Journal of Mosquito Research 2024, Vol.14, No.4, 172-183 http://emtoscipublisher.com/index.php/jmr 175 Emami et al. (2017) found that the type of host from which mosquitoes take a second blood meal significantly influences the parasite load in both the midgut and salivary glands of Anopheles mosquitoes. In their study, Anopheles arabiensis and Anopheles gambiae s.s. that fed on human blood exhibited higher oocyst and sporozoite loads compared to those that fed on cow blood. This effect was particularly pronounced in An. gambiae s.s., where a human blood meal led to significantly higher oocyst numbers and parasite genomes in the salivary glands, suggesting that human blood may enhance the development and transmission potential of malaria parasites. The study highlights the critical role of host selection in malaria transmission dynamics, emphasizing the need for targeted vector control strategies that consider host-mosquito interactions. 4 Pathogen Development and Persistence in Mosquitoes 4.1 Crossing the midgut barrier The midgut of mosquitoes serves as the initial barrier that pathogens must overcome to establish an infection. Upon ingestion of a blood meal containing pathogens, the pathogens encounter the midgut epithelium, which is a critical site for initial infection. The midgut epithelium is composed of various cell types that play roles in digestion, immunity, and maintaining the gut microbiome. Pathogens such as viruses and parasites must navigate through this complex environment to reach the hemocoel. For instance, the Zika virus (ZIKV) must persist in the midgut and then disseminate to secondary tissues, a process facilitated by structural modifications of the midgut basal lamina following blood meal ingestion (Cui et al. 2019). Similarly, the malaria parasite undergoes a series of transformations in the midgut, from gametocytes to ookinetes and then to oocysts, before producing sporozoites that invade the salivary glands. The ability of pathogens to cross the midgut barrier is influenced by their interactions with specific receptors and the mosquito's immune responses, which can limit pathogen propagation (Figure 2) (Hixson et al., 2021). Figure 2 Possible impacts of epithelial dynamics in the mosquito midgut on the hematophagous lifecycle, aging, interactions with gut flora, Plasmodium and arboviral infections (Adopted from Hixson et al., 2021) Image caption: (A) During the post-emergence maturation, JH could stimulate ISCs to proliferate and create new ECs or prompt ECs to endocycle to attain higher ploidy; blood-feeding stimulates the production of 20E, which could stimulate the proliferation of ISCs, the differentiation of new ECs, and transcriptional changes in ECs. (B) Normal microbiota could contribute to aging and basal turnover of EC populations; dying ECs could stimulate ISCs to effect homeostatic replacement; dysbiosis and/or infection with oral bacterial pathogens could accelerate the turnover of epithelial cells; ISC-mediated repair could serve as a disease tolerance mechanism, promoting mosquito survival. Invasion by Plasmodium (C) and/or arboviral pathogens (D) could prompt cell sacrifice mechanisms to limit pathogenic success; ISC proliferation and differentiation could help infected mosquitoes to tolerate epithelial damage (Adopted from Hixson et al., 2021)
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