Why the threat of mosquitoes should not be taken lightly
Amid the pandemic, another mosquito-borne disease outbreak is looming large on the horizon. In recent years (2017-2019), we are witnessing an upsurge of vector-borne disease outbreaks. Last year, Bangladesh recorded the deadliest outbreak of dengue since its emergence in the late 1990s. Officially, more than 100,000 dengue patients were hospitalized and perhaps many more cases were unreported. In 2017, Dhaka experienced a massive outbreak of chikungunya, causing widespread infection throughout the city.
During these two massive outbreaks caused by the same household mosquito species (Aedes spp), panic broke out in the city, affecting every age group and socio-economic stratum. The overall health care system of the city collapsed. All public and private clinics/hospitals were crowded with patients and mainstream news media and social networking platforms were inundated with outbreak-related news.
We don’t really care about a tiny mosquito buzzing around; we just let it go. But in fact, it is the deadliest animal in the world. These tiny insects kill at least 700,000 people every year, which accounts for 17% of the estimated global burden of infectious diseases. Mosquitoes threaten half of the global population and cause billions of dollars in lost productivity each year.
Around this time last year, I was struggling for my life in hospital due to a severe case of dengue. During the chikungunya crisis, Biomedical Research Foundation (BRF), a non-profit research organization based in Bangladesh, took a unique research effort to document the unbearable sufferings of the chikungunya patients, even without research funding. For better management and prevention of future outbreaks, a desperate research initiative was taken by this author from the sick bed while suffering from scathing pain.
Through an open Facebook call, a large team of 111 volunteer researchers were engaged to serve the nation. Researchers were affiliated from 34 institutes (including 22 universities from all over Bangladesh) consisting of clinicians, public health professionals, statisticians, and university students, with the work published in prestigious journals. Our study from Bangladesh represents one of the largest samples (1,326 patients were enrolled) studied so far around the world describing the clinical profile of chikungunya infection at acute phase (first two weeks since the onset of symptoms).
In Bangladesh, an estimated 2.4 million people are infected with dengue based on a seroprevalence study (observe the history of past infection) and these infections are largely confined to Dhaka, Chittagong, and Khulna. However, all previous outbreaks occurred mostly in Dhaka city. If the same person is infected by different versions (serotypes) of the dengue virus, this disease could be severe and fatal as well.
The 2019 dengue outbreak was deadly, since different serotypes were circulating in the environment. We conducted a hospital-based study to understand the severity of the 2019 dengue outbreak. A team of 85 volunteer researchers (mostly clinicians) contributed to this project.
More than 1,200 dengue cases were enrolled from eight different hospitals. Lack of research funds and the culture are the major challenges for quality research in Bangladesh. However, one encouraging experience is that, Techno Drug Ltd, a leading pharmaceutical company responded to the national crisis and supported our effort by providing a small research grant to conduct the study.
We have also investigated the transmission dynamics of the 2019 dengue outbreak (https://doi.org/10.1371/journal.pntd.0008349). This study has been recently published in the Plos Neglected Tropical Diseases journal. The 2019 outbreak peaked in August (more than 50% of the total cases reported over January to December) which coincided with Eid. During Eid, over 10 million people travel across the country, usually within a week.
In July 2019 (prior to Eid), over 81% of all dengue cases were reported within Dhaka city and the trend was reversed after Eid. Because of the sheer number of cases reported from Dhaka city, we hypothesized that mass mobility during Eid could change the dynamics of dengue transmission to other non-endemic areas of Bangladesh. Indeed, dengue increased nearly manifolds in some districts of Bangladesh. Conversely, it decreased 24-fold in Dhaka city as compared to pre-Eid week.
Previous studies showed that transport vehicles could disperse Aedes mosquitoes within a country and asymptomatic individuals can also transmit the virus in new areas. The transmission of dengue virus (or both virus and mosquito) during the mass migration before Eid was likely the most significant factor behind the geographical distribution of the diseases. Therefore, future dengue outbreaks are expected to spread across the country.
The most worrying aspect is that the same household Aedes mosquito species acts as a vector for four diseases including dengue, chikungunya, zika, and yellow fever. This mosquito species is abundant throughout the country since Bangladesh is a relatively small and flat country where climatic factors like temperature and rainfall usually do not vary significantly. The explosive growth of mosquitoes is induced by heavy rainfalls and hot temperature.
As a consequence of higher population density, unplanned urbanization (creates more breeding habitats) and the impact of climate change, Bangladesh is sitting on a ticking mosquito bomb. Dengue and chikungunya are already here while zika virus is also circulating in Bangladesh. Time is also ticking for inevitable yellow fever outbreaks in the coming years.
Since there is no effective vaccine against vector-borne diseases (dengue, chikungunya, zika), adequate public health preparedness remains the only viable option to prevent the deluge of upcoming mosquito-borne disease outbreaks in Bangladesh. Establishment of a comprehensive disease surveillance system, integrated vector control and management through community awareness, implementation of early diagnosis, and prompt treatment at all levels of health care must be in our priority list.
Mohammad Sorowar Hossain is Executive Director and Scientist, Biomedical Research Foundation, Dhaka, Bangladesh. He is also Associate Professor, School of Environment and Life Sciences, Independent University, Bangladesh. Email: [email protected] The author acknowledges the contribution to the chikungunya and dengue research team who served voluntarily during the crisis.