Mosquito ageing modulates the development, virulence and transmission potential of pathogens

Abstract

Host age variation is a striking source of heterogeneity that can shape the evolution and transmission dynamic of pathogens. Compared with vertebrate systems, our understanding of the impact of host age on invertebrate-pathogen interactions remains limited. We examined the influence of mosquito age on key life-history traits driving human malaria transmission. Females of Anopheles coluzzii, a major malaria vector, belonging to three age classes (4-, 8- and 12-day-old), were experimentally infected with Plasmodium falciparum field isolates. Our findings revealed reduced competence in 12-day-old mosquitoes, characterized by lower oocyst/sporozoite rates and intensities compared with younger mosquitoes. Despite shorter median longevities in older age classes, infected 12-day-old mosquitoes exhibited improved survival, suggesting that the infection might act as a fountain of youth for older mosquitoes specifically. The timing of sporozoite appearance in the salivary glands remained consistent across mosquito age classes, with an extrinsic incubation period of approximately 13 days. Integrating these results into an epidemiological model revealed a lower vectorial capacity for older mosquitoes compared with younger ones, albeit still substantial owing to extended longevity in the presence of infection. Considering age heterogeneity provides valuable insights for ecological and epidemiological studies, informing targeted control strategies to mitigate pathogen transmission.

Keywords: ageing; malaria; mosquito; pathogen transmission.

Figure 1.

Schematic of the design used in experiments 1 and 2. Mosquitoes belonging to one of three age classes (red: old; blue: middle-aged; black: young) were challenged with one of four parasite isolates 12 days following the emergence of the old mosquito batch. There was a 4 day difference among the three age classes. When mosquitoes received the infectious blood meal on day 12 (purple), the old batch was 12 days old, the middle-aged batch 8 days old and the young batch 4 days old. On day 20, corresponding to 8 days post-infectious blood meal, mosquitoes were dissected to count the number of developing oocyst in their midgut. On day 26, corresponding to 14 days post-infectious blood meal, the presence of parasite in mosquito head/thorax was assessed using qRT-PCR. On day 12 (infectious blood meal), 12-day-old mosquitoes were not only older than 8- and 4-day-old mosquitoes, they had also received more blood meals from rabbits (green). The old batch received a total of three blood meals, middle-aged two blood meals, and young a single infectious blood meal.

Figure 2.

Effects of mosquito age on competence for Plasmodium falciparum. (a) Oocyst prevalence, expressed as the proportion of mosquitoes exposed to an infectious blood meal and harbouring at least one oocyst in their midgut at 8 dpibm. (b) Oocyst intensity, expressed as the number of developing oocysts in the guts of infected females at 8 dpibm. (c) Sporozoite prevalence, expressed as the proportion of mosquitoes exposed to an infectious blood meal harbouring disseminated sporozoites in their heads/thoraces at 14 dpibm. (d) Sporozoite intensity, expressed as the estimated number of sporozoites. The y-axis in (b) is on a log₁₀ scale. Two experimental replicates were performed, each with two distinct wild parasite isolates. n = number of dissected mosquitoes (ca 50 for each of the four parasite isolates and age classes). Different letters (a,b) denote statistically significant differences based on multiple pairwise post hoc tests.

Figure 3.

Effects of mosquito age and infection status on mosquito survival. Survival was recorded daily from day 1 post-infectious blood meal until the death of all mosquitoes from a total of 48 paper cups containing ca 15 mosquitoes each. The sample sizes of uninfected control, exposed-infected and exposed-uninfected mosquitoes were 76, 99, 26 (4-day-old); 108, 101, 16 (8-day-old) and 111, 87, 33 (12-day-old), respectively. N total = 657. Solid, dashed and dotted lines show uninfected control, exposed-uninfected and exposed-infected mosquitoes, respectively.

Figure 4.

Effects of mosquito age on the parasite's EIP. (a) Proportion of infected mosquitoes with ruptured oocysts (± 95% CI) from 8 to 14 dpibm, expressed as the number of mosquitoes with at least one ruptured oocyst out of the total number of infected mosquitoes (i.e. harbouring intact and/or ruptured oocysts) for two age classes (4-day-old in black and 12-day-old mosquitoes in red). (b) Fraction of ruptured oocysts (± 95% CI), expressed as the number of ruptured oocysts out of the total number of oocysts (intact + ruptured). (c) Proportion of infected mosquitoes with sporozoites in their salivary glands (± 95% CI), expressed as the number of oocyst-infected mosquitoes harbouring sporozoites in their salivary glands out of the total number of infected mosquitoes. The lines represent best-fit logistic regression curves for each age class. Three parasite isolates were used over two experimental replicates. (a–c) Sample size = 21–31 mosquitoes per day per age class (median = 27.5). In panels (a) and (c) the sample sizes indicate total mosquito numbers while in panel (b) total oocyst number.

Figure 5.

Impact of mosquito age on vectorial capacity. The data for ‘4-day-old observed’ mosquitoes (black boxplot and line) and ‘12-day-old observed’ (red) reflect the age-dependent model of vectorial capacity, derived from actual survival, competence and EIP data in this study. The scenario ‘12-day-old with survival of uninfected controls' (blue) is a simulation where the survival values of infected 12-day-old mosquitoes were replaced with those of 12-day-old uninfected controls, while competence and EIP values remained unchanged. The scenario ‘12-day-old with survival of exposed-uninfected’ (green) is another simulation where the survival values of infected 12-day-old mosquitoes were replaced with those of 12-day-old exposed-uninfected individuals, while competence and EIP values remained unchanged.