Defense reaction of mosquitoes to filarial worms: role of the microfilarial sheath in the response of mosquitoes to inoculated Brugia pahangi microfilariae

The melanization response of Aedes trivittatus and A. aegypti (black-eyed Liverpool strain) against intrathoracically inoculated sheathed and chemically exsheathed Brugia pahangi microfilariae (mff) was assessed daily through 5 days postinoculation (PI). Response of A. aegypti against exsheathed mff was significantly reduced on all days compared with the response against sheathed mff, and a significantly greater percentage of exsheathed mff were alive through 4 days PI than were sheathed mff. The melanization response of A. trivittatus was nearly 100% effective against either sheathed or exsheathed mff by Day 2 PI. When mff were allowed to migrate through A. aegypti midguts in vitro before inoculation into intact A. aegypti, nearly 94% ($\text{120}\text{128}$) of the parasites recovered had avoided the response and were developing. Penetration of A. trivittatus midguts in vitro by mff before inoculation into intact A. trivittatus did not prevent a melanization response. Inoculation of mff into A. trivittatus following A. aegypti midgut penetration, however, resulted in almost 60% ($\text{98}\text{171}$) of the mff avoiding the response and developing as normal L₁ larvae after 5 days PI. The possibility of mff acquiring host antigens during midgut penetration and therefore avoiding recognition as nonself by mosquitoes, and (or) the possibility of the midgut environment modifying or stimulating mff to inhibit the response of mosquitoes are discussed.