Serotype DENV 2 has returned after a gap of a few years, which could be a reason for higher number of cases this year
More than 2,916 people have already been reported affected by dengue this year in Delhi. This has been the highest since 2010 when more than 3,782 cases were reported.
Small puddles of water created by intermittent rains are the main reason behind this surge as this provides places for the mosquito vector Aedes aegypti to breed, says A C Dhariwal, director of India's National Vector Borne Disease Control Programme, the nodal agency in the country for prevention and control of vector borne diseases.
But the bigger contributor to the increase in the number of cases has been the fact that mosquitoes are transmitting serotype DENV-2 this year. Till last year, the circulating virus was of serotype DENV-1, he reveals. There are four serotypes of dengue virus and while infection with one protects the person from being re-infected by this serotype, it does not protect from the remaining ones. The four serotypes are classified separately as each has different interactions with the antibodies in human blood serum. at present all serotypes circulate together in the environment and any one of them could become dominant.
During the past decade, such shifts in the serotype have been seen periodically in the capital. Overall, DENV-2 and DENV-3 were the predominant serotypes during 2003-2007, but DENV-1 replaced these strains in 2008, reported researchers from Department of Microbiology, Maulana Azad Medical College, New Delhi in the Indian Journal of Medical Microbiology. Another study published in Journal of Vector Borne Diseases says DENV-1 was circulating in Delhi in 2010-11.
DENV2 is more serious as it leads to more mortality, says Easwaran Sreekumar, scientist at the Rajiv Gandhi Centre for Biotechnology, Kerala. He studies dengue epidemics in Kerala where this serotype is leading to severe symptoms like shock syndrome and haemorrhages, leading to high mortality.
Another contributor to the high incidence of the disease could be that the serotypes themselves are showing some minor changes in the genetic make-up. For example, lineages have been observed in serotype 3 in Kerala. Sreekumar’s studies show the presence of lineage IV strains of serotype 3. Before this, lineage III strains were predominant in the area. Lineage shifts in DENV-3 strains have been attributed to dramatic increase in disease severity in many parts of the world, say researchers in Virology Journal. Similar shifts have been seen in Lucknow, too, where researchers reported an active circulation of a new lineage of DENV-3 in March issue of Iran Journal of Microbiology.
Though recent reports of circulation of new lineages in Delhi are not published, earlier studies show presence of two independent lineages of DENV-1 in Delhi during 2005 and 2006. Researchers in the study published in the International Journal of Infectious Diseases in September 2008 say that the introduction of an independent lineage of DENV-1 and its co-circulation with DENV-3 in Delhi could result in a more severe outbreak in the near future.
These lineages can be formed due to genetic changes or could be imported from outside. For example, lineage IV found in Kerala has been found predominantly in Singapore earlier. Sreekumar says that it is difficult to ascertain whether the strain originated in India and then was exported outside or whether, it originated in Singapore and was imported. The answers can come only through long-term studies on the virus, he says.
Studies on the virus are difficult as patients come in late and the virus is no longer circulating in the blood. The virus is present in the blood only during the first few days when fever is the predominant symptom. Patients come to doctors only after this when complications set in but at that time, the virus is no longer present.
Clinical management of the symptoms such as emphasis on oral rehydaration is important, says Sreekumar. Preventing breeding spaces and reducing mosquito bites is the key to tackling the menace, says Atul Gogia, consultant with the internal medicine department of Sir Ganga Ram Hospital, Delhi. “We are taking steps to control the vector,” says Dhariwal.
|Dengue returns in force after four years
|Number of cases
|2010 (1/1/10 - 9/10/10)
|2011 (1/1/11 - 8/10/11)
|2012 (1/1/12 - 6/10/12)
|2013 (1/1/13 - 5/10/13)
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