The effect of oral anthelmintics on the survivorship and re-feeding frequency of anthropophilic mosquito disease vectors

Abstract

In the Tropics, there is substantial temporal and spatial overlap of diseases propagated by anthropophilic mosquito vectors (such as malaria and dengue) and human helminth diseases (such as onchocerciasis and lymphatic filariasis) that are treated though mass drug administrations (MDA). This overlap will result in mosquito vectors imbibing significant quantities of these drugs when they blood feed on humans. Since many anthelmintic drugs have broad anti-invertebrate effects, the possibility of combined helminth control and mosquito-borne disease control through MDA is apparent. It has been previously shown that ivermectin can reduce mosquito survivorship when administered in a blood meal, but more detailed examinations are needed if MDA is to ever be developed into a tool for malaria or dengue control. We examined concentrations of drugs that follow human pharmacokinetics after MDA and that matched with mosquito feeding times, for effects against the anthropophilic mosquito vectors Anopheles gambiae s.s. and Aedes aegypti. Ivermectin was the only human-approved MDA drug we tested that affected mosquito survivorship, and only An. gambiae s.s. were affected at concentrations respecting human pharmacokinetics at indicated doses. Ivermectin also delayed An. gambiae s.s. re-feeding frequency and defecation rates, and two successive ivermectin-spiked blood meals following human pharmacokinetic concentrations compounded mortality effects compared to controls. These findings suggest that ivermectin MDA in Africa may be used to decrease malaria transmission if MDAs were administered more frequently. Such a strategy would broaden the current scope of polyparasitism control already afforded by MDAs, and which is needed in many African villages simultaneously burdened by many parasitic diseases.

Figure 1. Percent survivorship of mosquitoes that imbibed ivermectin (A) An. gambiae s.s. (B) Ae. aegypti

An. gambiae s.s. and Ae. aegypti were fed varied concentrations of IVM to determine if IVM reduced mosquito survivorship. IVM reduced the survivorship of both An. gambiae s.s. (LC₅₀ 22.4 ng/ml [18.0, 26.9]) and Ae. aegypti (LC₅₀ 601.3 ng/ml [506.6, 712.9]).

Figure 2. The pharmacokinetic curve of IVM with mosquito blood feeding times and interpolated IVM doses

The IVM pharmacokinetic curve was constructed by plotting mean data points (± SEM) from data previously published by Elkassaby (Elkassaby, 1991). IVM MDA was modeled to begin at 12:00 hrs. (Time = 0 hrs.). Mean mosquito feeding times were then matched to the curve and used to interpolate doses imbibed at these same times by fitting the decay portion of the pharmacokinetic curve to the Two-phase Exponential Decay model with non-linear regression (Goodness of Fit R² = 0.9999). Modeled times of An. gambiae s.s. blood feeding are designated by the red arrows (12 hrs. = 26.21 ng/ml, 36 hrs. = 11.73 ng/ml, 60 hrs. = 6.58 ng/ml, 84 hrs. = 4.03 ng/ml, 108 hrs. = 2.75 ng/ml, 132 hrs. = 2.1 ng/ml, 156 hrs. = 1.78 ng/ml).

Figure 3. Re-blood feeding frequency of An. gambiae s.s. after a primary blood meal that contained IVM

Re-blood feeding frequency was assessed for 2 days post emergence (left panels - A, C & E) and 8 days post emergence (right panels - B, D & F) An. gambiae s.s. that initially fed on 26.21 ng/ml (A & B), 11.73 ng/ml (C & D), and 6.58 ng/ml (E & F) IVM concentrations. Panels in which mosquitoes exhibited significantly altered re-feeding frequencies compared to controls are marked with an asterisk (Logrank Test).

Figure 4. Time to defecation of 2 day post emergence An. gambiae s.s. following a primary blood meal that contained IVM

Time to defecation was assessed for 2 days post emergence An. gambiae s.s. that initially fed on 26.21 ng/ml (A), 11.73 ng/ml (B), and 6.58 ng/ml (C) IVM concentrations. Panels in which mosquitoes exhibited significantly altered defecation times compared to controls are marked with an asterisk (Logrank Test).

Figure 5. Percent survivorship of An. gambiae s.s. that fed on interpolated IVM doses

Two day post emergence (A) and 8 days post emergence (B) An. gambiae s.s. survivorship after feeding on interpolated IVM doses. Data are presented as box and whiskers plots and represent 3–8 independent replicates per group, each containing between 10–100 mosquitoes/replicate.

Figure 6. Percent survivorship of An. gambiae s.s. after imbibing two consecutive blood meals that contained IVM

Two days post emergence An. gambiae s.s. were given two consecutive blood meals (3 days apart) that contained IVM concentrations corresponding to those interpolated from the IVM pharmacokinetic curve, and then monitored for survivorship over time. Panel A represents one experiment and Panels B and C represent the sum of two replicates. Mosquitoes that exhibited significantly reduced survival (Logrank Test) compared to controls (in which the 2º blood meal contained only PBS) are marked with an asterisk.