Anopheles vector distribution and malaria transmission dynamics in Gbêkê region, central Côte d’Ivoire
Alphonsine A. KoffiSoromane CamaraLudovic P. Ahoua AlouWelbeck A. OumboukeRosine Z. WolieInnocent Z. TiaEleanore D. SternbergFlorent H. A. YapoFernand Mathieu KoffiSerge-Brice AssiJackie CookMatthew B. ThomasRaphaël N’Guessan
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Abstract Background A better understanding of vector distribution and malaria transmission dynamics at a local scale is essential for implementing and evaluating effectiveness of vector control strategies. Through the data gathered in the framework of a cluster randomized controlled trial (CRT) evaluating the In2Care (Wageningen, Netherlands) Eave Tubes strategy, the distribution of the Anopheles vector, their biting behaviour and malaria transmission dynamics were investigated in Gbêkê region, central Côte d’Ivoire. Methods From May 2017 to April 2019, adult mosquitoes were collected monthly using human landing catches (HLC) in twenty villages in Gbêkê region. Mosquito species wereidentified morphologically. Monthly entomological inoculation rates (EIR) were estimated by combining the HLC data with mosquito sporozoite infection rates measured in a subset of Anopheles vectors using PCR. Finally, biting rate and EIR fluctuations were fit to local rainfall data to investigate the seasonal determinants of mosquito abundance and malaria transmission in this region. Results Overall, Anopheles gambiae , Anopheles funestus , and Anopheles nili were the three vector complexes found infected in the Gbêkê region, but there was a variation in Anopheles vector composition between villages. Anopheles gambiae was the predominant malaria vector responsible for 84.8% of Plasmodium parasite transmission in the area. An unprotected individual living in Gbêkê region received an average of 260 [222–298], 43.5 [35.8–51.29] and 3.02 [1.96–4] infected bites per year from An. gambiae, An. funestus and An. nili , respectively. Vector abundance and malaria transmission dynamics varied significantly between seasons and the highest biting rate and EIRs occurred in the months of heavy rainfall. However, mosquitoes infected with malaria parasites remained present in the dry season, despite the low density of mosquito populations. Conclusion These results demonstrate that the intensity of malaria transmission is extremely high in Gbêkê region, especially during the rainy season. The study highlights the risk factors of transmission that could negatively impact current interventions that target indoor control, as well as the urgent need for additional vector control tools to target the population of malaria vectors in Gbêkê region and reduce the burden of the disease.Keywords:
Anopheles gambiae
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Up to 1899 some one hundred and twenty-two definite species of the Culicidae were described.Since that time many collections of mosquitoes have been examined from all parts of the world.Mr. F. V. Theobald, at the close of last year, as the result of an exhaustive examination of past works and new material, has remodelled the classification of the Culicidae, which he has grouped into some twenty- two genera, and has described some one hundred and thirty-six distinct new species in his Monograph on the Culicidae of the World.To this number must be added some one hundred new species which he has not yet described.As yet only a few of the great number of mosquitoes have been investigatedwith regard to the important point of whether or no they can act as hosts for the malaria parasite.In fact only a few species of Culex have been shown to be incapable of transmitting the disease.When it was considered that not only malaria but other diseases were conveyed by mosquitoes, for example, filaria can develop in two genera, Culex and PanopliteSy and that yellow fever is certainly carried by one genus, Stegomyia, there arose a tendency to look upon all mosquitoes as harmful, and that their judicial destruction, as far as possible, was an object to be aimed at.How far this end can be attained is at present sub j.udice, but we have evidence that certain districts and towns lend themselves readily to this object.Experiments under this head are at present being carried on in West Africa and in Havana.On a smaller scale similar experiments have been carried out in Hong Kong and Statin Island, New York.Major Ross chose Freetown, Sierra Leone, for the scene for an experiment, to see how far mosquitoes can be diminished in a certain area, one of the most difficult places on the West Coast to tackle.He and Dr. Logan Taylor, who was to direct the work there, arrived on July 2, 1901, and at once commenced the campaign against all varieties of mosquitoes.As yet the experiment is not completed, but a great diminution in the numbers of these insects has already been brought about.In Havana the work of exterminating mosquitoes, by the destruction of their breeding places, was commenced soon after the discovery that yellow fever could be transmitted by the bites of the common mosquito, Stegomyia jasciata.Major and Surgeon W. C. Gorgas, in his January report to the Military Governor, states that out of seventeen thousand houses examined during the month by the ' Stegomyia Brigade,' in four hundred and eleven only were mosquito larvae found ; the preceding January, larvae would have been found in all of the houses.Not only have the mos- quitoes diminished, but Havana has been for the last four months, October, November, December, January, entirely free from yellow fever.This result is certainly very encouraging when we compare the prevalence of yellow fever there during the same period of the previous year, when the average number of deaths from this disease was 146-49.II. TOPOGRAPHY AND STATISTICSSt. Mary's Island and Bathurst St. Mary's Island is a long, low-lying island at the mouth of the River Gambia, extending roughly from north-east to south-west along the south bank of the river.It is about four miles long and half-a-mile across at its widest part.The island is only separated from the mainland by a small creek, which, at the north-west corner, is bridged across.One fairly good road runs the length of the island, from the town of Bathurst at the one end to the Creek Bridge.This road runs along the beach, separated from it for the most part by mounds of sand and scrub.On the south side of the road an extensive mangrove swamp occurs, and in places encroaches on to the road.The town of Bathurst is situated at the east corner of St. Mary's Island.It occupies an J-shaped area of land, the long arm of which is almost separated from the island by an encroachment of Oyster Creek at the back of the island and the mangrove swamp.There is only a distance of some two hundred yards between the swamp and the beach, so that this area is for the most part cut off from the island, and is surrounded by a broad expanse of water on all sides, to the north and west by the River Gambia, which is here about four miles across, to the south and west by the creek separating St. Mary's Island from the mainland.In this area the land scarcely reaches the height of four feet above sea-level, the greater part of it being situated below the sea-level.Altogether it is scarcely a square mile in extent.The formation is of light sand and loam on the surface, followed by denser loam which rests upon the water-bearing sand and silt, about eight feet under the sur- face.This area is well though not too thickly wooded ; some very fine trees occur in the town.The town of Bathurst is very well laid out.The chief streets are broad and run parallel and at right angles to one another ; in fact there are very tew narrow streets, even in the strictly native quarters.The chief houses and factories in the principal street, Wellington, face the mouth of the river.These houses are built of stone for the most part, the volcanic iron stone which occurs in great quantity across the river.These houses are very cool and airy, and the rooms are large ; they were probably built by the French.At the back of each house there is generally an enclosed piece of ground used as a garden.Unfortunately, in Bathurst, the houses of the Europeans are not segregated from the natives, many of the traders' and officials' houses, particularly, being sur- rounded by native compounds.
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This study was carried out between May and October, 2010 to morphologically identify Anopheles species in parts of Kano State. A total of 2374 Anopheles were collected and identified. 1782 (75.07%) were collected from Nassarawa Local Government Area while 592 (24.93%) were collected from Tarauni Local Government Area. Using Anopheles characters of Gilles and Coetzee (1987) under zeiss light microscope, 587 (24.75%) were Anopheles funestus, 1535 (64. 65%) were gambiae s.l. and 252 (10.60%) were An. maculipalpis. Nasssarawa Local Government had the higher of Anopheles identified. Anopheles gambiae s.l. ranked the highest among other species. Further molecular identification of sub-species complex of An. gambiae s.l and An. funestus is strongly recommended in the area.Keywords: Identification, malaria, vectors, anopheles, species.
Anopheles gambiae
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Trichuriasis
Strongyloidiasis
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Tropical disease
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Trichuris trichiura
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Accurate Anopheles species identification is key for effective malaria vector control. Identification primarily depends on morphological analysis of field samples as well as molecular species-specific identifications. During an intra-laboratory assessment (proficiency testing) of the Anopheles funestus group multiplex PCR assay, it was noted that Anopheles arabiensis can be misidentified as Anopheles leesoni, a zoophilic member of the An. funestus group. The aim of this project was, therefore, to ascertain whether other members of the Anopheles gambiae complex can also be misidentified as An. leesoni when using the standard An. funestus multiplex PCR.The An. funestus multiplex PCR was used to amplify DNA from An. gambiae complex specimens. These included specimens from the laboratory colonies and field samples from the Democratic Republic of Congo. Amplified DNA from these specimens, using the universal (UV) and An. leesoni species-specific primers (LEES), were sequence analysed. Additionally, An. leesoni DNA was processed through the diagnostic An. gambiae multiplex PCR to determine if this species can be misidentified as a member of the An. gambiae complex.Laboratory-colonized as well as field-collected samples of An. arabiensis, An. gambiae, Anopheles merus, Anopheles quadriannulatus, Anopheles coluzzii as well as Anopheles moucheti produced an amplicon of similar size to that of An. leesoni when using an An. funestus multiplex PCR. Sequence analysis confirmed that the UV and LEES primers amplify a segment of the ITS2 region of members of the An. gambiae complex and An. moucheti. The reverse was not true, i.e. the An. gambiae multiplex PCR does not amplify DNA from An. leesoni.This investigation shows that An. arabiensis, An. gambiae, An. merus, An. quadriannulatus, An. coluzzii and An. moucheti can be misidentified as An. leesoni when using An. funestus multiplex PCR. This shows the importance of identifying specimens using standard morphological dichotomous keys as far as possible prior to the use of appropriate PCR-based identification methods. Should there be doubt concerning field-collected specimens molecularly identified as An. leesoni, the An. gambiae multiplex PCR and sequencing of the internal transcribed spacer 2 (ITS2) can be used to eliminate false identifications.
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