Encapsulation of microfilariform and older larval stages of Wuchereria bancrofti was observed among seven species of mosquitoes in British Guiana over a 2-year period. In Mansonia humeralis and Psorophora ferox most of the microfilariform 1st-stage larvae were completely enclosed in a hard, thick, dark-brown capsule 12 to 24 hours after the infective blood-meal. In P. ferox encapsulation often occurred within the inflated sheath. In Aedes taeniorhynchus, A. scapularis, and A. serratus, progressive stages were found 12 hours after the blood-meal, from unencapsulated to fully encapsulated microfilariform larvae and, in the latter species, also two slightly encapsulated, dead 2nd-stage larvae. In all mosquitoes encapsulation was associated with the presence of varying amounts of tiny granules, globules, plaques and flake-like debris. These were especially evident in A. angustivittatus and A. scapularis.
Some encapsulated 1st-stage larvae were found, along with normal ones, in 2 (0.01%) of 14,405 Culex pipiens quinquefasciatus of the British Guiana strain and in 10 (9.8%) of 102 C. p. quinquefasciatus of the Barbados (Island) strain. This species is the major host of W. bancrofti in both localities. No encapsulation was ever seen in M. titillans, a secondary vector in British Guiana.
Most of the encapsulated microfilariform larvae were found in the hemocele, and fewer in the stomach. Rarely, encapsulated forms were found in the thorax, only in A. serratus and in the British Guiana and Barbados strains of C. p. quinquefasciatus. Various stages of encapsulation are illustrated by photomicrographs.
Present address: NIH-West Africa Research Laboratory (USPHS, NCI), c/o American Embassy, Accra, Ghana, West Africa.