Ivermectin inhibits the sporogony of Plasmodium falciparum in Anopheles gambiae

Abstract

Background: When ingested in a blood meal, ivermectin has been shown to reduce the survivorship of Anopheles gambiae in the laboratory and field. Furthermore, ivermectin mass drug administrations in Senegal have been shown to reduce the proportion of Plasmodium falciparum-sporozoite-containing An. gambiae. This study addresses whether ivermectin inhibits sporogony of P. falciparum in An. gambiae.

Methods: Anophele gambiae s.s. G3 strain were fed two concentrations of ivermectin (LC25 and LC5) along with P. falciparum NF54 in human blood meals at staggered intervals. Mosquitoes ingested ivermectin concurrent with parasites (DPI 0), or at three (DPI 3), six (DPI 6), and nine (DPI 9) days post parasite ingestion, or three days prior (DPI -3) to parasite ingestion. Mosquitoes were dissected at seven, twelve or fourteen days post parasite ingestion and either oocyst or sporozoite prevalence was recorded. To determine if P. falciparum sporozoite-containing An. gambiae were more susceptible to ivermectin than uninfected controls, survivorship was recorded for mosquitoes which ingested P. falciparum or control blood meal on DPI 0 and then a second blood meal containing ivermectin (LC25) on DPI 14.

Results: Ivermectin (LC25) co-ingested (DPI 0) with parasites reduced the proportion of An. gambiae that developed oocysts (χ2 = 15.4842, P = 0.0002) and sporozoites (χ2 = 19.9643, P < 0.0001). Ivermectin (LC25) ingested DPI 6 (χ2 = 8.5103, P = 0.0044) and 9 (χ2 = 14.7998, P < 0.0001) reduced the proportion of An. gambiae that developed sporozoites but not when ingested DPI 3 (χ2 = 0.0113, P = 1). Ivermectin (LC5) co-ingested (DPI 0) with parasites did not reduce the proportion of An. gambiae that developed oocysts (χ2 = 4.2518, P = 0.0577) or sporozoites (χ2 = 2.3636, P = 0.1540), however, when ingested DPI -3 the proportion of An. gambiae that developed sporozoites was reduced (χ2 = 8.4806, P = 0.0047). Plasmodium falciparum infection significantly reduced the survivorship of An. gambiae that ingested ivermectin (LC25) on DPI 14 compared to control mosquitoes that ingested a primary blood meal without parasites (χ2 = 4.97, P = 0.0257).

Conclusions: Ivermectin at sub-lethal concentrations inhibits the sporogony of P. falciparum in An. gambiae. These findings support the utility of ivermectin for P. falciparum transmission control.

Figure 1

Proportion of Anopheles gambiae that were infected with Plasmodium falciparum when ivermectin (LC₂₅) was co-ingested with parasites (DPI 0). When ivermectin (LC₂₅) was co-ingested with P. falciparum at DPI 0 it reduced the prevalence or proportion of An. gambiae that developed oocysts at DDPI 7 (χ² = 15.4842, P = 0.0002, n = 89) and sporozoites at DDPI 12 (χ² = 13.4741, P = 0.0003, n = 97) and at DDPI 14 (χ² = 19.9643, P < 0.0001, n = 136). * significantly different P < 0.05, DPI = days post infection, DDPI = dissected at day post infection.

Figure 2

Proportion of Anopheles gambiae that contained sporozoites when ivermectin (LC₂₅) was ingested DPIs 3, 6, 9. When ivermectin (LC₂₅) was fed at DPI 3 post parasites it did not reduce the proportion of An. gambiae that contained sporozoites at DDPI 12 (χ² = 4.3718, P = 0.0556, n = 112) or at DDPI 14 (χ² = 0.0113, P = 1, n = 145). However, when ivermectin (LC₂₅) was fed at DPIs 6 and 9 post parasites it reduced the proportion of An. gambiae that contained sporozoites on DPI 6 at DDPI 12 (χ² = 12.4919, P = 0.0006, n = 95) and at DDPI 14 (χ² = 8.5103, P = 0.0044, n = 135) and on DPI 9 at DDPI 12 (χ² = 16.2056, P < 0.0001, n = 132) and at DDPI 14 (χ² = 14.7998, P < 0.0001, n = 161). * significantly different P < 0.05, DPI = days post infection, DDPI = dissected at day post infection.

Figure 3

Proportion of Anopheles gambiae that were infected with Plasmodium falciparum when ivermectin (LC₅) was co-ingested with parasites (DPI 0). When ivermectin (LC₅) was co-ingested with P. falciparum on DPI 0 it did not reduce the proportion of An. gambiae that developed oocysts at DDPI 7 (χ² = 4.2518, P = 0.0577, n = 115) or sporozoites at DDPI 12 (χ² = 0.5937, P = 0.5101, n = 149) or at DDPI 14 (χ² = 2.3636, P = 0.1540, n = 156).

Figure 4

Proportion of Anopheles gambiae that contained sporozoites when ivermectin (LC₅) was ingested DPI −3. When ivermectin (LC₅) was ingested three days before parasites (DPI −3) it reduced the proportion of An. gambiae that contained sporozoites at DDPI 12 (χ² = 4.6333, P = 0.0406, n = 95) and at DDPI 14 (χ² = 8.4806, P = 0.0047, n = 155).

Figure 5

Survivorship of Anopheles gambiae that ingested ivermectin (LC₂₅) at DPI 14. Plasmodium falciparum infection significantly reduced the survivorship of An. gambiae that ingested ivermectin on DPI 14 compared to control mosquitoes that received a primary blood meal without parasites (χ² = 4.97, P = 0.0257, n = 450).