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30 July, 2019 11:48:00 AM
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Dengue menace and climate change

An early dengue warning system that allows time for successful mitigation will enhance the effectiveness of preventive measures
Mohammed Abul Kalam, PhD
Dengue menace and climate change

The dengue epidemic in the capital has assumed alarming proportions with the number of patients and the death toll climbing steadily. The situation is worsening in Dhaka with nearly 700 dengue patients being admitted to various city hospitals a day. There is not a single hospital in the capital without dengue patients.
World Health Organisation (WHO) delegates yesterday has termed the situation as alarming but said it was still under control. The WHO delegation assured the Dhaka South City Corporation (DSCC) to work in preventing diseases like dengue and chikungunya in the capital.The city authorities have taken several initiatives to combat the mosquito menace but there has been no significant outcome yet.
At the time of this writing, July 27, total of 10,528 people have been infected with the mosquito-borne disease. Last year, the total number was 10,148, according to the Directorate General of Health Services (DGHS)  The number of dengue patients exceeded the 10,000 mark only twice in the last 18 years: in 2018 and this year, according to a DGHS report. The previous highest was 6,232 in 2002. Meanwhile, authorities of different hospitals in the capital said at least 30 dengue patients have died so far. But the DGHS maintains that only eight have died of the disease. Explaining the disparity in the number of deaths, Prof Dr Meerjady Sa-brina Flora, director of Institute of Epidemiology, Disease Control and Research (IEDCR) and also the head of death review committee of the government, said after a patient dies, they collect information, as well as blood samples, of the deceased to confirm the cause of the death. Only after getting all those, they can declare whether it was dengue-related or not.

The climate change effects or disruptions lead to a serious outbreak of dengue in Bangladesh, no doubt. Climate ‘emergency’ is too strong; I think climate ‘breakdown’ makes it sound too mechanical. I think we’re looking at a systemic collapse gradually unfolding in front of us, and I think people can relate to that. For several years, dengue control has been a challenge for Bangladesh where dengue is endemic. Optimal timing of an early warning could help to bridge the forecast-mitigation gap between theoretical research predictions and practical control operations. Identifying the optimal lead time for dengue forecast and duration of local vector control could help to improve the functional aspects of a dengue forecasting model, reduce risk of dengue epidemic, increase the cost-effectiveness of control strategies, and encourage local authorities to adopt a model-based dengue early warning.  An early dengue warning system that allows time for successful mitigation will enhance the effectiveness of preventive measures. We know that a rise in weekly mean temperature and rainfall precedes risks of increasing dengue cases by 1 to 5 months with higher risks being evident at 3–4 months. The lag times could partly be explained by high desiccation resistance of Aedes mosquito's eggs which could survive several months without water. A study in Bangkok shows that temperature and rainfall precede dengue cases up to 6 months and 3 months, respectively. These results are consistent with studies in Singapore that analyzed the relationship between weekly temperature and dengue cases up to 20 weeks.

The usefulness of a dengue early warning may be reflected in the time between a forecast window and the time required by the local authority to mitigate an outbreak. A forecast-mitigation deficit occurs when the duration for successful mitigation exceeds the forecast window. In view of the maximum period required to curb transmission, a 3-month forecast is deemed appropriate. Incidentally, the highest risk of dengue as a function of mean temperature and rainfall occurs at lag week 12–16. Forecasting on the basis of the observed mean temperature and cumulative rainfall 12 to 16 weeks previously could, therefore, provide accurate warnings while allowing sufficient time for local authorities to mitigate, or even avoid, an impending outbreak.Climate change and emerging environmental risks will define much of the future context for environmental epidemiology. Mean surface temperature is expected to increase by 0.3 to 4.8 °C by 2100, leading to direct impacts on health from heat stress and flooding, as well as indirect health impacts mediated through infectious diseases, air quality, and food security.

The duration of dengue cluster management could be influenced by a complex spatiotemporal interplay of risk factors unique to respective dengue clusters: (1) Demographic characteristics, density, and herd immunity among host population or residents in clusters could have an influence on the number of dengue cases. A community with lower herd immunity could possibly experience dengue epidemic with a low mosquito-population density, (2) Environmental factors such as conditions, types and ages of structural buildings, construction activities, public drainage systems, and presence of parks possibly increase the challenges and prolong the duration of vector control operations in certain clusters, (3) Differential vector populations due to inaccessibility of larval breeding sites, larval indices, density of adult Aedes aegypti, and numbers of potential breeding sites complicate the dynamics of dengue transmission and require greater effort for outbreak control in respective clusters, (4) With the co-circulation of all four serotypes of dengue virus in Singapore, the circulating serotypes and herd immunity to the specific serotype of concern also influence the number of cases in clusters, (5) Community commitment to prevent dengue has a direct impact on the duration of vector control. Residents not granting permission to dengue officers to enter their premises to conduct mosquito larvae inspection and elimination could prolong the duration as well as reduce the effectiveness of vector control measures.

Since 2005, Singapore has monitored circulating dengue virus to detect switches in predominant serotype which could signal an impending outbreak. More recently, virus genetic data has shown that clade replacement without a switch in serotype, could also lead to an increase in cases. Determining that 3 months lead time could be optimal for a warning to be issued; and that temperature and rainfall data could provide a forecast in that timeframe, to allow for the preparation of control measures.

Intervention measures include public and stakeholders' engagement, gaining political support and systematic source reduction exercises. The findings in this study were geographically based because of the heterogeneity of the environment including local weather, circulating viral serotypes, and herd immunity in the respective study areas.

Owing to the fact that a model-based dengue early warning system has not previously been adopted for dengue surveillance in the study area, we utilized existing available vector control and cluster duration data as indicative references. Further studies could be undertaken to evaluate the forecast-mitigation gaps more accurately when such data are available in the future.

The study identified a short term forecast window of 3–4 months. In an environment where the long duration of response or mitigation is anticipated, the effectiveness of a dengue early warning can be improved by re-considering the forecast-mitigation gap. One approach is to forecast dengue outbreaks with a longer lead time using other predictors of weather. Several studies in various geographical areas have revealed the feasibility of forecasting dengue cases several months in advance using weekly or monthly weather predictors and up to 10 months ahead using Southern Oscillation Indices (SOI), El Niño Index, or El Niño Southern Oscillation indices (ENSO). Although long term forecast could possibly be compromised by lower forecast precision, the long lead time may be useful for longer-term planning such as allocation of resources and acquisition of control tools such as insecticides.

Generally, dengue epidemiology is influenced by a complex interplay of factors that include rapid urbanization and increase in population density, the capacity of healthcare systems, the effectiveness of vector control systems, predominant circulating dengue serotypes, herd immunity, and social behaviour of the population. Most dengue-endemic countries in the Asia Pacific have limited resources and/or lack of preparedness to contain dengue epidemic. Rising international and domestic trade and population movement contribute to the increases in domestic and cross border dengue transmission. As a result, the region is experiencing dengue epidemics with increasing frequency and magnitude. Until a vaccine or drug for dengue is available, vector control operations that eliminate adult mosquitoes and their larvae through breeding-source reduction remain the only effective method to curb dengue transmission. However, vector control can be resource and labour-intensive, which poses an economic burden on nations with limited resources. No other pandemics in the past century have approached the magnitudeof the 1918 influenza pandemic. If a pandemic of comparable scale and impact to the 1918 pandemic were to occur today, modeling estimates suggest that the impact could be similarly devastating—killing between50 and 80 million people worldwide, with expected annual global losses from pandemic risk of around $500 billion per year (Murray et al., 2006;Fan et al., 2018). However, there are opportunities to mitigate these grim scenarios.

Collective action at local, national, and global levels could strengthen preparedness for infectious disease outbreaks and pandemics by leveraging and intensifying the scientific and political momentum that has gathered over recent decades. A top priority, regardless of methods used in control programmes, is political commitment. Although the participation of those affected is crucial, there has never been a successful programme without enlightened, adequately funded, and well-organized leadership.

The writer is Former Head, Department of Medical Sociology, Institute of Epidemiology, Disease Control & Research (IEDCR),  Dhaka, Bangladesh

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Published by the Editor on behalf of Independent Publications Limited at Media Printers, 446/H, Tejgaon I/A, Dhaka-1215.
Editorial, News & Commercial Offices : Beximco Media Complex, 149-150 Tejgaon I/A, Dhaka-1208, Bangladesh. GPO Box No. 934, Dhaka-1000.

Editor : M. Shamsur Rahman
Published by the Editor on behalf of Independent Publications Limited at Media Printers, 446/H, Tejgaon I/A, Dhaka-1215.
Editorial, News & Commercial Offices : Beximco Media Complex, 149-150 Tejgaon I/A, Dhaka-1208, Bangladesh. GPO Box No. 934, Dhaka-1000.

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