Artificial nighttime lighting impacts Plasmodium falciparum mature stage V gametocytes infectivity in Anopheles stephensi

Abstract

Background: Malaria is one of the most important vector-borne diseases of humans with an estimated 241 million cases worldwide in 2020. As an urban and periurban mosquito species, Anopheles stephensi is exposed to artificial human stimuli like light that can alter many aspects of mosquito behaviour, physiology and metabolism. Therefore, fluctuations in the light environment may influence the host, parasite and/or mosquito biology and hence modulate risk for disease transmission. In this study, the effect of artifitial light at night on mosquito infectivity by Plasmodium falciparum during the first hours of blood digestion was tested.

Methods: A total of three independent standard membrane feeding assays were performed to artificially fed septic and aseptic mosquitoes with P. falciparum infected blood. After blood feeding, females were transferred to incubators with different photoperiod cycles, so digestion occurred under day artificial light or dark. At 7 and 16 days post blood feeding, mosquitoes were dissected for midguts and salivary glands, respectively. Percentage of mosquitoes fed, percentage of prevalence and P. falciparum oocyst intensity between septic and aseptic mosquitoes in the two different photoperiod regimes, were compared using a Kruskal-Wallis test followed by a Dunn´s multiple comparison test .

Results: The exposition of mosquitoes to light after they took an infected blood meal has a negative effect on the successful progression of P. falciparum in the mosquito midgut. Antibiotic treatment significantly incremented the number of oocysts per midgut. Photophase significantly reduced the median oocyst intensity in both septic and aseptic mosquitoes. The percentage of oocyst reduction, understood as the percentage of reduction in the mean oocyst intensity of the parasite in the mosquito midgut between photophase and scotophase, was 51% in the case of aseptic mosquitoes and 80% for septic mosquitoes, both in the photophase condition.

Conclusion: Although there are still many gaps in the understanding of parasite-mosquito interactions, these results support the idea that light can, not only, influence mosquito biting behaviour but also parasite success in the mosquito midgut. Hence, light can be considered an interesting additional mosquito-control strategy to reduce mosquito-borne diseases.

Keywords: Anopheles stephensi; Artificial light at night; Malaria; Plasmodium falciparum; Prevalence; SMFA.

Fig. 1

Total percentage of mosquitoes fed on infected blood with P. falciparum mature stage V gametocytes through the SMFA. Graphic shows the total percentage of mosquitoes fed in three independent experiments. The bars show the mean ± SD. Aseptic = Mosquitoes treated with antibiotics since emergence until 24 h before blood feeding; Septic = Mosquitoes non-treated with antibiotics; Photophase = After blood feeding, mosquitoes were transferred into an incubator in light phase; Scotophase = After blood feeding, mosquitoes were transferred into an incubator in dark phase. Kruskal–Wallis test followed by a Dunn Test for Multiple Comparisons was used to compare the different experimental groups (groups that were statistically significant are represented with an asterisk: * p < 0.1, ** p < 0.01, *** p < 0.001, **** p < 0.0001). Barless conditions were also subjected to analysis but were not significantly different

Fig. 2

Prevalence of infection of mosquitoes fed on infected blood with P. falciparum mature stage V gametocytes through the SMFA. Graphic shows the prevalence of infection of mosquitoes fed in three independent experiments. The bars show the mean ± SD. Aseptic = Mosquitoes treated with antibiotics since emergence until 24 h before blood feeding; Septic = Mosquitoes non-treated with antibiotics; Photophase = After blood feeding, mosquitoes were transferred into an incubator in light phase; Scotophase = After blood feeding, mosquitoes were transferred into an incubator in dark phase. Kruskal–Wallis test followed by a Dunn Test for Multiple Comparisons was used to compare the different experimental groups (groups that were statistically significant are represented with an asterisk: * p < 0.1, ** p < 0.01, *** p < 0.001, **** p < 0.0001). Barless conditions were also subjected to analysis but were not significantly different

Fig. 3

P. falciparum oocyst intensity of mature GC in the SMFA. Each dot represents the total number of oocysts in a single mosquito midgut. The black box shows the interquartile range, the line in the box depicts the median oocyst intensity of infection, and the top and bottom whiskers show the highest and the lowest value respectively. Figure represents results from three independent experiments. Kruskal–Wallis test followed but a Dunn´s Multiple Comparison Test was used to compare the statistical significance between the different experimental groups (groups which showed a statistically significant difference are represented with asterisks: p < 0.1, p < 0.01, p < 0.001, p < 0.0001 is represented by *, **, ***, **** respectively). Barless conditions were also subjected to analysis but were not significantly different. The table below the graph depicts the total number of full-fed mosquitoes dissected per condition and the % mean oocyst intensity, reduction in mean oocyst intensity and the % of block in transmission. Graphics represent results from three independent experiments