21 research outputs found
When a discriminating dose assay is not enough: measuring the intensity of insecticide resistance in malaria vectors
Background
Guidelines from the World Health Organization for monitoring insecticide resistance in disease vectors recommend exposing insects to a predetermined discriminating dose of insecticide and recording the percentage mortality in the population. This standardized methodology has been widely adopted for malaria vectors and has provided valuable data on the spread and prevalence of resistance. However, understanding the potential impact of this resistance on malaria control requires a more quantitative measure of the strength or intensity of this resistance.
Methods
Bioassays were adapted to quantify the level of resistance to permethrin in laboratory colonies and field populations of Anopheles gambiae sensu lato. WHO susceptibility tube assays were used to produce data on mortality versus exposure time and CDC bottle bioassays were used to generate dose response data sets. A modified version of the CDC bottle bioassay, known as the Resistance Intensity Rapid Diagnostic Test (I-RDT), was also used to measure the knockdown and mortality after exposure to different multipliers of the diagnostic dose. Finally cone bioassays were used to assess mortality after exposure to insecticide treated nets.
Results
The time response assays were simple to perform but not suitable for highly resistant populations. After initial problems with stability of insecticide and bottle washing were resolved, the CDC bottle bioassay provided a reproducible, quantitative measure of resistance but there were challenges performing this under field conditions. The I-RDT was simple to perform and interpret although the end point selected (immediate knockdown versus 24 h mortality) could dramatically affect the interpretation of the data. The utility of the cone bioassays was dependent on net type and thus appropriate controls are needed to interpret the operational significance of these data sets.
Conclusions
Incorporating quantitative measures of resistance strength, and utilizing bioassays with field doses of insecticides, will help interpret the possible impact of resistance on vector control activities. Each method tested had different benefits and challenges and agreement on a common methodology would be beneficial so that data are generated in a standardized format. This type of quantitative data are an important prerequisite to linking resistance strength to epidemiological outcomes
Adaptation and evaluation of the bottle assay for monitoring insecticide resistance in disease vector mosquitoes in the Peruvian Amazon
<p>Abstract</p> <p>Background</p> <p>The purpose of this study was to establish whether the "bottle assay", a tool for monitoring insecticide resistance in mosquitoes, can complement and augment the capabilities of the established WHO assay, particularly in resource-poor, logistically challenging environments.</p> <p>Methods</p> <p>Laboratory reared <it>Aedes aegypti </it>and field collected <it>Anopheles darlingi </it>and <it>Anopheles albimanus </it>were used to assess the suitability of locally sourced solvents and formulated insecticides for use with the bottle assay. Using these adapted protocols, the ability of the bottle assay and the WHO assay to discriminate between deltamethrin-resistant <it>Anopheles albimanus </it>populations was compared. The diagnostic dose of deltamethrin that would identify resistance in currently susceptible populations of <it>An. darlingi </it>and <it>Ae. aegypti </it>was defined. The robustness of the bottle assay during a surveillance exercise in the Amazon was assessed.</p> <p>Results</p> <p>The bottle assay (using technical or formulated material) and the WHO assay were equally able to differentiate deltamethrin-resistant and susceptible <it>An. albimanus </it>populations. A diagnostic dose of 10 μg a.i./bottle was identified as the most sensitive discriminating dose for characterizing resistance in <it>An. darlingi </it>and <it>Ae. aegypti</it>. Treated bottles, prepared using locally sourced solvents and insecticide formulations, can be stored for > 14 days and used three times. Bottles can be stored and transported under local conditions and field-assays can be completed in a single evening.</p> <p>Conclusion</p> <p>The flexible and portable nature of the bottle assay and the ready availability of its components make it a potentially robust and useful tool for monitoring insecticide resistance and efficacy in remote areas that require minimal cost tools.</p
Identification of Single Specimens of the Anopheles Gambiae Complex by the Polymerase Chain Reaction
Mixed-function oxidases and esterases associated with cross-r esistance between DDT and lambda-cyhalothrin in Anopheles darlingi Root 1926 populations from Colombia
In order to establish the insecticide susceptibility status for
Anopheles darlingi in Colombia, and as part of the National Network
on Insecticide Resistance Surveillance, five populations of insects
from three Colombian states were evaluated. Standardised WHO and CDC
bottle bioassays, in addition to microplate biochemical assays, were
conducted. Populations with mortality rates below 80% in the bioassays
were considered resistant. All field populations were susceptible to
deltamethrin, permethrin, malathion and fenitrothion. Resistance to
lambda-cyhalothrin and DDT was detected in the Amé-Beté
population using both bioassay methods with mortality rates of 65-75%.
Enzyme levels related to insecticide resistance, including mixed
function oxidases (MFO), non-specific esterases (NSE), glutathione
S-transferases and modified acetylcholinesterase were evaluated in all
populations and compared with a susceptible natural strain. Only
mosquitoes from Amé-Beté presented significantly increased
levels of both MFO and NSE, consistent with the low mortalities found
in this population. The continued use of lambda-cyhalothrin for An.
darlingi control in this locality has resulted in a natural resistance
to this insecticide. In addition, DDT resistance is still present in
this population, although this insecticide has not been used in
Colombia since 1992. Increased metabolism through MFO and NSE may be
involved in cross-resistance between lambda-cyhalothrin and DDT,
although kdr-type nerve insensitivity cannot be discarded as a possible
hypothesis. Additional research, including development of a kdr
specific assay for An. darlingi should be conducted in future studies.
Our data demonstrates the urgent need to develop local insecticide
resistance management and surveillance programs throughout Colombia