More people are suffering a dengue relapse this year than during any previous outbreak, a trend that has made it trickier for doctors to manage a disease that has already claimed more than 20 lives in Bengal.

Patients thought to have recovered from a bout of dengue are going down with fever again within a few days or weeks, which isn't unusual but certainly more pronounced this time.

Depending on the gap between fever subsiding and coming back, doctors term the resurgence short or long-time relapse. In either case, dengue management is a challenge.

The dengue outbreak this season started much earlier than the period when the mosquito-borne disease usual shows up. September to early November is the time when the virus is known to strike, but Calcutta has been battling a surge in infections this time since the first half of July.

More than 5,000 people across Bengal have been affected so far by the two most virulent serotypes of the dengue virus. On Friday morning, Anup Kar, a 44-year-old resident of Dasnagar in Howrah, died at Apollo Gleneagles of complications triggered by the disease. He had been shifted there at 7pm on Thursday from a private hospital in Howrah with swollen legs and high fever.

Officials at Apollo said Kar had been initially treated at home after testing positive for dengue. He was admitted to a nursing home when his symptoms aggravated and then moved to the Howrah hospital.

By the time he was shifted to Apollo, Kar had acute respiratory distress along with swelling of the legs.

Doctors said dengue manifesting itself with aggressive symptoms and sometimes relapsing had made management of the disease more challenging.

In the event of a short-term relapse, fever initially stays for two to three days before subsiding. After a gap of another two to three days, just when the patient appears to be recovering, there is another bout of fever.

Long-term relapse is characterised by the patient being apparently cured of the dengue infection after five to seven days. But body temperature is back to being high after three to four weeks from the first occurrence of fever.

"This time, we are seeing this happen in many more patients than during previous dengue outbreaks. Maybe one of the reasons is that antibodies are not effective in fighting the virus in some cases," said Amitabha Nandy, director of the Centre for Tropical Medicine and Parasitology.

Nandy said 30 to 40 per cent of the patients coming to the institute were those suffering a relapse.

"A relapse happens with complications like fluid accumulation in the abdomen and chest and also a sharp fall in a patient's platelet count," said paediatric critical care specialist Parthasarathi Bhattacharya.

People who haven't previously been infected by the dengue virus usually develop detectable antibodies three to five days after the onset of fever in 50 per cent of cases. In 80 per cent of the cases, antibodies are detectable by the fifth day.