Even before the nation's healthcare system has been able to fully recover from the blow of the Coronavirus pandemic, it is being hit hard by a recurrence of an outbreak of dengue, the disease that is endemic to Bangladesh.

With 7 more deaths from dengue confirmed on Wednesday (Oct. 19), the official death toll from the disease rose to 106 in 2022 - the second-highest on record after the 179 deaths recorded in 2019. In 2021, Bangladesh reported 28,429 dengue cases and 105 deaths, previously the second-highest number. The total number of cases this year stood at 27,802 as of Wednesday.

Even more worryingly, there have been 51 deaths from dengue in just the first 19 days of October, making it the second-deadliest month on record after August 2019, when 90 deaths occurred due to dengue. Apart from the ones in October, this year's deaths include 1 in June, 9 in July, 11 in August, and 34 in September, according to the latest figures reported by the Directorate General of Health Services (DGHS) under the Ministry of Health and Family Welfare.

The numbers reveal a seasonal shift in the incidence of dengue. Since dengue became endemic in Bangladesh in 2000, the highest number of dengue cases and high density of Aedes mosquitoes, the vector that carries the disease, were found during the monsoon season (July-September), with cases gradually declining from October. But this year the country has recorded over 11,710 monthly dengue hospitalisations in the first 19 days of October. It was 9,911 in September, 3,521 in August and 1,971 in July.

Dengue outbreaks in Bangladesh exceeded all previous records in 2019, mostly in the capital city of Dhaka. A total of 101,354 dengue cases with 179 dengue-related deaths were officially recorded.

According to a paper published by the US' National Library of Medicine, the true magnitude of dengue infection was possibly "concealed due to severely disrupted healthcare systems to tackle the ongoing COVID-19 crisis." The authors, all Bangladeshi disease specialists, also add that "misdiagnosis or delay in diagnosis of dengue is conceivable because of the similarities in clinical manifestations of these 2 diseases." This may be behind the relatively low number of infections detected since 2020.

But the spike in cases since September 2022 is undeniable, due to the accompanying spikes in death from dengue.

Speaking at a press conference in Dhaka on October 12, Institute of Epidemiology, Disease Control and Research (IEDCR) director Prof. Tahmina Shirin said that three serotypes - DEN 1, DEN 3 and DEN 4 - were dominating this year, causing higher fatalities.

"We found DEN 4 serotype for the first time in the country. About 10 percent of dengue patients are carrying this," she said. Shirin, however, added that 90 per cent were carrying DEN 3 while DEN 1 was found dominant in Rohingya camps.

The DGHS in their press conference revealed that higher dengue hospitalisation in Dhaka city were reported in the Mirpur, Uttara, and Mugdha areas, while Cox's Bazar district was reported as the most affected area outside the capital city. The National Malaria Elimination and Aedes Transmitted Diseases Control Programme deputy programme manager, Md Ekramul Haque, revealed the information.

Ekramul also mentioned other city areas, such as Jatrabari, Mohammadpur, Basabo, and Khilgaon, with high infection rates.

"Earlier, we warned of higher dengue cases through the pre-monsoon and monsoon surveys in the capital. We assumed higher dengue infections in Cox's Bazar and sent the message to the district civil surgeon," he said while presenting this year's dengue situation.

Epidemiology of the disease

According to the WHO, Dengue is a mosquito-borne viral disease that has rapidly spread to over 100 countries in recent years. Dengue virus is transmitted by female mosquitoes mainly of the species Aedes aegypti and, to a lesser extent, Aedes albopictus. These mosquitoes are also vectors of chikungunya, yellow fever and Zika viruses. Dengue is widespread throughout the tropics, with local variations in risk influenced by climate parameters as well as social and environmental factors.

Dengue causes a wide spectrum of disease. This can range from subclinical disease (people may not know they are even infected) to severe flu-like symptoms in those infected. Although less common, some people develop severe dengue, which can be any number of complications associated with severe bleeding, organ impairment and/or plasma leakage. Severe dengue has a higher risk of death when not managed appropriately.

Severe dengue was first recognised in the 1950s during dengue epidemics in the Philippines and Thailand. Today, severe dengue affects most Asian and Latin American countries and has become a leading cause of hospitalisation and death among children and adults in these regions.

Dengue is caused by a virus of the Flaviviridae family and there are four distinct, but closely related, serotypes of the virus that cause dengue (DENV-1, DENV-2, DENV-3 and DENV-4). Recovery from infection is believed to provide lifelong immunity against that serotype. However, cross-immunity to the other serotypes after recovery is only partial, and temporary. Subsequent infections (secondary infection) by other serotypes increase the risk of developing severe dengue.

Dengue has distinct epidemiological patterns, associated with the four serotypes of the virus. These can co-circulate within a region, and indeed many countries are hyper-endemic for all four serotypes. Dengue has an alarming impact on both human health and the global and national economies. DENV is frequently transported from one place to another by infected travellers; when susceptible vectors are present in these new areas, there is the potential for local transmission to be established, according to the WHO.

In Bangladesh, sporadic cases were first reported in the 1960s and a major outbreak occurred in 2000, with clinical cases reported annually since then. However, the burden of dengue is unclear. Researchers at the Institut Pasteur, an internationally renowned centre for biomedical research, have conducted a study to determine the burden of dengue in Bangladesh and identify key risk factors for infection.

In order to direct precious resources to tackle the virus in the most efficient way, the researchers at the Institut Pasteur, in collaboration with teams at Johns Hopkins University, icddr,b and IEDCR, wanted to know where the risk is the greatest and identify subsets of the population at increased risk. However, this is rarely known, especially in settings with limited surveillance capabilities. Prior to the study, there was essentially no understanding of where dengue virus circulated outside of the capital Dhaka.

"In this context, seroprevalence studies can help. Once infected, individuals develop long-lived antibodies that can be detected by specific tests" explains Henrik Salje, head of the Mathematical Modelling of Infectious Diseases Group at the Institut Pasteur. However, seroprevalence studies are nearly only done in single places, which means their findings are unlikely to be applicable to the wider population. By contrast, in this study, the researchers visited 70 different communities from all around Bangladesh, and invited over 5,000 individuals of all ages to provide blood samples, which were tested for evidence of dengue antibodies. They also asked individuals about themselves, including their age, sex and travel history, and also trapped mosquitoes as part of the study.

The researchers estimated that 24% of the Bangladesh population has been infected by dengue in their lifetime. However, this ranged from 3% in villages in the north of the country to close to 90% in the large urban hubs. They used mathematical models to estimate the number of annual infections and built maps that predicted where risk was concentrated. They estimated that there was an average of 2.4 million infections each year, mainly concentrated in the cities of Dhaka, Chittagong and Khulna. Outside these urban hubs, there was still some exposure to dengue, with risk concentrated in men, who tended to travel more. The main dengue mosquito, Aedes aegypti, was mainly found in urban cities, suggesting its current absence from many rural communities acts as a barrier to "broad nationwide epidemics".

The authors of the study, which came out in 2019, state that had they visited only a small number of communities, the resulting national estimates would have been very different, highlighting the danger in extrapolating findings from just a few communities to the whole country.

Are we fighting it right?

The day after the number of deaths in 2022 rose to the highest bar in 2019, Health Minister Zahid Maleque said that his ministry can provide treatment for patients suffering from dengue but cannot prevent the disease by killing mosquitoes.

"The Health Ministry has made adequate preparations to treat dengue patients if the cases rise further," Maleque said, while announcing that three hospitals ­- Dhaka North City Corporation Hospital, BSMMU's Super Specialised Hospital and Lalkuthi Hospital - have been kept ready for this purpose. The minister further said that the health sector is carrying out an advisory campaign for safety against mosquito bites, adding, "but killing mosquitoes is not our job."

It may come across as a crass statement at a moment when people are dying in droves. We often hear of the two city corporations in Dhaka carrying out special drives to eliminate the Aedes mosquito. One of the principal methods employed by the municipal bodies in their anti-dengue operations is 'fogging'. This uses foggers, sometime called "bug bombs." They are used to control cockroaches, fleas, and other insect pests. Most foggers contain pesticides, plus aerosol propellants that make a fog that fills the room. But according to architect and urban planner Mobassher Hossain, the city corporations are doing it wrong.

"An Aedes mosquito can upto 3 kilometres during its lifetime. But within Dhaka city there are some locations where the city corporations can't even spray insecticide," he says, referring to Pilkhana, the Cantonment and some other restricted zones.

"If we are to really battle dengue, there needs to be a coordinated effort that leaves no stone unturned. Just spraying insecticide, and publishing news on how many litres have been sprayed, and how many people have been fined for allowing larvae on their properties, will not do," Dr Mobassher adds.

His more sensational claim is that the method of fogging employed by the city corporations only has the effect of driving mosquitoes that were outdoors, around drains, indoors and therefore, even nearer to the people.

Kabirul Bashar, Jahangirnagar University zoology professor and vector management expert, said in the DGHS briefing that city corporations and municipalities should ensure mosquito breeding control. Referring to the high dengue cases in Uttara, Kabirul advised the city authorities to kill flying mosquitoes by spraying adulticide (a type of insecticide used to kill adult mosquitoes) through 100 fogging machines at a time to control infections.

He also urged the formation of a National Vector Containment Cell to fight harder against mosquito-borne diseases. In April, the health directorate in a pre-monsoon survey found a higher density of Aedes mosquitoes in the capital compared to 2021, predicting a worsening dengue situation this year in the city unless preventive steps were taken. The DGHS revealed its monsoon survey in September and found mosquito densities twice as high in Dhaka city as in the pre-monsoon survey.

Nazmul Islam, director of the disease control wing of the DGHS, has said they are expecting dengue cases may decrease from early next month. He said this on October 20 after an event organised by the National Malaria Eradication and Aedes Transmitted Diseases Control Program at the Westin Hotel in Dhaka.

While talking to journalists, Nazmul Islam said, "We feel that there are mosquito breeding sites and if residents and everyone who is responsible for trash management fulfill their tasks effectively, the situation will be manageable."

Responding to a question, he said, "We anticipate a decrease (in dengue cases) between the end of this month and the beginning of next month."

He also said many issues have worked behind the increase in dengue: "This year the monsoon is late. There has been intermittent rain and barely any torrential rain, then there has been repeated rain due to low pressure. These things have played a very important role."

Time for novel methods?

There is no specific treatment for dengue fever. Patients should rest, stay hydrated and seek medical advice. Depending on the clinical manifestations and other circumstances, patients may be sent home, be referred for in-hospital management, or require emergency treatment and urgent referral. Supportive care such as fever reducers and pain killers can be taken to control the symptoms of muscle aches and pains, and fever.

The best options to treat these symptoms are acetaminophen or paracetamol. NSAIDs (non-steroidal anti-inflammatory drugs), such as ibuprofen and aspirin should be avoided. These anti-inflammatory drugs act by thinning the blood, and in a disease with risk of haemorrhage, blood thinners may exacerbate the prognosis, advises WHO.

For severe dengue, medical care by physicians and nurses experienced with the effects and progression of the disease can save lives - decreasing mortality rates to less than 1% in majority of the countries.

As for existing vaccine options, there is currently only one, which was developed by the French multinational Sanofi-Pasteur in 2015 and is marketed in 20 countries around the world. But its use is limited by the fact that, when administered to individuals who have never been infected with dengue, it actually increases the risk of developing the more severe version of the disease. Because of this, it is only recommended for people between the ages of 9 and 45 who have had at least one previous instance of infection.

In a handful of cities around the world, mosquitoes have been armed with a microscopic weapon against disease. The bacterium Wolbachia pipientis blocks insects' ability to spread fearsome viruses such as dengue, Zika, and chikungunya. Since 2011, researchers have been injecting Wolbachia into the eggs of Aedes aegypti mosquitoes and releasing the hatched insects, which spread this protection to their offspring.

Wolbachia is present in more than 60% of all insects, including dragonflies, butterflies and moths. But the Aedes aegypti mosquito is an anomaly, with no naturally occurring Wolbachia. When the bacterium is introduced into Aedes aegypti eggs, the dengue virus is unable to replicate in the modified mosquitoes that hatch. The exact mechanisms for this are unclear, but some experts suggest that Wolbachia outcompetes the virus for resources such as lipids, or turbocharges the host's immune response, according to a paper published in Nature. Regardless, the Wolbachia-modified mosquito is prevented from spreading dengue through future bites.

There are two approaches to tackling dengue with Wolbachia. The first involves releasing only modified male mosquitoes. Since 2015, this strategy has been successfully adopted in Singapore and Guangzhou, China, and in parts of the United States, such as Miami, Texas and California. Because eggs produced from unmodified females that mate with modified males do not hatch, the number of mosquitoes in the community is greatly reduced.

The second approach, used by some cities in Vietnam, Indonesia, Malaysia, Brazil and Australia, among others, involves releasing modified mosquitoes of both sexes. The infected females pass the bacteria to their offspring. Over time (several months to years, depending on characteristics of the release site), the modified mosquitoes replace the native population.

"The use of Wolbachia as a tool for reducing the capacity of mosquitoes to transmit dengue is a proven technology," says Leo Braack, a vector-control specialist at the University of Pretoria in South Africa, as quoted in Nature. "Its efficacy has been demonstrated in large-scale studies in multiple countries."

According to a paper published in the journal Genetics, dengue fever incidence has been reduced between 40% and 96% in 4 different regions of the world where Wolbachia-infected Aedes aegypti have been established in the field. "It is not yet clear how sustainable such control programs will prove to be, but there is good reason for optimism," the paper states.

There should be nothing to hold us back in Bangladesh from adapting this method.