prescription pediculicides, a reassessment of pediculicide safety is warranted. The safety and efficacy of commonly used over-the-counter and prescription pediculicide products are discussed, along with the safety and efficacy of other treatments, such as ivermectin, that are not indicated for the treatment of head lice but are being used increasingly.

Pediculosis

The 3 major lice that infest humans are Pediculus humanus capitis (head louse), Pthirus pubis (crab louse), and Pediculus humanus humanus (body louse). Patients with louse infestation present with scalp pruritus, excoriations, cervical lymphadenopathy, and conjunctivitis. A hypersensitivity rash, or pediculid, may mimic a viral exanthem. Head lice infestation crosses all economic and social boundaries, whereas body lice infestation preferentially affects the homeless and displaced. Body lice are major vectors of diseases such as typhus, trench fever, and relapsing fever. Pubic lice infestation often is acquired as a sexually transmitted disease and may be a marker to screen for other sexually transmitted diseases. Treatment of louse infestation can be challenging. Mechanical measures, such as combing, are helpful as adjunctive measures, but most studies suggest they are not as effective as chemical agents. Resistance to chemical agents is a growing problem. Major types of resistance include knock-down resistance, glutathione-S-transferase-based resistance, and monooxygenase-based resistance. Research is needed to identify new effective treatments.

Human lice and their control

Current research on human louse biology has focused on the long-standing debate about speciation of head and body lice but using new tools of DNA and enzyme analysis. These studies have indicated that head and body lice from the same geographical zone may be more closely allied than insects inhabiting the same ecological niche in other regions. However, the majority of research over the past decade has involved clinical aspects including transmission, treatment, and the appearance and identification of resistant strains within populations of lice. Despite advances, there is a need for a better understanding of louse biology, as existing therapies fail and lice remain potential vectors of disease for millions of people.