EVERY YEAR, as the monsoon begins to wane, it stirs up an old scourge: dengue fever. But what has startled the country and public health experts this time is the timing when cases of dengue fever began to appear and the rate at which the illness has swept the country. “Usually, the monsoon lasts from June to September, and we see a surge in dengue cases around October. The number of cases starts declining as soon as humidity and mercury dip in November,” says Atul Gogia, an internal medicine specialist at Sir Ganga Ram Hospital in New Delhi. But this year, dengue season began much earlier, in July, and peaked by the end of August. Certain pockets in the country also reported the breakbone fever as early as January. A longer-than-usual dengue season ensured an unusually high number of dengue cases. What made the yearly scourge so virulent and unmanageable this time?
Monsoon with dry spells
The unusual susceptibility of the country this time is due to an unusual monsoon, which was characterised by intense wet spells followed by long dry spells, explains A C Dhariwal, director of the National Vector Borne Disease Control Programme. This created favourable breeding conditions for <g data-gr-id="113">Aedis</g> <g data-gr-id="114">aegypti</g> and <g data-gr-id="115">Aedis</g> <g data-gr-id="116">albopictus</g>, the two mosquito species that transmit dengue virus.
The National Capital Region (NCR), which is worst-hit by the illness, received almost the normal rainfall this year (with three percent deficit until September 20), but it was not uniform across the season. The region has not received any rain since the third week of August. Long dry spells in the rainy season resulted in a hot and humid condition, suitable for the growth of mosquito larvae. This year, even though the monsoon became weak in the NCR in late August, humidity continued to stay above 60 percent and the maximum temperature hovered around 39°C to 40°C, which is 5-6°C higher than the normal temperature for the period. Besides, explains Dhariwal, when rainfall distribution is uniform, rainwater flushes away stagnant puddles preferred for breeding by mosquitoes. But this year, freshwater remained accumulated in places like tyres and pots, allowing dengue larvae to grow. This has been the case in most parts of the country this year. Studies conducted in other tropical countries corroborate this observation. A study that analysed 23 years of Mexico’s weather data shows that the risk of dengue is almost zero at temperatures below 5°C and modest between 5°C and 18°C. The risk increases as temperature rises above 20°C; and declines beyond 32°C as adult mosquitoes gradually die above 36°C. The researchers also found a link between dengue and rainfall pattern. The risk of dengue increases as precipitation rises to about 550 mm, beyond which the risk declines. This is due to the creation of rain-filled breeding sites, the researchers noted in journal plus Neglected Tropical Diseases in 2013. The risks declined at high levels of rainfall, which may be due to <g data-gr-id="134">washing</g> out of such breeding sites, conclude the authors.
Another study in 2013 led by A Manoharan of Christian Medical College, Vellore, however, shows that in states like Tamil Nadu where temperatures are conducive to vector proliferation throughout the year, high incidences of dengue are reported in years that receive poor rainfall as well as in years of excess rainfall. Chennai experienced high incidences of dengue during the monsoons of 2001 and 2005 when it received excess rainfall.
In 2003, it experienced poor rainfall, yet reported a high incidence of dengue. The study was published in Indian Journal of Medical Microbiology. While the researchers of the Tamil Nadu study say climatic factors in isolation cannot dictate rate of dengue incidence, others point at a unique character of the dengue virus: it is the master of adaptation and knows how to fool the human immune system.
Too many types made dengue virulent
As of now, the scientific community is aware of four varieties or serotypes of dengue virus—Type 1, (denv1), Type 2 (denv2), Type 3 (denv3) and Type 4 (denv4). “Each serotype has a different bearing on transmission and fatality of the disease,” says <g data-gr-id="112">Dhariwal</g>. But an analysis of historical data reveals that the virus transmits at a faster pace and becomes virulent when multiple serotypes are in circulation. This is exactly how the dengue virus succeeded in causing isolated outbreaks across India this year.
Usually, when the virus enters the bloodstream, the human immune system triggers the release of antibodies that are effective in halting its entry. But different serotypes are largely similar in their genetic make-up and have very similar antigens. So, after thwarting infection by one serotype, if another serotype enters the bloodstream, the body fails to recognise it as a foreign body. This helps the virus give the human immune system a slip and spread the infection, according to research published in <g data-gr-id="123">plos</g> Pathogens on January 30, 2015. This year, Delhi has seen a predominance of Type 2 and Type 4 dengue viruses, which together handle a high rate of transmission of the disease, says Dhariwal.
It has also been observed that anyone who survives an infection by one of the four serotypes gains lifelong immunity from it. For evasion, there is an observed pattern of a yearly shift in the predominance of the virus serotypes in different regions of the country, he adds.
According to a theory doing the rounds in the media, the dengue virus might have undergone mutation this year. A newspaper article reported that researchers at AIIMS in Delhi suspect the prevalence of a new strain through its analyses of 50 samples. While AIIMS was expected to submit its final findings to the Union Ministry of Health and Family Welfare by September 18, no announcement was made regarding the findings till the time the magazine went to press.
If the suspicion turns out to be true, this wouldn’t be the first time that dengue virus has undergone mutation. In a study paper published in 2013 in <g data-gr-id="118">plos</g> Pathogens, scientists have reported a crucial link between a mutation in the Type 1 dengue virus and the high rates of transmission during a major outbreak in Myanmar in 2001 when more than 200 people died from the disease. In India too, doctors in Mumbai suspect that mutation of the virus in 2012 and 2014 was responsible for the high rates of infection. But not everyone agrees with this theory. According to P K Das, former Director, Vector Control Research Centre in Puducherry, the virus keeps mutating and it makes no difference in the transmission rate of the disease. What went wrong this year is our lack of preparedness and the fact that there has been a lot of noise about this in media, creating panic. The municipal corporation should have been prepared to prevent mosquito breeding and to create awareness about the fever, Das said. The reason for the unpreparedness, though, lies not in the unpredictable nature of the virus, but in the lack of societal responsibility, says a scientist with the Indian Council for Medical Research. The spread of dengue can be easily prevented with adequate and routine steps like regular flushing and covering of stagnant water sources. The administration alone cannot do it.
Changing with climate
The one message that is clear from this year’s outbreak is that the situation is going to worsen in the coming years—all thanks to climate change. In fact, not just dengue, climate change is going to make the environment conducive to the spread of all vector-borne diseases. As per the World Health Organization (WHO), “one of the major consequences of climate change is the rise in the incidence of vector-borne diseases (<g data-gr-id="106">vbds</g>)”, which include dengue, malaria, filaria, Japanese encephalitis and chikungunya.
Explaining the link between climate change and <g data-gr-id="107">vbds</g>, Ramesh Dhiman, a scientist at the Delhi-based National Institute of Malaria Research, says, “Insects are cold-blooded. They cannot regulate their body temperatures. That’s why any change in temperature affects the developmental cycle of vectors as well as the development of pathogens in their bodies.” According to A K Ghosh, director of the Centre for Environment and Development, a Kolkata-based non-profit, temperature rise can lead to floods, and this may lead to an increase in <g data-gr-id="108">vbds</g>.
According to the June 2015 report of the Lancet Commission on Health and Climate Change, vector-borne disease risks increase as sea level rises and creates new breeding grounds along the coast.
Spreading to new areas
“Globalisation, unplanned urbanisation and environmental challenges such as climate change are having a significant impact on disease transmission in recent years. Some diseases, such as dengue, chikungunya and West Nile virus, are emerging in countries where they were previously unknown,” says WHO. The Intergovernmental Panel on Climate Change (IPCC) also says climate change is likely to expand the geographical distribution of several <g data-gr-id="105">vbds</g> and extend transmission seasons in some locations.
IPCC’s fourth assessment report in 2007 says India will witness a “2°C to 4°C increase by 2050 compared with current climate”. The report also predicts that the “geographical range (for malaria is) projected to shift away from central regions towards south-western and northern states. The duration of the transmission window is likely to widen in northern and western states and shorten in southern states”. Experts also warn that the projected temperature increase in colder regions, such as the Himalayas, can trigger the breeding of mosquitoes and the rate of transmission. The epidemiology of dengue in India has already changed over the years in terms of strain, location and severity. And there is more to come, warn experts. “By 2030-50, India will become the malaria capital of the world,” says Ghosh. In fact, the Indian Network for Climate Change Assessment Report, 2010, shows that the climate in 14 districts in Assam will be conducive for malaria to spread throughout the year by 2030 because of global warming. At present, just one district in the state has a climate conducive to malaria to spread the entire year. The report also predicts that Uttarakhand, Arunachal Pradesh, Mizoram and Meghalaya will suffer because of climate change.