Human onchocerciasis in the Amazonian area of southern Venezuela: Spatial and temporal variations in biting and parity rates of black fly (Diptera : Simuliidae) vectors

We investigated some entomological factors underlying altitudinal prevalenc e 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 oy- a pockense s.l. Floch & Abonnenc approximate to S. incrustatum Lutz much less than S, guianense s.l. Wise). Yanomami villages were selected among sentin el communities of the ivermectin control program, representing hypo hyperen demicity conditions of infection. Spatial variation was explored via increa sing village altitude on two river systems (A: Ocamo-Putaco and B: Orinoco- Orinoquito). Temporal variation was studied between 1995 and 1999 by sampli ng the biting population during dry and rainy months. Environmental variabl es included monthly rainfall and maximum river height. Simuliid species com position itself varied along the altitudinal and prevalence gradient. S, oy aporkense 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 coll ected, along A but not along B, where S, incrustatum remained absent. The d aily biting rate of S. oyapockense s.l, was higher during the dry season al ong A, whereas tile converse took place along B. Daily biting rate of S, in crustatum was lowest during early rains. By contrast, the daily biting rate of S. guianense s.l. was highest during this period. There was a significa nt negative cross-correlation between proportion of parous of S. oyapockens e 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 f or S. incrustatum. Monthly parous biting rate values suggest that the month s contributing: most to onchocerciasis transmission in the area are likely to be the dry season and the transition periods between seasons.