Resistant Lice Research Articles

The Efficacy and Biopharmaceutical Properties of a Fixed-Dose Combination of Disulfiram and Benzyl Benzoate.

Scabies and hair lice are parasitic diseases that affect human skin and hair, respectively. The incidence and resistances of these infections are increasing. Tenutex® (disulfiram and benzyl benzoate emulsion) is an alternative to standard insecticides to avoid resistances. The aim of the work is to evaluate the transdermal absorption and the in vitro efficacy against scabies and hair lice after different exposition times. Dermatomed human skin was used to assess the dermal absorption using a validated High Performance Liquid Chromatography (HPLC) method. HEK001 keratinocytes were used to evaluate the cytotoxicity of benzyl benzoate. Only benzyl benzoate was able to cross the skin, but it did not show cytotoxicity at any of the tested concentrations. The product efficacy was tested on Psoroptes ovis after direct contact and after administration on sheep skin explants at different contact times. Permethrin/malathion-resistant strains of Pediculus humanis capitis adults and eggs were directly exposed to Tenutex, and the vitality and hatchability, respectively, were evaluated. The anti-scabies study demonstrated that exposure for 6 or 24 h completely eradicated the parasite. The pediculicidal activity of Tenutex exhibited superior efficacy than standard treatment on resistant lice. The positive results obtained suggest that Tenutex® is a good treatment option, especially in drug resistance situations.

Candidate genes for monitoring hydrogen peroxide resistance in the salmon louse, Lepeophtheirus salmonis

BackgroundHydrogen peroxide (H2O2) is one of the delousing agents used to control sea lice infestations in salmonid aquaculture. However, some Lepeophtheirus salmonis populations have developed resistance towards H2O2. An increased gene expression and activity of catalase, an enzyme that breaks down H2O2, have been detected in resistant lice, being therefore introduced as a resistance marker in the salmon industry. In the present study the aim was to validate the use of catalase expression as a marker and to identify new candidate genes as additional markers to catalase, related to H2O2 resistance in L. salmonis.MethodsA sensitive and an H2O2 resistant laboratory strain (P0 generation, not exposed to H2O2 for several years) were batch crossed to generate a cohort with a wide range of H2O2 sensitivities (F2 generation). F2 adult females were then exposed to H2O2 to separate sensitive and resistant individuals. Those F2 lice, the P0 lice and field-collected resistant lice (exposed to H2O2 in the field) were used in an RNA sequencing study.ResultsCatalase was upregulated in resistant lice exposed to H2O2 compared to sensitive lice. This was, however, not the case for unexposed resistant P0 lice. Several other genes were found differentially expressed between sensitive and resistant lice, but most of them seemed to be related to H2O2 exposure. However, five genes were consistently up- or downregulated in the resistant lice independent of exposure history. The upregulated genes were: one gene in the DNA polymerase family, one gene encoding a Nesprin-like protein and an unannotated gene encoding a small protein. The downregulated genes encoded endoplasmic reticulum resident protein 29 and an aquaporin (Glp1_v2).ConclusionsCatalase expression seems to be induced by H2O2 exposure, since it was not upregulated in unexposed resistant lice. This may pose a challenge for its use as a resistance marker. The five new genes associated with resistance are put forward as complementary candidate genes. The most promising was Glp1_v2, an aquaglyceroporin that may serve as a passing channel for H2O2. Lower channel number can reduce the influx or distribution of H2O2 in the salmon louse, being directly involved in the resistance mechanism.

Caligus rogercresseyi acetylcholinesterase types and variants: a potential marker for organophosphate resistance

BackgroundControl of the sea louse Caligus rogercresseyi in the Chilean salmonid industry is reliant on chemical treatments. Azamethiphos was introduced in 2013, although other organophosphates were previously used. In 2014, reduced sensitivity to azamethiphos was detected in the Los Lagos Region using bioassays. The main target of organophosphates is the enzyme acetylcholinesterase (AChE). Mutations in the AChE gene are the main cause of organophosphate resistance in arthropods, including other sea lice. In the present study, we aimed to characterize C. rogercresseyi AChE(s) gene(s) and to study the association between AChE variants and azamethiphos resistance in this sea louse species.MethodsSamples of adult male and female C. rogercresseyi were collected in the Los Lagos Region in 2014. Twenty-four hour exposure bioassays with azamethiphos were performed to select sensitive and resistant lice. The full-length cDNA coding sequences encoding for two AChEs in C. rogercresseyi were molecularly characterized. One of the AChE genes was screened by direct sequencing in the azamethiphos-selected lice to search for variants. An additional louse sampling was performed before and after an azamethiphos treatment in the field in 2017 to validate the findings.ResultsThe molecular analysis revealed two putative AChEs in C. rogercresseyi. In silico analysis and 3D modelling of the protein sequences identified both of them as invertebrate AChE type 1; they were named C. rogercresseyi AChE1a and 1b. AChE1a had the characteristics of the main synaptic AChE, while AChE1b lacked some of the important amino acids of a typical AChE. A missense change found in the main synaptic AChE (1a), F318F/V (F290 in Torpedo californica), was associated with survival of C. rogercresseyi at high azamethiphos concentrations (bioassays and field treatment). The amino acid change was located in the acyl pocket of the active-site gorge of the protein.ConclusionsThe present study demonstrates the presence of two types of AChE1 genes in C. rogercresseyi. Although enzymatic assays are needed, AChE1a is most probably the main synaptic AChE. The function of AChE1b is unknown, but evidence points to a scavenger role. The AChE1a F/V318 variant is most probably involved in organophosphate resistance, and can be a good marker for resistance monitoring.

A selection study on a laboratory-designed population of salmon lic (Lepeophtheirus salmonis) using organophosphate and pyrethroid pesticides.

Resistance towards antiparasitic agents in the salmon louse (Lepeophtheirus salmonis) is a widespread problem along the Norwegian coast, reducing treatments efficacies and slowing down the envisioned expansion of Norwegian salmon production. The present study was conducted in order to assess the efficacies of two of the most widely used anti-parasitic substances–azamethiphos and deltamethrin–as well as assessing the benefit of having a resistant genotype compared to being fully sensitive when exposed to one of these substances. Atlantic salmon were exposed to a mix of salmon lice copepodids from a fully sensitive, a double resistant and a multi-resistant strain. Once the lice reached pre-adult stages, one group was exposed to 100 μg/L azamethiphos for 60 minutes, the other to 2 μg/L deltamethrin for 30 minutes, and the last was kept in a seawater control. Detached lice were collected at a series of time points following exposure, and all lice (immobilized and surviving) were analysed for both pyrethroid (sensitive “S” and resistant “R”) and azamethiphos (fully sensitive “SS”, heterozygous resistant “RS” and fully resistant “RR”) resistance markers. We found that the efficacies of deltamethrin on parasites with genotype S and R were 70.3 and 13.2%, respectively. The overall efficacy of the deltamethrin treatment was 32.3%. The efficacies of azamethiphos on parasites with genotype SS, RS and RR were 100, 80 and 19.1%, respectively. The overall efficacy of the azamethiphos treatment was 80.4%. Survival analyses revealed that the median survival time in deltamethrin-sensitive and–resistant parasites were 16.8 and >172 hours, respectively. The differences were even more pronounced in the azamethiphos-treated group, where SS, RS and RR parasites survived for 0.26, 6.6 and >172 hours, respectively. The substantial differences in survival between sensitive and resistant lice following treatment demonstrate the ability of medicinal treatments to drive genetic selection towards a much more resistant salmon lice population within a very short time span if there is no influx of sensitive genotypes.

A simple model to assess selection for treatment-resistant sea lice

Sea lice are damaging marine copepod parasites that infest wild and farmed salmon. Lice are controlled largely by the application of medicines; however resistance has evolved to several such treatments. A simple model is used to explore situations under which treatment-resistant lice are likely to emerge. The model consists of farmed and wild populations of hosts that are infected with lice that exist in treatment-sensitive and treatment-resistant morphs. Resistance is assumed to impose costs on lice fitness, so the sensitive morphs have a selective advantage in the absence of treatment. Larval lice are exchanged between treated farmed hosts and untreated wild hosts by variable water currents. The model finds that resistance is most strongly selected under moderate levels of treatment on farms. High levels of treatment remove both sensitive and resistant lice from farms and, leave the wild untreated fish as a source of lice. The treatment per fish required to eradicate the resistant morphs increases as hydrodynamic mixing rates increase and so controlling emergence of resistance becomes less cost effective when mixing rates are high.

Pediculicide Resistance in Head Lice: A Survey

The head louse is an obligate human parasite. Evidence points to the growing resistance of head lice to nonprescription pesticides (eg, synergized pyrethrins, permethrin). To gather data, recent pharmacy graduates were surveyed about treatment failures and resistant lice. The response rate was 28.4%, with 173 of 609 surveys returned. One hundred and thirty-one pharmacists provided information that they had gathered during counseling sessions or other contacts with patients. Patients who remained infested after using either synergized pyrethrins or permethrin were encountered by 81.7% and 78.6% of pharmacists, respectively; these treatment failures occurred once or twice weekly in 58.1% of pharmacists' practices. Patients treating themselves more frequently or in higher doses with pesticides in an attempt to rid themselves of lice were reported by 62.6% of pharmacists. Patients reported the use of gasoline, kerosene, or other dangerous treatments once weekly or more in the experience of 42% of pharmacists. Resistant head lice had been a problem in their areas for 1 or 2 years for 42.8% of pharmacists. A smaller percentage (5.3%) of pharmacists had received official confirmation that resistant head lice were present in their areas. Pharmacists recommended nontoxic options (eg, combing and vacuuming) 19.8% of the time with positive results. Resistance seems to be a widespread, growing phenomenon and is not entirely related to patients' inability to use pesticides correctly. Pharmacists should consider recommending nontoxic alternatives that do not pose a threat to the patient or environment and do not contribute to resistance.

Pyrethroid resistance mechanisms in the head louse Pediculus capitis from Israel: implications for control.

In Israel, the head louse, Pediculus capitis, developed resistance to DDT through the extensive use of this insecticide until the 1980s. In 1991, permethrin was introduced for control of DDT resistant P. capitis in Israel, leading to control failure of this pyrethroid insecticide by 1994. Pyrethroid resistance of P. capitis in Israel extends to phenothrin, which has not been used for louse control. We identified a glutathione S-transferase(GST)-based mechanism of DDT resistance in the Israeli head lice. This GST mechanism occurred before 1989, while permethrin resistance in P. capitis developed after 1994, suggesting that the main GST resistance mechanism selected by DDT use does not confer any pyrethroid cross-resistance. Esterase activity levels were equivalent in pyrethroid resistant and susceptible P. capitis field-collected in Israel, and in a susceptible strain of P. humanus, the body louse, indicating no involvement of any esterase-based mechanism in resistance. A weak monooxygenase-based permethrin metabolism resistance mechanism was the only factor identified which could account for any of the observed pyrethroid resistance in P. capitis. However, the lack of synergism of phenothrin resistance by piperonyl butoxide suggests that a non-oxidative mechanism is also present in the resistant lice. Therefore it seems probable that pyrethroid resistance in Israeli P. capitis is due to a combination of nerve insensitivity (knockdown resistance or 'kdr') and monooxygenase resistance mechanisms.

Biochemical investigations on DDT-resistance in the human body louse, Pendiculus humanus humanus.

Summary DDT was found to be absorbed very slowly from acetone or benzene solutions when applied topically to susceptible and DDT-resistant human body lice. The resistant strain could tolerate larger quantities of applied DDT without any appreciably harmful effect. The feeding of lice through dissected chick skins on citrated human blood containing various concentrations of radioactive DDT showed that the resistant strain could tolerate 100 ppm of DDT in the blood whereas 10 ppm caused 53 per cent mortality in the susceptible strain. Only resistant lice metabolized DDT in vivo to a relatively nontoxic material unextractable with common organic solvents but partly extractable with methanol. Determination of the metabolite by the Schechter-Haller method yielded a nitrated product which was removed from solution by dilute alkali, unlike DDT, DDE, or DDA. Both susceptible and DDT-resistant lice metabolized DDT in vitro at approximately the same rate. The nontoxic, water-soluble metabolite gave a red-colored complex in the Schechter-Haller procedure and had solubility characteristics similar to that obtained in vivo. It appears that an intrinsic biochemical factor which, presumably, interferes with the detoxifying mechanism, might be operative in the susceptible strain. This interference is manifested in vivo but not in vitro.