Malaria Transmission In Africa Research Articles

Difficulties in organizing first indoor spray programme against malaria in Angola under the President’s Malaria Initiative

Successful attempts to control malaria require understanding of its complex transmission patterns. Unfortunately malaria transmission in Africa is often assessed using routine administrative reports from local health units, which are plagued by sporadic reporting failures. In addition, the lack of microscopic analyses of blood slides in these units introduces the effects of many confounding diseases. The danger of using administrative reports was illustrated in Angola, the first country in which malaria control was attempted under the President's Malaria Initiative, a development programme of the Government of the United States of America. Each local health unit submitted monthly reports indicating the number of suspected malaria cases to their municipality. The identification of the disease was based on clinical diagnoses, without microscopic examination of blood slides. The municipal and provincial reports were then passed on to the national headquarters, with sporadic reporting lapses at all levels. After the control effort was completed, the defective municipal reports were corrected by summarizing only the data from those health units which had submitted reports for every month during the evaluation period. The corrected data, supplemented by additional observations on rainfall and mosquito habitats, indicated that there had probably been no malaria transmission before starting the control operations. Thus the expensive malaria control effort had been wasted. It is unfortunate that WHO is also trying to plan and evaluate its malaria control efforts based on these same kinds of inadequate administrative reports.

Window screening, ceilings and closed eaves as sustainable ways to control malaria in Dar es Salaam, Tanzania

BackgroundMalaria transmission in Africa occurs predominantly inside houses where the primary vectors prefer to feed. Human preference and investment in blocking of specific entry points for mosquitoes into houses was evaluated and compared with known entry point preferences of the mosquitoes themselves.MethodsCross-sectional household surveys were conducted in urban Dar es Salaam, Tanzania to estimate usage levels of available options for house proofing against mosquito entry, namely window screens, ceilings and blocking of eaves. These surveys also enabled evaluation of household expenditure on screens and ceilings and the motivation behind their installation.ResultsOver three quarters (82.8%) of the 579 houses surveyed in Dar es Salaam had window screens, while almost half (48.9%) had ceilings. Prevention of mosquito entry was cited as a reason for installation of window screens and ceilings by 91.4% (394/431) and 55.7% (127/228) of respondents, respectively, but prevention of malaria was rarely cited (4.3%, 22/508). The median cost of window screens was between US $ 21-30 while that of ceilings was between US $301-400. The market value of insecticide-treated nets, window screening and ceilings currently in use in the city was estimated as 2, 5 and 42 million US$. More than three quarters of the respondents that lacked them said it was too expensive to install ceilings (82.2%) or window screens (75.5%).ConclusionHigh coverage and spending on screens and ceilings implies that these techniques are highly acceptable and excellent uptake can be achieved in urban settings like Dar es Salaam. Effective models for promotion and subsidization should be developed and evaluated, particularly for installation of ceilings that prevent entry via the eaves, which are the most important entry point for mosquitoes that cause malaria, a variety of neglected tropical diseases and the nuisance which motivates uptake.

Complexity of the Malaria Vectorial System in Cameroon: Contribution of Secondary Vectors to Malaria Transmission

Malaria transmission in Africa is a dynamic and complex system that is so far superficially understood. Further knowledge is required to improve control of the disease. In the present report, we highlight the contribution of the so-called "secondary" malaria vectors to the overall parasite transmission intensity in several sites across Cameroon, through a retrospective analysis of surveys from the Organisation de Coordination pour la lutte Contre les Endémies en Afrique Centrale database. In total, 48,490 female anophelines belonging to 21 different species were collected between October 1998 and March 2003. Anopheles gambiae Giles, Anopheles arabiensis Patton, Anopheles funestus Giles, Anopheles nili (Theobald), and Anopheles moucheti Evans represented 89% of the total anopheline fauna. Beside these major vectors, malaria parasites or their circumsporozoite proteins were found in nine secondary malaria vectors: Anopheles ovengensis Awono-Ambene et al., Anopheles carnevalei Brunhes et al., Anopheles coustani Laveran, Anopheles hancocki Edwards, Anopheles marshallii (Theobald), Anopheles paludis Theobald, Anopheles pharoensis Theobald, Anopheles wellcomei Theobald, and Anopheles ziemanni Grtünberg. The mean infection rate of secondary vectors (1.36%) was significantly (P < 0.001) lower than that of major vectors (3.08%). An. pharoensis and An. ovengensis were repeatedly found infected by Plasmodium falciparum Welch and contributed substantially to the total malaria transmission intensity in some areas where they were abundant. Both species have strong exophilic and/or exophagic habits such that they might elude vector control directed against endophilic and endophagic malaria vectors.

Sulfadoxine-pyrimethamine plus artesunate versus sulfadoxine-pyrimethamine plus amodiaquine for treating uncomplicated malaria.

Artemisinin-based combination treatments are strongly advocated, but supplies are limited. Sulfadoxine combined with amodiaquine is an alternative non-artemisinin combination. To compare sulfadoxine-pyrimethamine plus amodiaquine (SP plus AQ) with sulfadoxine-pyrimethamine plus artesunate (SP plus AS) for treating uncomplicated Plasmodium falciparum malaria. We searched the Cochrane Infectious Diseases Group Specialized Register (October 2005), CENTRAL (The Cochrane Library 2005, Issue 4), MEDLINE (1966 to October 2005), EMBASE (1988 to October 2005), LILACS (October 2005), and reference lists. We also contacted researchers and organizations working in this field. Randomized controlled trials comparing SP plus AS with SP plus AQ for treating uncomplicated P. falciparum malaria. Two authors independently applied the inclusion criteria, extracted data, and assessed methodological quality. The primary outcome measure was treatment failure (parasitological or clinical evidence of treatment failure between start of treatment and day 28). We calculated the relative risk (RR) with 95% confidence intervals (CI) for dichotomous data. Four trials (775 participants) met the inclusion criteria. All were from areas of high and seasonal malaria transmission in Africa. Fewer participants using SP plus AQ failed treatment by day 28 (RR 0.59, 95% CI 0.42 to 0.83; 652 participants, 3 trials). Even excluding new infections, SP plus AQ performed better (RR 0.62, 95% CI 0.40 to 0.96; 649 participants, 3 trials). There was no statistically significant difference between the two treatments for treatment failure at day 14 (RR 1.14, 95% CI 0.47 to 2.78; 775 participants, 4 trials). SP plus AS was more effective at reducing gametocyte carriage at day seven (RR 2.31, 95% CI 1.36 to 3.92; 220 participants, 1 trial). One trial reported that one person - in the SP plus AQ group - developed severe malaria. Adverse events were poorly reported, but did not seem to differ in type and number between the two treatment combinations. SP plus AQ performed better at controlling treatment failure at day 28, but was not as good as SP plus AS at reducing gametocyte carriage at day seven. Careful consideration of local resistance patterns is required because resistance to sulfadoxine-pyrimethamine and amodiaquine are high in many areas. In order to delay development of resistance to artesunate, the combination with sulfadoxine-pyrimethamine should only be considered where both drugs are known to be effective. Data on adverse events are still lacking.

Reducing Malaria Transmission in Africa

Reducing Malaria Transmission in Africa

A highly conserved candidate chemoreceptor expressed in both olfactory and gustatory tissues in the malaria vector Anopheles gambiae.

Anopheles gambiae is a highly anthropophilic mosquito responsible for the majority of malaria transmission in Africa. The biting and host preference behavior of this disease vector is largely influenced by its sense of smell, which is presumably facilitated by G protein-coupled receptor signaling [Takken, W. & Knols, B. (1999) Annu. Rev. Entomol. 44, 131-157]. Because of the importance of host preference to the mosquitoes' ability to transmit disease, we have initiated studies intended to elucidate the molecular mechanisms underlying olfaction in An. gambiae. In the course of these studies, we have identified a number of genes potentially involved in signal transduction, including a family of candidate odorant receptors. One of these receptors, encoded by GPRor7 (hereafter referred to as AgOr7), is remarkably similar to an odorant receptor that is expressed broadly in olfactory tissues and has been identified in Drosophila melanogaster and other insects [Krieger, J., Klink, O., Mohl, C., Raming, K. & Breer, H. (2003) J. Comp. Physiol. A 189, 519-526; Vosshall, L. B., Amrein, H., Morozov, P. S., Rzhetsky, A. & Axel, R. (1999) Cell 96, 725-736]. We have observed AgOr7 expression in olfactory and gustatory tissues in adult An. gambiae and during several stages of the mosquitoes' development. Within the female adult peripheral chemosensory system, antiserum against the AgOR7 polypeptide labels most sensilla of the antenna and maxillary palp as well as a subset of proboscis sensilla. Furthermore, AgOR7 antiserum labeling is observed within the larval antenna and maxillary palpus. These results are consistent with a role for AgOr7 in both olfaction and gustation in An. gambiae and raise the possibility that AgOr7 orthologs may also be of general importance to both modalities of chemosensation in other insects.

Climatic suitability for malaria transmission in Africa, 1911-1995.

Time series analysis of a climate-driven model of malaria transmission shows limited evidence for an increase in suitability during the last century across Africa. Outside areas where climate was always or never suitable, <17% of the continent showed significant trends in malaria transmission. Of these areas, 5.7% showed positive deterministic trends, 6.1% had negative deterministic trends, and 5.1% exhibited stochastic trends. In areas with positive trends, precipitation, rather than temperature, was the primary forcing variable. This analysis highlights the need to examine the relationship between climate and malaria more closely and to fully consider nonclimatic factors as drivers of increased malaria transmission across the continent.

Potential effect of climate change on malaria transmission in Africa.

Climate change is likely to affect transmission of vector-borne diseases such as malaria. We quantitatively estimated current malaria exposure and assessed the potential effect of projected climate scenarios on malaria transmission. We produced a spatiotemporally validated (against 3791 parasite surveys) model of Plasmodium falciparum malaria transmission in Africa. Using different climate scenarios from the Hadley Centre global climate model (HAD CM3) climate experiments, we projected the potential effect of climate change on transmission patterns. Our model showed sensitivity and specificity of 63% and 96%, respectively (within 1 month temporal accuracy), when compared with the parasite surveys. We estimate that on average there are 3.1 billion person-months of exposure (445 million people exposed) in Africa per year. The projected scenarios would estimate a 5-7% potential increase (mainly altitudinal) in malaria distribution with surprisingly little increase in the latitudinal extents of the disease by 2100. Of the overall potential increase (although transmission will decrease in some countries) of 16-28% in person-months of exposure (assuming a constant population), a large proportion will be seen in areas of existing transmission. The effect of projected climate change indicates that a prolonged transmission season is as important as geographical expansion in correct assessment of the effect of changes in transmission patterns. Our model constitutes a valid baseline against which climate scenarios can be assessed and interventions planned.

Hpall endonuclease distinguishes between two species in the Anopheles funestus group.

The Anopheles funestus group consists of at least eight species that are currently identified mainly on morphological criteria. Until recently, only An. funestus s.s. was implicated in the transmission of malaria in Africa, but recent work in Tanzania has shown that An. rivulorum is also involved, albeit to a lesser degree than An. funestus. The constraints in the identification of the species and the need to clarify better their epidemiological role have led to the development of a PCR-RFLP method for the identification of two anthropophilic members of the group. Using PCR primers developed from the D3 region in the 28S ribosomal gene, amplified products were digested with the restriction endonuclease Hpall. This produced two distinct fragments on an agarose gel that could be used to separate An. funestus from An. vaneedeni. The technique needs to be tested on natural populations of these two species as well as on other members of the An. funestus group.

Intraleucocytic malaria pigment and clinical severity of malaria in children

Intraleucocytic malaria pigment has been suggested as a measure of disease severity in malaria. We have tested this hypothesis by studying 146 children aged 6 months to 14 years in 4 categories—cerebral malaria, mild malaria, asymptomatic malaria and ‘no malaria’—in Ibadan, Nigeria, an area of intense malaria transmission in Africa. Children with cerebral malaria were studied at the university hospital, those with mild malaria at 2 primary health centres and the other 2 groups were studied in a primary school.The proportion of pigment-containing neutrophils showed a clear rise across the spectrum no malaria—asymptomatic malaria—mild malaria—cerebral malaria (median values 2·0%, 6·5%, 9·0% and 27·0%, respectively; P < 0·0001).The proportion of pigment-containing monocytes did not differ significantly between the mild malaria, asymptomatic malaria and no malaria groups but the cerebral malaria group had a higher median value than the other 3 groups. The ratio of pigment-containing neutrophils to pigment-containing monocytes showed the same trend across the groups of subjects as was observed with the number of pigmentcontaining neutrophils. It is concluded that the pigment-containing neutrophil count is a simple marker of disease severity in childhood malaria in addition to the parasite count.