Dengue, chikungunya may strike Delhi early this year
Till 3 March, Delhi hospitals tracked by the South Delhi Municipal Corporation reported 15 cases of dengue, five more than last year when the mosquito-borne disease infected 9,271 people and killed 10.delhi Updated: Mar 06, 2018 07:11 IST
A warmer-than-usual winter, earlier-than-usual summer, and very early trends of the disease have prompted experts to call for immediate measures to prevent the spread of dengue in the national Capital.
Meanwhile, scientists warn that this year could see more chikungunya cases, courtesy the medical equivalent of the so-called base effect, where the previous year’s data affects this year’s.
While there is no cause for alarm yet, given the weather conditions, efforts to contain the breeding of the mosquitoes responsible for transmitting both diseases should have begun by now, if not earlier, the experts add.
Till 3 March, Delhi hospitals tracked by the South Delhi Municipal Corporation reported 15 cases of dengue, five more than last year when the mosquito-borne disease infected 9,271 people and killed 10.
“The high numbers at this time of the year are most likely because it was a warm winter and the temperatures did not drop enough to end mosquito breeding and virus transmission,” said Dr BK Hazarika, municipal health officer, Delhi. “It is important for the temperature to remain below at least 16 degrees for 10 to 15 days at a time for the mosquitoes and the virus to die,” he explained. That hasn’t happened this year.
The spike in day temperatures, already 1 to 1.5 degrees Celsius above normal, is also creating breeding grounds for the Aedes mosquitoes -- Aedes (Stegomyia) aegypti and the Aedes Alpopictus – that spread dengue and chikungunya.
“Even small environmental changes of 1-2 degrees in temperature and in humidity matter,” said Dr Neena Valecha, director, National Institute of Malaria Research, New Delhi. According to her, mosquitoes can breed indoors, changing their behaviour. “High temperature increases the transmission window,” Dr Valecha added.
Dengue causes flu-like illness, with symptoms of headache, pain behind the eyes, muscle and joint pains, nausea, vomiting, swollen glands or rashes lasting for two to seven days. Death, though rare, is from potentially fatal complications, such as dengue haemorrhagic fever and dengue shock syndrome.
In the case of chikungunya, there’s an additional risk this year. Unlike dengue, chikungunya offers herd immunity – resistance to the spread of an infection from a large number of people being immune to it – and fewer cases in 2017 could lead to more cases this year, experts said.
“Once a huge chunk of the population gets chikungunya, it develops immunity against the virus and hence in the subsequent year the number of infections is less. It is only when the herd immunity of the population goes down again that an outbreak can happen again,” said Dr P Jambulingam, director, Vector Control Research Centre, Puducherry.
Delhi’s first big chikungunya outbreak occurred in 2016, when close to 12,300 people were infected. That resulted in there being less than 1,000 infections in 2017. “Since much of the population has no herd immunity against chikungunya, the risk remains,” said Dr Valecha.
Summer also results in human behaviour that’s conducive to the spread of the diseases.
“Warm weather leads to people using coolers and water-storage cans and pots, which become breeding grounds for the mosquitoes. Two cases of malaria and three cases of chikungunya have also been reported in February this year, which marks the start of the mosquito-borne diseases that plague the Capital from April to mid-December,” said Dr Hazarika. In 2017, 1,142 malaria and 940 chikungunya cases were reported, with no deaths.
The Aedes aegypti thrives in urban and peri-urban environments and can breed in a thin film of fresh water in containers, tanks, buckets, vases and even bottle caps. “Our studies have shown that mosquito-control anti-larvael measures should begin in January, much before the transmission period, for breeding to be contained,” said Dr Valecha.
With increasing urbanisation, the number of dengue cases has risen 30-fold globally over the past five decades, with 50 to 100 million infections and 20,000 deaths each year in more than 100 countries, says the World Health Organization.
“The best time to start mosquito control and surveillance measures is now. Right now, the temperature is conducive to breeding, add to it enough breeding grounds and there will be an explosion of the mosquito population,” said Dr Sujatha Sunil, group leader, Vector Borne Diseases Group, International Centre for Genetic Engineering and Biotechnology, New Delhi.
There are four distinct but closely related virus serotypes that cause dengue (DEN-1, DEN-2, DEN-3 and DEN-4), with DEN-3 being the strain in circulation in Delhi in 2016 and 2017. “For two years now, the DEN 3 strain has been in circulation and there is no way of telling whether there will be a change this year. But if there is a serotype switch, there will be an increase in severity and incidence of complicated dengue,” said Dr Sunil.