Increase in Aedes breeding combined with danger of secondary dengue infection appears to pose much bigger future threat than a mere outbreak
As the unprecedented dengue outbreak in the country continues to unfurl, data indicate that a bigger threat might be lurking in the shadow of current crisis.
The outbreak has direct correlation with the relentless unplanned urbanization, which caused “an imbalance in the existing ecology that has led to increase in dengue cases in 2016 and the emergence of the chikungunya virus for the first time in Bangladesh in 2017,” a study published this year in Canadian Journal of Infectious Diseases and Medical Microbiology stated.
The study, (‘Dengue Situation in Bangladesh: An Epidemiological Shift in terms of Morbidity and Mortality’, authored by Pulak Mutsuddy, Sanya Tahmina Jhora, Abul Khair Mohammad Shamsuzzaman, S M Golam Kaisar, and Md Nasir Ahmed Khan), found that dengue cases have been happening even in pre-monsoon seasons, straying from the typical proliferation period: monsoon and post-monsoon.
This means the proliferation of the disease will no longer be limited to monsoon (May–August) and post-monsoon seasons (September–December), essentially making it a year-long phenomenon.
Recent survey data by the Communicable Disease Control (CDC) unit of Directorate General of Health Services (DGHS) showed that density of adult Aedes mosquitoes has increased 13 folds compared to pre-monsoon.
The density in March, before the rainy season, was 36, which increased to 487 in July. The survey was carried out in 98 wards in two city corporations.
The Breteau Index (BI) count, in the north (DNCC) and south (DSCC) city corporations, was found to be over 20 in 58% and 79% households respectively. BI measures the number of positive containers per 100 houses inspected.
Worryingly, the CDC report mentions that lower BI in any place doesn’t necessarily mean “lower risk of getting affected in that place,” owing to the vast number of people moving around in the overpopulated city.
However, not just the adult Aedes mosquitoes, but the density of larvae was also found to be much higher than before.
Discarded tires and trapped water in the ground floors/basements of buildings were found to be the largest breeding grounds for these larvae, culpable for containing 22.90% and 11.29% of the larvae respectively.
The findings are hardly surprising, given the trend of a steady increase of dengue for the last two decades. During the pre-monsoon season in 2015–2017, dengue cases were reported to be more than seven times higher compared to the previous 14 years, the 2019 study said.
The recent outbreak has so far claimed at least 23 lives, according to DGHS data, which is widely considered lower than actual number due to the government organization’s sinuous process for affirming the cause of death.
It does not accept any third party data. DGHS has to get the data from Institute of Epidemiology, Disease Control and Research (IEDCR), which does not take into account any death unless it collects blood sample from the deceased.
The unofficial tally reported by mass media put the death toll over 60, as of August 6.
The danger of the exponentially higher breeding of Aedes is even much more threatening when secondary infection is taken into account.
DGHS’s death review found that 60 percent of the victims were second-time dengue patient.
Meerjady Sabrina Flora, director of IEDCR and head of the death review committee said that most of the deaths resulted from what is known as the ‘shock syndrome’, which occurs when a patient contracts the dengue virus for the second time.
This raises the worrying question that with over 22 thousand reported cases only this year, what happens when the dengue season returns next year.
To make matters much worse, not only the breeding increased 13-fold since January, but as the ‘Dengue Situation in Bangladesh’ study reveals, the Aedes mosquitoes are likely to be active throughout the year, instead of only the monsoon and post-monsoon periods.
The situation seems infinitely bleaker as scientists promise an exacerbation due to climate change.
A recent study, published in March in the journal PLOS Neglected Tropical Diseases Preventing, finds that up to a billion additional people could be exposed to mosquito-carried viruses including dengue and Zika virus by 2080 if the climate continues to warm at current rates.
The researchers of the study said that putting a stop to the expansion of potentially fatal diseases will require quick action to curb climate change and limit warming.
The effect of such proliferation “...will lead to economic decline in areas where they take off,” Sadie Ryan, a lead author of the study told Reuters.
The ‘Dengue Situation in Bangladesh’ study similarly found that “...climate changes after 2014 may have caused some ecological imbalances in the environment that leads to occurrence of more dengue cases in the pre-monsoon season.”
The study concludes that dengue “will remain in Bangladesh and will continue to constitute a serious public health problem as is happening worldwide.” It recommends long-term investment to achieve “behavioral changes in the urban population.”
The authors also recommended gaining clearer understanding of the evolution of the disease (“changing epidemiology”), and “constant monitoring including extending the surveillance areas and addressing the challenges to reduce the impact of the disease on public health and the economy of the country.”