Dengue mosquito spreads its wings across the country
LETHAL STING Erratic rainfall, temperature and humidity patterns, rapid urbanisation, increased travel and poor vectorcontrol create new dengue hotspots every year
NEW DELHI: A surge in dengue infections and deaths in West Bengal have doubled the number of cases in the state in one week, leading to protests in Kolkata’s streets against chief minister Mamata Banerjee, who is also the state health minister. Dengue cases in India crossed 100,000 in the week ending October 22, with 172 deaths, with this mosquito-borne infection that sickens and sometimes kills causing infections in all states.
Three states — Kerala, Karnataka and Tamil Nadu — account for close to half of India’s 100,794 confirmed dengue cases and 177 deaths, shows national data collated till October 22.
Between October 15 and 22, cases in West Bengal shot up from 5,389 to 10,697, and deaths from 13 to 19. Chief minister Mamata Banerjee blamed garbage, private hospitals and path labs for spreading panic, while Kolkata municipal corporation’s chief vector-control officer Debasish Biswas said dengue deaths were the result of “pre-existing conditions”.
They got it wrong. The Aedes aegypti that spreads the viral fever breeds in fresh water and dengue infections aggravate pre-existing conditions, leading to complications and multi-organ failure. Since people get symptoms four-10 days after getting bitten by an infected mosquito, the outbreak is likely to have started in West Bengal during October festivities. “In well-lit areas, the ‘day-biting’ Aedes aegypti also bites at night. Yet the government’s awareness campaigns remain outdated, so people don’t use protection,” said Dr KK Aggarwal, president, Indian Medical Association.
GOING VIRAL
Dengue is a viral infection that causes severe joint pain, nausea, abdominal cramps, fever lasting several days, and in n some cases, bleeding from the gums, nose or ears, and rashes. It has no cure but most people recover with symptomatic treatment for fever and pain.
In severe cases, people develop haemorrhagic fever, which may cause death from shock and multi-organ failure.
Before 1970, only nine countries experienced severe dengue outbreaks but by 2016, it spread across the tropics, putting more than 3.5 billion people at risk of infection. Around 390 million people in more than 100 countries get infected with dengue each year, estimates the World Health Organisation.
Over the past 50 years, variations in rainfall, temperature and relative humidity, urbanisation, rising population, increased travel and poor vector-control have increased the infection thirty-fold.
“The Aedes aegypti mosquito is a ‘synanthropic’ species – much like house flies and cockroaches -- that benefit from artificial habitats created where people live,” said Dr Veena Valecha, director, India’s National Institute of Malaria Research. Since Aedes aegypti breeds in fresh water collected in hard surfaces, such as containers, it is found in and around homes.
“This partly explains why more cases are reported from densely populated urban areas unlike malaria, which causes disease near fields and forests, too, because the anopheles mosquito breeds in freshwater ponds as well,” said Dr Valecha.
Early management of symptoms prevents complications. Among states with recorded deaths, Karnataka has more than 13,600 cases and five deaths, which makes the death rate a low 0.036%. Uttar Pradesh has India’s highest death rate of 1.06%, with 2,225 cases and 24 deaths.
“Of the four dengue serotypes, DEN-2 and DEN-4 are more virulent and cause severe disease. This year, DEN-3 is causing infection in India, which leads to fewer hospitalisations,” said Dr Aggarwal. Since DEN-3 was also the dominant strain last year, many people developed “herd immunity” against it and fewer people needed hospitalisation.
“After recovery, cross immunity to other serotypes is temporary, so if a person gets subsequent infections by other serotypes, the risk of developing severe dengue is higher,” warned Dr Aggarwal.
CASES OF UNDERDIAGNOSIS
Dengue hit India the hardest in 2016, when close to 130,000 cases and 245 deaths were reported. For the first time, every state reported cases of the viral fever, National Vector Borne Disease Control Programme data shows.
As in the past, more cases and deaths this year are in the states with better pub- lic health infrastructure than the underserved states, raising fears of underdiagnosis and under-reporting.
Highly populated states like Bihar, for example, has reported 1,036 cases and no deaths, and Madhya Pradesh has 1,141 cases and five deaths.
International studies have alleged under-reporting happens at an unprecedented scale in India. A Sanofi Pasteurfunded study in the American Journal of Tropical Medicine and Hygiene in 2014 said India had nearly six million annual clinically-diagnosed dengue cases between 2006 and 2012, which is about 282 times higher than the officially reported data by the health ministry. Sanofi Pasteur’s dengue vaccine, Dengvaxia, is awaiting regulatory approval in India.
The discrepancies are possible and often not intentional. “Often, people get treated symptomatically and don’t get
Dengue cases Dengue deaths
tested. Also, testing is more complicated than, say malaria, which can be tested in the field,” said Dr Valecha.
Another hurdle is that IGM and IGG rapid card tests for dengue sometimes throw up false negative results. A positive IGG but a low or negative IGM, for example, indicates past infection and can be misread by a less informed physician.
The confirmatory NS1 Elisa-based antigen test is expensive and costs between Rs 2,000 and Rs 3,000 in private clinics. “Dengue has no cure and since the treatment is symptomatic, many people say no to tests, adding to underdiagnosis and under-reporting,” Dr Valecha said.
“Erratic rainfall, temperature and relative humidity patterns are changing epidemiological patterns, and unless realtime tracking is improved, new infection hotspots will keep appearing every year,” Dr Aggarwal said.
(Figures for 2017 till Oct 22)