Symptomatic pattern changes and climatic factors in dengue outbreaks in Bangladesh

Epidemiology of the outbreak in Bangladesh

Dengue first appeared in the world in 1780. In 1950, the dengue virusspread to Thailand and the Philippines extending to Kolkata, India in 1963 and finally spreading to Dhaka, Bangladesh in 1964. At the time, dengue was called the Dhaka Fever.⁶ The presence of dengue was officially acknowledged in Bangladesh in 2000. That year, 5,500 patients infected with the dengue virus were admitted to hospitals, 93 of whom died. For two decades, the Bangladeshi government has been working on dengue control, patient management and eradication of Aedes mosquitoes.⁷ However, due to lapses in mosquito control and lack of patient management, the disease could not be brought under control. In 2000, globally, 500,000 people were infected with dengue, and in 2019, that number increased ten times to 5.2 million. According to the Pan American Health Organization (PAHO) under the WHO, 9,569,467 people were infected with dengue in Brazil in 2024, of these, 5,303 died. This was the highest death record due to dengue in a single year wan any country in the world.⁸ The mortality, as compared to the infection, is 0.05 per cent. Argentina held the second place. In 2024, 574,620 people were infected with dengue in the country, of these, 408 people died. The mortality, as compared to the infection, was 0.07 per cent. 284,173 people were infected with dengue in Paraguay. The number of deaths from dengue in the country was 121, which was 0.04 per cent of the total infected people. In Colombia, 262,440 people were infected with dengue,⁹ of these, 131 people died. The mortality rate in the country was 0.05 per cent. In Peru, the mortality rate was 0.09 percent. 234 out of 261,415 people died in the country.¹⁰

In 2024, the number of dengue infections detected worldwide exceeded 13 million. According to data from the WHO and the European Center for Disease Prevention and Control (ECDC), most of these infections occurred in a few countries in North and South America.¹¹ The highest number of infections was seen in Brazil, Argentina, Peru, Colombia and Paraguay. More than 9.5 million infections were detected in Brazil alone. The number of infections in Argentina, Peru, Colombia and Paraguay was between 200,000 and 600,000. Although the number of infections detected in these countries was high, Bangladesh had already risen to the top of the list in terms of the number of deaths.¹² According to the data of the Directorate General of Health Services, as of December 9, 2024, the number of dengue infections detected in Bangladesh was 96,062, of which 531 people had died [Figure 1]. Accordingly, Bangladesh was second to Brazil and Argentina in the number of dengue deaths.¹³ In 2019, a total of 101,354 people were infected with dengue across the country, and 164 people died.¹⁴ Whereas, in 2022, 62,382 people were infected in Bangladesh, and 281 people died [Figure 2]. The mortality rate was 0.45 percent.¹⁵

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Figure 1:

Comparisum between dengue outbreak in (a) 2024 and (b) 2023. Source: https://dashboard.dghs.gov.bd/.

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Figure 2:

Current epidemiology of dengue in Bangladesh (2024). (a) Month dengue cases by Month, (b) Monthly dengue confirmed death by Month, (c) Dengue affected (Admitted) by year, (d) Death by age group in 2024. Source: https://dashboard.dghs.gov.bd/.

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In 2023, 321,179 dengue patients were identified. Due to the sudden increase in dengue patients, there was a saline crisis in some places [Figure 3]. At that time, the price of saline had also increased. Meanwhile, it was that 60 per cent of the total dengue cases this year were men. Men in Bangladesh wear scantier clothing than women,¹⁶ so they had more risk of being bitten by mosquitoes.¹⁷ Moreover, there are fewer working women in Bangladesh. This would directly reduce the exposure of women to the outdoors.¹⁸ Although more men were infected with dengue, mortality in women was higher¹⁹ From January 1 to December 7 of this year, 60,393 or 63.20 per cent of those infected with dengue were men and 35,239 or 36.80 per cent were women. But of those who died from dengue, 270 or 51.70 per cent were women, and 252 or 48.30 per cent were men.²⁰ An analysis of age-based data on dengue-infected men and women showed that people aged 16 to 40 were the most affected. A total of 95,632 people were infected between January 1 and December 7. Of these, 57,724 or 60.36 per cent were aged 16 to 40. Of the infected in this age group, 37,756 or 65.41 per cent were men and 19,968 or 34.59 percent are women. 55 per cent of the patients admitted to hospitals due to dengue were outside Dhaka. Although there were more adult dengue patients, the number of children s also relatively high 30 per cent. of the total patients are children.²¹ Outside the City Corporation area, Chittagong division recorded 14,703 infections and 50 deaths, Barisal division recorded 8276 infections and 57 deaths, Khulna division recorded 9401 infections and 30 deaths, Mymensingh division recorded 3194 infections and 13 deaths. In addition to these, Rangpur division had 1474 cases, Rajshahi division had 3604 cases and Sylhet division had 318 cases.^8,22 Nine people died in Rajshahi and Rangpur. Currently, more people between 20 and 40 years of age are dying from dengue in Dhaka. However, the mortality rate for the children and elderly is relatively high in Chittagong.²³

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Figure 3:

Number of dengue cases and deaths in Bangladesh by year. Source: https://dashboard.dghs.gov.bd/.

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Environment involvement in dengue outbreak

Climate change and unplanned urbanization has increased the number of mosquitoes carrying dengue and chikungunya viruses in Bangladesh. This has been compounded by the weakness of the mosquito eradication system.²⁴ So far, some mosquito-borne diseases have been identified in Bangladesh, apart from Japanese encephalitis, dengue, chikungunya, malaria and Zika virus. Among them, the increasing number has made it difficult to control dengue cases and deaths. Overall, the outbreak of these mosquito-borne diseases has become a cause of extreme concern for the overall public health of the country. There was a time when it was believed that the rainy season meant dengue season.²⁵ Experts say that now is the time to change that belief. Because now the disease is not only prevalent in the monsoon, but also in the winter and summer seasons.^26,27 In Bangladesh, the post-monsoon period, i.e., August to October, is usually the ‘peak season’ of dengue. But although the number of dengue infections and deaths has decreased since October this year, it is not completely under control. This December as well, deaths due to dengue have been recorded almost every day. Between 2019 and 2022, a total of 34 people died in December alone. But in the December, 2023, 76 people have died of dengue, which is twice the total number of deaths in December of the last 5 years and in 2024, 43 people have died until December 9, 2024.

Mosquitoes survive by adapting to various hostile environments.²⁸ Earlier, Aedes mosquitoes laid eggs exclusively in clean water. They fed only in the mornings and evenings. Aedes albopictus mosquitoes now breed in polluted and dirty water. They also feed irrespective of the time of day. Many homes in cities remain dark even during the day because sunlight can’t reach them. Light pollution occurs due to excessive artificial lighting in the evening and at night. As a result, mosquitoes have also changed their behavior.²⁹ Pesticides are not working. Earlier, although the Culex mosquitoe infestation increased in the winter seasons, it was not seen for Aedes mosquitoes. But for the past few years, climate change has caused an increase in the population of Aedes mosquitoes in winter.³⁰ This is because of increase in temperatures. If the temperature drops below 13°C, then the larvae do not hatch from mosquito eggs. But this winter in Bangladesh, the temperature did not drop below 13°C. It may have dropped in Panchagarh and Dinajpur; but not across the country.³¹ The reason for widespread dengue in 2023 - 2024 was the favorable weather for mosquitoes all year round. There was high temperature and humidity for a long period. Along with this, there was intermittent rain.³² The temperature in the areas with prevalent dengue was found to be relatively high during winter. For example, in Old Dhaka Aedes mosquitoes could breed because the minimum temperature for mosquito breeding is 18 degrees Celsius. But Aedes mosquitoes do not breed in Ramna Park, because the temperature there is low due to the presence of many trees.³³

The dengue situation is constantly changing. Dengue outbreaks are seen to be confined to small pockets in different areas or neighborhoods.³⁴ It is very important to identify these pockets first. When it rains, the breeding ground for mosquitoes is not limited to any pocket. Wherever it rains, water accumulates and becomes a breeding ground. In the absence of rain, pockets where water accumulates contain dengue-carrying mosquitoes or patients. To identify these pockets, it is necessary to talk to dengue patients and find out their exact addresses.³⁵ Generally, dengue is more likely to be contracted from someone’s home or workplace. It is necessary to identify these and find dengue patients from there as well. In this way, the control system should be strengthened by identifying pockets. If hotspot management is done properly, good results can be achieved with minimal effort.

Changes pattern in Bangladesh

Due to climate change, dengue infection no longer stays limited winter and summer summer seasons, resulting in constant outbreaks of dengue disease. Especially in the last few years, more and more dengue patients have been found. Dengue outbreaks have continued throughout 2024.³⁶ The number of patients and deaths from dengue fever is increasing every day. Although the dengue outbreak was not high in the district cities, the situation in Dhaka and Chittagong was dire. This time, the mortality rate was increasing in ‘expanded dengue syndrome’. Usually, dengue outbreaks in the country start before the monsoon. The period from May to September is considered the dengue season.³⁷ However, due to climate change, dengue virus-carrying mosquitoes have been breeding and spreading throughout the year. One way to track the spread of infectious diseases is to determine the monthly ‘growth factor’ or growth rate. This shows how many people will be infected or will die in the current month compared to the previous month, or the next month compared to the current month. In Bangladesh, the dengue growth rate (2.42) was at its highest in June 2024—, meaning that the infection rate may increase by about two and a half times by July. The number of patients infected in October was half of the number of those in September. The growth rate decreased from 0.82 to 0.45 in October. According to data from the last 23 years, mortality rate among dengue patients in Bangladesh risen from 0.45 to 0.53. Considering the number of dengue patients in August 2024 (6,521), the number of patients in September was expected to be around 8,000. But in reality, the number of patients in September was more than 18,000. This is more than double the expected number.³⁸

Dengue fever is spread by the bite of a dengue mosquito. When a mosquito bites a person infected with the dengue virus, the virus enters the mosquito. Then, when the infected mosquito bites another person, the virus enters that person’s bloodstream and causes the infection. Symptoms usually appear 3 to 8 days after infection. These include a high temperature (40°C/104°F), severe headache, pain behind the eyes, muscle and joint pain, nausea, vomiting, and a distinctive rash.³⁹ Additionally, platelet counts usually begin to fall from the fourth day of dengue fever. They continue to fall until the seventh day, reaching their lowest point. Recovery begins on day seven, with platelet counts usually returning to normal within ten days [Table 1]. According to the Centers for Disease Control and Prevention (CDC), there are three stages of dengue. The first is the febrile phase, the second is the critical phase, and the third is the convalescent phase. This year, dengue patients in Bangladesh were showing atypical symptoms. Among those being admitted to the hospital, the number of patients who were infected with dengue for the second or third time was high.⁴⁰ Previously, they were infected with a different variant, so the symptoms were not typical. This is called “complicated dengue syndrome or extended dengue syndrome.” The symptomsare:

• 1.

  Prolonged diarrhea: Diarrhea that has been present for three or four days without improvement. This is one of the new symptoms of dengue that was seen.

• 2.

  Absence of fever: Dengue patients were being admitted to the hospital without a. That is, even though they did not have a fever, they were diagnosed with dengue after the test. In addition, many people may have suffered from a mild fever for a couple of days due to the heat but did not pay attention to it. But it later progressed to severe dengue.

• 3.

  Persistent vomiting: Vomiting starts after a few days of fever, and does not subside. Also, vomiting may contain blood that looks like coffee grounds.

• 4.

  Severe brain inflammation: This symptom is very similar to those of meningitis. It causes severe headaches. After a slight fever, there is a severe headache, stiff neck, stiff hands and feet or severe convulsions.

• 5.

  Swelling of hands and feet.

• 6.

  Water accumulation in different parts of the body - especially in the chest and abdomen.

• 7.

  Unbearable stomach pain.

• 8.

  Multiple organ failure: Multiple organ failure was slightly more common in children with dengue when compared to adults. Dengue also affects the liver and kidneys. That is why multi-organ failure was more common in children.

• 9.

  Even after dengue fever had subsided, some patients were found to have chest pain, shortness of breath, and even irregular heartbeat. In medical terms, this is called ‘myocarditis’.

• 10.

  A serious aspect of dengue is ‘macrophage activation syndrome’. In this case, liver enzymes increase. Again, the levels of LDH and triglycerides also increase. Similarly, the levels of ferritin in the blood also increase significantly. As a result, a ‘cytokine storm’ is created in the body. In such cases, the patient has to be given steroids.

Moreover, there are four types of dengue. After recovering from the type of dengue virus that infected you, you will develop long-term immunity. However, if you are infected with any of the other three types again, there is no immunity. This means that you can be infected again by one of the other three viruses in the future. If you get dengue fever a second, third, or fourth time, your risk of death is much higher.⁴¹

Author contributions

RAM & SA: Web-analysis design, supervised the data collection process, checked writing, approved methodology, manuscript editing and supervised all steps; PR: Researched literature, questionnaire design, coordinated and monitored the process, wrote the first draft of The manuscript; AAR: Organizing and data arrangement, checked writing; MFA: Organizing and data arrangement, checked writing; NS: Final paper revision.