A deadly spread

An un­usual mon­soon this year helped the highly adap­tive virus to pro­lif­er­ate, caus­ing out­breaks

Down to Earth - - COVER STORY -

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

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