Vector biology and malaria transmission in western Venezuela

The status of all anopheline species reported to occur in western Venezuela is reviewed. A longitudinal study was conducted in three villages in western Venezuela to assess the malaria risk factors determined by the abundance, parous rate, biting activity, sporozoite rate and human blood index of the various potential vector species in relation to weather and human habits. The main method of mosquito sampling was on human baits; three other methods tested did not prove to be effective substitutes. The collections yielded 14 anopheline species, the most abundant being those belonging to the subgenus Nyssorhynchus. Because species identification of adult females with available keys proved to be difficult, linked rearings were undertaken. An. nuneztovari, comprising over 70% of the total anophelines collected, was the most abundant species, followed by An. triannulatus, An. albitarsis s. l. and An. oswaldoi. The anopheline populations showed fluctuations which correlated positively with rainfall and humidity. The four most abundant species showed different diel patterns of biting. The diel peak for An. nuneztovari was close to midnight indoors and outdoors, for An. triannulatus between 1900 and 2000 hours outdoors, for An. albitarsis mainly before midnight indoors and outdoors and for An. oswaldoi outdoors at 1900 hrs, there being an additional smaller peak indoors at midnight. Most of the human population use bed nets, go to bed before 2200 hrs and wake up before 0700 hrs: they are therefore most exposed to the bites of those species that bite early in the night outdoors. All anopheline species in the study area are exophilic. Some anophelines were collected resting on vegetation around houses between 0600 and 0800 hrs but very few An. nuneztovari were found there. The source of blood meals in resting mosquitoes was determined by the ELISA technique. The human blood index for the different species collected showed variations among villages that could not be explained by variation in the ratio of humans to cows in each village. 2 Over 61,000 anophelines were assayed by ELISA to detect P. vivax circumsporozoite protein. The six specimens confirmed as positive belonged to three species: nuneztovari, albitarsis s. l. and oswaldoi. The estimated overall sporozoite rate was 0.0098% (95% confidence limits 0.0036 to 0.0214%). Multiplying this rate by the mean number of bites on the catchers suggests a sporozoite inoculation rate of 10.5 positive bites per person per year. Recommendations for possible improvements in malaria vector control in this area are made taking into account the endophagic and exophilic behaviour of the incriminated vectors, their diel patterns of biting and some aspects of the behaviour of the human population revealed by questionnaires.