C.M.M. Ferreira

Prospective risk of morbidity in relation to multiplicity of infection with Plasmodium falciparum in São Tomé.

The prospective risk of acute morbidity was analysed in relation to multiplicity of Plasmodium falciparum infection in 491 individuals in a peri-urban community in São Tomé. In an initial cross-sectional survey, 40.5% of individuals were recorded by microscopy as infected with P. falciparum, and by PCR 60.5%, with the maximum prevalence in children aged 5-10 years. PCR-RFLP typing of the msp-2 gene of P. falciparum found a mean of 2.4 parasite genotypes per infected person, with little age dependence in this multiplicity and a total of 43 different msp-2 alleles identified. None of these were unique for São Tomé. Study participants were encouraged to report to a project worker whenever they suffered a febrile illness. During the 3 months following the parasitological survey the recorded incidence rates decreased with increasing baseline msp-2 multiplicity, both for P. falciparum-positive episodes and for fever without parasitaemia. While this is consistent with suggestions that multiple P. falciparum infections may protect against super-infecting parasites, confounding by patterns of health service usage is an alternative explanation. The incidence of clinical malaria episodes was only a little higher in children than in adults. This weak age-dependence in clinical immunity might be a consequence of a cohort effect resulting from resurgence of the disease after the breakdown of malaria control programs in the 1980s.

Malaria in São Tomé and Príncipe: parasite prevalences and vector densities.

A cross-sectional survey was carried out in 16 localities on the island of São Tomé and three on the island of Príncipe, at the end of the rainy season of 1997, to determine malaria prevalence and vector densities. Blood samples from 664 inhabitants of all ages were examined by optical microscopy (OM) and PCR. Mosquito collections were made by outdoor landing captures from 21:00-23:00 h. Great differences were found between OM and PCR readings. OM had a sensitivity of 66%, a specificity of 79% and failed to reveal any mixed-infections. Overall prevalence, determined by PCR, was higher in São Tomé (53%) than in Príncipe (35%). It was highest in children below 16 years-old. All four human Plasmodium species occurred in São Tomé but P. ovale was not detected in Príncipe. The human population was largely asymptomatic. Bednet users had lower prevalence than did non-users. The FOREST form of Anopheles gambiae s.s., identified by PCR and cytogenetics, was the only vector on the islands. The sporozoite rate in São Tomé, assessed by ELISA, was 0.5%. Parasite prevalence and vector densities were positively correlated in São Tomé, where malaria transmission must occur predominantly in the more populated coastal areas.

Raised houses reduce mosquito bites

BackgroundIn many parts of continental Africa house construction does not appear to impede entry of malaria vectors and, given their generally late biting cycle, the great majority of transmission takes place indoors. In contrast, many houses in São Tomé, 140 km off the coast of Gabon, are raised on stilts and built of wooden planks. Building on stilts is a time-honoured, but largely untested, way of avoiding mosquito bites. Exposure may also be affected by mosquito activity times and age composition of host-seeking females. A study was therefore undertaken on the island of São Tomé to determine if exposure to Anopheles gambiae, the only vector on the island, varied with house construction or time of the night.MethodsA series of all-night landing collections were undertaken out of doors at ground level, inside houses at ground level, on the verandas of, and inside houses built on stilts. The gonotrophic age of an unselected sample of insects from the first three hours of landing collection (18:00–21:00) was determined by dissection. In addition, 1,149 miniature light-trap collections were obtained from 125 houses in the study area. Numbers collected were related to house construction.ResultsBiting of An. gambiae took place primarily outside at ground level. Less than one third of biting occurred inside houses. Houses built on stilts had half the number of An. gambiae in them compared to those built at ground level. Conversely houses with an eaves gap had more An. gambiae in them than houses without such a gap. Gonotrophic age did not affect house entry rates in An. gambiae. House construction affected Culex quinquefasciatus less than An. gambiae. Mean density per house, derived from a series of 1,490 randomly assigned light-trap collections, was over-dispersed with 18% of houses having 70% of the vectors.ConclusionHouse construction plays an important role in determining exposure to malaria vectors in São Tomé. Neighbours can have very different exposure levels. Recommendations for improvement in control are given.

In vitro assessment of artesunate, artemether and amodiaquine susceptibility and molecular analysis of putative resistance-associated mutations of Plasmodium falciparum from São Tomé and Príncipe

Objective  To evaluate the basal in vitro responses of Plasmodium falciparum isolates collected in The Democratic Republic of São Tomé and Príncipe to artemether (ATH), artesunate (ATN) and amodiaquine (AMQ).

Dogs as a favored host choice of Anopheles gambiae sensu stricto (Diptera: Culicidae) of São Tomé West Africa.

Abstract The host source and human blood index (HBI) of an exophilic population of the “forest” cytoform of Anopheles gambiae Giles sensu stricto, from a peri-urban area of the island of São Tomé, were assessed. Blood meals of 434 An. gambiae females from all-night indoor light-trap collections, 193 from indoor and 422 from outdoor resting collections, were determined by ELISA. Significant differences were found in the HBI estimates from insects collected indoors (0.93) and outdoors (0.27). Blood-fed insects collected resting outdoors provided the most representative sample for host determination. Dogs were the predominant hosts, followed by humans and pigs. Of all human feeds, it was estimated that 81.5% were taken inside houses. The low HBI of 0.27 for the An. gambiae population explains the low sporozoite rate and the meso-endemicity of malaria in the island.

Malaria and its possible control on the island of Príncipe

BackgroundMalaria can be eradicated from islands. To assess the prospects for eradication of malaria from the island of Príncipe in the Gulf of Guinea, we fitted a mathematical model to age-prevalence curves and thus obtained estimates of the vectorial capacity and of the basic reproductive number (R0) for malaria.MethodsA cross-sectional malariological survey was carried out, in mid-1999, in six communities, comprising circa 17% of the total 6,000 population of the island. All houses in these communities were registered and their mode of construction recorded. Thick and thin blood films were prepared from all consenting individuals. Each individual was asked whether they possessed a mosquito net, whether they had slept under a mosquito net the previous night, whether they were allergic to chloroquine, and whether they had visited the main island of São Tomé since the beginning of the year. Outpatient records from March 1999 until the end of December 2000 were also examined and the age and place of residence of diagnosed cases noted.Results203 (19.8%) of the 1,026 individuals examined were found to be infected with Plasmodium falciparum. By fitting the mathematical model of the Garki project to the age-prevalence curve we estimate that the basic reproductive number, R0, on the island is approximately 1.6. Over a period of one year, a total of 1,792 P. falciparum cases reported to an outpatient facility at the islands hospital. Overall, 54% of the people interviewed slept under mosquito nets and were at reduced risk of infection. Conversely, people living in houses with openings between the top of the wall and the roof had higher risk of infection.ConclusionThis high incidence suggests that most of the malaria cases on the island attend the hospital and that treatment of these cases is an important factor reducing the effective rate of transmission. Providing that clinical cases are effectively treated, endemic malaria can probably be eliminated from the island mainly by reducing exposure to the vector with simple measures such as insecticide-treated nets and mosquito-proofing of dwellings. In contrast to traditional malaria eradication strategies, this would avoid the risk of malaria epidemics because the reduction in R0 should be sustainable.

Genetic structure of Anopheles gambiae (Diptera: Culicidae) in São Tomé and Príncipe (West Africa): implications for malaria control.

The impact of a vector eradication programme, conducted in the 1980s, on Anopheles gambiae populations from the islands of São Tomé and Príncipe, was evaluated by microsatellite DNA analysis. Significant genetic differentiation was observed within and between the two islands and between the islands and a population from Gabon, suggesting a degree of isolation between them. Large estimates of long‐term Ne suggested that the control programme did not affect the effective population size of the vector. Heterozygosity tests were also not consistent with a recent bottleneck.

Tracing the origins and signatures of selection of antifolate resistance in island populations of Plasmodium falciparum.

BackgroundResistance of the malaria parasite Plasmodium falciparum to sulfadoxine-pyrimethamine (SP) has evolved worldwide. In the archipelago of São Tomé and Principe (STP), West Africa, although SP resistance is highly prevalent the drug is still in use in particular circumstances. To address the evolutionary origins of SP resistance in these islands, we genotyped point mutations at P. falciparum dhfr and dhps genes and analysed microsatellites flanking those genes.MethodsBlood samples were collected in July and December 2004 in three localities of São Tomé Island and one in Principe Island. Species-specific nested-PCR was used to identify P. falciparum infected samples. Subsequently, SNPs at the dhfr and dhps genes were identified through PCR-RFLP. Isolates were also analysed for three microsatellite loci flanking the dhfr gene, three loci flanking dhps and four loci located at putative neutral genomic regions.ResultsAn increase of resistance-associated mutations at dhfr and dhps was observed, in particular for the dhfr/dhps quintuple mutant, associated with clinical SP failure. Analysis of flanking microsatellites suggests multiple independent introductions for dhfr and dhps mutant haplotypes, possibly from West Africa. A reduced genetic diversity and increased differentiation at flanking microsatellites when compared to neutral loci is consistent with a selective sweep for resistant alleles at both loci.ConclusionsThis study provides additional evidence for the crucial role of gene flow and drug selective pressures in the rapid spread of SP resistance in P. falciparum populations, from only a few mutation events giving rise to resistance-associated mutants. It also highlights the importance of human migration in the spread of drug resistant malaria parasites, as the distance between the islands and mainland is not consistent with mosquito-mediated parasite dispersal.

Non-immune patients in the Democratic Republic of São Tomé e Principe reveal a high level of transmission of P. ovale and P. vivax despite low frequency in immune patients

Malaria parasite species circulating in immune inhabitants of the Democratic Republic of São Tomé e Principe (DRSTP) and those acquired by non-immune travellers returning from this country have been compared. Using sensitive detection and species identification by PCR, Plasmodium parasites were found in the blood of 16 of the 43 travellers, who reported during the first 8 months of 1995 to a clinical diagnosis laboratory in Lisbon. Plasmodium vivax was found in four (25%) and Plasmodium ovale in ten (63%) of these patients. The observed prevalence of these two species in infected patients of DRSTP during 1995/1996 was <2%.

Contingency screening and ranking algorithm using two different sets of security performance indices

In this paper it is studied and analyzed the impact of different security performance indices on a contingency screening and ranking algorithm. The proposed filtering technique consists of three modules, with different complexity levels. The contingencies are ranked in accordance with severity indices that allow assessing the influence of the overloads and voltage limit violations in the power network. The developed software package was applied to the IEEE 118 test power system. The simulation results that were produced using two different sets of security indices were compared, showing a satisfactory agreement. The simulation was carried out considering a monotonous increase of the load level as well as a random one. For the different solutions the computing times were also obtained. Finally, some conclusions that provide a valuable contribution to the understanding of the electric power system security analysis are highlighted.