Effect of seasonal malaria chemoprevention plus azithromycin on Plasmodium falciparum transmission: gametocyte infectivity and mosquito fitness

Abstract

Background: Seasonal malaria chemoprevention (SMC) consists of administration of sulfadoxine-pyrimethamine (SP) + amodiaquine (AQ) at monthly intervals to children during the malaria transmission period. Whether the addition of azithromycin (AZ) to SMC could potentiate the benefit of the intervention was tested through a double-blind, randomized, placebo-controlled trial. The effect of SMC and the addition of AZ, on malaria transmission and on the life history traits of Anopheles gambiae mosquitoes have been investigated.

Methods: The study included 438 children randomly selected from among participants in the SMC + AZ trial and 198 children from the same area who did not receive chemoprevention. For each participant in the SMC + AZ trial, blood was collected 14 to 21 days post treatment, examined for the presence of malaria sexual and asexual stages and provided as a blood meal to An. gambiae females using a direct membrane-feeding assay.

Results: The SMC treatment, with or without AZ, significantly reduced the prevalence of asexual Plasmodium falciparum (LRT X²₂ = 69, P < 0.0001) and the gametocyte prevalence (LRT X²₂ = 54, P < 0.0001). In addition, the proportion of infectious feeds (LRT X²₂ = 61, P < 0.0001) and the prevalence of oocysts among exposed mosquitoes (LRT X²₂ = 22.8, P < 0.001) was reduced when mosquitoes were fed on blood from treated children compared to untreated controls. The addition of AZ to SPAQ was associated with an increased proportion of infectious feeds (LRT X²₁ = 5.2, P = 0.02), suggesting a significant effect of AZ on gametocyte infectivity. There was a slight negative effect of SPAQ and SPAQ + AZ on mosquito survival compared to mosquitoes fed with blood from control children (LRTX²₂ = 330, P < 0.0001).

Conclusion: This study demonstrates that SMC may contribute to a reduction in human to mosquito transmission of P. falciparum, and the reduced mosquito longevity observed for females fed on treated blood may increase the benefit of this intervention in control of malaria. The addition of AZ to SPAQ in SMC appeared to enhance the infectivity of gametocytes providing further evidence that this combination is not an appropriate intervention.

Keywords: Azithromycin; Gametocytes; Seasonal malaria chemoprevention; Transmission.

Fig. 1

Summary of the transmission trial. The numbers represent the number of volunteers included at each step. DMFA: Direct membrane feeding assay

Fig. 2

Effects of SPAQ and SPAQ + AZ on Plasmodium falciparum infection in humans. A The prevalence (± 95% CI) of asexual stage parasitaemia for each treatment and the control group. B The density of asexual stage parasites (number of parasites/µl of blood in infected blood collections) for each treatment and the control group. Each point represents a blood sample from P. falciparum-positive slides. The horizontal line represents the median value of parasite density for each of the three groups, and the upper and lower boundaries of the box indicate the 75th and 25th percentile, respectively. Note that the y-axis is on a log10 scale. C The prevalence (± 95% CI) of gametocyte for each treatment and the control group as measured by microscopy; D The density of gametocyte (number of gametocytes/µl of blood in gametocyte-positive blood collections) for each group. Note that the y-axis is on a log₁₀ scale. The presence and number of asexual stages or gametocytes in samples was determined by microscopic observation. Different lowercase letters above the bars denote statistically significant differences based on multiple pair-wise post-hoc tests. NS: Not significant

Fig. 3

Effects of SPAQ and SPAQ + AZ on Plasmodium falciparum infectivity from humans to mosquitoes. A The proportion (± 95% CI) of feeds resulting in at least one successful mosquito infection (i.e., a minimum of one mosquito harbouring a minimum of one oocyst) for each treatment group. There were 171 feeds for the control, 182 for SPAQ and 190 for the SPAQ-AZ treatment and an average of 38.87 ± 0.48 (median = 41, range = 5–51) mosquitoes were dissected per feed (n total = 21,108 mosquitoes). B Oocyst prevalence (± 95% CI) defined as the total number of mosquitoes fed with blood drawn from each study group harbouring at least one oocyst in their midgut out of the number of dissected mosquitoes from this group. C Oocyst density defined as the number of oocysts observed in infected mosquitoes for each group. The horizontal line represents the median value of density for each of the three study groups, and the upper and lower boundaries of the box indicate the 75th and 25th percentile, respectively. D Oocyst density following infectious feeds on samples from the control group, SPAQ group and SPAQ + AZ group. Note that the y-axis on panels (C) and (D) is on a log₁₀ scale. Different lowercase letters above the bars denote statistically significant differences based on multiple pair-wise post-hoc tests. NS: Not significant

Fig. 4

Effect of SPAQ and SPAQ + AZ on mosquito life history traits. A Mosquito blood-feeding rate, expressed as the proportion (+ 95% CI) of engorged mosquitoes out of the total number exposed to the blood meal; B Mosquito blood-meal size, expressed as the quantity of haematin in µg excreted by individual mosquitoes. Note that the y-axis is on a log₁₀ scale. C The daily survival of mosquitoes individually kept in drosophila tubes. D The relationship between mosquito lifespan and blood meal size. Note that the x-axis is on a log₁₀ scale