Evaluation of standard field and laboratory methods to compare protection times of the topical repellents PMD and DEET

In the arm-in-cage test, topical repellents activate mosquitoes to disengage upon contact instead of repelling them at distance

Topical repellents provide protection against mosquito bites and their efficacy is often assessed using the arm-in-cage test. The arm-in-cage test estimates the repellent's protection time by exposing a repellent-treated forearm to host-seeking mosquitoes inside a cage at regular intervals until the first confirmed mosquito bite. However, the test does not reveal the repellents' behavioural mode of action. To understand how mosquitoes interact with topical repellents in the arm-in-cage test, we used a 3D infrared video camera system to track individual Aedes aegypti and Anopheles stephensi females during exposure to either a repellent-treated or an untreated forearm. The repellents tested were 20% (m/m) ethanolic solutions of N, N-diethyl-meta-toluamide, p-menthane-3,8-diol, icaridin and ethyl butylacetylaminopropionate. All four repellents substantially reduced the number of bites compared to an untreated forearm, while the flight trajectories indicate that the repellents do not prevent skin contact as the mosquitoes made multiple brief contacts with the treated forearm. We conclude that, in the context of the arm-in-cage test, topical repellents activate mosquitoes to disengage from the forearm with undirected displacements upon contact rather than being repelled at distance by volatile odorants.

Repelling Aedes aegypti mosquitoes with electric fields using insulated conductor wires

BACKGROUND

The control and prevention of mosquito-borne diseases is mostly achieved with insecticides. However, their use has led to the rapid development and spread of insecticide resistance worldwide. Health experts have called for intensified efforts to find new approaches to reduce mosquito populations and human-mosquito contact. A promising new tool is the use of electrical fields (EFs), whereby mosquitoes are repelled by charged particles in their flight path. Such particles move between two or more conductors, and the use of uninsulated copper or aluminum plates as conductors has been proven to be effective at repelling mosquitoes. Here, for the first time, we assess if EFs generated using a single row of insulated conductor wires (ICWs) can also successfully repel mosquitoes, and whether mosquitoes are equally repelled at the same EF strength when the electrodes are a) orientated differently (horizontal vs. vertical placement), and b) spaced more apart.

METHODOLOGY/PRINCIPAL FINDINGS

Over a period of 23 hours, the number of host-seeking female Aedes aegypti mosquitoes that were successfully repelled by EFs, using ICWs, at EF strengths ranging from 0 kV/cm (control) to 9.15 kV/cm were quantified. Mosquitoes were released inside a 220×220×180 cm room and lured into a BG-Pro trap that was equipped with a BG-counter and baited with CO2 using dry ice. Mosquitoes had to pass through an EF window, that contained a single row of ICWs with alternating polarity, to reach the bait. The baseline interaction between EF strength and repellency was assessed first, after which the impact of different ICW orientations and ICW distances on repellency were determined. Over 50% of mosquitoes were repelled at EF strengths of ≥ 3.66 kV/cm. A linear regression model showed that a vertical ICW orientation (vertical vs. horizontal) had a small but insignificant increased impact on mosquito repellency (p = 0.059), and increasing ICW distance (while maintaining the same EF strength) significantly reduced repellency (p = 0.01).

CONCLUSIONS/SIGNIFICANCE

ICWs can be used to generate EFs that partially repel host-seeking mosquitoes, which will reduce human-mosquito contact. While future studies need to assess if (i) increased repellency can be achieved, and (ii) a repellency of 50-60% is sufficient to impact disease transmission, it is encouraging that EF repellency using ICWs is higher compared to that of some spatial repellent technologies currently in development. This technology can be used in the housing improvement toolkit (i.e. preventing mosquito entry through eaves, windows, and doors). Moreover, the use of cheap, over-the-counter ICWs will mean that the technology is more accessible worldwide, and easier to manufacture and implement locally.

Repellent activity of Vitex negundo and Curcuma longa herbal extract against Simulium species in India

BACKGROUND OBJECTIVES

At present, use of synthetic pesticides to manage insects and other arthropods creates a number of issues that are related to the environment and public health. The goal of the present study is to find repellent activity of Vitex negundo and Curcuma longa herbal extracts against the wild species Simulium (blackfly) bite.

METHODS

We have studied Simulium biting behavior and tested the repellency of herbal extract obtained from Vitex negundo L. (Lamiales: Lamiaceae) and Curcuma longa L. (Zingiberal: Zingiberaceae) along with their essential oils in three locations in Arunachal Pradesh, India on human volunteers' resistance to Simulium (blackflies). The reported herbal extracts were incorporated to topical drug delivery vehicle as a cream and gel.

RESULTS

The methanolic extract of Vitex negundo cream and gel formulation showed >2 h safety at 5% concentration and >5 h safety at 10% concentration in all the testing sites followed by ethyl acetate extract. Whereas, chloroform extract of Curcuma longa cream and gel formulation provided >1 h safety at 5% concentration and >4 h safety at 10% concentration.

INTERPRETATION CONCLUSION

At an optimum concentration of 10%, the methanolic extract of Vitex negundo cream provided complete protection times (CPTs) 320.4, 358.6, and 346.4 min, respectively. These findings offer confirmation of the methanolic extract and chloroform extract potential for use in new blackfly repellents.

Human landing catches provide a useful measure of protective efficacy for the evaluation of volatile pyrethroid spatial repellents

BACKGROUND

The human landing catch (HLC) method, in which human volunteers collect mosquitoes that land on them before they can bite, is used to quantify human exposure to mosquito vectors of disease. Comparing HLCs in the presence and absence of interventions such as repellents is often used to measure protective efficacy (PE). Some repellents have multiple actions, including feeding inhibition, whereby mosquitoes may be unable to bite even if they land on a host. A comparison was made between the PE of the volatile pyrethroid spatial repellent (VPSR) transfluthrin determined using a landing method (HLC) and a biting method (allowing the mosquitoes that landed to blood-feed) to evaluate whether HLC is a suitable method for the estimation of the personal PE of a VPSR.

METHODS

A fully balanced, two-arm crossover design study was conducted using a 6 × 6 × 2-m netted cage within a semi-field system. Hessian strips (4 m × 0.1 m) treated with a 5-, 10-, 15-, or 20-g dose of transfluthrin were evaluated against a paired negative control for three strains of laboratory-reared Anopheles and Aedes aegypti mosquitoes. Six replicates were performed per dose using either the landing or the biting method. The number of recaptured mosquitoes was analysed by negative binomial regression, and the PEs calculated using the two methods were compared by Bland-Altman plots.

RESULTS

For Anopheles, fewer mosquitoes blood-fed in the biting arm than landed in the landing arm (incidence rate ratio = 0.87, 95% confidence interval 0.81-0.93, P < 0.001). For Ae. aegypti, biting was overestimated by around 37% with the landing method (incidence rate ratio = 0.63, 95% confidence interval 0.57-0.70, P = 0.001). However, the PEs calculated for each method were in close agreement when tested by the Bland Altman plot.

CONCLUSIONS

The HLC method led to underestimation of mosquito feeding inhibition as a mode of action of transfluthrin, and there were species- and dose-dependent differences in the relationship between landing and biting. However, the estimated PEs were similar between the two methods. The results of this study indicate that HLC can be used as a proxy for personal PE for the evaluation of a VPSR, especially when the difficulties associated with enumerating blood-fed mosquitoes in a field setting are taken into consideration.

The fabrication and assessment of mosquito repellent cream for outdoor protection

Mosquito-borne infections like dengue, malaria, chikungunya, etc. are a nuisance and can cause profound discomfort to people. Due to the objectional side effects and toxicity associated with synthetic pyrethroids, N,N-diethyl-3-methylbenzamide (DEET), N,N-diethyl phenylacetamide (DEPA), and N,N-di ethyl benzamide (DEBA) based mosquito repellent products, we developed an essential oil (EO) based mosquito repellent cream (EO-MRC) using clove, citronella and lemongrass oil. Subsequently, a formulation characterization, bio-efficacy, and safety study of EO-MRC were carried out. Expression of Anti-OBP2A and TRPV1 proteins on mosquito head parts were studied by western blotting. In-silico screening was also conducted for the specific proteins. An FT-IR study confirmed the chemical compatibility of the EOs and excipients used in EO-MRC. The thermal behaviour of the best EOs and their mixture was characterized by thermogravimetric analysis (TGA). GC-MS examination revealed various chemical components present in EOs. Efficacy of EO-MRC was correlated with 12% N,N-diethyl benzamide (DEBA) based marketed cream (DBMC). Complete protection time (CPT) of EO-MRC was determined as 228 min. Cytotoxicity study on L-132 cell line confirmed the non-toxic nature of EO-MRC upon inhalation. Acute dermal irritation study, acute dermal dose toxicity study, and acute eye irritation study revealed the non-toxic nature of EO-MRC. Non-target toxicity study on Danio rerio confirmed EO-MRC as safer for aquatic non-target animals. A decrease in the concentration of acetylcholinesterase (AChE) was observed in transfluthrin (TNSF) exposed Wistar rats. While EO-MRC did not alter the AChE concentrations in the exposed animals. Results from western blotting confirmed that Anti-OBP2A and TRPV1 proteins were inhibited in TNSF exposed mosquitoes. Mosquitoes exposed to EO-MRC showed a similar expression pattern for Anti-OBP2A and TRPV1 as the control group. In silico study revealed eight identified compounds of the EOs play significant roles in the overall repellency property of the developed product. The study emphasizes the mosquito repellent activity of EO-MRC, which could be an effective, eco-friendly, and safer alternative to the existing synthetic repellents for personal protection against mosquitoes during field conditions.

Two New Alternatives to the Conventional Arm-in-Cage Test for Assessing Topical Repellents

European guidelines for testing attractant and repellent efficacy (i.e., Product type 19 [PT19]) have been in revision since 2017. A key topic of discussion is the current approach to evaluating topical repellents. The European Chemical Agency has stated field testing should be avoided because of mosquito-borne disease risks. However, the most common laboratory method, the arm-in-cage (AIC) test, may limit the reliable extrapolation of lab results to field conditions. This study's main goal was to assess alternative laboratory methods for evaluating topical mosquito repellents that use mosquito landing rates more representative of those in the field. The study took place at three European testing labs using 30 study participants per test and the mosquito, Aedes albopictus (Skuse, 1894, Diptera: Culicidae). In phase 1, a conventional AIC test and a sleeved AIC test were performed. Respectively, the arm area exposed was 600 and 100 cm2, and cage volume was 0.040 and 0.064 m3. Mosquito density was the same for both: 1 female/840 cm3. In phase 2, room-based testing (40 ± 5 mosquitoes in 25-30 m3) was used as a proxy for field testing. The mosquito repellent employed was 15% N,N-diethyl-m-toluamide in ethanol at two doses: 1 and 0.5 g/600 cm2. The protection times measured at each laboratory were analyzed both separately and together using nonparametric (Kruskal-Wallis) test. The two alternatives methods showed to be potential alternatives to the current AIC method recreated field mosquito landing rates and achieved reproducible protection times across laboratories.

From the Field to the Laboratory: Quantifying Outdoor Mosquito Landing Rate to Better Evaluate Topical Repellents

Vector-borne diseases are a worldwide threat to human health. Often, no vaccines or treatments exist. Thus, personal protection products play an essential role in limiting transmission. The World Health Organization (WHO) arm-in-cage (AIC) test is the most common method for evaluating the efficacy of topical repellents, but it remains unclear whether AIC testing conditions recreate the mosquito landing rates in the field. This study aimed to estimate the landing rate outdoors, in an area of Europe highly infested with the Asian tiger mosquito (Aedes albopictus (Skuse, 1894, Diptera: Culididae)), and to determine how to replicate this rate in the laboratory. To assess the landing rate in the field, 16 individuals were exposed to mosquitoes in a highly infested region of Italy. These field results were then compared to results obtained in the laboratory: 1) in a 30 m3 room where nine volunteers were exposed to different mosquito abundances (ranges: 15-20, 25-30, and 45-50) and 2) in a 0.064 m3 AIC test cage where 10 individuals exposed their arms to 200 mosquitoes (as per WHO requirements). The highest mosquito landing rate in the field was 26.8 landings/min. In the room test, a similar landing rate was achieved using 15-20 mosquitoes (density: 0.50-0.66 mosquitoes/m3) and an exposure time of 3 min. In the AIC test using 200 mosquitoes (density: 3,125 mosquitoes/m3), the landing rate was 229 ± 48 landings/min. This study provides useful reference values that can be employed to design new evaluation standards for topical repellents that better simulate field conditions.

Evaluating putative repellent 'push' and attractive 'pull' components for manipulating the odour orientation of host-seeking malaria vectors in the peri-domestic space

Background

Novel malaria vector control approaches aim to combine tools for maximum protection. This study aimed to evaluate novel and re-evaluate existing putative repellent ‘push’ and attractive ‘pull’ components for manipulating the odour orientation of malaria vectors in the peri-domestic space.

Methods

Anopheles arabiensis outdoor human landing catches and trap comparisons were implemented in large semi-field systems to (i) test the efficacy of Citriodiol^® or transfluthrin-treated fabric strips positioned in house eave gaps as push components for preventing bites; (ii) understand the efficacy of MB5-baited Suna-traps in attracting vectors in the presence of a human being; (iii) assess 2-butanone as a CO₂ replacement for trapping; (iv) determine the protection provided by a full push-pull set up. The air concentrations of the chemical constituents of the push–pull set-up were quantified.

Results

Microencapsulated Citriodiol^® eave strips did not provide outdoor protection against host-seeking An. arabiensis. Transfluthrin-treated strips reduced the odds of a mosquito landing on the human volunteer (OR 0.17; 95% CI 0.12–0.23). This impact was lower (OR 0.59; 95% CI 0.52–0.66) during the push-pull experiment, which was associated with low nighttime temperatures likely affecting the transfluthrin vaporisation. The MB5-baited Suna trap supplemented with CO₂ attracted only a third of the released mosquitoes in the absence of a human being; however, with a human volunteer in the same system, the trap caught < 1% of all released mosquitoes. The volunteer consistently attracted over two-thirds of all mosquitoes released. This was the case in the absence (‘pull’ only) and in the presence of a spatial repellent (‘push-pull’), indicating that in its current configuration the tested ‘pull’ does not provide a valuable addition to a spatial repellent. The chemical 2-butanone was ineffective in replacing CO₂. Transfluthrin was detectable in the air space but with a strong linear reduction in concentrations over 5 m from release. The MB5 constituent chemicals were only irregularly detected, potentially suggesting insufficient release and concentration in the air for attraction.

Conclusion

This step-by-step evaluation of the selected ‘push’ and ‘pull’ components led to a better understanding of their ability to affect host-seeking behaviours of the malaria vector An. arabiensis in the peri-domestic space and helps to gauge the impact such tools would have when used in the field for monitoring or control.

Characterisation of actions of p-menthane-3,8-diol repellent formulations against Aedes aegypti mosquitoes

BACKGROUND

Characterisation of mosquito repellents using arm-in-cage tests are performed by assessing the 95% effective dose (ED95), half-life and complete protection time (CPT). This study fully characterizes these properties for p-menthane-3,8-diol (PMD), which has not been widely studied, and a long-acting formulation containing a PMD-vanillin composite.

METHODS

A series of arm-in-cage tests against Aedes aegypti (Diptera: Culicidae) mosquitoes were devised using 6 volunteers to estimate CPT or 10 to estimate the ED95 and half-lives for three repellents: 20% N,N-diethyl-3-methylbenzamide (DEET), 30% PMD and a novel 30% PMD-vanillin formulation. Non-linear regression analysis was used to characterize the relationship between applied dose and CPT.

RESULTS

PMD and DEET showed a very similar log dose relationship to CPT; however, the PMD-vanillin formulation exhibited a sigmoidal 'S-shaped' relationship. This resulted in a 1.5-fold higher CPT for PMD-vanillin compared with that of 20% DEET when applied at a dose of 1.6 mg/cm2, but little difference was observed at lower doses of 0.8-1 mg/cm2. The ED95 value for the 30% PMD and PMD-vanillin formulations were 0.25 and 0.24 mg/cm2, respectively, these being higher than that for 20% DEET (0.09 mg/cm2). The half-lives for 30% PMD and 20% DEET were similar (2.23 vs. 2.74 h), but longer for the PMD-vanillin formulations (3.8 h).

CONCLUSIONS

A full characterisation for other repellent formulations, particularly those claiming extended longevity, should be conducted in order to identify differences at various applied doses.

Management of Travel-Related Infectious Diseases in the Emergency Department

Purpose of Review

Emergency physicians generally have limited exposure to internationally acquired illnesses. However, travelers can present quite ill, and delays in recognition and treatment can lead to increased morbidity and mortality. This paper aims to summarize typical presentations of common international diseases and provide the emergency physician with a practical approach based on current guidelines.

Recent Findings

In the treatment of traveler’s diarrhea, azithromycin has become the treatment of choice due to the growing antibiotic resistance. Intravenous artesunate was approved in 2019 under investigational new drug protocol for the treatment of severe malaria, and artemisinin-based combination therapies (ACTs) have become the first-line treatment for most cases of uncomplicated malaria. Since the 2015 outbreak, Zika has become a concern to many travelers, but the current treatment is supportive.

Summary

Clinicians should be aware of a few noteworthy updates in the treatment of internationally acquired illnesses, but more importantly, they must recognize warning signs of severe illness and treat promptly. Future research on workup and disposition could help emergency physicians identify which patients need admission in well-appearing febrile travelers.