Current Perspectives on Plague Vector Control in Madagascar: Susceptibility Status of Xenopsylla cheopis to 12 Insecticides.

Plague is a rodent disease transmissible to humans by infected flea bites, and Madagascar is one of the countries with the highest plague incidence in the world. This study reports the susceptibility of the main plague vector Xenopsylla cheopis to 12 different insecticides belonging to 4 insecticide families (carbamates, organophosphates, pyrethroids and organochlorines). Eight populations from different geographical regions of Madagascar previously resistant to deltamethrin were tested with a World Health Organization standard bioassay. Insecticide susceptibility varied amongst populations, but all of them were resistant to six insecticides belonging to pyrethroid and carbamate insecticides (alphacypermethrin, lambdacyhalothrin, etofenprox, deltamethrin, bendiocarb and propoxur). Only one insecticide (dieldrin) was an efficient pulicide for all flea populations. Cross resistances were suspected. This study proposes at least three alternative insecticides (malathion, fenitrothion and cyfluthrin) to replace deltamethrin during plague epidemic responses, but the most efficient insecticide may be different for each population studied. We highlight the importance of continuous insecticide susceptibility surveillance in the areas of high plague risk in Madagascar

Xenopsylla cheopis (Siphonaptera: Pulicidae) susceptibility to Deltamethrin in Madagascar.

The incidence of bubonic plague in Madagascar is high. This study reports the susceptibility of 32 different populations of a vector, the flea Xenopsylla cheopis (Siphonaptera: Pulicidae), to the insecticide Deltamethrin. Despite the use of Deltamethrin against fleas, plague epidemics have re-emerged in Madagascar. The majority of the study sites were located in the Malagasy highlands where most plague cases have occurred over the last 10 years. X. cheopis fleas were tested for susceptibility to Deltamethrin (0.05%): only two populations were susceptible to Deltamethrin, four populations were tolerant and 26 populations were resistant. KD50 (50% Knock-Down) and KD90 (90% Knock-Down) times were determined, and differed substantially from 9.4 to 592.4 minutes for KD50 and 10.4 min to 854.3 minutes for KD90. Susceptibility was correlated with latitude, but not with longitude, history of insecticide use nor date of sampling. Combined with the number of bubonic plague cases, our results suggest that an immediate switch to an insecticide other than Deltamethrin is required for plague vector control in Madagascar

Insecticides used in the bioassays with their concentration and the diagnostic exposure times.

<p>Insecticides used in the bioassays with their concentration and the diagnostic exposure times.</p

Mean mortality rates of flea populations per insecticides through the exposure time.

<p>Mean mortality rates of flea populations per insecticides through the exposure time.</p

Box-and-whisker plot of mortality rate after 24 hours for each in secticide for all populations study sites.

<p>Black Diamond-shaped points inside the boxes are mean values. Horizontal bars in boxes are the 50^th percentiles (medians), and the bottom and the top of the box represent the 25^th and the 75^th percentiles, respectively. The two limits of vertical lines above and at the bottom of the box are the wiskers and represent the maximum and the minimum values of the data. Points outside the limit of vertical line are “outlier”, which are values outside 95% the confidence interval.</p

Mean of mortality rate (24 hours) per insecticide for each population.

<p>X-axis indicates populations and Y-axis indicates mortality rate in percent. Error bars represents standard errors. Diagrams color codes: in red are resistant populations, in yellow, tolerant and in green, susceptible according to WHO. Letters code (a, b, and c) above and on the side of each bar plot indicate significant difference between the mortality for each population according to the Tukey’s b test.</p

Current Perspectives on Plague Vector Control in Madagascar: Susceptibility Status of <i>Xenopsylla cheopis</i> to 12 Insecticides

<div><p>Plague is a rodent disease transmissible to humans by infected flea bites, and Madagascar is one of the countries with the highest plague incidence in the world. This study reports the susceptibility of the main plague vector <i>Xenopsylla cheopis</i> to 12 different insecticides belonging to 4 insecticide families (carbamates, organophosphates, pyrethroids and organochlorines). Eight populations from different geographical regions of Madagascar previously resistant to deltamethrin were tested with a World Health Organization standard bioassay. Insecticide susceptibility varied amongst populations, but all of them were resistant to six insecticides belonging to pyrethroid and carbamate insecticides (alphacypermethrin, lambdacyhalothrin, etofenprox, deltamethrin, bendiocarb and propoxur). Only one insecticide (dieldrin) was an efficient pulicide for all flea populations. Cross resistances were suspected. This study proposes at least three alternative insecticides (malathion, fenitrothion and cyfluthrin) to replace deltamethrin during plague epidemic responses, but the most efficient insecticide may be different for each population studied. We highlight the importance of continuous insecticide susceptibility surveillance in the areas of high plague risk in Madagascar.</p></div

Map showing location of the eight sites where fleas were collected.

<p>Source: Institut Pasteur de Madagascar, OCHA.</p

Schematic illustration of correlation between insecticide mortality rates (mean of 24h mortality per insecticide for all stations).

<p>Positive correlations are in blue and negative correlations are in red. The intensity of colors and the size of circles are proportional to the correlation coefficient. In the right, the color legend shows the correlation coefficient values with corresponding colors. Numbers followed by star are significant p values (p<0.05). Positive correlations between insecticides are represented surrounded with red rectangle.</p

Correlations results between the mortality values parameters and between the times spent by fleas in the insectariums (Pearson's correlation Test).

<p>Correlations results between the mortality values parameters and between the times spent by fleas in the insectariums (Pearson's correlation Test).</p