Estivation of Malaria Vector Mosquitoes in the Sahelian Region of Mali
Condition: Malaria
MeSH terms: Malaria
Study type: Observational

Summary: 
      Background:

        -  P. falciparum, one of the most virulent forms of malaria, causes more than 300 million
           episodes of malaria and 1 million deaths each year. The spread of drug-resistant
           parasites, insecticide-resistant mosquitoes, and persistent socioeconomic conditions of
           poverty compound the difficulties of malaria as a major global health problem. New means
           of disease and vector control are vitally needed.

        -  Several promising strategies rely on targeting mosquito populations when they are most
           vulnerable, such as during the dry season when mosquitoes find it difficult to
           reproduce. Large regions of the West African country of Mali have prolonged dry seasons
           (up to 8 months), during which mosquito populations dramatically decline within a month
           after the rainfall ceases. Clearly, mosquitoes can survive the dry season (as evident
           from their robust numbers during the wet season) but the process that enables them to do
           so remains unknown. Targeting the small and fragile mosquito population at the end of
           the dry season could reduce or eliminate the numbers of mosquitoes in certain regions,
           providing great benefits for communities in dry regions.

      Objectives:

        -  To determine if common malaria-carrying mosquitoes survive the dry season in the Mali
           village of Thierola by estivation (going dormant, or hibernating, during dry periods).

        -  To identify and examine mosquitoes that were marked with special paint during a previous
           protocol, if these marked mosquitoes are captured during the investigation.

      Eligibility:

        -  All activities in this protocol will take place in Thierola village, Banamba district,
           Koulikoro region, Mall, West Africa. The village was chosen because it is isolated from
           other communities by at least 6 km and is a small community of less than 300 inhabitants
           living in 90 houses.

        -  Participants will be healthy adult men between 18 and 65 years of age.

      Design:

        -  Thirty adult men who live in Thierola will be recruited to participate as mosquito
           collectors for the human-baited trapping method and will work in teams of two.

        -  The first collector will expose his lower legs to attract human-seeking mosquitoes.
           Using a mouth aspirator, the second collector will collect the mosquitoes as they land
           on the first collector's legs.

        -  The collections will be conducted both indoors and outdoors from 6 p.m. to 6 a.m. the
           following morning for 14 consecutive days.

        -  All study volunteers will be trained in proper collection technique and supervised
           throughout the study by a mobile team led by the study investigators. Volunteers will be
           monitored for signs of malaria and treated accordingly if they develop symptoms of the
           disease.

        -  Researchers will collect mosquito samples at the end of the dry season (April-May) and
           at the start of the rainy season (May-June).

        -  Mosquitoes collected in the study will be analyzed by NIH researchers to learn more
           about how they survive during the dry season.
    

Detailed description: 
      Malaria causes over one million deaths annually. A promising control strategy relies on
      targeting mosquito populations when they are vulnerable, such as during the dry season.
      Prolonged dry season during which larval sites are scarce and mosquito populations decline
      prevail in the Sahel which covers a large part of Mali. How mosquitoes withstand the dry
      season remains unknown. Some evidence supports the adult estivation hypothesis whereby
      mosquitoes remain in protected sites, except for sporadic journeys to obtain blood-meals to
      sustain themselves rather than to produce eggs. Alternatively, mosquitoes do not survive the
      dry season locally; rather, they migrate from areas where reproduction continues at permanent
      water bodies. We aim to determine if An. Gambiae estivate during the dry season in the Sahel;
      and if so, identify their hidden resting sites. As mosquito populations decline, standard
      sampling techniques yield zero mosquitoes. Because estivating females take blood meals
      intermittently, the most promising sampling technique is by human landing catch (HLC).
      Finding a few mosquitoes during the dry season have been reported, but it could not be
      determined if they were estivating or were migrants from villages, where larval sites
      persist. This study can resolve this because we captured marked and released over 5000
      mosquitoes just before the dry season (November 2008). We aim to collect mosquitoes during
      the end of the dry season (April 2009) and the onset of the rainy season (June 2009) and find
      out if they bear marks from the previous wet season. Finding a single marked specimen will be
      the ultimate proof for the estivation hypothesis. Experiments will take place in the Sahelian
      village Thierola, Koulikoro Region, Mali, where the dry season exceeds 7 months. The village
      was chosen because it has no permanent larval sites and is a small community (< 300
      inhabitants) living in 90 houses, allowing collection in all houses in a single day. Thirty
      adult male residing in Thierola will be recruited as collectors for the HLC. A collector will
      expose his lower legs to attract mosquitoes. Using a mouth aspirator, mosquitoes will be
      collected as they land on the collector's legs, ideally, before a blood meal is taken.
      Collection will be conducted 6pm to 6am for 14 consecutive days in April 2009 and again after
      the rains begin in June 2009. Collectors will be informed of the proper collecting procedures
      during the consenting procedure. During the time of year we propose to conduct this work,
      mosquito populations will be at their minima and any surviving mosquitoes will take blood
      meals infrequently. Therefore, the HLCs will be conducted when malaria transmission is
      extremely low and the risk to volunteers essentially nil. In summary, knowledge of how
      mosquitoes survive the dry season would open novel ways for vector and malaria control.
    

Time perspective: Prospective
Keywords: Mosquito; Human Landing Collection; Malaria