Transmission of malaria and genotypic variability of Plasmodium falciparum on the island of Annobon (Equatorial Guinea).

BACKGROUND: Malaria transmission in Equatorial Guinea and its space-time variability has been widely studied, but there is not much information about the transmission of malaria on the small island of Annobon. In 2004, two transversal studies were carried out to establish the malaria transmission pattern on Annobon and analyse the circulating Plasmodium falciparum allelic forms. METHODS: A blood sample was taken from the selected children in order to determine Plasmodium infection by microscopical examination and by semi-nested multiplex PCR. The diversity of P. falciparum circulating alleles was studied on the basis of the genes encoding for the merozoite surface proteins, MSP-1 and MSP-2 of P. falciparum. RESULTS: The crude parasite rate was 17% during the dry season and 60% during the rainy season. The percentage of children sleeping under a bed net was over 80% in the two surveys. During the rainy season, 33.3% of the children surveyed were anaemic at the time of the study. No association was found between the crude parasite rate, the use of bed nets and gender, and anaemia. However, children between five and nine years of age were five times less at risk of being anaemic than those aged less than one year. A total of 28 populations of the three allelic families of the msp-1 gene were identified and 39 of the msp-2 gene. The variability of circulating allelic populations is significantly higher in the rainy than in the dry season, although the multiplicity of infections is similar in both, 2.2 and 1.9 respectively. CONCLUSION: Based on the high degree of geographical isolation of the Annobon population and the apparent marked seasonality of the transmission, it is feasible to believe that malaria can be well controlled from this small African island.

Spatial and temporal variability of the Glossina palpalis palpalis population in the Mbini focus (Equatorial Guinea).

BACKGROUND: Human African Trypanosomiasis is a vector-borne parasitic disease. The geographical distribution of the disease is linked to the spatial distribution of the tsetse fly. As part of a control campaign using traps, the spatial and temporal variability is analysed of the glossina populations present in the Mbini sleeping sickness foci (Equatorial Guinea). RESULTS: A significant drop in the annual mean of the G. p. palpalis apparent density was noted from 2004 to 2005, although seasonal differences were not observed. The apparent density (AD) of G. p. palpalis varies significantly from one biotope to another. The fish dryers turned out to be zones with the greatest vector density, although the AD of G. p. palpalis fell significantly in all locations from 2004 to 2005. CONCLUSION: Despite the tsetse fly density being relatively low in fish dryers and jetties, the population working in those zones would be more exposed to infection. The mono-pyramidal traps in the Mbini focus have been proven to be a useful tool to control G. p. palpalis, even though the activity on the banks of the River Wele needs to be intensified. The application of spatial analysis techniques and geographical information systems are very useful tools to discriminate zones with high and low apparent density of G. p. palpalis, probably associated with different potential risk of sleeping sickness transmission.

Predicted distribution and movement of Glossina palpalis palpalis (Diptera: Glossinidae) in the wet and dry seasons in the Kogo trypanosomiasis focus (Equatorial Guinea).

The aim of this study was to predict the distribution and movement of populations of the tsetse fly, Glossina palpalis palpalis (Diptera: Glossinidae), in the wet and dry seasons and to analyze the impact of the use of mono-pyramidal traps on fly populations in the Kogo focus in 2004 and 2005. Three Glossina species are present in Kogo: Glossina palpalis palpalis, major HAT vector in West-Central Africa, Glossina caliginea, and Glossina tabaniformis. The apparent density (AD) of G. p. palpalis clearly fell from 1.23 tsetse/trap/day in July 2004 to 0.27 in December 2005. A significant reduction in the mean AD for this species was noted between seasons and years. The diversity of Glossina species was relatively low at all the sampling points; G. p. palpalis clearly predominated over the other species and significantly dropped as a consequence of control activities. The predictive models generated for the seasonal AD showed notable differences not only in the density but in the distribution of the G. p. palpalis population between the rainy and dry season. The mono-pyramidal traps have proven to be an effective instrument for reducing the density of the tsetse fly populations, although given that the Kogo trypanosomiasis focus extends from the southern Equatorial Guinea to northern Gabon, interventions need to be planned on a larger scale, involving both countries, to guarantee the long-term success of control.

Spatial variability in the density, distribution and vectorial capacity of anopheline species in a high transmission village (Equatorial Guinea).

BACKGROUND: Malaria transmission varies from one country to another and there are also local differences in time and space. An important variable when explaining the variability in transmission is the breeding behaviour of the different vector species and the availability of breeding sites. The aim of this study was to determine the geographical variability of certain entomological parameters: human biting rate (HBR), sporozoitic index (SI) for Plasmodium falciparum and entomological inoculation rate (EIR). METHODS: The study was carried out in a small village in the mainland region of Equatorial Guinea. Adult mosquitoes were collected by CDC light traps. Polymerase Chain Reaction was employed to identify the species within the Anopheles gambiae complex and to detect P. falciparum sporozoites. The geographical position of all the dwellings in the village were taken using a global positioning system receiver unit. Data relating to the dwelling, occupants, use of bednets and the mosquitoes collection data were used to generate a geographical information system (GIS). This GIS allowed the minimum distance of the dwellings to the closest water point (potential breeding sites) to be determined. RESULTS: A total of 1,173 anophelines were caught: 279 A. gambiae s.l. (217 A. gambiae s.s. and one Anopheles melas), 777 Anopheles moucheti and 117 Anopheles carnevalei. A. moucheti proved to be the main vector species and was responsible for 52.38 [95% IC: 33.7-71] night infective bites during this period. The highest SI was found in A. carnevalei (24%), even though the HBR was the lowest for this species. A significant association was found between the distance from the dwellings to the closest water point (River Ntem or secondary streams) and the total HBR. CONCLUSION: A clear association has been observed between the distance to potential breeding sites and the variability in the anopheline density, while the other parameters measured do not seem to condition this spatial variability. The application of GIS to the study of vector-transmitted diseases considerably improves the management of the information obtained from field surveys and facilitates the study of the distribution patterns of the vector species.

Malaria Panel Assay versus PCR: detection of naturally infected Anopheles melas in a coastal village of Equatorial Guinea.

BACKGROUND: A study was carried out in a village of the mainland region of Equatorial Guinea in order to ascertain a) which members of Anopheles gambiae complex could be involved in malaria transmission and b) the rate of infectivity for Anopheles melas comparing two different methods, a PCR able to detect sporozoite-DNA and an immunochromatographic assay MPR (Malaria Rapid Dipstick Panel Assay). METHODS: Mosquitoes were sampled at night by indoor captures in two houses of a coastal village in Equatorial Guinea (Ayantang). Collected mosquitoes were identified as An. gambiae s.l. These were individually dried into silica-gel. The head-thorax of the An. gambiae s.l. mosquitoes were analysed by PCR to verify that the species was of the gambiae complex. Individual head-thorax and pools (5 pools) of homogenized mosquitoes employed in Malaria Rapid Panel assay (MRP assay) were lysed and DNA was extracted. PCR was designed from the 753 base pair insert of pBRKl-14 and DNA was amplified. The relationship between dipstick and PCR to detect Plasmodium falciparum sporozoites was measured in terms of sensitivity, specificity and test association (Cohen's kappa value). RESULTS: Two hundred and sixty-four An. gambiae s.l. females were studied (214 individually and five pools with 10 mosquitoes in each). PCR analysis showed that 207 mosquitoes were An. melas, 3 An. gambiae s.s. and 4 could not be identified. By using PCR as the gold standard method when dipstick assay was compared, matching results were obtained for 6 mosquitoes and, in one case MRP was positive while PCR was not reactive. MRP assay showed a low sensitivity (3.3%) when compared with falciparum-DNA detection (17,7% and 14,3%, series A and B respectively). Agreement between the two test formats was low (kappa = 0,224). CONCLUSION: It was determined that An. melas is the main anopheline vector involved in malaria transmission in Ayantang, a coastal village in mainland Equatorial Guinea. A comparison of PCR and Vec-Test Assay, concluded that the PCR method proved to be a more sensitive and useful tool than the dipstick assay to determine the malarial infection rate in mosquitoes in an area of stable and high malaria transmission like Equatorial Guinea.