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"Malaria - epidemiology"
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The changing epidemiology of malaria elimination: new strategies for new challenges
Malaria-eliminating countries achieved remarkable success in reducing their malaria burdens between 2000 and 2010. As a result, the epidemiology of malaria in these settings has become more complex. Malaria is increasingly imported, caused by Plasmodium vivax in settings outside sub-Saharan Africa, and clustered in small geographical areas or clustered demographically into subpopulations, which are often predominantly adult men, with shared social, behavioural, and geographical risk characteristics. The shift in the populations most at risk of malaria raises important questions for malaria-eliminating countries, since traditional control interventions are likely to be less effective. Approaches to elimination need to be aligned with these changes through the development and adoption of novel strategies and methods. Knowledge of the changing epidemiological trends of malaria in the eliminating countries will ensure improved targeting of interventions to continue to shrink the malaria map.
Journal Article
Influence of environmental, geographic, socio-demographic, and epidemiological factors on presence of malaria at the community level in two continents
The interactions of environmental, geographic, socio-demographic, and epidemiological factors in shaping mosquito-borne disease transmission dynamics are complex and changeable, influencing the abundance and distribution of vectors and the pathogens they transmit. In this study, 27 years of cross-sectional malaria survey data (1990–2017) were used to examine the effects of these factors on
Plasmodium falciparum
and
Plasmodium vivax
malaria presence at the community level in Africa and Asia. Monthly long-term, open-source data for each factor were compiled and analyzed using generalized linear models and classification and regression trees. Both temperature and precipitation exhibited unimodal relationships with malaria, with a positive effect up to a point after which a negative effect was observed as temperature and precipitation increased. Overall decline in malaria from 2000 to 2012 was well captured by the models, as was the resurgence after that. The models also indicated higher malaria in regions with lower economic and development indicators. Malaria is driven by a combination of environmental, geographic, socioeconomic, and epidemiological factors, and in this study, we demonstrated two approaches to capturing this complexity of drivers within models. Identifying these key drivers, and describing their associations with malaria, provides key information to inform planning and prevention strategies and interventions to reduce malaria burden.
Journal Article
A Prospective Comparative Study of Knowlesi, Falciparum, and Vivax Malaria in Sabah, Malaysia: High Proportion With Severe Disease From Plasmodium Knowlesi and Plasmodium Vivax But No Mortality With Early Referral and Artesunate Therapy
Background. Plasmodium knowlesi commonly causes severe malaria in Malaysian Borneo, with high case-fatality rates reported. We compared risk, spectrum, and outcome of severe disease from P. knowlesi, Plasmodium falciparum, and Plasmodium vivax and outcomes following introduction of protocols for early referral and intravenous artesunate for all severe malaria. Methods. From September 2010 to October 2011 we prospectively assessed nonpregnant patients aged ≥12 years admitted to Queen Elizabeth Hospital (QEH), Sabah, with polymerase chain reaction—confirmed Plasmodium monoinfection. Standardized referral and prereferral intravenous artesunate were instituted at district hospitals. Results. Severe malaria occurred in 38 of 130 (29%) patients with P. knowlesi, 13 of 122 (11%) with P. falciparum, and 7 of 43 (16%) with P. vivax. The commonest severity criteria in knowlesi malaria included parasitemia >100 000/μL (n = 18), jaundice (n = 20), respiratory distress (n = 14), hypotension (n = 13), and acute kidney injury (n = 9). On multivariate analysis, P. knowlesi was associated with a 2.96-fold (95% confidence interval, 1.19–7.38-fold) greater risk of severity than P. falciparum (P = .020); only parasitemia and schizontemia >10% independently predicted knowlesi severity. Risk of severe knowlesi malaria increased 11-fold with parasitemia >20 000/μL, and 28-fold with parasitemia >100 000/μL. Nearly all (92%) knowlesi malaria patients received oral artemisinin therapy; 36 of 38 (95%) and 39 of 92 (42%) with severe and nonsevere disease, respectively, also received ≥1 dose of intravenous artesunate. No deaths occurred from any species. Conclusions. Plasmodium knowlesi is the commonest cause of severe malaria at QEH, with parasitemia the major risk factor for severity. Early referral and treatment with artesunate was highly effective for severe malaria from all species and associated with zero mortality.
Journal Article
Lower Microscopy Sensitivity with Decreasing Malaria Prevalence in the Urban Amazon Region, Brazil, 2018–2021
Malaria is increasingly diagnosed in urban centers across the Amazon Basin. In this study, we combined repeated prevalence surveys over a 4-year period of a household-based random sample of 2,774 persons with parasite genotyping to investigate the epidemiology of malaria in Mâncio Lima, the main urban transmission hotspot in Amazonian Brazil. We found that most malarial infections were asymptomatic and undetected by point-of-care microscopy. Our findings indicate that as malaria transmission decreases, the detection threshold of microscopy rises, resulting in more missed infections despite similar parasite densities estimated by molecular methods. We identified genetically highly diverse populations of Plasmodium vivax and P. falciparum in the region; occasional shared lineages between urban and rural residents suggest cross-boundary propagation. The prevalence of low-density and asymptomatic infections poses a significant challenge for routine surveillance and the effectiveness of malaria control and elimination strategies in urbanized areas with readily accessible laboratory facilities.
Journal Article
Sustained Malaria Control Over an 8-Year Period in Papua New Guinea
Continuous malaria control in Papua New Guinea has resulted in a marked decline of Plasmodium falciparum and P. vivax prevalence. Yet, an increasing proportion of submicroscopic infections, many of them carrying gametocytes, demands for novel strategies to target residual transmission.
Abstract
Background
The scale-up of effective malaria control in the last decade has resulted in a substantial decline in the incidence of clinical malaria in many countries. The effects on the proportions of asymptomatic and submicroscopic infections and on transmission potential are yet poorly understood.
Methods
In Papua New Guinea, vector control has been intensified since 2008, and improved diagnosis and treatment was introduced in 2012. Cross-sectional surveys were conducted in Madang Province in 2006 (with 1280 survey participants), 2010 (with 2117 participants), and 2014 (with 2516 participants). Infections were quantified by highly sensitive quantitative polymerase chain reaction (PCR) analysis, and gametocytes were quantified by reverse-transcription qPCR analysis.
Results
Plasmodium falciparum prevalence determined by qPCR decreased from 42% in 2006 to 9% in 2014. The P. vivax prevalence decreased from 42% in 2006 to 13% in 2010 but then increased to 20% in 2014. Parasite densities decreased 5-fold from 2006 to 2010; 72% of P. falciparum and 87% of P. vivax infections were submicroscopic in 2014. Gametocyte density and positivity correlated closely with parasitemia, and population gametocyte prevalence decreased 3-fold for P. falciparum and 29% for P. vivax from 2010 to 2014.
Conclusions
Sustained control has resulted in reduced malaria transmission potential, but an increasing proportion of gametocyte carriers are asymptomatic and submicroscopic and represent a challenge to malaria control.
Journal Article
Prevalence and associated risk factors of malaria among adults in East Shewa Zone of Oromia Regional State, Ethiopia: a cross-sectional study
Background
Malaria is one of the most important causes of morbidity and mortality in sub-Saharan Africa. The disease is prevalent in over 75% of the country’s area making it the leading public health problems in the country. Information on the prevalence of malaria and its associated factors is vital to focus and improve malaria interventions.
Methods
A cross-sectional study was carried out from October to November 2012 in East Shewa zone of Oromia Regional State, Ethiopia. Adults aged 16 or more years with suspected malaria attending five health centers were eligible for the study. Logistic regression models were used to examine the effect of each independent variable on risk of subsequent diagnosis of malaria.
Results
Of 810 suspected adult malaria patients who participated in the study, 204 (25%) had microscopically confirmed malaria parasites. The dominant
Plasmodium
species were
P. vivax
(54%) and
P. falciparum
(45%), with mixed infection of both species in one patient. A positive microscopic result was significantly associated with being in the age group of 16 to 24 years [Adjusted Odds Ratio aOR 6.7; 95% CI: 2.3 to 19.5], 25 to 34 years [aOR 4.2; 95% CI: 1.4 to 12.4], and 35 to 44 years [aOR 3.7; 95% CI: 1.2–11.4] compared to 45 years or older; being treated at Meki health center [aOR 4.1; 95% CI: 2.4 to 7.1], being in Shashemene health center [aOR = 2.3; 95% CI: 1.5 to 4.5], and living in a rural area compared to an urban area [aOR 1.7; 95% CI: 1.1 to 2.6)].
Conclusion
Malaria is an important public health problem among adults in the study area with a predominance of
P. vivax
and
P. falciparum
infection. Thus, appropriate health interventions should be implemented to prevent and control the disease.
Journal Article
Submicroscopic malaria in pregnancy and associated adverse pregnancy events: A case-cohort study of 4,352 women on the Thailand–Myanmar border
Malaria in pregnancy detected by microscopy is associated with maternal anaemia, reduced fetal growth, and preterm birth, but the effects of lower density (i.e., submicroscopic) malaria infections are poorly characterised. This analysis was undertaken to investigate associations between submicroscopic malaria at the first antenatal care (ANC) visit and these adverse pregnancy events on the Thailand-Myanmar border.
Blood samples taken from refugee and migrant pregnant women presenting for their first ANC visit were analysed retrospectively for malaria using ultrasensitive PCR (uPCR, limit of detection 22 parasites/mL). The relationships between submicroscopic malaria and subsequent microscopically detectable malaria, anaemia, birth weight, and preterm birth were evaluated using inverse probability weighting for stratified random sampling.
First ANC visit samples from 4,352 asymptomatic women (median gestational age 16.5 weeks) attending between October 1st 2012 and December 31st 2015 were analysed. The weighted proportion of women with submicroscopic malaria infection was 4.6% (95% CI 3.9-5.6), comprising 59.8% (49.5-69.4) Plasmodium vivax, 6.5% (4.0-10.5) Plasmodium falciparum, 1.8% (0.9-3.6) mixed, and 31.9% (22.2-43.5) infections which could not be speciated. Submicroscopic parasitaemia at first ANC visit was associated with subsequent microscopically detected malaria (adjusted hazard ratio [HR] 12.9, 95% CI 8.8-18.8, p < 0.001) and lower birth weight (adjusted predicted mean difference -275 g, 95% CI -510 to -40, p = 0.022). There was no association with preterm birth. Submicroscopic P. falciparum mono-infection (adjusted HR 2.8, 95% CI 1.2-6.6, p = 0.023) and coinfection with P. falciparum and P. vivax (adjusted HR 10.3, 95% CI 2.6-40.4, p = 0.001) was associated with increased risk of maternal anaemia, but submicroscopic P. vivax mono-infection was not. That uPCR was conducted for only a part of the cohort due to cost constraints is a limitation.
In low transmission settings, uPCR identifies substantially more malaria infections at antenatal screening than conventional diagnostic methods. On the Thailand-Myanmar border, submicroscopic malaria at first antenatal consultation was associated with higher risks of microscopically diagnosed malaria later in pregnancy, anaemia, and reduced birth weight.
Journal Article
Vectorial drivers of malaria transmission in Jabi Tehnan district, Amhara Regional State, Ethiopia
Among the factors affecting the effectiveness of malaria control is poor knowledge of the entomologic drivers of the disease. We investigated anopheline populations as part of a baseline study to implement house screening of windows and doors as a supplementary malaria control tool towards elimination in Jabi Tehnan district, Amhara Regional State of Ethiopia. The samples were surveyed monthly using CDC light traps between June 2020 and May 2021. Mosquito trap density (< 3 mosquitoes/trap) was low, however, with a high overall
Plasmodium
sporozoite rate (9%; indoor = 4.3%, outdoor = 13.1%) comprising
P. falciparum
(88.9%) and
P. vivax
(11.1%).
Anopheles gambiae
s.l., mostly
An. arabiensis
, comprised > 80% of total anopheline captures and contributed ~ 42% of
Plasmodium
-infected mosquitoes. On the other hand, morphologically scored
Anopheles funestus
s.l., constituting about 6% of anopheline collections, accounted for 50% of sporozoite-infected mosquitoes. Most of the infected
An. funestus
s.l. specimens (86.7%) were grouped with previously unknown or undescribed
Anopheles
species previously implicated as a cryptic malaria vector in the western Kenyan highlands, confirming its wider geographic distribution in eastern Africa. Other species with
Plasmodium
infection included
An. longipalpis
C,
An. theileri
,
An. demillioni,
and
An. nili.
Cumulatively, 77.8% of the infected mosquitoes occurred outdoors. These results suggest efficient malaria parasite transmission despite the low vector densities, which has implications for effective endpoint indicators to monitor malaria control progress. Additionally, the largely outdoor infection and discovery of previously unknown and cryptic vectors suggest an increased risk of residual malaria transmission and, thus, a constraint on effective malaria prevention and control.
Journal Article
Epidemiological characteristics of imported malaria related to international travel in the Republic of Korea from 2009 to 2018
Malaria, transmitted by mosquitoes infected with
Plasmodium
parasites, remains a significant health issue with global travel increasing the risk of imported malaria. This study investigates imported malaria cases in the Republic of Korea from 2009 to 2018 using data from the Korea National Infectious Disease Surveillance System. During this period, 601 imported cases were reported, with 82.4% male patients and a median age of 39.1 years. Most cases (76.5%) involved Korean residents returning from malaria-endemic areas, mainly Africa and Asia.
Plasmodium falciparum
(55.7%) and
Plasmodium vivax
(30.3%) were the predominant species. The annual percent change in incidence rate was 6.45%. Notably, 71.5% of the patients did not receive prophylactic chemotherapy, and 18% of those who did still developed malaria. Median diagnostic delays were 4 days for
P. falciparum
and 7 days for
P. vivax
. The case fatality rate was 2.3%, with all deaths occurring in travelers who contracted
P. falciparum
in Africa. This study emphasizes the ongoing risk of imported malaria in the ROK and highlights the need for better awareness and preventive measures among travelers. Enhancing surveillance and educating travelers on anti-malaria chemoprophylaxis are crucial.
Journal Article