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1 March 2014 Survival of the Immature Stages of the Malaria Vectors Anopheles pseudopunctipennis and Anopheles argyritarsis (Diptera: Culicidae) in Northwestern Argentina
Guillermina B. Galante, Mirta Santana, Cecilia A. Veggiani Aybar, María J. Dantur Juri
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Abstract

In order to optimally time the application of control measures to reduce populations of malaria vectors, program managers need to know precisely when the vulnerable larval stage will be most abundant at each specific breeding site. Therefore at 4 different breeding sites of the malaria vectors, Anopheles pseudopunctipennis Theobald and Anopheles argyritarsis Robineau-Desvoidy in northwestern Argentina, we recorded the calendar dates during spring and summer when different life stages appeared, and in each of these 2 seasons, we measured the duration of each life stage and the probability that it would transition to the subsequent stage or die. Larval samples were collected during the spring and summer of 2008-2009 at 4 localities in northwestern Argentina. These larvae were reared individually in plastic containers in which the volume of water was kept constant, temperature and photoperiod were controlled, and a standard amount of food was provided each day. The data were analyzed by multistate models, a nonparametric model of survival without covariates, a survival model with covariates, a Cox-type survival model with specific co-variates, and models of reduced rank. We collected 1,643 larvae of which 1,404 reached adulthood. Of these 1,119 were An. pseudopunctipennis, and 285 were An. argyritarsis. Both An. pseudopunctipennis and An. argyritarsis were abundant in autumn (55.3% and 66.7%, respectively). Considerably more individuals transitioned from larvae to pupae than from pupae to adults. The probability of an individual remaining in the larval stage for the first 2 days was close to 100% and then decreased. The transition from the larval stage to death was significant in the summer. The breeding site at Rosario de la Frontera exhibited a particularly significant effect on the transition from the larval stage to death, i.e., greatly increased larval mortality. The results obtained in the present study are substantial contributions to the bionomics of An. pseudopunctipennis and An. argyritarsis. According to our results, mosquito source management programs should be focused on the larval stage during the summer season and principally at Rosario de la Frontera River. These actions could substantially reduce the production of the adult vectors and potentially reduce transmission of malaria in northwestern Argentina.

Malaria remains one of the most important tropical infectious diseases that affect people worldwide. In fact, 219 million cases of malaria and 660,000 deaths were reported in 2010 (Paaijmans et al. 2007; World Health Organization 2012). In the Americas, at least 30% of the population of the 21 countries is living at some degree of risk, and approximately 8% of the population is at high risk. Argentina, El Salvador, Mexico, and Paraguay remain in the pre-elimination phase of malaria eradication (World Health Organization 2012).

The number of confirmed cases in the Americas decreased from 1.18 million in 2000 to 490.000 in 2011 (World Health Organization 2012). Brazil and Colombia accounted for 68% of the cases in 2011. Reductions in the number of confirmed cases and in the case incidence rates of more than 75% were recorded in 13 countries (Argentina, Belize, Bolivia, Costa Rica, Ecuador, El Salvador, French Guiana, Guatemala, Honduras, Mexico, Nicaragua, Paraguay, and Suriname) between 2000 and 2011 (World Health Organization 2012).

Anopheles pseudopunctipennis Theobald (Diptera: Culicidae) is the primary mosquito incriminated in malaria transmission in the foothills of South America, including Argentina (Zimmerman 1992). The larval habitats of this species along their wide geographical distribution have been characterized as freshwater stream pools that are partially shaded and contain clean water and filamentous green algae. The few studies that have focused on the biological and ecological aspects of immature forms of An. pseudopunctipennis reported that larvae could also be found in artificial containers (tanks, fountains, rice paddies, and marshy meadows) and that environmental variables, including filamentous green algae, altitude, shallow water, annual rainfall, and water temperature, influence larval abundance (Shannon 1930; Hoffmann 1931; Hoffmann & Samano 1938; Rozeboom 1941; Hackett 1945; Downs et al. 1948; Savage et al. 1990; Rejmankova et al. 1991; Fernández-Salas et al. 1994; Berti et al. 1993; Manguin et al. 1996; Grillet 2000; Dantur Juri et al. in press).

Since the beginning of the 20th century, An. pseudopunctipennis was reported as the main vector of malaria in northwestern Argentina, and this pioneering work was based on the characterization of the larval habitats of the species during different seasons (Shannon & Davis 1927). The densities of the immature stages of An. pseudopunctipennis have been shown to fluctuate from the end of the spring season to the autumn season, when malaria cases occur (Dantur Juri et al. in press).

In general, the availability of aquatic habitats depends on precipitation (Koenraadt et al. 2004; Fillinger et al. 2004; Paaijmans et al. 2007); howe