The future dispersal of onchocerciasis in Ecuador is dependent on the
distribution of cytotypes of the vector species complex Simulium exiguum.
Over the last 14 years, collections of larvae have been made from over 25
rivers, between 80-1600 m altitude, from various sites on both sides of the
Andes. Analysis of larval polytene chromosomes was used to determine the
distributions of each cytotype. On the western side of the Andes, the
Cayapa cytotype (the only cytotype directly incriminated as a vector) has a
distribution from Santo Domingo de los Colorados northwards. The Quevedo
and Bucay cytotypes occur from Santo Domingo de los Colorados southwards.
On the eastern side of the Andes, the Aguarico cytotype occurs in the Rio
Aguarico and a new cytotype is present in the tributaries of the Rio Napo.
Whether the disease will spread south of Santo Domingo and on the eastern
side of the Andes depends on vector capacity of the cytotypes and the
dispersal patterns of individuals infected with onchocerciasis. At present
the Aguarico, Bucay and Quevedo cytotypes are known to be efficient hosts,
but their biting preferences and biting densities have not yet been