Dengue: Heavy rains during July 2023 led to outbreaks across India

Monsoon in July 2023 was 13 per cent above the long-term average. While many regions across India reported flooding due to torrential downpours, some districts saw seasonal outbreaks of vector-borne diseases like dengue fever.

Down To Earth (DTE) analysed the correlation between vector-borne disease outbreaks and rainfall departure swing in India for July; the findings suggested a positive link between rainfall deficit and the virus spread.

Source: Pulaha Roy

As seen in the chart, a total of 22 districts in India reported dengue outbreaks in July. In 18 of the 22 districts, DTE found a correlation between surge in outbreaks and positive swing in rainfall departure.

Though monsoon resumed in India on June 8, 2023, it recorded a cumulative deficit of 10 per cent.

According to a research titled Lag effect of climatic variables on dengue burden in India, dengue outbreaks happen 0-3 weeks after the breeding of the Aedes aegypti mosquito, the carrier of the dengue virus. 

According to the World Health Organization, epidemics like dengue are a common phenomenon post-flooding, where stagnant water aids in the breeding process. 

Keeping the delayed occurrence of dengue outbreaks from the time of breeding in mind, DTE, for the analysis, observed the rainfall status from June 1 to June 19 and from June 1 to July 30, 2023. All the 22 districts listed had experienced deficient rain till then, according to the India Meteorological Department’s rainfall data.

However, by the end of July, 18 of the 22 districts reported a positive swing in rainfall departure, with Palghar in Maharashtra reporting the highest rainfall surge — to an excess spike of 62 per cent by the end of July from 72 per cent deficient rain till June 19, 2023.

Apart from Palghar, Raigad in Maharashtra and Rajnandgaon in Chhattisgarh were the other districts to have reported massive swings in rainfall between June and July. 

However, DTE also found that in 11 of the 22 districts, rainfall remained deficient or lower than the long-term average. So, that raises the question: How come deficient rainfall leads to dengue outbreaks?

While the above observation is true, a more granular analysis of the rainfall data suggests that 18 of the 22 districts also reported a positive swing in precipitation footfall between July 19 and July 30, 2023 in various degrees.

Some places like South Goa and Osmanabad in Maharashtra saw extreme rainfall days — more than 60 millimetres in a day’s time.

On the other hand, a gradual rise in precipitation levels was witnessed by districts like Ernakulam (Kerala). The deficit in rainfall here was brought down to -27 per cent by July 30, 2023, from -46 per cent on June 19.

Interestingly, Ernakulam, along with Malappuram, also in Kerala, reported multiple instances of dengue outbreaks in July with a cumulative cases load of 82 and 281, respectively.

State-wise, Kerala had the highest caseload (363), followed by Maharashtra (323) and Tripura (259) in July.

While DTE’s analysis is only based on a positive correlation between dengue and a positive swing in rainfall in almost all the districts which reported an outbreak in July, triggers for such outbreaks can be multifaceted. 

In fact, according to the research mentioned above, reasons could be climactic, like temperature playing a role in the breeding process or a positive correlation between an upsurge in dengue cases and El Nino and Indian Ocean Dipole years.

It could also be impacted by socio-economic factors like population density, migration or whether the outbreak happens in an urban or rural setting. 

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