New Delhi: On November 10, 2019, Delhi Chief Minister Arvind Kejriwal claimed Delhi’s fight against dengue has been successful as there has been a sharp decline in the number of dengue cases and no deaths have been reported this year.

“In 2015, there were more than 15,000 cases and 60 deaths and till last week, there have been fewer than 1,100 dengue cases and no deaths,” Kejriwal said in a video uploaded on Twitter, on what was the last Sunday of a "10 hafte, 10 baje, 10 minute" campaign launched on September 1, 2019. The 10-week campaign involved checking household breeding sites for 10 minutes at 10 am each Sunday.

This decrease and the public awareness campaign are worth lauding, public health experts said, but emphasised that it is too early to say whether Delhi has defeated dengue for good.

“We won’t have the same number of dengue cases each year,” said G Arun Kumar, director, Manipal Centre for Virus Research, Mangalore, who was instrumental in detecting the Nipah virus infection in Kerala in 2018. “Dengue is cyclical and seasonal in nature, it depends on many factors like weather, rainfall and wind. We have to consistently observe a reduction in cases to say that we have defeated the virus.”

Three-year cycles

There is a dengue outbreak every three years or so in the Indian subcontinent, show studies such as this paper published in The Journal of Infection in Developing Countries in 2011 and this 2018 paper in Annals of Clinical Microbiology and Antimicrobials, a global journal.

“I would be wary of calling this [case reduction] as an outright success, because that would lead to complacency,” said Rakhal Gaitonde, professor at the Achutha Menon Centre for Health Science Studies, Thiruvananthapuram.

“Also, in case there is an outbreak in the future, that could be interpreted as a failure of this initiative which it would not be, given the complexity of determinants of epidemics of dengue,” Gaitonde added.

Since dengue has four subtypes--DEN1, DEN2, DEN3 and DEN4--there are years when only one subtype is more prominent and the population has already been exposed to it and has developed immunity for it, reducing the number of cases that year, said another infectious disease expert who did not wish to be named.

While preventive measures such as controlling breeding of mosquitoes by community members are very important and play a vital role in reducing the spread of the disease, this alone cannot be proven to have made all the difference, said Kumar.

Season not over yet, variations common

The dengue season in North India is usually post-monsoon, from August to November. The dengue figures are still provisional and may yet change, experts said.

When Kejriwal made the announcement, only 1,100 dengue cases had been detected; within a week, the numbers had more than doubled.

“Dengue season is not yet over, patients are coming with dengue, last week I have treated over 50 cases of dengue,” said Atul Gogia, senior consultant, diabetes and infectious disease, Sri Ganga Ram Hospital, a private hospital in Delhi, on November 13, 2019.

Source: National Vector Borne Disease Control Programme, National Health Profile 2012, 2015

By the end of October, there were 2,289 cases in Delhi, according to figures posted in the National Vector Borne Disease Control Programme (NVBDCP), which maintains a state-wise database of the number of deaths and cases of all vector-borne diseases including dengue.

While this is 67% lower than the number of dengue cases in 2018, the number of dengue cases has varied widely in Delhi in the past. Numbers of cases ranged between 2,000 and 3,000 from 2009 to 2010 and then increased to 6,000 in 2010 and peaked at 15,867 in 2015. Numbers fell to 4,431 in 2016 (72% decrease) and then doubled (100% increase) to 9,271 in 2017 before falling to 7,136 in 2018 (23% decrease).

Nationally, in 2008, 12,561 dengue cases were reported, which increased to more than 90,000 in 2015 and peaked at 188,401 in 2017. There have been around 100,000 cases of dengue reported each year since 2015.

Also dengue is underreported despite being a disease that needs mandatory notification. For example, right to information applications by the non-profit Praja revealed there were 317 deaths due to dengue in Delhi while the NVBDCP shows there were just 10 deaths in 2017.

Dengue is here to stay, needs constant vigilance

While a reduction in the number of cases reported is good news, the disease is not easy to control or defeat. This is because the dengue virus is spread by the Aedes Aegypti mosquito, which lives very close to human dwellings, breeding in the clean water stored in overhead tanks, under houseplants, in buckets and disused tyres, for instance.

“As urbanisation increases and people move from rural areas to urban areas, we will see more of dengue,” said Kumar.

“Aedes Aegypti mosquito bites in the day time, biting more people as they move around unlike the falciparum mosquito that bites in the dark and gets its share of blood from one person,” said Rajni Kant, who is part of the research management, policy, planning and coordination cell at the Indian Council of Medical Research.

Climate change plays an important role in the spread of dengue. “As the temperatures warm, the dengue mosquito gets longer duration to breed, infrequent rain increases breeding sites for the mosquito and due to overall warming even higher mountainous regions are seeing a rise in the number of dengue cases,” said Kant.

Spurred on by climate change, dengue has become the most rapidly spreading mosquito-borne viral disease in the world, with half of the world’s population at risk, said The Lancet Countdown report published in The Lancet, a medical journal as IndiaSpend reported in November 2019. Nine of the 10 most hospitable years for dengue transmission have occurred since 2000, it said.

The dengue vaccine is not yet widely accepted. Therefore, experts point out, the only effective way to prevent the disease is to control the breeding of the mosquito through community awareness, just as Delhi government has done.

(Yadavar is a special correspondent with FactChecker.in.)

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Swagata Yadavar