Why did the country experience such a big outbreak last year?
Bangladesh experienced its largest dengue virus outbreak in 2019, with 101,354 patients admitted to hospital in contrast to the cumulative number of dengue patients in the previous 19 years -- 50,674 cases.
Dengue is transmitted by the bites of female Aedes mosquitoes. After a period of virus development known as the “extrinsic incubation period,” the mosquito becomes infectious and is able to transmit the dengue virus to other people. Therefore, one dengue patient can potentially spread the virus to a large number of people within a very short period of time.
The question is: Why did Bangladesh experience such a large outbreak in 2019?
A recent study published in the renowned entomological journal Journal of Medical Entomology explained the potential reasons behind the unprecedented 2019 dengue outbreak in Bangladesh. The main reasons according to the authors are:
i) Introduction of a new serotype (Dengue virus serotype 3, DENV- 3) into a largely susceptible population
ii) Unusual rain during the pre-monsoon season which provided two additional months of mosquito reproduction
iii) Resistance to insecticides There are four different serotypes of dengue viruses (DENV-1 to DENV- 4) that are transmitted by Aedes mosquitoes. Infection with one serotype does not protect against another, except for the first few (four to six) months after infection.
In Bangladesh, all four serotypes of DENV have been detected, with DENV-3 as the predominant virus until 2002. Surveillance conducted by Institute of Epidemiology Disease Control and Research (IEDCR) identified DENV-3 in Dhaka city again in 2017.
This serotype caused a moderate outbreak in 2018 and probably reached peak prevalence in the subsequent year when DENV-3 was the predominant dengue virus circulating in the country. Earlier studies suggested that nationally 24% of people have antibodies against any one serotype of dengue virus.
This figure is >80% for the resident in Dhaka and 3% in the northern part of the country. This study was conducted during 2015- 2016 when DENV-1 and DENV-2 were circulating in the country. The absence of DENV-3 for 16 years (2002-2017) provided a large population susceptible to this serotype.
Thus, the circulation of DENV-3 in 2019 contributed to a major epidemic in the country. Concurrently, DENV-3 was circulating in neighbouring India, Nepal, Myanmar, and multiple countries in Southeast Asia including Thailand with whom Bangladesh has strong trade and travel links.
The second plausible reason for the increase in cases was the unusually high rainfall before the normal 2019 monsoon season. In Bangladesh, the dry season extends from November to February. However, during 2019, record rainfall was documented during February in Dhaka city by the Bangladesh Meteorological Department.
After that initial heavy rainfall, intermittent to heavy rainfall continued until October 2019. Therefore, the vector season started earlier compared to previous years, which may have triggered early season Aedes population growth and dengue virus transmission until the end of the year.
Early rainfall might have contributed to the increased number of dengue cases in the country. Previous reports indicated that the Aedes population peaked in July and August in Bangladesh. Thus, in 2019, the mosquitoes had two to three additional months to increase the population. The third factor contributing to the size of the 2019 DENV outbreak was insecticide resistance. Globally, insecticide resistance is a rising problem.
In a study conducted by researchers at the International Centre for Diarrhoeal Disease Research, Bangladesh on Aedes aegypti mosquitoes found high levels of resistance against the permethrin, the insecticide used by Dhaka City Corporation.
In this study, the mortality rate among Ae aegypti females with permethrin ranged from 0-14.8% at the dose of 15 μg/bottle; doubling the dose increased the mortality rate to 5.1-44.4% after 30 minutes of exposure. According to WHO guidelines, susceptibility to an insecticide requires the death of 98-100% of the mosquitoes at 30 minutes following standardized protocols, whereas with a mortality rate of <90%, populations should be considered resistant.
Thus, a combination of introduction of new serotype, unusual higher rainfall in pre- monsoon and post-monsoon season and insecticide resistance contributed a large outbreak of dengue virus in 2019 in Bangladesh.
What is the solution?
We suggest a robust local community- level surveillance system for the timely identification of dengue cases and the regular destruction of breeding sites of Aedes mosquitoes. In each union/ward, the lowest administrative level of the country, there should be a community volunteer team under the leadership of a local representative (commissioner/ member/ chairman).
The locals should arrange for an assessment of the medical condition of the caller/patient(s) by registered general physicians and, upon prescription from the physician, the committee would arrange testing for DENV using non-specific protein 1 (NS-1) kit, an antigen detection kit currently used as a diagnostic tool in Bangladesh in local diagnostic centres/hospitals.
There should be a central control and an efficient data collection system. The team would collate the results of DENV diagnostic tests daily from all the diagnostic centres and hospitals within the ward/union. When a case cluster is identified (more than three cases in a 400m radius area within three consecutive days), the team will activate mosquito control using adulticides and larval surveillance/ source destruction activities.
Regular destruction of Aedes mosquito breeding sites is the key to control dengue outbreaks. The common breeding sites of Aedes mosquitoes are the unused tires, plastic drums, buckets, containers, coconut shells, flower pots, polythene sheets or bags, and any discarded materials or objects that can hold clean water. The building construction sites are especially rich as breeding habitats of Aedes mosquitoes, and need regular inspections.
Dr Najmul Haider is an Epidemiologist and Post-doctoral researcher at Royal Veterinary College, University of London, UK. Atik Ahsanis a Medical anthropologist and consultant of The Lifesaving Early Actions Responding to Nationwide Dengue Outbreak (LEARN-DO) Project of SEEP, a UKAID funded project of The Start Fund Bangladesh.