The mosquito-borne dengue virus has mutated several times over the years,leading to five outbreaks of the disease in the Capital since 1996,doctors said.
This comes at a time when scientists from All India Institute of Medical Sciences (AIIMS) and National Centre for Disease Control (NCDC) were preparing tests to identify this years dengue strain.
There are four strains of the virus,commonly categorised as dengue-I,II,III and IV. It is a myth that the virulence of the disease is determined by the strain. Any strain can lead to aggressive forms of the disease. Since most people dont have immunity against new strains,a greater number are affected, Dr Shobha Broor,the head of microbiology in AIIMS,said.
She said type-II has been responsible for most cases of dengue haemorrhagic fever (DHF) the world over. This is the most aggressive form of the virus and usually causes dengue shock syndrome.
The four strains can show a wide range of symptoms from simple fever to haemorrhages and shock syndrome,which can prove fatal if not treated properly.
Dengue-I,which is most common,is termed as the classic,while type-II is known for haemorrhagic fever and shock syndrome.
A patient having type-III or IV dengue shows severe symptoms,but generally dont suffer from shock or haemorrhagic situations.
Analysis of dengue cases from 1960 in the country,including sporadic incidents in the 1970s and 1980s,reveal that type-I and III have been the most common strains,particularly in Delhi. It is important to understand that the severity of symptoms can be caused by any strain,if ignored, a senior virologist in NCDC said.
Delhi health officials said two of the three deaths recorded in the city this year were from the shock syndrome. They said haemorrhagic fever was also seen in the three cases.
The first case of haemorrhagic dengue confirmed through serological tests was recorded in 1988,an official said. The first virological analysis was done in 1996 and dengue-II was found to be the most prevalent strain in samples screened in AIIMS.
Though there were reports of outbreaks in 1970,1982 and 1988,no culture confirmed or laboratory recorded cases of haemorrhage or shock syndrome were recorded. The next widespread incidence of dengue was in 2003 when all four strains of the virus were detected in patients.
In 2005,isolates of type-I and III were seen,while combined infection where both strains were present in a patient was also detected. A year later,doctors and virologists found type-III as the most common strain.
In 2010,when a spurt in dengue cases were reported,type-I was found to be the common strain.
This year,AIIMS has already tested 1,000 samples of which 194 have tested positive for the IgM antibody. Biochemical polymerase chain reaction (PCR) tests are currently being conducted on the samples.
These strains mutate into sub-types over the years. In 1996,a different genotype of dengue strain was detected. It was genotype 4. In some 1,967 samples,genotype 5 was found, Dr Broor said.
Concurrent strains infections of more than one strain at the same time were observed in 2006. This led to different manifestations of the disease.