Publications
109 results found
Winskill P, Slater HC, Griffin JT, et al., 2017, THE IMPORTANCE OF US FOREIGN AID FOR GLOBAL MALARIA CONTROL AND ELIMINATION, 66th Annual Meeting of the American-Society-of-Tropical-Medicine-and-Hygiene (ASTMH), Publisher: AMER SOC TROP MED & HYGIENE, Pages: 217-217, ISSN: 0002-9637
Walker PG, Griffin JT, Ferguson NM, et al., 2016, Estimating the most efficient allocation of interventions to achieve reductions in Plasmodium falciparum malaria burden and transmission in Africa: a modelling study, Lancet Global Health, Vol: 4, Pages: e474-e484, ISSN: 2214-109X
BACKGROUND: Reducing the burden of malaria is a global priority, but financial constraints mean that available resources must be allocated rationally to maximise their effect. We aimed to develop a model to estimate the most efficient (ie, minimum cost) ordering of interventions to reduce malaria burden and transmission. We also aimed to estimate the efficiency of different spatial scales of implementation. METHODS: We combined a dynamic model capturing heterogeneity in malaria transmission across Africa with financial unit cost data for key malaria interventions. We combined estimates of patterns of malaria endemicity, seasonality in rainfall, and mosquito composition to map optimum packages of these interventions across Africa. Using non-linear optimisation methods, we examined how these optimum packages vary when control measures are deployed and assessed at national, subnational first administrative (provincial), or fine-scale (5 km(2) pixel) spatial scales. FINDINGS: The most efficient package in a given setting varies depending on whether disease reduction or elimination is the target. Long-lasting insecticide-treated nets are generally the most cost-effective first intervention to achieve either goal, with seasonal malaria chemoprevention or indoor residual spraying added second depending on seasonality and vector species. These interventions are estimated to reduce malaria transmission to less than one case per 1000 people per year in 43·4% (95% CI 40·0-49·0) of the population at risk in Africa. Adding three rounds of mass drug administration per year is estimated to increase this proportion to 90·9% (95% CI 86·9-94·6). Further optimisation can be achieved by targeting policies at the provincial level, achieving an estimated 32·1% (95% CI 29·6-34·5) cost saving relative to adopting country-wide policies. Nevertheless, we predict that only 26 (95% CI 22-29) of 41 countries could reduce transmissio
Ghani AC, Walker PG, 2015, Provision of malaria treatment for Ebola case contacts., Lancet Infectious Diseases, Vol: 16, Pages: 391-392, ISSN: 1473-3099
Slater HC, Ross A, Ouedraogo AL, et al., 2015, Assessing the impact of next-generation rapid diagnostic tests on Plasmodium falciparum malaria elimination strategies, Nature, Vol: 528, Pages: S94-S101, ISSN: 0028-0836
Mass-screen-and-treat and targeted mass-drug-administration strategies are being considered as a means to interrupt transmission of Plasmodium falciparum malaria. However, the effectiveness of such strategies will depend on the extent to which current and future diagnostics are able to detect those individuals who are infectious to mosquitoes. We estimate the relationship between parasite density and onward infectivity using sensitive quantitative parasite diagnostics and mosquito feeding assays from Burkina Faso. We find that a diagnostic with a lower detection limit of 200 parasites per microlitre would detect 55% of the infectious reservoir (the combined infectivity to mosquitoes of the whole population weighted by how often each individual is bitten) whereas a test with a limit of 20 parasites per microlitre would detect 83% and 2 parasites per microlitre would detect 95% of the infectious reservoir. Using mathematical models, we show that increasing the diagnostic sensitivity from 200 parasites per microlitre (equivalent to microscopy or current rapid diagnostic tests) to 2 parasites per microlitre would increase the number of regions where transmission could be interrupted with a mass-screen-and-treat programme from an entomological inoculation rate below 1 to one of up to 4. The higher sensitivity diagnostic could reduce the number of treatment rounds required to interrupt transmission in areas of lower prevalence. We predict that mass-screen-and-treat with a highly sensitive diagnostic is less effective than mass drug administration owing to the prophylactic protection provided to uninfected individuals by the latter approach. In low-transmission settings such as those in Southeast Asia, we find that a diagnostic tool with a sensitivity of 20 parasites per microlitre may be sufficient for targeted mass drug administration because this diagnostic is predicted to identify a similar village population prevalence compared with that currently detected using polym
Wu L, van den Hoogen LL, Slater H, et al., 2015, Comparison of diagnostics for the detection of asymptomatic Plasmodium falciparum infections to inform control and elimination strategies, Nature, Vol: 528, Pages: S86-S93, ISSN: 0028-0836
The global burden of malaria has been substantially reduced over the past two decades. Future efforts to reduce malaria further will require moving beyond the treatment of clinical infections to targeting malaria transmission more broadly in the community. As such, the accurate identification of asymptomatic human infections, which can sustain a large proportion of transmission, is becoming a vital component of control and elimination programmes. We determined the relationship across common diagnostics used to measure malaria prevalence — polymerase chain reaction (PCR), rapid diagnostic test and microscopy — for the detection of Plasmodium falciparum infections in endemic populations based on a pooled analysis of cross-sectional data. We included data from more than 170,000 individuals comparing the detection by rapid diagnostic test and microscopy, and 30,000 for detection by rapid diagnostic test and PCR. The analysis showed that, on average, rapid diagnostic tests detected 41% (95% confidence interval = 26–66%) of PCR-positive infections. Data for the comparison of rapid diagnostic test to PCR detection at high transmission intensity and in adults were sparse. Prevalence measured by rapid diagnostic test and microscopy was comparable, although rapid diagnostic test detected slightly more infections than microscopy. On average, microscopy captured 87% (95% confidence interval = 74–102%) of rapid diagnostic test-positive infections. The extent to which higher rapid diagnostic test detection reflects increased sensitivity, lack of specificity or both, is unclear. Once the contribution of asymptomatic individuals to the infectious reservoir is better defined, future analyses should ideally establish optimal detection limits of new diagnostics for use in control and elimination strategies.
Walker PGT, 2015, Tracking malaria transmission at the antenatal clinic, LANCET GLOBAL HEALTH, Vol: 3, Pages: E581-E582, ISSN: 2214-109X
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Cairns ME, Walker PGT, Okell LC, et al., 2015, Seasonality in malaria transmission: implications for case-management with long-acting artemisinin combination therapy in sub-Saharan Africa, Malaria Journal, Vol: 14, ISSN: 1475-2875
Background: Long-acting artemisinin-based combination therapy (LACT) offers the potential to prevent recurrentmalaria attacks in highly exposed children. However, it is not clear where this advantage will be most important, anddeployment of these drugs is not rationalized on this basis.Methods: To understand where post-treatment prophylaxis would be most beneficial, the relationship betweenseasonality, transmission intensity and the interval between malaria episodes was explored using data from six cohortstudies in West Africa and an individual-based malaria transmission model. The total number of recurrent malariacases per 1000 child-years at risk, and the fraction of the total annual burden that this represents were estimated forsub-Saharan Africa.Results: In settings where prevalence is less than 10 %, repeat malaria episodes constitute a small fraction of thetotal burden, and few repeat episodes occur within the window of protection provided by currently available drugs.However, in higher transmission settings, and particularly in high transmission settings with highly seasonal transmis‑sion, repeat malaria becomes increasingly important, with up to 20 % of the total clinical burden in children estimatedto be due to repeat episodes within 4 weeks of a prior attack.Conclusion: At a given level of transmission intensity and annual incidence, the concentration of repeat malariaepisodes in time, and consequently the protection from LACT is highest in the most seasonal areas. As a result, thedegree of seasonality, in addition to the overall intensity of transmission, should be considered by policy makers whendeciding between ACT that differ in their duration of post-treatment prophylaxis.
Walker PGT, White MT, Griffin JT, et al., 2015, Malaria morbidity and mortality in Ebola-affected countries caused by decreased health-care capacity, and the potential effect of mitigation strategies: a modelling analysis, Lancet Infectious Diseases, Vol: 15, Pages: 825-832, ISSN: 1473-3099
BackgroundThe ongoing Ebola epidemic in parts of west Africa largely overwhelmed health-care systems in 2014, making adequate care for malaria impossible and threatening the gains in malaria control achieved over the past decade. We quantified this additional indirect burden of Ebola virus disease.MethodsWe estimated the number of cases and deaths from malaria in Guinea, Liberia, and Sierra Leone from Demographic and Health Surveys data for malaria prevalence and coverage of malaria interventions before the Ebola outbreak. We then removed the effect of treatment and hospital care to estimate additional cases and deaths from malaria caused by reduced health-care capacity and potential disruption of delivery of insecticide-treated bednets. We modelled the potential effect of emergency mass drug administration in affected areas on malaria cases and health-care demand.FindingsIf malaria care ceased as a result of the Ebola epidemic, untreated cases of malaria would have increased by 45% (95% credible interval 43–49) in Guinea, 88% (83–93) in Sierra Leone, and 140% (135–147) in Liberia in 2014. This increase is equivalent to 3·5 million (95% credible interval 2·6 million to 4·9 million) additional untreated cases, with 10 900 (5700–21 400) additional malaria-attributable deaths. Mass drug administration and distribution of insecticide-treated bednets timed to coincide with the 2015 malaria transmission season could largely mitigate the effect of Ebola virus disease on malaria.InterpretationThese findings suggest that untreated malaria cases as a result of reduced health-care capacity probably contributed substantially to the morbidity caused by the Ebola crisis. Mass drug administration can be an effective means to mitigate this burden and reduce the number of non-Ebola fever cases within health systems.
Cairns M, Walker PG, 2015, Monthly malaria chemoprevention shows potential in an area of very high, perennial malaria transmission, Evidence-Based Medicine, Vol: 20, ISSN: 1473-6810
Walker PGT, Jost C, Ghani AC, et al., 2015, Estimating the Transmissibility of H5N1 and the Effect of Vaccination in Indonesia, TRANSBOUNDARY AND EMERGING DISEASES, Vol: 62, Pages: 200-208, ISSN: 1865-1674
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- Citations: 5
Walker PGT, Cairns M, 2015, Value of additional chemotherapy for malaria in pregnancy, LANCET GLOBAL HEALTH, Vol: 3, Pages: E116-E117, ISSN: 2214-109X
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- Citations: 1
Slater HC, Walker PGT, Bousema T, et al., 2014, The Potential Impact of Adding Ivermectin to a Mass Treatment Intervention to Reduce Malaria Transmission: A Modelling Study, JOURNAL OF INFECTIOUS DISEASES, Vol: 210, Pages: 1972-1980, ISSN: 0022-1899
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- Citations: 71
Walker PGT, ter Kuile FO, Garske T, et al., 2014, Estimated risk of placental infection and low birthweight attributable to Plasmodium falciparum malaria in Africa in 2010: a modelling study, Lancet Global Health, Vol: 2, Pages: E460-E467, ISSN: 2214-109X
Walker PGT, Griffin JT, Cairns M, et al., 2013, A model of parity-dependent immunity to placental malaria, Nature Communications, Vol: 4, ISSN: 2041-1723
Plasmodium falciparum placental infection during pregnancy is harmful for both mother andchild. Protection from placental infection is parity-dependent, that is, acquired over consecutivepregnancies. However, the infection status of the placenta can only be assessed atdelivery. Here, to better understand the mechanism underlying this parity-dependence, wefitted a model linking malaria dynamics within the general population to observed placentalhistology. Our results suggest that immunity resulting in less prolonged infection is a greaterdeterminant of the parity-specific patterns than immunity that prevents placental sequestration.Our results also suggest the time when maternal blood first flows into the placenta isa high-risk period. Therefore, preventative strategies implementable before or early inpregnancy, such as insecticide-treated net usage in women of child-bearing age or any futurevaccine, could substantially reduce the number of women who experience placental infection.
Walker P, Cauchemez S, Hartemink N, et al., 2012, Outbreaks of H5N1 in poultry in Thailand: the relative role of poultry production types in sustaining transmission and the impact of active surveillance in control, JOURNAL OF THE ROYAL SOCIETY INTERFACE, Vol: 9, Pages: 1836-1845, ISSN: 1742-5689
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- Citations: 21
Fournie G, Walker P, Porphyre T, et al., 2012, Mathematical Models of Infectious Diseases in Livestock: Concepts and Application to the Spread of Highly Pathogenic Avian Influenza Virus Strain Type H5N1, HEALTH AND ANIMAL AGRICULTURE IN DEVELOPING COUNTRIES, Editors: Zilberman, Otte, RolandHolst, Pfeiffer, Publisher: SPRINGER, Pages: 183-205, ISBN: 978-1-4419-7076-3
Walker PGT, Cauchemez S, Metras R, et al., 2010, A Bayesian Approach to Quantifying the Effects of Mass Poultry Vaccination upon the Spatial and Temporal Dynamics of H5N1 in Northern Vietnam, PLOS COMPUTATIONAL BIOLOGY, Vol: 6, ISSN: 1553-734X
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- Citations: 26
Walker PT, Hallett TB, White PJ, et al., 2008, Interpreting declines in HIV prevalence: impact of spatial aggregation and migration on expected declines in prevalence, SEXUALLY TRANSMITTED INFECTIONS, Vol: 84, Pages: II42-II48, ISSN: 1368-4973
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- Citations: 19
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