MUMBAI: When a 75-year-old retired engineer from Thane, Madhav Paranjape, got Covid-19 in April 2021, he and his daughter Mridula were happy it was a mild infection.But, 15 days later, Paranjape developed a fever that crossed 103 degrees and his blood pressure dropped to such dangerously low levels that he was rushed back to hospital.

Since then, he has been to hospitals at least 18 times with the same symptoms: sudden high fever and hypotension.He has undergone PET scans, bone marrow biopsy and hundreds of blood tests to detect the cause for his periodic fevers, but no answers have been forthcoming.

Dr Kirti Sabnis, the infectious disease specialist who has been treating his for a year now and overseen his last 10 hospitalisations at Fortis Hospital in Mulund, has reached a diagnosis by the process of elimination: "It is long Covid." She has worked out a prescription, including steroids, for Paranjape to control the "inflammatory response" that the patient's 2021 Covid-19 infection started.

Long Covid is considered a post-acute-infection syndrome in which patients suffer from a series of debilitating symptoms for months or years.

Studies say long Covid lasts for a year on average, but Paranjape seems to be an exception.

He got discharged from Fortis on Saturday after a dengue infection was followed by his "mystery fever".

"Steroids are avoided for dengue patients, but it's the only medicine that brought his fever under control,'' said Mridula.

It is estimated that 10% of Covid patients developed long Covid, but there is still no concrete explanation why it occurs.

One theory is that fragments of the Covid-causing SARS-CoV-2 virus stay embedded in tissues and trigger an auto-immune response.

As there is no clear-cut test to determine long Covid, doctors are often divided over what constitutes long Covid infection.

Dr Vasant Nagvekar from Lilavati Hospital, Bandra, said long-Covid patients suffer from a range of symptoms, most of them minor, such as gastrointestinal discomfort, brain fog or higher heart rate but they don't require hospitalisation.

A senior doctor said Paranjape's repeated fevers are not typical of long Covid.

But Paranjape isn't the only one.

A doctor said one of his patients develops palpitation and suffers an anxiety attack every time he gets fever or a cold.

"He keeps testing himself at home for Covid and has left his job since he got Omicron 18 months back," the doctor said.

Infectious diseases specialist Dr Anita Mathew from Fortis Hospital, Mulund, said she has a few patients who have muscular weakness and brain fogging more than a year after their Covid infections.

"One of my patients developed lung fibrosis after a severe Covid infection during which he needed a ventilator.

He has to use a wheelchair now because of his oxygen requirement,'' she said.

Long Covid, said Dr Mathews, could have a physical or a psychological component.

"The most common refrain of patients is that they cannot carry out day-to-day tasks easily any more,'' she said.

An article in 'Nature Reviews Microbiology' by Dr Eric J Topol, one of the most cited researchers in medicine, in April said that long Covid has already debilitated millions of individuals worldwide, and "that number is continuing to grow".

Diagnostic and treatment options are currently insufficient, and many clinical trials are urgently needed to rigorously test treatments that address underlying biological mechanisms, including viral persistence, the article concluded.