Disease Risk: Mapping the emerging burden of dengue
The first nationally-representative survey of dengue has revealed the growing burden of the disease in Bangladesh.
- London School of Hygiene & Tropical Medicine, United Kingdom
Dengue is a viral disease spread by mosquitoes and found in more than 120 countries around the world (Bhatt et al., 2013). Because the species that transmits the disease, Aedes aegypti, lives in dense man-made environments, recent urbanization throughout the tropical world has accelerated the spread of dengue (Bhatt et al., 2013). Aedes aegypti is also the primary vector for a number of other diseases, including Zika, chikungunya and Yellow Fever, so understanding how to control the global emergence of dengue could help to prevent the spread of these diseases.
However, getting accurate data on who is being infected is surprisingly difficult. Only a small proportion of people (11–32%) are likely to have symptoms after contracting dengue, with few being sick enough to require formal medical care (Bhatt et al., 2013; Undurraga et al., 2013). And even if they receive medical care, misdiagnosis and under-reporting are common. This means that maps based on clinical case counts are not always useful, and may just reflect differences in access to healthcare, diagnostics and ability to report cases.
The gold standard for measuring who has been infected with dengue is a seroprevalence survey, where serum (blood) samples are taken from a representative group of people and tested for antibodies against the dengue virus. Now, in eLife, Henrik Salje and colleagues (who are based at institutes in France, the United States and Bangladesh) report how they have used this approach, which is usually reserved for local studies, on a scale never seen before to provide new insights into the transmission of dengue at a national level in Bangladesh (Salje et al., 2019).
Salje et al. visited 70 randomly selected communities across Bangladesh, between 2014 and 2016. During these visits, randomly selected residents were asked to contribute a blood sample and to answer a range of questions about behaviours thought to increase or decrease the risk of dengue. Overall, they found that 24% of participants showed evidence of previous dengue infection: this was much lower than the figures in other affected regions, such as Indonesia (~70%), where dengue has been around much longer (Tam et al., 2018). They also found that males were more likely to be infected than females, and that regular travellers were almost twice as likely to be infected as people who did not travel regularly.
However, the biggest factor in determining the risk of infection was location: over 80% of individuals in big cities, such as Dhaka, had been infected with the virus at some stage in their life, whereas fewer than 5% of people in rural areas showed signs of any previous infection. Using the results of their survey, Salje et al. were able to construct models that could predict seroprevalence in areas where data had not been collected, and then use this information to produce an updated risk map for dengue across Bangladesh (Figure 1).
Figure 1
Mapping dengue risk in Bangladesh.
A cross-sectional seroprevalence map (right) of dengue risk in Bangladesh, based on data collected from 70 communities throughout the country (Salje et al., 2019), shows that the risk is highest in the cities of Chittagong (in the south east), Khulna (south west) and Dhaka (in the middle of the country). Previous maps, such as this map based on data from Bhatt et al. (2013) (left), have predicted more widespread risk throughout the country, but these maps were largely based on less detailed data and often relied on extrapolation from nearby countries. This new study by Sajle et al. may suggest that there are unique, as yet unknown, factors that constrain the current distribution of dengue in Bangladesh.
By comparing seroprevalence between different age groups, Salje et al. were able to estimate the rate at which infections accumulate. Using standard 'catalytic models' (and making some assumptions about how long dengue has been in Bangladesh) they estimated that about 2.4 million people, out of a population of ~160 million, are infected with dengue virus in Bangladesh each year. In contrast fewer than 6,000 dengue cases per year were reported over the same time period, mostly in Dhaka (Mutsuddy et al., 2019). The results from this survey have the potential to play an important role in expanding and adapting dengue surveillance practices in Bangladesh.
Salje et al. also suggest that the current distribution of mosquito species in Bangladesh may be the cause of the limited spread of dengue. The mosquito that carries dengue, Aedes aegypti, was found in higher abundance in urban areas, whilst Aedes albopictus, a mosquito species that carries and transmits the virus less effectively, was more common in rural areas.
Together, these results paint a picture of a country part way through the emergence of dengue. Transmission is already high in urban centres but – unlike many countries in South East Asia and Latin America – regular, continuous transmission has yet to spread to more rural areas. However, more work is needed to understand how human movement and lower transmission potential in rural areas interact in shaping the current distribution of dengue in Bangladesh (Wesolowski et al., 2015). Genetic analyses of circulating dengue viruses can be used to identify the origins of outbreaks (Salje et al., 2017), which could help with efforts to contain the spread of the virus.
By proving the feasibility and value of nationally-representative seroprevalence surveys for dengue, it is hoped that the results of Salje et al. will renew interest in mass disease surveillance for dengue. Such community-representative surveys have proven instrumental for targeting and evaluating interventions for successful malaria control (Bhatt et al., 2015). A range of new dengue vaccines are currently becoming available, but some of these will only be beneficial if targeted to the areas of highest transmission (Flasche et al., 2016). Therefore, having regularly updated maps that accurately show the national spread of dengue will be critical in the ongoing fight against further expansion of the disease.
References
-
Dengue situation in Bangladesh: An epidemiological shift in terms of morbidity and mortalityCanadian Journal of Infectious Diseases and Medical Microbiology 2019:1–12.https://doi.org/10.1155/2019/3516284
-
Geographic variation in dengue seroprevalence and force of infection in the urban paediatric population of IndonesiaPLOS Neglected Tropical Diseases 12:e0006932.https://doi.org/10.1371/journal.pntd.0006932
-
Use of expansion factors to estimate the burden of dengue in Southeast Asia: A systematic analysisPLOS Neglected Tropical Diseases 7:e2056.https://doi.org/10.1371/journal.pntd.0002056
Article and author information
Author details
Publication history
- Version of Record published: May 13, 2019 (version 1)
Copyright
© 2019, Brady
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 3,426
- views
-
- 318
- downloads
-
- 7
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Disease Risk: Mapping the emerging burden of dengue
eLife 8:e47458.
https://doi.org/10.7554/eLife.47458
Further reading
-
- Epidemiology and Global Health
- Microbiology and Infectious Disease
Background:
Few national-level studies have evaluated the impact of ‘hybrid’ immunity (vaccination coupled with recovery from infection) from the Omicron variants of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Methods:
From May 2020 to December 2022, we conducted serial assessments (each of ~4000–9000 adults) examining SARS-CoV-2 antibodies within a mostly representative Canadian cohort drawn from a national online polling platform. Adults, most of whom were vaccinated, reported viral test-confirmed infections and mailed self-collected dried blood spots (DBSs) to a central lab. Samples underwent highly sensitive and specific antibody assays to spike and nucleocapsid protein antigens, the latter triggered only by infection. We estimated cumulative SARS-CoV-2 incidence prior to the Omicron period and during the BA.1/1.1 and BA.2/5 waves. We assessed changes in antibody levels and in age-specific active immunity levels.
Results:
Spike levels were higher in infected than in uninfected adults, regardless of vaccination doses. Among adults vaccinated at least thrice and infected more than 6 months earlier, spike levels fell notably and continuously for the 9-month post-vaccination. In contrast, among adults infected within 6 months, spike levels declined gradually. Declines were similar by sex, age group, and ethnicity. Recent vaccination attenuated declines in spike levels from older infections. In a convenience sample, spike antibody and cellular responses were correlated. Near the end of 2022, about 35% of adults above age 60 had their last vaccine dose more than 6 months ago, and about 25% remained uninfected. The cumulative incidence of SARS-CoV-2 infection rose from 13% (95% confidence interval 11–14%) before omicron to 78% (76–80%) by December 2022, equating to 25 million infected adults cumulatively. However, the coronavirus disease 2019 (COVID-19) weekly death rate during the BA.2/5 waves was less than half of that during the BA.1/1.1 wave, implying a protective role for hybrid immunity.
Conclusions:
Strategies to maintain population-level hybrid immunity require up-to-date vaccination coverage, including among those recovering from infection. Population-based, self-collected DBSs are a practicable biological surveillance platform.
Funding:
Funding was provided by the COVID-19 Immunity Task Force, Canadian Institutes of Health Research, Pfizer Global Medical Grants, and St. Michael’s Hospital Foundation. PJ and ACG are funded by the Canada Research Chairs Program.
-
- Epidemiology and Global Health
Background:
The Zanzibar archipelago of Tanzania has become a low-transmission area for Plasmodium falciparum. Despite being considered an area of pre-elimination for years, achieving elimination has been difficult, likely due to a combination of imported infections from mainland Tanzania and continued local transmission.
Methods:
To shed light on these sources of transmission, we applied highly multiplexed genotyping utilizing molecular inversion probes to characterize the genetic relatedness of 282 P. falciparum isolates collected across Zanzibar and in Bagamoyo district on the coastal mainland from 2016 to 2018.
Results:
Overall, parasite populations on the coastal mainland and Zanzibar archipelago remain highly related. However, parasite isolates from Zanzibar exhibit population microstructure due to the rapid decay of parasite relatedness over very short distances. This, along with highly related pairs within shehias, suggests ongoing low-level local transmission. We also identified highly related parasites across shehias that reflect human mobility on the main island of Unguja and identified a cluster of highly related parasites, suggestive of an outbreak, in the Micheweni district on Pemba island. Parasites in asymptomatic infections demonstrated higher complexity of infection than those in symptomatic infections, but have similar core genomes.
Conclusions:
Our data support importation as a main source of genetic diversity and contribution to the parasite population in Zanzibar, but they also show local outbreak clusters where targeted interventions are essential to block local transmission. These results highlight the need for preventive measures against imported malaria and enhanced control measures in areas that remain receptive to malaria reemergence due to susceptible hosts and competent vectors.
Funding:
This research was funded by the National Institutes of Health, grants R01AI121558, R01AI137395, R01AI155730, F30AI143172, and K24AI134990. Funding was also contributed from the Swedish Research Council, Erling-Persson Family Foundation, and the Yang Fund. RV acknowledges funding from the MRC Centre for Global Infectious Disease Analysis (reference MR/R015600/1), jointly funded by the UK Medical Research Council (MRC) and the UK Foreign, Commonwealth & Development Office (FCDO), under the MRC/FCDO Concordat agreement and is also part of the EDCTP2 program supported by the European Union. RV also acknowledges funding by Community Jameel.
