High malaria transmission intensity in a village close to Yaounde, the capital city of Cameroon

A 2-yr longitudinal malaria study was undertaken in a suburb of Yaounde, the capital city of Cameroon, in the village of Simbock, approximately 2 km from the city limits. This study allowed assessment of malaria transmission intensity and dynamics in this region before implementation of pyrethroid impregnated bed nets through the national vector control program. Anophelines were captured on human volunteers by pyrethrum spray collections and in resting sites outdoors. Malaria vectors were Anopheles funestus Giles, Anopheles gambiae s.s. Giles (M and S forms), Anopheles moucheti Evans, and Anopheles nili Theobald. An. moucheti was the most abundant mosquito captured during the study, accounting for >54% of total anophelines caught. The annual Plasmodium falciparum Welch entomological inoculation rates measured by enzyme-linked immunosorbent assay were 277 infected bites per human for the first year and 368 for the second year. An. gambiae s.s., An. funestus, An. moucheti, and An. nili were responsible for 23.8%, 26.8%, 39.2%, and 10.2% of malaria transmission, respectively. Malaria transmission is perennial throughout the year. All these vectors were highly anthropophagous because only two out of 566 mosquitoes blood-meal tested were not taken on humans.     

Anopheles paludis: important vector of malaria in Zaire]

An entomological study was carried out on the transmission of malaria in the Bandundu region, Za?re, during the dry season (July and August 1991). Five Anopheles species were recorded: Anopheles paludis, A. gambiae, A. funestus, A. moucheti and A. nili. A paludis was the dominant species and represents 55.1% of the total; the average number of A. paludis bites per man/day was 4.2. The sporozoite index was 6.2%; the inoculation rate due to A. paludis was h = 0.26, 1 infective bite each 4 days. A. paludis plays an important role with A. gambiae and is considered as one of the main vectors of malaria in this region.     

The Anopheles fauna and the transmission of human malaria in Kinshasa (Zaire)]

A longitudinal epidemiological study of malaria and its vectors was conducted in Kinshasa. 264 night-bite collections on human bait (1,056 man nights) and 384 collections of the house-resting fauna were carried out from April 1989 to October 1990. The anophelian fauna was identified and inventoried, 7 Anopheles species were found: Anopheles gambiae, An. funestus, An. paludis, An. hancocki, An. counstani, An. brunnipes, and An. nili. A single species, An. gambiae s. l. is responsible for the transmission of malaria, it represents 93.27% of the anopheline fauna. The average number of anophele bites man day was 16.28 bites/man/night, it varied between 1 b/m/n in urban area to 26.05 b/m/n in semi-rural area. The average of the sporozoite index for An. gambiae was 3.3%, but it varied from 0% in the urban area to 6.52% in the semi-rural area. The entomological inoculation rate (h) was 197 infective bites per year. This rate fluctuated from 1 infective bite each 128 nights in urban area to 1.7 infective night-bite in semi-rural area. Other epidemiological index were also determined: the level of daily survival rate (p = 8.75 days), the vectorial capacity of 17.97 and the Macdonald's stability 3.5 bites on man taken by a vector during its entire lifetime.     

High malaria transmission intensity due to Anopheles funestus (Diptera: Culicidae) in a village of savannah-forest transition area in Cameroon

An entomological survey was conducted on vectors of malaria in a village of the forest-savannah transition area in Cameroon from February 1999 to October 2000. A total of 2,050 anopheline mosquitoes belonging to eight species were caught 1) after landing on human volunteers, 2) by using pyrethrum spray collections in human dwellings, and 3) in resting sites outdoors. Anopheles funestus Giles was the most abundant species (accounting for 91% of anophelines caught) followed by Anopheles gambiae Giles (7%). Applying polymerase chain reaction led to the identification of all specimens of the An. funestus group as An. funestus sensu stricto and mosquitoes from the An. gambiae complex were mostly An. gambiae sensu stricto of the S molecular form. Malaria transmission was perennial with an entomological inoculation rate estimated at 172 infective bites per person during the period of study. An. funestus was responsible for 88% of the total malaria transmission, with a Plasmodium falciparum circumsporozoite rate of 6.8% and an anthropophilic rate of 99.3%. These results confirm that in high agricultural activity areas, An. funestus can be, by far, the major malaria vector.     

Bionomics of Anopheles gambiae Giles,An. arabiensis Patton,An. funestus Giles and An. nili (Theobald) (Diptera: Culicidae) and transmission of Plasmodium falciparum in a Sudano-Guinean zone (Ngari,Senegal)

An entomological study was conducted in a village of Sudano-Guinean savanna in Senegal, during the rainy season from July to November 2001, to investigate the biology and the involvement of each anopheline species in malaria transmission. Mosquitoes were captured when landing on human volunteers and by pyrethrum spray catches. Twelve anopheline species were captured. Four species amounted to 97% of human-bait sampling: Anopheles gambiae molecular form S, An. arabiensis, An. funestus, and An. nili s.s. All An. gambiae and An. nili females were fed on human, whereas the anthropophilic rate was 94.5% for An. funestus and 88.9% for An. arabiensis. Plasmodium falciparum was the only malaria parasite found, and infecting only An. gambiae, An. arabiensis, An. funestus, and An. nili. The circumsporozoite rate was 4.5% for An. gambiae, 1.6% for An. arabiensis, 3.9% for An. funestus, and 2.1% for An. nili. During the period of study, the entomological inoculation rate was estimated to 264 infected bites. An. gambiae, An. arabiensis, An. funestus, and An. nili were responsible respectively of 56, 3, 20, and 21% of malaria transmission. This study shows for the first time the implication of An. nili in malaria transmission in this area and the complexity of the malaria vectorial system that should be taken into account for any malaria control strategy.     

Malaria transmission and insecticide resistance of Anopheles gambiae (Diptera: Culicidae) in the French military camp of Port-Bouët, Abidjan (Côte d'Ivoire): implications for vector control

An important vector control program is ongoing to lower the risk of malaria transmission in the French military camp of Port-Bou?t, Abidjan (C?te d'Ivoire). However, some autochthonous malaria cases are regularly suspected. An entomological survey was conducted in June 2004 in the camp to assess malaria transmission and evaluate the pyrethroid and organophosphate resistance of the malaria vectors. The average mosquito biting rate was 178.0 bites per person per night. Mosquitoes belonging to the Anopheles gambiae (Diptera: Culicidae) complex and the Anopheles funestus group were collected. An. gambiae s.s. molecular form M was the only species of the An. gambiae complex present. The average number of An. gambiae bites was approximately 44.3 per person per night. The circumsporozoite index was 0.38% and the entomological inoculation rate estimated to be 1.2 infective bites per week for the study period. The kdr and ace1 gene frequencies in the An. gambiae population were 0.70 and 0.15, respectively. Personnel living in the French barracks of Port-Bou?t are thus at high risk of being bitten by parasite-infected mosquitoes. Such an entomological inoculation rate, usually found in African peri-urban environments, was unexpected considering the extensive effort deployed to control mosquitoes in the camp. Insecticide resistance could explain the inefficacy of the vector control program but the spraying strategy is also questionable.     

Population dynamics of malaria vectors in western Kenya highlands

Studies were carried out at three sites in the highlands of western Kenya (Iguhu and Mbale in Kakamega and Vihiga districts, respectively, and Marani in Kisii district) and at one site in the western Kenya lowlands (Kombewa in Kisumu district) to determine the spatial-temporal dynamics of malaria vectors and intensity of malaria transmission from June 2003 to June 2004. At the highland sites, Anopheles gambiae Giles predominated, constituting >80% of the vector species, whereas An. funestus Giles made up <20%. In contrast, at the lowland site, An. funestus made up 68% of the vector species. The mean annual indoor resting densities of An. gambiae at Iguhu were 5.0 female mosquitoes per house per night, 14.2- and 26.3-fold greater than those at Mbale and Marani. During the main transmission season, the indoor resting densities of An. gambiae increased 4.1-, 10.1-, and 5.0-fold over the dry season period in Iguhu, Mbale, and Marani, respectively. The estimated annual entomological inoculation rate (EIR) at Iguhu was 16.6 infectious bites per person per year (ib/p/yr), 1.1 at Mbale, and 0.4 at Marani. This suggests high spatial variation in vector abundance and malaria transmission intensity. At the lowland site, Kombewa, the total annual EIR was 31.1 ib/p/yr and the indoor resting densities during the transmission season increased 7.1-fold in An. funestus and 18.5-fold in An. gambiae sensu lato over the dry season. The low level of transmission in the highlands suggests that it may be disrupted by vector control methods such as residual spraying.     

Epidemiology of malaria in a village of Sudanese savannah area in Mali (Bancoumana). 2. Entomo-parasitological and clinical study

We carried out five cross sectional surveys between 1993 and 1994 to assess the epidemiology of malaria in the village of Bancoumana, located in the Sudanese savannah areas of Mali. Each survey included a collection of entomological, clinical, parasitological and immunological data. The study population involved 1600 children from six months to 9 years of age. The main vector was Anopheles gambiae s.l., man bite rate and entomological inoculation rate were maximum respectively in August (peak of the transmission season) and October (end of transmission season). Plasmodium. falciparum was the main parasite species observed. Spleen enlargement rate, parasite rate, gametocyte rate and parasite density varied significantly with age and season. The parasite rate, gametocyte rate and parasite density were significantly low in October 1994 compared with October 1993 while the entomologic parameter did not show any variation over the two years. This reduction of parasitologic index between 1993 and 1994 may be related to an increase of anti-malarial drug use in the population. Our results show that malaria is hyperendemic in the village of Bancoumana.     

Plasmodium falciparum: epidemiology and man-mosquito transmission and infection in the vector

The level of malaria transmission is usually estimated by some entomological parameters (entomological inoculation rate or reproductive rate and its seasonal variations). However, only one aspect of the malaria transmission is explored by this way, i.e. the transmission of Plasmodium from mosquito to man. The transmission from man to mosquito, the development of parasite in the mosquito midgut, and the role of transmission blocking immunity remain poorly documented. Recent studies on vaccination with gamete antigens showed that transmission blocking immunity, and the natural infectiousness of gametocytes after treatment underlined the need for taking into account a new aspect of malaria epidemiology concerning the transmission of Plasmodium from man to mosquito. In this paper, authors propose and discuss different new indicators and methods to improve our knowledge on malaria epidemiology.     

Malaria transmission in an area of future vaccination in equatorial forest of south Cameroon

In order to describe malaria transmission in a future antigamete vaccine trial area, a longitudinal entomological study was conducted, together with parasitological and immunological surveys, from June 1997 to May 1998 in two nearby villages in a tropical rain forest area 100 km east of Yaoundé. Koundou is located along the main road in an open and degraded environment combining cultivated lands and forests; Ebolakounou is located 5 km from the road in forest surroundings. Indoor mosquito night catches no human volunteers were performed twice a month, in ten houses. We determined the entomological infection rate as 176 infected bites per human per year in Koundou (47.7% for An. moucheti, 47.3% for An. gambiae and 5% for An. funestus) and only 17.7 infected bites/human/year in Ebolakounou, with An. gambiae only. Transmission appears to be ten times higher in the village situated in a degraded environment than in the village situated in the rainforest.     

Ecoethology of vectors and transmission of malaria in the lowland rice growing region of Mandritsara, Madagascar

To evaluate the determining factors of the malaria transmission in the northwest region of Madagascar, transversal studies were carried out through one year, from March 1997 to April 1998, in two villages located near Mandritsara, at less than 300 meters above sea level. The rice-growing region forms an intermediate zone between the central highlands with epidemic and instable malaria and the coastal zone with endemic and stable malaria. Mosquitoes were collected when landing on humans during the night and by pyrethrum spray catches as regards endophilic mosquitoes. Three vectors were identified: Anopheles arabiensis, An. gambiae and An. funestus. An. arabiensis and An. gambiae were exophagic and zoophilic. An. funestus was endo-exophagic and anthropophilic but this species shows also a zoophilic trophic deviation. In both villages, An. funestus is the main vector of human malaria. Malaria transmission was estimated to be around 50 and 70 infective bites/person/year in each village.     

Impact of the use of larvivorous fish Poecilia reticulata on the transmission of malaria in FIR of Comoros

Field tests were conducted in the Grande Comore Island, Federal Islamic Republic of Comoros, in order to evaluate the potential of the larvivorous fish Poecilia reticulata for the control of the malaria vector Anopheles gambiae s.s. Due to the high permeability of soil, Anopheles breeding sites in all island occurs only in the man-made water reservoirs. The study was carried out from November 1987 to November 1988 within a framework of a malaria and filariasis control programme, supported by WHO and UNDP in collaboration with the Government of the FIR of Comoros. All larval breeding places of An. gambiae existing in the village of Hantsambou were recorded (59 ablutions basins and 61 cisterns) and provided initially with 3-5 specimens of P. reticulata/m3 in November 1987, after the importation of the larvivorous species from Mayotte Island. The percentage of breeding places positive for An. gambiae decreased from 41% to 6% after one year. Pyrethrum spray catch showed a reduction of indoor resting density from 5.5 to 0.3, while the ma value, number of Anopheles bites/man/night, obtained by night-biting catches, decreased from 6.3 to 1.2. At the same time of the reduction of entomological indices parasite index for P. falciparum and spleen rate drop steadily in 5-9 years school children. The tested vector control method, well accepted by the community, could be implemented in malaria control through primary health care, being the ecological conditions in the entire island very peculiar.     

Anopheles mascarensis (De Meillon, 1947): main vector of malaria in the region of Fort-Dauphin (south-east of Madagascar)]

Anopheles funestus and Anopheles gambiae s.l. have been considered until now the major vectors of malaria everywhere in Madagascar. Anopheles mascarensis, a mosquito native to Madagascar, has been identified in Sainte-Marie island as a secondary vector only. In 1997, an entomological study was carried out to identify the malaria vectors in the area of Fort-Dauphin, South-East of Madagascar. Every month, mosquitoes were collected from landing catches on human volunteers (from 7:00 am to 5:00 pm inside dwellings and from 7:00 am to 0:00 pm outside) and from knockdown spray-collections indoors. An mascarensis was the most abundant mosquito, the average number of An. mascarensis bites per man/night was 7.6. The sporozoite index was 0.89%. Despite the presence of An. funestus and An. gambiae s.l., An. mascarensis was found to be responsible for 2/3 of the infectious bites (25 infectious bites per man/year). An. mascarensis is widely distributed ih Madagascar but only specimens from the east coast have been found to carry sporozoites of human malaria. Further arguments are thus advanced for the hypothesis according to which a sibling species of An. mascarensis is present in Madagascar.     

Agricultural activities and malaria transmission in two eco-epidemiological settings in central Côte d'Ivoire

We comparatively studied the dynamics of malaria transmission in the villages of Zatta (located in close proximity to an irrigated rice perimeter) and Tiémélékro (rural area with traditional agriculture), central C?te d'Ivoire. In the former village, the irrigated rice farming had been interrupted in 2003/2004 due to a farmers' conflict over land. In each village, mosquitoes were collected by human landing catches at night in sentinel houses. Anopheles gambiae was the predominant malaria vector, followed by An. funestus. In Zatta, the return to an irrigated rice farming in January 2005 was paralleled by a significant increase of the entomological inoculation rate (EIR) ranging from 38 infective bites per person per year (ib/ply) in 2003 to 295 ib/ply in 2005. In Tiémélékro high EIRs were found in 2003 (342 ib/ply) and 2005 (572 ib/ply). Our findings confirm that changes in irrigated rice agriculture influence malaria transmission dynamics, and call for control measures that are readily adapted to local eco-epidemiological settings.     

Malaria in the urban highland area of Antananarivo, Madagascar: bioecology of Anopheles arabiensis

An entomological study was performed to document the transmission of Plasmodium, agents of human malaria in Antananarivo, capital of Madagascar. Human landing mosquitoes were collected at night during two years, between May 2003 and September 2005, in the two sites of Ambohimiandra-Manakambahiny and Ambolokandrina. The genuses of collected mosquitoes were, in order of abundance, Culex, Mansonia and Anopheles. The only potential vector was Anopheles arabiensis. Its maximal abundance was observed in January (22 and 15 bites per man per night, outdoors, respectively in the two sites), during the rainier month of the austral summer This anopheles was biting indoors, in bedrooms, but its agressivity was always higher outdoors than indoors. Its maximal agressivity was observed indoors between 00 and 01 am. The absence of An. gambiae and An. funestus in the catches on the one hand, and the absence of An. arabiensis infected by Plasmodium falciparum on the other hand, are discussed.     

The epidemiology of malaria in the southwestern forests of the Ivory Coast (Tai region)

An epidemiological study on malaria was undertaken between July 1995 and July 1996 in two villages (Za?pobly and Gahably) and their encampments (Kouassikro, Hamanikro and Konankro), in the south-western forest area of C?te d'Ivoire (region of Ta?). The parasitological scheme comprised a total of 2023 tests performed on children aged from 0 to 14 years. The species found were Plasmodium falciparum, P. malariae and P. ovale with a proportion of 84%, 14% and 2% respectively. The global parasite prevalence of all Plasmodium species was 85% and malaria was holoendemic. The average parasitic density decreased progressively as the age increased, in contrast to the plasmodic index which did not vary. All the malarial indexes were similar in the villages and their encampments. Only overall fever prevalence was permanent and in all age groups it was higher in the encampments than in the villages. The entomological findings showed that transmission was permanent and intense throughout the year, with a recrudescence during the rainy season. Transmission was attributed to Anopheles gambiae s.l. in 85% of the cases whereas An. funestus played a secondary role. The average sporozo?tic index was 7.6% and varied between 1.1% and 16.7%. The entomological inoculation rate was of 400 infected bites per person-year for An. gambiae s.l. In such conditions of intense transmission, acquisition of premunition starts at a very early age. This assertion is verified by the average parasite density and the frequency of high parasitic densities which were at their maximum between 1 and 4 years of age and decreased thereafter as the age increased. The paludometric and entomologic indexes obtained are the most elevated ever to have been observed in C?te d'Ivoire, as a result of considerable ecological changes linked to the deterioration of the forest environment over the past 30 years. This deterioration has probably been caused by demographic pressure resulting from internal and foreign immigration to the Ta? region and more especially by the influx of Liberian refugees.     

The effect of 2 chloroquine-based drug strategies (prevention and therapy of febrile cases] on malaria transmission

A 3 years study was decided in 12 villages of the South-West Burkina Faso to compare the chemoprophylaxis and the chemotherapy of febrile cases as potential malaria control strategies. During the first year pretreatment data were collected. During the two following years a programme carried out (I) prophylaxis (10 mg chloroquine/kg body weight) was given weekly to all children under 14 years old in 5 villages, and (II) therapy (10 mg chloroquine/kg body weight) was given in a single dose to all febrile cases in 7 other villages. Chloroquine tablets were distributed by health workers belonging to the community. Both prophylaxis and therapy reduced the gametocyte rate in children (2-9 years) respectively of 63% and 45%. The analysis of the evolution after the first year of the sporozoite rate of anopheline was made difficult by concomitant natural variations of mosquitoes longevity and by mosquitoes displacements. Significant variations of sporozoite rate can be explained by natural variations of mosquitoes longevity. But data from the rice field villages support evidence that reduction of the pool of parasite infective for vectors induced the decrease of Anopheles gambiae s. l. sporozoite rate. Therefore our results reflect a trend more than a strict reduction of malaria transmission.     

Situation épidémiologique avant la mise en eau du barrage hydroagricole de cinq villages de Bouaké, centre Côte-d’Ivoire

From June 2007 to June 2008, entomological, malacological and parasitological investigations were carried out in five villages in close proximity to a small dam of Raffierkro in Bouaké, central Côte-d’Ivoire. The objective of the study was to identify vectors and intermediate host snails of parasitic diseases related to water, and to assess the prevalence of malaria, schistosomiasis and soil-transmitted helminthiasis before dam construction. Mosquitoes were caught by human landing catches, snails were collected in water bodies, and microscopic analyses of blood, stool and urine samples were carried out. Two malaria vector species were identified: Anopheles gambiae and Anopheles funestus. The average infective rate of An. gambiae ranged between 3.1 and 4.5 %. The infective rate of An. funestus was 1 % in all sites. The entomological inoculation rate ranged between 343.1 and 427.1 infective bites per person per year (ib/p/y) for An. gambiae and between 14.6 and 40.1 ib/p/y for An. funestus. Three species of were found: Plasmodium falciparum (predominant species), Plasmodium ovale and Plasmodium malariae. There were no intermediate host snails of schistosomiasis with the exception of Biomphalaria pfeifferi, intermediate host snail of Schistosoma mansoni in Ahougui. The prevalence of urinary and intestinal schistosomiasis and soil-transmitted helminthiasis was low. Our studies revealed important transmission of malaria, with the presence of three plasmodial species, whereas schistosomiasis and soil-transmitted helminthiasis were present, but only at low frequencies.     

Malaria infection,morbidity and transmission in two ecological zones Southern Ghana

A one year survey was conducted in 1992 to compare malaria infection, morbidity and transmission patterns between a coastal savannah community (Prampram) and a community (Dodowa) in the forest zone in southern Ghana. The study population of 6682 at Prampram and 6558 at Dodowa were followed up in their homes once every two weeks and all episodes of clinical malaria recorded. Blood films for microscopy were prepared from 600 participants randomly selected in each community in April and in August representing dry and wet seasons respectively. Mosquitoes biting humans between 1800 hrs and 0600 hrs, as well as indoor and outdoor resting mosquitoes were collected weekly. All mosquitoes collected were classified into species and examined for sporozoites by dissection and ELISA. The incidence rate of clinical malaria was higher in Dodowa (106.6/1000 pop.) than in Prampram (68.5/1000 pop.) It was highest in < 10 year age groups in both communities. It was also higher in the wet season than in the dry season. The prevalence of patent parasitaemia at Prampram and Dodowa in April in the dry season. The prevalence of patent parasitaemia at Prampram and Dodowa in April 1992 was 19.8% (117/590) and 42.2% (253/599) respectively. The corresponding figures for August were 26.6%(160/602)at Prampram and 51.3% (309/602) at Dodowa. Plasmodium falciparum infection contributed 78-85% of the parasitaemia in April and 93-99% in August. The average man-biting rate for Anopheles gambiae s.l was higher at Prampram than at Dodowa (1.54 vs 0.79 bites/man/night) but the average sporozoite rate was higher at Dodowa than at Prampram (2% vs 0.7%). The peak of biting density at Prampram occurred in June whilst that of Dodowa occurred in November.     

Dynamics of malaria transmission in Kafiné, a rice growing village in a humid savannah area of Côte d'Ivoire

A study on malaria transmission based on samples of mosquitoes caught on human subjects was conducted from February to August 1995 in the rice growing area of Kafine, a village located in the Niakaramandougou district of northern C?te d'Ivoire. The village is surrounded by 117,500 acres of rice fields. Irrigation is sub-permanent in the rice field and harvests number two a year. During the 6 months of the study, 12.353 mosquitoes were caught. The average biting rate was 118.8 bites per man per night (b/m/n). Mansonia, Culex and Aedes represented only 17.5% of the total number of mosquitoes caught. Anopheles accounted for 82.5% of the number of anthropophilic mosquitoes. Anopheles gambiae s.s. represented 83.7% of the total Anopheles species. As a whole, the average biting rate recorded for the Anopheles was 98 b/m/n. The average biting rate of An. gambiae was 90.4 b/m/n. The highest rate (121.5 b/m/n) was recorded in April. During the dry and rainy season, the indoor biting cycle per hour of An. gambiae s.s. was studied from 6 p.m. to 6 a.m. In both seasons, a marked biting activity was noticed between 10 p.m. and 5 a.m. The average annual parity rate reached 40.2% but it ranged from 59.8% (n = 82) in February to 19% (n = 63) in May. The mean sporozoitic index of An. gambiae throughout the study period was 1.1% (14 positive salivary glands/1.251 dissected). The index ranged from 0 in April, May and June to 6.2 (n = 192) in July. The rate did not vary with rainfall but with the different stages of rice growing. The non-synchronisation of agricultural practises for each growing cycle seemed to be a conclusive factor in the transmission of malaria in this locality. Malaria transmission at Kafine can be characterised by 3 main elements: transmission is intensive with an estimated inoculation rate of 1 ib/m/n; transmission is more related to double phase rice cultivation (regardless of synchronisation on plots) than to rainfall; transmission shows particular variations linked to rice cultivation cycles with an increase during periods of ripeness and harvest. The nuisance caused by higher mosquito density has brought people to comply with use of pyrethroid impregnated bed nets advocated for wide use by the National Malaria Control Program.