Asia’s “twindemic”: dengue fever cases rocket as covid-19 rages on

Pre-pandemic threat

Dengue epidemics tend to occur in a three to five year cycle, says Duane J Gubler, emeritus professor at Duke-NUS Medical School, depending on the strain of virus that is dominant; the mosquito population; and the herd immunity, density, and behaviour of the human population, which may vary by country and year.

In 2019, the World Health Organization highlighted dengue as one of 10 major threats to global health, with several regions seeing the highest number of cases on record at the time.3 Then covid-19 arrived.

A study looking at the relation between covid-19 and dengue in 2020 across 67 countries, including Brazil, Indonesia, and Vietnam, found an “inverse relation between the two.”4 “When covid-19 cases were on [the] rise, dengue cases dropped and vice versa,” lead author Miguel Ângelo Laporta Nicolelis, professor emeritus at Duke University Medical School’s department of neurobiology, tells The BMJ.

As people now re-emerge from two years of disruptions, so has dengue. A total of 2.59 million cases and 2065 deaths have been reported globally this year, with most cases in Vietnam, the Philippines, and Indonesia (Brazil and Peru are also common dengue hotspots).5

The Philippines recorded 102 619 cases in the first seven months of this year, a 131% rise from the same period last year.6 Between January and mid-August, Malaysia reported over 33 911 dengue cases, 89.5% more than the same period last year.7 From January to June, India reported 14 077 cases, a 3.6% rise from the same period last year. Indonesia reported 68 903 dengue cases and 640 deaths up to mid-August.5 And Singapore declared a “dengue emergency” in June, having seen more than 25 591 cases already this year, far greater than the 5258 cases throughout 2021. WHO reported on 23 June that South East Asia saw a 45% rise in covid cases (though only a 4% rise in deaths).8

These rises are placing a strain on hospitals and health systems when even “mild” cases may need treatment.

Difficult distinction

Distinguishing dengue from covid-19 in endemic areas is difficult because in both diseases patients may show symptoms of fever that are difficult to differentiate.9For instance, a 2021 study described a Filipino patient treated as a suspected case of covid-19 but whose symptoms of body ache, retro-orbital pain, and headache could also be indications of dengue.10 In Singapore, two patients were first incorrectly diagnosed as having dengue owing to a false positive dengue rapid diagnostic test, and were later confirmed positive for covid-19. A few cases of coinfection have been reported globally too.10

Thang Nguyen-Tien, a public health researcher at CGIAR (the Consortium of Agricultural Research Centres), says that as both covid-19 and dengue fever have similar symptoms to flu, people in Vietnam often take over-the-counter medicines from pharmacies instead of testing for covid or dengue.

Asked why dengue is undergoing such a rise this year, he cites the failure of mosquito control programmes and falling standards around preventive practices, including sanitation and hygiene, particularly in cities with a high population density.11 In Vietnam, he says, covid-19 had raised people’s awareness of sanitation and hygiene generally but it was “still not enough” to instil behaviours such as handwashing and routinely clearing standing water permanently.

Dicky Budiman, a researcher in global health security at Australia’s Griffith University, says the breeding sites for mosquitoes in high rise buildings often remain undetected, unlike in rural areas, where people are far more conscious about dealing with stagnant water.

Wenbiao Hu, a researcher at Queensland University of Technology’s School of Public Health and Social Work in Australia, also links the current dengue explosion with extreme weather conditions, such as heat waves and La Niña.

“Weather variability can affect viral amplification, mosquito reproduction and survival, and human biting rate,” Hu says, “A large change in temperature and rainfall after extreme weather conditions may provide further opportunities for the development of mosquito larvae or pupa. The flood seasons and wet weather contribute to significantly higher mosquito abundance.”

Since mosquitoes thrive when temperatures rise, Budiman warns that climate change will make dengue fever more of a threat globally over the next few decades.

Monsoon measures

Things will also get worse in the short term: the September monsoon season brings standing water that provides fertile breeding grounds for mosquitoes. “We are just coming into the high transmission season in countries north of the equator,” warns Gubler.

The region must prepare, experts tell The BMJ. Do Duy Cuong, an infectious diseases expert at a state run hospital at Hanoi, says that “with a shortage of healthcare workers especially infectious diseases specialists and a lack of medical equipment for diagnosis and treatment, the government in Vietnam needs to gear up to deal with covid-19 and dengue cases together.”

Researchers have emphasised the need for better tests to differentiate between dengue and covid-19.12 Hu says improving covid-19 vaccination coverage in regions at high risk of dengue, as well as developing integrated early warning systems based on weather and socio-environmental factors for covid-19 and dengue fever are a must. Simply identifying and eliminating standing water and tracking mosquito breeding could serve as effective dengue control measures. Entomologist Appadurai Daniel Reegan, from India’s National Centre for Disease Control, said that there should be routine entomological surveillance to identify water containers, which could be possible breeding sites in areas where dengue cases are high. “Once identified, they should be removed on a regular basis,” Reegan said.

But what’s needed most is vigilance and surveillance, says Rana Jawad Asghar, chief executive officer at the Islamabad based consulting firm Global Health Strategists and Implementers, which specialises in infectious disease prevention. Governments have long taken surveillance for granted once the initial crisis passes, a pattern that’s being repeated with covid-19.

He cites his home country, Pakistan, as a cautionary tale. To control cases reported from August onwards, Pakistan needs to start aggressive vector control surveillance as early as March to eliminate mosquito breeding sites, he says. But only one province, Punjab, had an effective system working with health workers and the community, and this has stalled following the change of government in 2018. After Pakistan reported over 53 498 dengue cases and 95 deaths in 2019, a national plan was drafted to improve surveillance, preparedness, and response—including health education and community awareness—for mosquito borne diseases across all provinces, to which Asghar contributed.

But urgency evaporated again once cases subsided in 2020. Provinces which have tried to implement better surveillance and prevention measures such as “fogging” (which disperses a liquid fog of insecticide into outdoor areas) have lapsed because of a lack of funding and staff. With one third of Pakistan having been ravaged by floods that have affected over 33 million people, health experts fear a rise in dengue cases once the water starts to recede.

Asghar fears surveillance systems are in stasis just when we need to be most vigilant.

“Wars can’t be won without good intelligence,” he warns. “We can’t have effective dengue control without strong health intelligence.”