A deadly spread
An unusual monsoon this year helped the highly adaptive virus to proliferate, causing outbreaks
Eas the monsoon begins to wane, VERY YEAR, 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 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 Aedis aegypti and Aedis albopictus, 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 per cent deficit till 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 per cent 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 the 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. 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 plos Neglected Tropical Diseases in 2013. The risk declined at high levels of rainfall, which may be due to washing 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