Exsheathment and midgut invasion of nocturnally subperiodic Brugia malayi microfilariae in a refractory vector, Aedes aegypti (Thailand strain)

Abstract

Exsheathment and midgut invasion of nocturnally subperiodic Brugia malayi microfilariae were analyzed using light and scanning electron microscopy in a refractory vector, Aedes aegypti (Thailand strain). Results showed that exsheathed microfilariae represented only approximately 1% of the total microfilaria midguts dissected at 5-min post-infected blood meal (PIBM). The percentage of exsheathed microfilariae found in midguts progressively increased to about 20, 60, 80, 90, and 100% at 1-, 2-5-, 6-12-, 18-36-, and 48-h PIBM, respectively. Importantly, all the microfilariae penetrating the mosquito midguts were exsheathed. Midgut invasion by the exsheathed microfilariae was observed between 2- and 48-h PIBM. SEM analysis revealed sheathed microfilariae surrounded by small particles and maceration of the microfilarial sheath in the midguts, suggesting that the midguts of the refractory mosquitoes might have protein(s) and/or enzyme(s) and/or factor(s) that induce and/or accelerate exsheathment. The microfilariae penetrated the internal face of the peritrophic matrix (PM) by their anterior part and then the midgut epithelium, before entering the hemocoel suggesting that PM was not a barrier against the microfilariae migrating towards the midgut. Melanized microfilariae were discovered in the hemocoel examined at 96-h PIBM suggesting that the refractory mosquitoes used melanization reactions against this parasite. This study provided evidence that A. aegypti (Thailand strain) has refractory mechanisms against B. malayi in both midgut and hemocoel.

Immune responses of Aedes togoi, Anopheles paraliae and Anopheles lesteri against nocturnally subperiodic Brugia malayi microfilariae during migration from the midgut to the site of development

“…Our results agree with previous studies of refractory vectors [ 19 ]. Intakhan et al [ 19 ] found that B . malayi cast off their sheaths only in the midgut of refractory Ae .…”

“…paraliae . However, during 48–72 hours post-blood meal, all exsheathed microfilariae successfully migrated out of the midgut which supports the belief that PM does not serve as a physical barrier to nocturnally B. malayi in these three mosquito species and corresponds with previous studies [ 19 , 22 , 42 ]. Kato et al [ 42 ] used RNAi to knock-down chitin synthase and demonstrated that PM does not affect the development of B. pahangi or the dissemination of dengue virus as well as infectivity of Plasmodium gallinaceum in Ae .…”

“…The midgut is one of the first tissue barriers preventing pathogens from entering the body cavity. Numerous studies have suggested that the exsheathment of mf either occurred exclusively in the lumen of the midgut [ 17 – 19 ] or the haemocoel as well as the midgut lumen [ 20 – 22 ]. Likewise, the study by Chen & Shih [ 23 ] demonstrated that the exsheathment of B. pahangi microfilariae occurred in the lumen of midgut and haemocoel of susceptible and refractory strains of Aedes aegypti .…”

Brugia malayi microfilariae transport alphaviruses across the mosquito midgut

“…However, a few studies have shown that in certain strains of Ae . aegypti , exsheathment of ingested MF may occur prior to midgut penetration [ 17 , 26 ]. The degree to which MF exsheath before or after midgut penetration within a particular mosquito species or strain may affect the magnitude of MF enhancement that occurs and could explain the species differences observed in our earlier studies ( Table 1 ).…”

Wolbachia depletion blocks transmission of lymphatic filariasis by preventing chitinase-dependent parasite exsheathment

“…Lymphatic filarial nematodes release their mf encased within a chitinous sheath, the remains of the nematode eggshell. Shedding of the microfilarial sheath is known to occur both in the mosquito midgut and hemocoel and is essential for further larval development ( 11 , 12 ). Successful colonization of a permissible vector requires these nematodes to penetrate the mosquito gut wall ( 12 , 13 ) and migrate to flight muscle tissues, where they undergo two molts to develop into infective third-stage larvae (L3) ( 14 ).…”