Is climate change increasing dengue risk?

As temperatures rise and monsoon unleashes its fury, dengue could shift towards areas untouched by it.
Published September 25, 2019
A child is examined at the dengue ward at the Benazir Bhutto Hospital, Rawalpinidi – Photo: Sara Farid
A child is examined at the dengue ward at the Benazir Bhutto Hospital, Rawalpinidi – Photo: Sara Farid

This past week, thousands of Pakistani youth, energised through Climate Action Pakistan's calls for a climate strike, marched through the country's major cities to draw attention to a critical challenge.

Climate change is affecting countries all around the world; developing nations, though, bear the brunt of extreme climate events due to a combination of complex inter-linked factors. One of the impacts of climate change that has recently been more pronounced is an increase in vector-borne illnesses.

According to the World Health Organization (WHO) vectors, pathogens and hosts survive and reproduce within optimal climatic conditions, chief among which are temperature and rainfall. The WHO goes on to state that “rainfall can influence the transport and dissemination of infectious agents [including anthroponosis like dengue fever], while temperature affects their growth and survival”.

The spread of dengue has expanded, and the WHO reports that between 50-100 million infections occur annually in over 100 countries. Around 45 per cent of the world’s population, or 3.6 billion people, live in countries with a high risk of dengue. Pakistan is one of them and has once again been hit by dengue this year, with over 10,000 cases reported so far across the country.

How dengue is spread

Dengue itself is a flavivirus that is transmitted through the Aedes aegypti mosquito. Symptoms include high fever, headache, muscle and joint pain and vomiting accompanied by a characteristic skin rash. In severe cases, it can lead to haemorrhaging, organ failure, and eventually, death.

The Aedes aegypti is a small, dark coloured mosquito with visible white markings (dots or stripes) on its body and legs. The maturity cycle of this mosquito, from egg to adult, is as little as a week, which makes it an extremely effective vector that can turn dengue into an epidemic.

Unlike other types of mosquitoes, Aedes mosquitoes are most active during the day; peak biting periods are the early mornings and hours before sunset. Preferring to stay in households and indoors, this mosquito is generally unnoticed because it approaches people from behind, usually biting the ankles and elbows.

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The Aedes mosquito thrives in the shade and areas with poor sanitation. The United State's Centers for Disease Control and Prevention adds that standing water is essential for the Aedes mosquito to lay its eggs in. The eggs can survive for as long as six months, but they do need water to hatch. The monsoon season, thus, creates an ideal environment for colonies of this mosquito to thrive and breed.

Daily rains result in puddles and humid spots in parks, graveyards, junkyards and rooftops, creating a perfect atmosphere for the Aedes mosquito to live and breed in. Standing water as a result of urban or riverine flooding can also promote vector population and propagation.

Thus, the ideal larval habitats for this mosquito are puddles, open water storage containers, particularly dark coloured containers, desert coolers, water dispenser trays, abandoned water reservoirs, coal tar drums, tree holes, flower and plant pots, broken or unused earthen pots, discarded tyres, trash and garbage that accumulates water (such as plastic bags and construction waste), buckets, fountains, underground water and sewerage tanks, wells, drains, etc.

As the weather becomes cooler, the Aedes mosquito disappears; it cannot survive at temperatures below 14 degree Celsius.

Previous outbreak and lessons

In 2011, more than 21,000 people were infected with dengue in Punjab, and 354 succumbed to the illness. A research by the Provincial Disaster Management Authority Punjab, presented in the book Dengue Prevention & Control: The Lahore Model of Success, concluded that precipitation is a more influencing factor than temperature in the spread and breeding of the Aedes family mosquitoes.

The research goes on to add that rainfall was the biggest contributor to the dengue outbreak; heavy rain in Lahore had caused urban flooding which resulted in large pools of standing water, particularly in parks and graveyards. This created an ideal environment for the vector-borne illness to gain a strong foothold in the city, and within a matter of weeks Aedes mosquitoes were not only breeding, but thriving in exponential numbers.

Adequate interventions by the government in a timely manner ensured that the epidemic was contained, and did not break out with remotely comparable intensity in the following years. Predictive modelling had estimated over 55,000 infections and nearly 1,000 deaths, but conscientious efforts at various administrative levels upended the virus as it set in.

The same book details how the government of Punjab set about tackling this behemoth challenge; the incredible amount of coordination required; and the magnanimous efforts of every part of government machinery to contain dengue.

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Some of the best practices that helped the containment of dengue in 2011 include the astute utilisation of data to create GIS-based maps of areas with high rates of infection. This allowed decision makers to pinpoint problem spots such as parks and graveyards early on in the process. For example, in Lahore, Model Town and localities surrounding Miani Sahib graveyard had a disproportionately high number of dengue cases.

In the meantime, the health department reported addresses of patients to the concerned Town Municipal Administrations, which fumigated the patient’s home and surrounding houses, thus eliminating a confirmed source of dengue infection and larvae. A three-tiered system based on an initial surveillance team, a second fumigation team and a follow-up evaluation team was implemented to ensure larvae eradication.

An expansive vector control programme was launched by the City District Government, in collaboration with the health department. Vector control was largely achieved through chemical means, which included fogging, internal residual spray and larvicide. As workers cranked up howling fog machines, others filled up ponds and drained standing water from parks and vacant plots.

All major parks were chemically treated to control the spread of Aedes mosquito and larvae, and green belts and nurseries were inspected to ensure elimination of any possible mosquito breeding sites. Over a 100,000 Tilapia, Grass Carp and Mosquitofish were introduced in more than 100 large ponds in order to eradicate Aedes larvae. At shrines, water reservoirs and open ablution areas were fumigated.

Efforts were also ramped up to ensure daily removal of garbage and waste, which could otherwise accumulate water. Additional sanitary workers were deployed in selected areas and garbage dumping on vacant plots was minimised. Nearly 4,000 tons of solid waste were disposed from the city on a daily basis in addition to the regular garbage.

The Auqaf officials engaged ulema and imams to increase public awareness through Friday sermons and at other religious gatherings. The government also engaged stakeholders of private housing societies for dengue awareness events and ensured fumigation was carried out regularly. Fumigation was also done in all government schools, colleges and universities. Many of these educational institutes were closed for a period of 10 days at the peak of the dengue outbreak to accommodate spraying and decontamination.

Involvement of university and college students in various social action campaigns served as a crucial part of the mass awareness campaign, which was also promoted through print, electronic and digital media. Two training workshops were organised in each high school and college in order to efficiently mobilise the youth.

These workshops were followed by student-led activities such as surveys of their schools and localities for identification of breeding sites and door-to-door awareness campaigns under the supervision of teachers. Such activities, in combination with the abundant use of information, education and communication materials such as banners, leaflets, posters and panaflexes increased basic awareness about dengue.

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The Punjab Information Technology Board (PITB) established a dedicated helpline manned by 150 doctors around the clock to address queries related to dengue. The PITB also established the Electronic Complaint Routing System which was a successful example of community involvement in dengue eradication efforts.

Citizens were able to report relevant complaints through a single telephone number; these complaints were then sorted and forwarded to the concerned department for immediate redressal. This effort allowed identification of empty and vacant plots requiring fumigation, abandoned properties, junkyards and other possible habitats for Aedes breeding. 

NGOs and welfare trusts were mobilised to supplement dengue eradication efforts and improve community-level awareness. Volunteers were trained to conduct dengue awareness seminars around Lahore. Hospitals operated by NGOs and trusts played a key role in treating dengue patients, and many private hospitals provided free dengue treatment as well.

Finally, all of the aforementioned efforts were supplemented by a high-ranking committee of public representatives, secretaries of all concerned departments and health experts for effective coordination of efforts and taking day-to-day operational decisions which would have otherwise been hampered by red tape.

Further adaptation

Historically, dengue has been seasonal. Normally, cases would begin to fall off in November with the conclusion of the rainy season and pick up again in June. However, in Central America, the virus has resurged with a vengeance in January, indicating a possible change in the nature of the Aedes mosquito, or contribution of other vectors in the spread of dengue.

Predictive modelling can be used to understand changing climate patterns in Pakistan. As temperatures continue to rise, and severe monsoon rains unleash their fury, it is likely to shift dengue risk towards those areas which have been untouched by it as yet.

Rapid urbanisation and high internal migration in Pakistan is paving the way for mushroom growth of squatter settlements. These localities are scattered throughout urban areas, from the city-centre to peripheral, suburban or peri-urban areas, and typically lack adequate sanitation and solid waste management infrastructure.

Following periods of heavy rainfall, slums are typically the worst hit, with accumulated water standing for several days. Lack of access to healthcare facilities for slum residents creates an ideal environment for an epidemic outbreak.

Based on information from other countries facing the epidemic, for example Bangladesh and Honduras, which reported maximum infections in five- to nine-year-olds, a school-level strategy needs to be re-energised to increase awareness among children and parents about the risks and signs of dengue. Children have fewer defenses and greater exposure due to attending schools and playing outside. Vulnerabilities among street children also need to be factored into the equation.

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A level of complacency among the population may be contributed to the fact that limited dengue cases were reported in the past few years. But that is often the way this virus acts, a few years of calm followed by an epidemic outbreak. When an outbreak does occur, health experts report that patients do not seek immediate treatment which further exacerbates the situation.

This underscores the need for multiple behaviour change communication campaigns through schools, colleges, universities and the mass media to promote positive behaviours in the local populace.

While no definite relationship between dengue larvae with temperature or humidity has been established, frequent rains during the monsoon season do promote Aedes breeding, and thus preventive and chemical control measures must be organised during this period.

An annual fumigation drive before the onset of the monsoon season should be an essential item on the government’s checklist. Repeated incidents of urban flooding necessitate coordinated measures for draining standing water and puddles, and filling up ponds and water bodies where drainage may not be practicable.

Finally, the government must consider constituting an emergency response committee or joint task force, chaired by the chief executive of the province, with decision makers from key departments, sector specialists and representatives from allied sectors to formalise sustainable long-term measures to combat dengue.

The government needs to be better prepared for epidemics, floods and other events in this age of climate change – no one should have to suffer as a consequence for lack of foresight.

This article was prepared through conscientious review of information from a combination of resources. It's meant for information only; the opinions expressed herein are not meant for any medical or clinical use, or for field implementation without the advice or consent of such individual (s) as required by local laws and regulations.

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