Human Onchocerciasis in the Amazonian Area of Southern Venezuela: Spatial and Temporal Variations in Biting and Parity Rates of Black Fly (Diptera: Simuliidae) Vectors

Abstract

We investigated some entomological factors underlying altitudinal prevalence variation in the Venezuelan Amazonia human onchocerciasis focus. Spatial and temporal variation in relative abundance, daily biting rate, proportion of parous flies, and monthly parous biting rate were studied for the three main simuliid vectors (based on their vectorial competence: Simulium oyapockense s.l. Floch & Abonnenc approximately = S. incrustatum Lutz << S. guianense s.l. Wise). Yanomami villages were selected among sentinel communities of the ivermectin control program, representing hypo- to hyperendemicity conditions of infection. Spatial variation was explored via increasing village altitude on two river systems (A: Ocamo-Putaco and B: Orinoco-Orinoquito). Temporal variation was studied between 1995 and 1999 by sampling the biting population during dry and rainy mouths. Environmental variables included monthly rainfall and maximum river height. Simuliid species composition itself varied along the altitudinal and prevalence gradient. S. oyapockense s.l. prevailed below 150 m. Above this altitude and up to 240 m, S. incrustatum and S. guianense s.l. became more frequently and evenly collected along A but not along B, where S. incrustatum remained absent. The daily biting rate of S. oyapockense s.l. was higher during the dry season along A, whereas the converse took place along B. Daily biting rate of S. incrustatum was lowest during early rains. By contrast, the daily biting rate of S. guianense s.l. was highest during this period. There was a significant negative cross-correlation between proportion of parous of S. oyapockense s.l. and river height (2 and 3 mo lagged), whereas this variable (1 and 2 mo lagged) was positively correlated with the proportion of parous flies for S. incrustatum. Monthly parous biting rate values suggest that the months contributing most to onchocerciasis transmission in the area are likely to be the dry season and the transition periods between seasons.

Rapid Suppression of Onchocerca volvulus Transmission in Two Communities of the Southern Chiapas Focus, Mexico, Achieved by Quarterly Treatments with Mectizan

“…The geometric mean and the CIs (as they are in the same units as the means) were multiplied by 10 to obtain the CIs for the daily biting rates. To estimate the confidence interval of the ATP, the seasonal biting rate was multiplied by the CI of the proportion of infective flies 17,18…”

Human infection patterns and heterogeneous exposure in river blindness

“…in the Guatemalan fincas (29) and reported for Simulium guianense s.l. and Simulium incrustatum in mesoendemic and hyperendemic Amazonian areas (30)(31)(32).…”

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex

“…In Neotropical America, knowledge of adult species distributions, biting rates and parous rates recently increased [31][32][33][34], motivated by the necessity of the Onchocerciasis Elimination Program for the Americas (OEPA) to understand where and when to conduct entomological monitoring to evaluate elimination efforts. Similarly detailed data on vector distribution and bionomics will be crucial for understanding the epidemiology of infection and transmission patterns in parts of Africa shifting towards elimination goals [17].…”

“…Similarly detailed data on vector distribution and bionomics will be crucial for understanding the epidemiology of infection and transmission patterns in parts of Africa shifting towards elimination goals [17]. Diurnal and locality-specific variations in simuliid biting densities have been associated with variations in air temperature, relative humidity, and river levels as well as with forest versus savannah ecology [32][33][34][35][36][37].…”