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Dengue in numbers: a sting in the tale

Monsoon and dengue transmission have a strong link.

The number of Dengue cases in Kerala has crossed 18,000 this year, with around 75 days still remaining. The closest peak was in 2013 with 8000 cases.

With 18,727 cases recorded till October 8, Kerala has handled 24% of all dengue cases so far this year in India. In the last 10 years this is just the sixth such instance — a single State handling more than 20% of all cases — and a first for Kerala.

But the State’s robust health infrastructure has helped to contain fatalities. There have been 35 deaths related to dengue — highest among States this year. But the case-fatality ratio this year was 0.19% — close to national average of 0.16% .

Monsoon link

In July, former Health and Family Welfare secretary C.K. Mishra had attributed the sudden surge of dengue in Kerala to the early arrival of monsoon. Monsoon and dengue transmission have a strong link.

According to a study, "Dengue burden in India: recent trends and importance of climatic parameters" by S.R. Mutheneni et al published in in 2017, while temperature influences mosquito’s reproductive behavior, precipitation provides water that serves as its habitat.

The study used extrinsic incubation period (EIP) to establish the link. EIP is the viral incubation period between the time when a mosquito takes a viremic bloodmeal and the time when that mosquito becomes infectious.

The study calculated EIP in five dengue-endemic States — Punjab, Haryana, Rajasthan, Gujarat and Kerala — and concluded that Kerala experiences a high incidence of Dengue during the monsoon period as it showed the lowest EIP among the studied States during rainy season.

Thus a suitable temperature range (23.5–30°C), an increased availability of breeding grounds and a higher percentage of infected mosquitoes makes Kerala vulnerable to vector borne diseases during monsoon, the study said.

The study also pointed out that Mexico reported a similar prevalence of dengue during rainy season when the temperature ranged between 17-30°C.

Entomological surveys done by the Kerala Health Department earlier this year confirmed this theory. It exposed plenty of breeding sites for Aedes species of mosquitoes inside homes across the State.

Kalpana Baruah, Joint Director, National Vector-Borne Disease Control Programme (NVBDCP), had in June pointed out that public apathy to appeals from government led to high mosquito density.

Dengue’s geographical pattern

This year, 55% of all dengue cases were recorded in the three southern States — Kerala, Tamil Nadu and Karnataka. Karnataka with 13,016 cases (16.5% to overall) until October 8, has registered a historical high for the State in a year. The previous high was 6,408 cases in 2013.

Tamil Nadu with 11,552 cases (14.6% to overall) so far is fast approaching its previous high of 12,826 cases registered in 2012. Whereas, West Bengal which saw its worst dengue incidence last year, with 22,865 cases (17.7% to overall) - highest ever in recorded history for any State in a year has seen only 5,389 cases so far this year.

With 15,033 cases (11.6% to overall) last year, Uttar Pradesh also experienced its worst dengue year in 2016. But this year only 1,461 dengue cases have been recorded so far.Thus the disease incidence shifted from the east in 2016 to South this year.

In 2015, Delhi (15.88% cases to overall) and Punjab (14.14% cases to overall) — in north India — were the top two States which shared the dengue burden. In 2014, it was Maharashtra and Odisha.


Vignesh Radhakrishnan OCTOBER 16, 2017 17:45 IST
UPDATED: OCTOBER 16, 2017 20:58 IST

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