The dissemination potential of Microsporidia MB in Anopheles arabiensis mosquitoes is modulated by temperature

Abstract

Microsporidia MB, a vertically transmitted endosymbiont of Anopheles mosquitoes, shows strong potential as a malaria control agent due to its ability to inhibit Plasmodium development within the mosquito host. To support its deployment in malaria transmission reduction strategies, it is critical to understand how environmental factors, particularly temperature, influence its infection dynamics. In this study, we investigated the impact of four temperature regimes (22 °C, 27 °C, 32 °C, and 37 °C) on Microsporidia MB prevalence and infection intensity by rearing mosquito larvae under controlled laboratory conditions. Our results demonstrate that elevated temperatures, especially 32 °C, significantly enhance both larval growth and Microsporidia MB infection rates. Population growth modeling further indicates that at 32 °C, an infected mosquito population can reach 1000 offspring within 15-35 days, representing a 4.7-, 1.3-, and 1.7-fold increase in dissemination potential compared to 22 °C, 27 °C, and 37 °C, respectively. Although mortality at 32 °C was approximately 20% higher than at 27 °C, this temperature emerged as the most favorable for mass-rearing Microsporidia MB-infected larvae. These findings provide the first insights into temperature-mediated dynamics of Microsporidia MB and support its potential for scalable implementation in malaria-endemic regions.

Keywords: Microsporidia MB; Computational biology; Dissemination potential; Endosymbiosis; Malaria control; Temperature.

Fig. 1

Panel representing the average (A) Pupation rate: The pupation rates of the offspring from MB+ female An. arabiensis differed significantly by temperature, with all other temperatures significantly different from 27 °C. The highest pupation rate was observed at 27 °C, 74.4 (70.14–79.56) %; then 22 °C, 68.4 (63.37–73.37) %; then 32 °C, 55.8 (49.84–60.95) % and low at 37 °C, 14.6 (10.77–18.40) % (Tukey for MB + female An. arabiensis: p_{22 °C–27 °C} = 0.001; p_{27 °C–32 °C} = 0.137, p_{27 °C–37°C} < 0.001; p_{32 °C–37°C} < 0.001; p_{22 °C–32 °C} > 0.05; χ² = 29.39 df = 3, p < 0.001). (B) Age at death: MB− female An. arabiensis’ offspring died two days earlier on average than the offspring from the MB+ female An. arabiensis regardless of the temperature [MB− female An. arabiensis: 4.2 (3.62–4.86) days; MB+ female An. arabiensis: 6.4 (6.07–6.66) days] (χ² = 18.66, df = 1, p < 0.001), except at 27 °C where we found no significant differences between the female An. arabiensis’ groups (Tukey 27 °C: p_MB+=/MB− > 0.05); χ² = 9.98, df = 3, p = 0.02). (C) Time to pupation: In offspring coming from MB- female An. arabiensis, the time to pupation did not vary between 22 and 27 °C (Tukey: p_{22 °C–27 °C} > 0.05), but we observed a two days difference in offspring coming from MB+ female An. arabiensis reared in these temperatures [22 °C: 12.0 (11.70–12.31) days; 27 °C: 10.2 (9.93–10.48) days; 32 °C: 7.4 (6.89–7.42) days; 37 °C: 7.6 (7.25–8.04) days, (Tukey: p_{22 °C–27 °C} < 0.001; χ² = 21.89, df = 3, p < 0.001). (D) Infection rate: The highest infection rate was recorded at 37 °C (52.1%) followed by 27 °C (48.7%), 32 °C (46.7%) and 22 °C (30.8%), there was no significant difference in pupation across all temperature regimes (Tukey: estimate_{27 °C–22°C} = 0.68, p_{27 °C–22 °C} = 0.001; p_{27 °C–32°C} and p_{27 °C–37 °C} > 0.05; χ² = 9.99, df = 1, p = 0.001) (E) Microsporidia MB intensity: The lowest Microsporidia MB intensity in the offspring was observed at 27 °C which was significantly different from the intensity in those maintained at 37 °C [22 °C: 2.2 (0.34–4.01) ratio_MB18S/S7; 27 °C: 1.0 (0.55–1.43) ratio_MB18S/S7; 32 °C: 2.2 (1.27–3.11) ratio_MB18S/S7; 37 °C: 2.55 (1.28–3.75) ratio_MB18S/S7; (Tukey: p_{22 °C–37 °C} < 0.001; p_{27 °C–37 °C} = 0.001; χ² = 19.40, df = 3, p < 0.001)]. At 27 °C and 37 °C, the development time was negatively correlated to Microsporidia MB intensity in the offspring. At 27 °C faster pupation led to a 45% increase in Microsporidia MB intensity, while at 22 °C, delayed pupation increased intensity by the same amount [y₍₂₇₎ = 9.7 − 0.467x, r² = 0.12; y₍₂₂₎ = 11.4 + 0.311x, r² = 0.13; y₍₃₇₎ = 7.75 − 0.252x, r² = 0.23]. The intensity at 32 °C was unaffected by pupation time [y₍₃₂₎ = 7.33 + 0.00623x, r² < 0.01]. (data log transformed for better data visualization) in offspring coming from non-infected (MB-, lighter colours) and infected female An. arabiensis (MB+ , darker colours) reared in 4 temperature treatments: 22 °C (blue bars), 27 °C (tan bars), 32 °C (green bars) or 37 °C (emerald bars). Average pupation rates are calculated out of the total count of offspring, average larval death ages are calculated for larvae that died, average times to pupation were calculated for all individuals that pupated, offspring infection rates were calculated for all individuals coming from infected female An. arabiensis, and average Microsporidia MB intensities were calculated for all MB infected offspring. The error bars show 95% confidence intervals.

Fig. 2

Panel representing the effects of 4 different temperature treatments: 22 °C (blue colour), 27 °C (tan colour), 32 °C (green colour) or 37 °C (emerald colour) on (A) The probabilities modelled using a gaussian function that an offspring is infected, survives to age x, and pupates at age x and (B–D) represent the population growth of MB+ offspring starting with an initial population of 10 MB+ female An. arabiensis and progressing over a 100-day period across the 4 four temperature treatments considering different fecundity levels; 33, 66 and 99 offspring per female An. arabiensis, respectively. Each growth curve incorporates a sex ratio of 0.5 (indicating an equal proportion of female offspring), a generation cycle and is capped by a carrying capacity K = 1000, illustrating how temperature affects the speed and likelihood of reaching the target population over time.