Transmission-blocking activities of artesunate, chloroquine, and methylene blue on Plasmodium vivax gametocytes

Abstract

Plasmodium vivax is now the main cause of malaria outside Africa. The gametocytocidal effects of antimalarial drugs are important to reduce malaria transmissibility, particularly in low-transmission settings, but they are not well characterized for P. vivax. The transmission-blocking effects of chloroquine, artesunate, and methylene blue on P. vivax gametocytes were assessed. Blood specimens were collected from patients presenting with vivax malaria, incubated with or without the tested drugs, and then fed to mosquitos from a laboratory-adapted colony of Anopheles dirus (a major malaria vector in Southeast Asia). The effects on oocyst and sporozoite development were analyzed under a multi-level Bayesian model accounting for assay variability and the heterogeneity of mosquito Plasmodium infection. Artesunate and methylene blue, but not chloroquine, exhibited potent transmission-blocking effects. Gametocyte exposures to artesunate and methylene blue reduced the mean oocyst count 469-fold (95% CI: 345 to 650) and 1,438-fold (95% CI: 970 to 2,064), respectively. The corresponding estimates for the sporozoite stage were a 148-fold reduction (95% CI: 61 to 470) and a 536-fold reduction (95% CI: 246 to 1,311) in the mean counts, respectively. In contrast, high chloroquine exposures reduced the mean oocyst count only 1.40-fold (95% CI: 1.20 to 1.64) and the mean sporozoite count 1.34-fold (95% CI: 1.12 to 1.66). This suggests that patients with vivax malaria often remain infectious to anopheline mosquitos after treatment with chloroquine. Use of artemisinin combination therapies or immediate initiation of primaquine radical cure should reduce the transmissibility of P. vivax infections.

Keywords: Anopheles dirus; Plasmodium vivax; Thailand; antimalarials; gametocytes; membrane-feeding assay; transmission.

Fig 1

Effects of chloroquine, methylene blue, and artesunate on the development of P. vivax in An. dirus mosquitos. (A–C) Median oocyst count, (D–F) oocyst index, (G–I) median sporozoite count, and (J–L) sporozoite index. Values in the control and treated replicates were collated by assay run.

Fig 2

Relationship between the mean parasite count and proportion of Plasmodium-infected mosquitos in the assay. (A) Paired mean number of oocysts per mosquito and oocyst index determined in the control and treated replicates; (B) paired mean number of sporozoites per mosquito and sporozoite index. The black line and shaded area show the model-fitted relationship plotted using a₀ and a₁ estimates given by the model output and the corresponding 95% credible interval, respectively.

Fig 3

Inter-experiment variability. Mean estimated oocyst (top panel) and sporozoite (bottom panel) counts (in the untreated state) under the Bayesian multi-level model; points and error bars show the median and 95% credible interval of posterior draws, respectively.

Fig 4

Temporal trend in intra-assay variability. Random variation of the mean oocyst (top panel) and sporozoite (bottom panel) count under the Bayesian multi-level model; points and error bars show the median and 95% credible interval of posterior draws, respectively.