| Vol. 102(3) June 2007 |
 |
|
Vector bionomics and malaria transmission in the Upper Orinoco River, Southern Venezuela Vol. 102(3): 303-311, June 2007 Magda Magris, Yasmin Rubio-Palis+, Cristóbal Menares, Leopoldo Villegas Ministerio de Salud, Maracay, Venezuela A longitudinal epidemiological and entomological study was carried out in Ocamo, Upper Orinoco River, between January 1994 and February 1995 to understand the dynamics of malaria transmission in this area. Malaria transmission occurs throughout the year with a peak in June at the beginning of the rainy season. The Annual Parasite Index was 1,279 per 1,000 populations at risk. Plasmodium falciparum infections accounted for 64% of all infections, P. vivax for 28%, and P. malariae for 4%. Mixed P. falciparum/P. vivax infections were diagnosed in 15 people representing 4% of total cases. Children under 10 years accounted for 58% of the cases; the risk for malaria in this age group was 77% higher than for those in the greater than 50 years age group. Anopheles darlingi was the predominant anopheline species landing on humans indoors with a biting peak between midnight and dawn. A significant positive correlation was found between malaria monthly incidence and mean number of An. darlingi caught. There was not a significant relationship between mean number of An. darlingi and rainfall or between incidence and rainfall. A total of 7295 anophelines were assayed by ELISA for detection of Plasmodium circumsporozoite (CS) protein. Only An. darlingi (55) was positive for CS proteins of P. falciparum (0.42%), P. malariae (0.25%), and P. vivax-247 (0.1%). The overall estimated entomological inoculation rate was 129 positive bites/person/year. The present study was the first longitudinal entomological and epidemiological study conducted in this area and set up the basic ground for subsequent intervention with insecticide-treated nets. Key words: Plasmodium falciparum - Plasmodium vivax - Plasmodium malariae - malaria incidence - Anopheles darlingi - biting activity - sporozoite rate - entomological inoculation rate - Venezuela
Malaria is a public health problem in some areas of Venezuela, particularly in the state of Amazonas and among Amerindians of the Yanomami ethnic group who inhabit the Upper Orinoco River in the South of Venezuela near the border with Brazil (Fig. 1) (MSAS 1995, MS 2005). Cross-sectional surveys carried out in this area during 1991 determined that malaria was hyperendemic based on parasitological, clinical, and immunulogi-cal parameters (Marcano et al. 2004). Anopheles (Nyssorhynchus) darlingi Root is considered the most efficient vector of malaria parasites throughout its range of distribution, especially in the Amazon basin because of its high degree of anthro-pophilism and susceptibility to infection with Plasmodium falciparum, P. vivax, and P. malariae (Rachou 1958, Deane 1986, 1989, Rozendaal 1990, Arruda et al. 1986, Lourenço-de-Oliveira et al. 1989, Klein et al. 1991a, b, Tadei et al. 1998, Rubio-Palis 2000, Flores-Mendoza et al. 2004, Grieco et al. 2005, Póvoa et al. 2006). In the southern states of Amazonas and Bolívar, in Venezuela, An. darlingi has been implicated as the principal vector and is responsible for 84% of total cases reported in the country (DER 1998, MS 2005). Nevertheless, limited information exists on this species regarding seasonal abundance, biting rate, biting activity, relative abundance in relation to other species, and infection rates with malaria parasites. Although some epidemiological studies were conducted in the Upper Orinoco River, there are no longitudinal entomological studies directed toward the understanding of the dynamics of malaria transmission. Vector control measures applied in this area have been sporadic, using DDT or fenitrothion to spray dwellings with incomplete or no walls, without the basic knowledge on the bionomics of An. darlingi in this particular remote area. At present the malaria control program is confronted by several technical and administrative difficulties that reduce the effectiveness of the program. In Amazonas, and in particular in the Upper Orinoco River, the control methods applied have failed because of several factors, such as: (a) geographic situation, with difficult access; (b) socio-cultural characteristics of the Yanomami population (Lizot 1988,1998), with frequent migration, belief in supernatural etiology of the disease, intra-ethnic conflicts and housing with incomplete or no walls; (c) circulation of P. falciparum strains with multiple drug resistance (Magris 1996, Riggione et al. 1998); and (d) biting and resting behavior of the principal vector, An. darlingi (Rubio-Palis 1995). To understand the dynamics of malaria transmission in the Upper Orinoco River and suggest control measures, a longitudinal epidemiological and entomological study was carried out in Ocamo between January 1994 and February 1995. ACKNOWLEDGMENTS To the Yanomami population which permitted us to do the mosquito catches in theirs "shabonos". To Clemencia González, Hernán Guzmán, and Marina García who assisted in laboratory activities. To Henry Rupp for correcting the English language. The map of the study sites was produced by Teresa Méndez. To Smith Kline Beecham for the Pf and Pv-210 positive controls, and the following laboratories that produced the MAb cell lines: New York University (Pf), Naval Medical Research Institute (Pv-210), Walter Reed Army Institute of Research (Pv-247), and Centers for Disease Control, Atlanta (Pm). This study used reagents produced with support from the UNDP/WORLD BANK/WHO Special Programme for Research and Training in Tropical Diseases. To Robert Wirtz for sending reagents and materials for the ELISAs. Fig. 1 | Fig. 2 | Fig. 3 | Fig. 4 | Fig. 5 | Fig. 6 | Table
Financial support: Convenio Antimalárico CVG/MSAS/Gobernación estado Amazonas, the Pan American Health Organization +Corresponding author: rubiopalis@yahoo.com Received 30 October 2006 Accepted 8 May 2007 |
