Search PubMed⌕ Search

Biomedical subjects

J D Charlwood

Publications and source records attributed to J D Charlwood.

At least 19 recordsLinked to original sources
PubMed2003

Observations on the swarming and mating behaviour of Anopheles funestus from southern Mozambique.

BACKGROUND: Control of malaria by the release of genetically modified mosquitoes refractory to transmission is now becoming a possibility. In many areas of Africa, Anopheles gambiae is found together with an equally important vector, An. funestus. Given their sympatry and the likelihood of a similar mating period some aspects of the mating behaviour of An. gambiae s.l. and An. funestus are likely to differ. We therefore attempted to characterise the swarming behaviour of An. funestus and to determine if any aspects of the observed behaviour differed from that recorded for the M form of An. gambiae from São Tomé. METHODS: In March - May 2002 the swarming, mating, house exiting and resting behaviour of Anopheles funestus was studied by direct observation in Mozambique. Swarming males and insects in copula were collected by sweep net. Wing lengths of males collected resting, exiting houses, swarming and mating were measured and the wingbeat frequency distribution of individual insects, in free flight confined inside netting covered paper cups, was also determined. RESULTS: Mono-specific swarms occurred at sunset in relatively open areas close to houses used for resting. Mating pairs were seen 11 +/- 3.7 min after the start of swarming. The number of total pairs observed being inversely proportional to the time difference between the start of swarming and the first pairing. The great majority of females mated before feeding. Male or female size did not appear to affect mating success or other behaviours. During the study, ambient temperatures decreased and female, but not male, wing size increased. At 516 Hz, the flight tone of female An. funestus was similar to the 497 Hz of the local An. gambiae. Males dispersed if light or dark artificial horizontal markers were placed underneath naturally occurring swarms. CONCLUSION: Differential response to markers would be sufficient for swarming in An. funestus and An. gambiae s.l. to occur in distinct sites.

Activity Cycles↗
PubMed2003

An island within an island: genetic differentiation of Anopheles gambiae in São Tomé, West Africa, and its relevance to malaria vector control.

Islands are choice settings for experimental studies of vector control strategies based on transgenic insects. Before considering this approach, knowledge of the population structure of the vector is essential. Genetic variation at 12 microsatellite loci was therefore studied in samples of the malaria vector Anopheles gambiae s.s., collected from six localities of São Tomé island (West Africa). The objectives were (i) to assess the demographic stability and effective population size of A. gambiae from these sites, (ii) to determine population differentiation and (iii) to relate the observed patterns of population structure with geographic, ecological and historical aspects of the vector on the island. Significant population differentiation, revealed by FST and RST statistics, was found between the southernmost site, Porto Alegre, and northern localities. The observed patterns of population substructure are probably a result of restrictions to gene flow in the less inhabited, more densely forested and mountainous south. In all localities surveyed, A. gambiae appeared to be experiencing a demographic expansion, consistent with a relatively recent (ca. 500 years) founder effect. The results are discussed with respect to current and future prospects of malaria vector control.

Africa, Western↗
PubMed2003

Mating does not affect the biting behaviour of Anopheles gambiae from the islands of São Tomé and Príncipe, West Africa.

To determine if mating or gonotrophic age influenced the biting behaviour of Anopheles gambiae s.s., a series of all-night landing captures was performed on the islands of São Tomé and Príncipe in the Gulf of Guinea. On São Tomé 49% and on Príncipe 56% of the newly emerged An. gambiae taking their first bloodmeal were virgins. On each island, with the exception of recently mated insects on Príncipe, all age-groups had similar biting cycles. The biting cycle on Príncipe resembled that observed on continental Africa, with a peak in the latter part of the night. Peak biting on São Tomé, however, occurred before midnight. Estimated daily survival rates were 0.77 and 0.29 for São Tomé and Príncipe, respectively. Mating does not affect the biting behaviour of An. gambiae on these islands.

Age Factors↗
PubMed2002

Male size does not affect mating success (of Anopheles gambiae in São Tomé).

For malaria control, the utility of transgenic vector Anopheles mosquitoes (Diptera: Culicidae) refractory to Plasmodium transmission, will depend on their interbreeding with the wild vector population. In many species, larger males are more successful in obtaining mates. In São Tomé island, we determined that size did not affect mating success of male Anopheles gambiae Giles sensu stricto, the main malaria vector in tropical Africa. Also we showed that larval intraspecific competition is probably insignificant in this population of An. gambiae. Thus, the potential success of transgenic An. gambiae is unlikely to be affected by size selection under field conditions.

Animals↗
PubMed2002

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 N(e) 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.

Africa, Western↗
PubMed2002

The swarming and mating behaviour of Anopheles gambiae s.s. (Diptera: Culicidae) from São Tomé Island.

The swarming and mating behaviours of the forest cytoform of Anopheles gambiae s.s. were investigated on 194 evenings and 14 mornings between April 1997 and November 1999 in a peri-urban area of the island of São Tomé, West Africa. Males swarmed 2-3 m above markers of horizontal contrast such as those formed between grass areas and footpaths, or bushes. Evening swarms started 2 min before sunset in sheltered sites and a minute or two later in exposed ones. It took approximately 5 mins from the arrival of the first male for the swarm to reach estimated maximum numbers. Mating pairs were first seen 7 min after the start of swarming. Maximum numbers of pairs in copula were observed 8 min later. Up to 270 pairings were seen in the 20 min period before darkness. Removal of males had no effect on the number of females arriving at the swarm. Males were attracted to sounds that approximated the female flight tone but not to filter paper samples of squashed virgin females swung through the swarm. A much-reduced amount of swarming and a single mating were recorded at dawn. The same locations for swarming were used at different times and at different heights by ants, Culicoides sp. and Culex quinquefasciatus.

Africa, Western↗
PubMed2001

The impact of indoor residual spraying with malathion on malaria in refugee camps in eastern Sudan.

An exploratory trial of the efficacy of indoor spraying with malathion on morbidity and mortality in refugee camps in eastern Sudan was conducted during the rainy season of 1997. The interior walls of houses from a randomly selected group of five camps were sprayed with malathion in mid-September and morbidity and mortality rates in the camps for the months October to December compared with rates in five controls. Pyrethrum spray collection and human landing catches were performed in two collection rounds. An exophagic but endophilic population of Anopheles arabiensis was the most common mosquito collected. The mean human blood index of 242 mosquitoes from eight camps was 0.51. Only two of 1040 mosquitoes examined harboured sporozoites. Blood samples of 83 putative malaria patients were examined for parasites by PCR. Mortality rates in the 3 months following spraying were significantly lower in sprayed camps although differences in clinical malaria incidence between sprayed and non-sprayed camps were not significant.

Animals↗
PubMed2001

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.

Adolescent↗
PubMed2001

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

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.

Africa, Western↗
PubMed2000

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.

Adolescent↗
PubMed2000

Dry season refugia of malaria-transmitting mosquitoes in a dry savannah zone of east Africa.

Dry season survival of Anopheles funestus, Anopheles gambiae and Anopheles arabiensis in the Kilombero valley a dry savannah zone of east Africa, was investigated with over 400 collections from 23 areas, covering 300 sq km of the valley. Anopheles gambiae was found only in association with humans, in forested areas of high annual rainfall, while An. funestus occurred at high densities at the valley edge where large non-moving bodies of water remained. A large population of An. arabiensis was present along the river system throughout the middle of the valley, and mosquitoes probably derived from this population were occasionally caught in villages bordering the valley. No evidence was obtained of aestivation in any mosquito species. Anopheles gambiae was the most long lived, 6.3% compared to 2.0% of the An. arabiensis and 4% of the An. funestus surviving for four or more gonotrophic cycles, the approximate duration of the extrinsic cycle of most malaria parasites. Oocysts of malaria parasites were found in 5.4% of An. funestus and 2.3% of An. arabiensis from villages. Oocyst rates in An. funestus differed significantly between areas but not between houses within areas. Anopheles funestus is the most important dry season malaria vector in the valley, and remains in foci closely associated with groups of houses. All three species survive at high densities but as otherwise hidden refugia populations.

Animals↗
PubMed1999

Population structure in the malaria vector, Anopheles arabiensis patton, in East Africa.

The population structure of the malaria vector Anopheles arabiensis was investigated using data from six microsatellite loci in samples from localities in Mozambique and Tanzania. Genotype frequencies were neither significantly different between houses in a village in Tanzania nor between villages within a 20-km radius in Mozambique. Thus a deme has an area greater than 20 km in radius. At five of the six loci the heterozygosity of the population from Mozambique was lower than that from Tanzania, implying a lower effective population size (Ne) at this southern edge of the species range. There were significant differences in genotype frequencies between the Tanzanian and Mozambique populations at five of the six loci (P<0.05). Values for both FST (mean=0.069) and RST (mean=0.025) were significantly different from zero (P<0.05) at four and three out of five loci, respectively, but there was no significant correlation between the two statistics. The wide variation in values of FST and RST across loci suggests that care should be taken in interpreting values derived from averaging across loci. Whether the variation results from sampling effects or selectional constraints on some loci is unclear. Although there is evidence for significant differentiation between these populations, estimates of gene flow (Nm) calculated from mean FST and RST statistics were relatively high, 3.4 and 4.9, respectively. We argue that this is more likely to reflect recent separation of these populations and/or large effective population size rather than large-scale present day migrations.

Animals↗
PubMed1998

Genetic characterization of the malaria parasite Plasmodium falciparum in the transmission from the host to the vector.

The present study followed a Plasmodium falciparum population through its life-cycle at the level of individual hosts and vectors. The aim was to determine to what extent genotypes of the parasite that were found in the host were transmitted to the vector. Mosquitoes were collected that had fed on people sleeping under mosquito nets with holes. Genes determining 2 highly polymorphic merozoite surface proteins, MSP-1 and MSP-2, were used to characterize the parasite by PCR at different stages of the life-cycle. The parasite genotypes found in the host were compared to the genotypes of the oocyst stages after transmission to the mosquitoes. The results show that there was no significant correlation in the rate of oocyst-positive mosquitoes and the presence of parasites in the blood samples. For MSP-1, most of the parasites characterized in the human blood by PCR fragment size were also found afterwards in the oocyst-stage (22 of 29; MSP-2: 17 of 36). This study indicates that there is no selective transmission of distinct genotypes to the vector. In addition, the frequencies of the allelic families of both genes are similar in the blood samples and in the oocysts for the whole population.

Adolescent↗
PubMed1998

Incidence of Plasmodium falciparum infection in infants in relation to exposure to sporozoite-infected anophelines.

The relationship of the incidence of Plasmodium falciparum infection to entomologic inoculation rates (EIRs) was studied in 163 children less than one year of age in a Tanzanian village to determine likely effects of transmission-reducing interventions on infection incidence. A total of 66,727 Anopheles gambiae s.l. and 17,620 An. funestus mosquitoes were caught in 1,056 light trap collections from 139 houses over a period of more than two years. Time period-specific human biting rates were estimated for 11 village neighborhoods. Sporozoites were detected by ELISA in 4.4% of the An. funestus and 2.5% of the An. gambiae s.l. Eight hundred seventeen pairs of blood slides with approximately two-week intervals between slides were used to estimate incidence of parasitemia by fitting reversible catalytic models to parasite positivity data. Estimated EIRs during the four weeks preceding each intersurvey interval averaged 1.6 (SD = 2.1) per adult per night. Parasites were present at the end of 31% of the 443 intervals that commenced with a parasite-negative slide. Attack rates were comparable with those in western Kenya, and the proportion of bites resulting in human infections was strongly dependent on mosquito density. Incidence of infection increased with the EIR up to approximately one bite from a sporozoite-carrying mosquito per adult per night. However, higher levels of transmission observed locally in the wet season did not result in a correspondingly higher incidence. These data suggest that transmission-reducing measures cannot be expected to reduce incidence of infection at the highest levels of EIR.

Animals↗
PubMed1998

Relationships of malaria morbidity with exposure to Plasmodium falciparum in young children in a highly endemic area.

To study incidence of clinical Plasmodium falciparum malaria in relation to exposure to parasites, attendance of children less than eighteen months old at a village dispensary in a highly endemic area of Tanzania was recorded. Entomologic inoculation rates (EIRs), estimated as a function of time period and place of residence, exceeded one sporozoite positive bite per adult per night in some village neighborhoods during the wet season. Incidence of clinical P. falciparum malaria, defined either as fever with parasitemia or as fever with hyperparasitemia, increased with the EIR over the whole range of exposures. Each 10-fold increase in the EIR corresponded to a 1.6-fold increase in incidence of fever plus parasitemia (95% confidence interval = 1.4-2.0). Therefore reduction of human-vector contacts will probably reduce morbidity incidence even at very high exposures. Incidence showed little relationship to estimated cumulative numbers of inoculations since birth, but decreased steeply with estimated cumulative time infected with trophozoites. This suggests that clinical immunity depends mainly on the extent of exposure to blood-stage antigens, not on the diversity of inocula seen, and thus temporary reductions in human-vector contacts are unlikely to result in subsequent increases in morbidity.

Animals↗
PubMed1997

Heterogeneities in the transmission of infectious agents: implications for the design of control programs.

From an analysis of the distributions of measures of transmission rates among hosts, we identify an empirical relationship suggesting that, typically, 20% of the host population contributes at least 80% of the net transmission potential, as measured by the basic reproduction number, R0. This is an example of a statistical pattern known as the 20/80 rule. The rule applies to a variety of disease systems, including vector-borne parasites and sexually transmitted pathogens. The rule implies that control programs targeted at the "core" 20% group are potentially highly effective and, conversely, that programs that fail to reach all of this group will be much less effective than expected in reducing levels of infection in the population as a whole.

Animals↗
PubMed1996

Absence of relationships between selected human factors and natural infectivity of Plasmodium falciparum to mosquitoes in an area of high transmission.

The effects of sex, age of the human host, patency of asexual and sexual stages and seasonality on infectiousness of Plasmodium falciparum to mosquitoes were investigated in a rural village in southern Tanzania between 1992 and 1994. Villagers from randomized subgroups of households were surveyed for malaria parasites. Gametocyte and trophozoite prevalences were age dependent and fluctuated without any clear pattern of seasonality. A sample of 107 participants, selected to include an excess of gametocyte carriers, slept under bednets with holes cut into the sides for 3 weeks. A total of 3837 Anopheles gambiae s.l. and 5403 A. funestus recovered from these bednets, was examined for all oocysts 5-7 days after feeding or for oocysts less than 17.5 microns in diameter 2-3 days after feeding. Additional blood slides from participants were taken twice weekly. The 5-7 day oocyst rates were 12.1% in A. gambiae s.l. and 10.9% in A. funestus and 2-3 day rates were 3.6 and 4.9%, respectively. The higher rates using the former method were attributed to previous infection. There were strong correlations in the levels of infection in both vectors when they fed on the same hosts. However, patent gametocytaemia was only weakly associated with the development of oocysts in the mosquito. Infectiousness was not related to host age, sex, or the season.

Adolescent↗