Part One: Causes, Symptoms, and Present Trends of Dengue Fever

Due to dramatic climate change, a rise in average temperatures nationwide, erratic rainfall schedules, and unplanned urbanization, the severity of dengue in Bangladesh is no longer confined to a specific season. Previously, the monsoon was considered the sole dengue season, but mosquitoes now find favorable breeding environments year-round. Continuous or intermittent rainfall, high atmospheric humidity, and stagnant clean water in and around households create ideal breeding grounds for Aedes mosquitoes. Consequently, the number of cases increases exponentially at the onset of the monsoon, placing an abnormal strain on hospitals across the country.

Primary Causes and Transmission Process of Dengue Fever

Dengue is primarily a viral disease spread through the bites of two species of Aedes mosquitoes: Aedes aegypti and Aedes albopictus. Among these, Aedes aegypti is more commonly found in human residences, balconies, urban areas, and densely populated neighborhoods. On the other hand, Aedes albopictus typically breeds in villages, bushes, or natural environments. When an infected Aedes mosquito bites a healthy person, that individual contracts dengue fever after an incubation period of 4 to 6 days. Similarly, if an uninfected mosquito bites a dengue patient, it becomes a carrier of the virus. In this way, the disease spreads rapidly from human to human through mosquitoes.

Types and Complexities of the Dengue Virus

The dengue virus has four distinct serotypes or strains: DEN-1, DEN-2, DEN-3, and DEN-4. A person can contract dengue multiple times in their lifetime. Infection with a specific serotype grants lifelong immunity against that particular strain, but it does not provide permanent protection against the other three. As a result, a person remains at risk of contracting dengue up to four times in their life. From a medical perspective, individuals infected for the second or third time with a different serotype face a significantly higher risk of severe physical complications and mortality compared to their first infection.

Classic Symptoms and the Critical Period

The classical or initial symptoms of dengue fever primarily include a sudden high fever, which can reach up to 105°F, accompanied by severe body aches. Intense pain occurs in the spine, lower back, joints, muscles, head, and behind the eyes. The severity of the pain is so intense that it feels as though the bones are breaking, which is why this fever has historically been called "Breakbone Fever." Children, unable to express their pain, often cry when touched and become extremely irritable.

Typically, 4 to 5 days after the onset of fever, a reddish rash or allergic skin eruption appears all over the body. This is accompanied by nausea, persistent vomiting, loss of appetite, and extreme physical weakness. In medical science, this is sometimes referred to as a "Biphasic Fever," as the fever may temporarily subside after 4–5 days only to resurface for another 2–3 days.

Most patients and relatives assume that once the fever drops, the patient is recovering. However, the real danger begins at this exact stage, known in medicine as the "Critical Period." During this phase, blood platelet counts rapidly decline, and fluid leaks from the capillary network of blood vessels, leading to "Dengue Hemorrhagic Fever."

In this state, bleeding can occur from various delicate parts of the body—such as red spots under the skin, bleeding from the nose and gums, coughing up blood, vomiting blood, or passing fresh blood or dark, tarry stools. In women, premature or excessive menstrual bleeding may occur. Most dangerously, uncontrolled internal bleeding can happen inside and outside the eyes, as well as in the brain and heart. This leads to an alarming drop in blood pressure, a condition known as "Hypovolemic Shock." Additionally, fluid accumulation in the lungs and abdomen, liver involvement leading to jaundice, and acute renal failure can rapidly push the patient toward death.

In some cases, the circulatory system fails completely, resulting in "Dengue Shock Syndrome" (DSS). Symptoms include an extremely weak and rapid pulse, cold and clammy extremities, decreased urine output, and loss of consciousness.

Evolving Symptoms and Mosquito Behavior

In recent years, significant changes have been observed in the patterns and symptoms of the dengue virus. Many patients no longer exhibit classical signs such as high fever, severe bone-breaking pain, or skin rashes. Instead, they present with mild initial symptoms like low-grade fever, sudden clearance of fever, cough, mild abdominal pain, vomiting, loose stools, or swelling of the hands and feet. Blood pressure and urine output drop dangerously, and inflammation of the liver or brain develops before patients even consult a doctor. Due to the lack of distinct symptoms, patients often mistake it for a routine seasonal fever and delay seeking care, arriving at the hospital at a very late and critical stage.

Similarly, entomological research reveals significant shifts in the behavior of Aedes mosquitoes. It was previously believed that Aedes mosquitoes only bite during sunrise and sunset—namely in the morning and late afternoon. However, it is now proven that in the presence of artificial or bright lighting, Aedes mosquitoes can bite at any time, day or night. This behavioral shift has made the threat of dengue ubiquitous.

Author: Professor Dr. A. B. M. Abdullah, Ekushey Padak-winning renowned medicine specialist and former Dean, Faculty of Medicine, BSMMU.