Karachi: Major cities of Sindh especially Karachi witnessed unusually severe dengue illness this season not because of a new or mutated virus, but due to the dominance of a dengue serotype that had not been circulating widely in recent years, health experts say.
Doctors and virologists at Dow University of Health Sciences (DUHS) and Aga Khan University confirm that dengue virus serotype-1 is responsible for nearly 90 percent of cases reported this season, a major shift from the past four to five years when serotype-2 was the predominant form in Karachi and other cities of Sindh.
Dengue virus has four distinct serotypes namely DENV-1, DENV-2, DENV-3 and DENV-4, they said adding that infection with one serotype provides lifelong immunity against that specific serotype, but only partial and temporary protection against the others. This scientific reality, experts say, explains why many patients are experiencing more severe disease this year.
According to infectious disease specialists, a large proportion of Karachi’s population was exposed to serotype-2 during earlier outbreaks, particularly between 2019 and 2024. While those individuals developed antibodies against serotype-2, those antibodies offered little protection against serotype-1, which became dominant this season.
More concerning, experts explain, is that in dengue, antibodies from a previous infection can sometimes worsen the illness during a second infection with a different serotype. This phenomenon, known as antibody-dependent enhancement, allows the virus to enter cells more efficiently, leading to higher viral loads and a greater risk of complications such as dengue hemorrhagic fever, shock and organ failure.
“This is why patients who had dengue in previous years are presenting with more severe symptoms this season,” clinicians at DUHS and AKU said, adding that several fatal cases involved people who had a documented history of dengue infection.
Official data from the Sindh health department puts the death toll at 39 so far, mostly from Karachi. However, senior clinicians believe the actual number could be many times higher, as dengue deaths are underreported. Only a limited number of hospitals formally notify dengue fatalities, and many deaths are recorded under complications such as bleeding, pneumonia or organ failure rather than dengue itself.
Health experts also allege that underreporting helps mask administrative failures. “There is a misplaced belief that controlling numbers means controlling the disease,” one public health expert said, warning that inaccurate data undermines response planning.
Weather conditions have further aggravated the situation. Experts point to prolonged summer-like temperatures, shorter winters and sustained humidity, which extended the dengue transmission season well beyond its usual timeframe. Instead of tapering off by October, cases peaked between late November and early December, and doctors fear infections could continue into January 2026.
The Pakistan Meteorological Department had earlier warned that warmer-than-normal temperatures, combined with stagnant water from monsoon rains and flooding, would create ideal breeding conditions for Aedes mosquitoes, which transmit dengue. The department highlighted that dengue risk remains high when temperatures stay elevated and winters are mild.
Health experts say they had flagged the likelihood of a severe season months ago once surveillance data showed serotype-1 gaining ground, but preventive measures remained inadequate, particularly in densely populated urban areas.
Despite the changing serotype pattern, doctors stress that preventive measures remain the same. People are advised to protect themselves from mosquito bites by using repellents, wearing long-sleeved clothing, keeping doors and windows screened, using bed nets, and eliminating standing water in and around homes. Water storage containers should be properly covered, and blocked drains cleared to prevent mosquito breeding.
Doctors also urge the public to seek immediate medical attention if warning signs appear, including persistent vomiting, severe abdominal pain, bleeding, extreme weakness or difficulty breathing.
As the season stretches longer than usual, clinicians warn that complacency could cost lives. “This year’s outbreak is a reminder that dengue is not a one-time illness,” experts said. “A second infection with a different serotype can be far more dangerous than the first.”
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