As the capital battles with rising number of dengue cases, the type 4 strain of the disease has emerged as the dominant type for the first time in the capital, along with dengue type 2, serological tests conducted on patients at AIIMS have revealed.
While symptoms of type 4 dengue include fever with shock and a drop in platelets, type 2 causes a severe drop in platelets, haemorrhagic fever, organ failure and dengue shock syndrome. Barring stray cases in 2003, the type 4 strain of the virus has never been isolated in Delhi, said microbiologists from AIIMS.
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“We were expecting the strain to change this year, given the large number of cases. We thought it would be one of the other three strains, apart from type 2. But we were not expecting type 4 because it has never been actively circulating in Delhi.We have never managed to grow type 4 strains in our cultures except in very limited cases in 2003,” said Dr Lalit Dar, professor of microbiology at AIIMS.
When the dominant strain remains the same for a long period, a significant population develops immunity to it, and fewer patients are diagnosed with the virus. However the type 4 strain had never even shown a significant presence, said Dr Dar.
Type 4 and type 2 have emerged as the dominant strains this year, confirmed AIIMS spokesperson Dr Amit Gupta.
In two of the six strains where cultures were identified, type 4 of the dengue virus was identified. In four, type 2 continued to be the dominant strain.
“Two out of six samples show the virus type 4 has a significant presence this year. We are analysing another batch soon but these findings are enough to give us an indication of the virus this year,”said Dr Dar.
The institute had conducted antigen tests for dengue on 738 patients, of which 314 were found positive.
Microbiologists had anticipated a change in the dominant strain of the virus, considering the spike in cases this year — over 830 cases till August 29.
This year, with few dengue patients showing severe symptoms like drop in platelets and haemorrhagic fever in the first three weeks of August, doctors were expecting the relatively milder strains to be dominant. But several patients have been showing these symptoms in the last two weeks, according to doctors.
Dr Vanashree Singh, director of blood banks at the Indian Red Cross Society, said the demand for platelets had seen a sharp spike in the last ten days.
According to Dr Dar, dengue type 4 symptoms are less severe compared to type 2, which has remained the only dominant type since 2013.
“Every strain carries the risks of hemorrhagic fever, but type 4 is less virulent than type 2,” he said.
Each of the four types of dengue has its own characteristic symptoms.Type 1 and type 3, considered to be relatively mild strains, have been the most common strains in the capital since 1960, according to studies conducted by the National Centre for Disease Control and AIIMS.
Type 4 is identified as one of the stronger strains of the virus; symptoms include fever with shock and a drop in platelets.
Type 2, which emerged as the dominant strain in Delhi in 1996, causes thrombocytopenia or a severe drop in platelets, hemorrhagic fever, organ failure and dengue shock syndrome.
In 1996, when an outbreak of the disease was reported in Delhi with over 10,000 cases, the relatively severe type 2 strain of the virus was identified as the most common one.
In 2003, when a sharp rise in dengue cases was again reported, type 3, a mild strain, emerged as the most common type. In 2013, with over 5,500 cases, type 2 returned as the common strain in the capital.