The dangers of the dengue virus

There have been few days without rain this April and May in Kathmandu. That means more disease for Kathmandu. Not COVID-19, but the virus we used to talk about -- last year's scary unknown disease, dengue.

The year 2019 saw Nepal's worst ever outbreak of dengue, with 15,000 people infected, many times the previous high. Dengue was particularly bad in the Kathmandu Valley. At least six people died.

The medical journal The Lancet has warned that Kathmandu could see a dengue ‘explosion’ because it has become ‘an ideal breeding ground’ for the mosquito that carries dengue -- the notorious Aedes aegypti. Already in 2020, four cases have been reported in Kathmandu, and four in Lalitpur.

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Dengue spiked in 2019 with outbreaks around the developing world. In recent years, Delhi, Dhaka, Ho Chi Minh City, Senegal and Singapore have faced bad outbreaks. A dengue outbreak in Delhi in 2015 inundated hospitals and clinics across the city.

Dengue likes warm humid conditions. Before corona, dengue was the world's fastest growing dangerous virus, increasing four-fold in just over a decade. Two of every five people around the world, nearly 3 billion people, live in dengue risk areas where it is a leading cause of illness. Each year approximately 100 million people contract the disease, and over 20,000 die.

Like malaria, dengue is actually several different diseases that we give a single name. There are four dengue viruses: Dengue 1, 2, 3, and 4. A person can catch each one, and get sick as many as four times.

A first infection generally brings only mild flu symptoms, if any at all. But a second infection can cause ‘severe dengue’ with plasma leakage, bleeding, and shock. This can kill. No treatment exists for dengue, but generally symptoms can be managed.

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Mosquitoes move up mountains as the earth warms, Sonia Awale

Dengue's main mosquito host, Aedes aegypti, is no ordinary mosquito. In a species known for death and deviousness, it has few rivals. Not many mosquitoes have so successfully defied human efforts at control, including in recent decades.

Long before dengue, Aedes aegypti carried a far deadlier disease: yellow fever. During the 16^th and 17^th, slave ships brought yellow fever carrying A. aegypti from Africa to the Americas, devastating indigenous populations and reshaping the region's balance of power. In the early twentieth century, the A. aegypti -- yellow fever combination almost blocked the construction of the Panama Canal.

Most wildlife avoid humans. Not A. aegypti. It loves us, particularly the ecological disorder we often create. The more humans, the more disturbed ecosystems, the more A. aegypti. Cities, especially disorganised cities, create ideal habitat: lots of standing water for breeding and lots of humans for feeding.

Dengue's other mosquito carrier is A. albopictus.

The Lancet points to dengue's major causes: climate change, relentless and haphazard urbanisation, brisk trade and transit from dengue-infested areas, and poor health infrastructure. As Kathmandu grows and gets more connected to other parts of Asia, dengue spreads.

Climate change has been overrated as a cause. Warming temperatures can extend the range of Aedes carriers but other causes deserve more attention. ‘Last summer’s explosion of dengue in Kathmandu Valley was blamed on climate change,’ the Nepali Times recently noted, ‘but it was actually a perfect storm of crowded and squalid living conditions, poor drainage, greater mobility of people, and a particularly wet monsoon.’

The real problem is disorganised urbanisation. The A. aegypti carrier thrives in small collections of clean water in flower pots, old car and truck tyres, uncovered water containers, and even puddles. It bites people early morning and at dusk.

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Nepal, like much of the developing world, has witnessed dramatic and disorderly urban growth in recent years, much in the lowlands along highways. ‘Nepal's Tarai," Mukesh Pokhrel recently wrote also in this newspaper, "is turning into one endless elongated metropolis.’

Kathmandu itself grew four-fold between 1980 and 2010, and is one of the fastest growing cities in one of the world's fastest urbanising regions.

'Cities like Kathmandu are perfect breeding grounds for the Aedes mosquito,' says Jeremy Farrar of the Wellcome Trust in an interview with this paper. 'Kathmandu’s climate is changing, the city is growing exponentially in a haphazard way and the mosquito loves highly dense populations of people.'

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Dengue fever, Buddha Basnyat

The Lancet highlights what might happen in a dengue explosion in Kathmandu. ‘A tug-of-war might occur between the worried well and the truly sick,’ it says. ‘Hospitals might be overwhelmed and many needy patients with other diseases could be turned away.’

Doctors inexperienced with the disease pose a particular worry. They could misunderstand its dynamics and cause unnecessary deaths by over-treating with unneeded antibiotics, transfusions, and excessive intravenous fluids. This has happened elsewhere.

The dengue-carrying Aedes aegypti mosquito is difficult to control. Brazil eradicated it twice, in the 1950s and in the 1970s, and twice it returned. Several factors explain why: a lack of sustained careful epidemiological surveillance, insecticide resistance, the high costs of materials and wages, insufficient community participation and coordination with the health sector, and, crucially, the spread of haphazard urban development. Narrow control programs inevitably run into trouble.

The World Health Organisation (WHO) stresses both micro and macro efforts to fight dengue. Individual families can put tight lids on water containers. They can eliminate breeding spots such as empty cans, bottles, and tires. It also says community engagement and building country capacity for environmental management and disease surveillance is important.

“Locally adapted vector control measures such as household water storage management and, more importantly, engaging individuals and communities to manage their immediate environment are essential to controlling the spread of the disease,” says Raman Velayudhan, head of WHO’s Vector and Ecology Management Unit.

School-based awareness programs, The Lancet authors noted, can help block dengue's spread. ‘Students are good learners and can be taught to effectively use repellents and mosquito nets and get rid of water collections near their homes and schools; students will then spread this information to their parents.’ Several countries have implemented successful school programs.

Historian Frank Snowden sums it up: "Dengue takes full advantage of societal neglect and the absence or cessation of vector control programs."

Tom Robertson, Ph D, is an environmental historian who writes about the history of malaria in Nepal. And just wrote a five-part series on smallpox for Nepali Times as part of his History of Disease column.