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Will dengue get us if corona doesn't?

  • Published at 05:18 pm April 17th, 2020
mosquito dengue

Some of the lessons learned from Covid-19 should be applied to other outbreaks

If a survey is done today in Dhaka city to find families which have not had a dengue case among their loved ones in the last 20 years, I doubt we will find any. 

As we fight Covid-19, we also wait apprehensively for the impending dengue season. 2019 saw the highest number of dengue cases globally -- and Bangladesh was no exception. There were 101,354 reported cases within the country, out of which 179 people died. 

The rise of uncommon serotypes produced severe atypical symptoms which led to complications like internal hemorrhage or Dengue Shock Syndrome (DSS). Compared to previous years, by this time in 2020, we have already had 241 reported cases, and this is high, considering it isn’t the season yet. 

Intermittent rainfall, temperature variation, and higher humidity as a result of climate change are affecting the change in disease incidence.

In the tropical region, dengue is one of the worst vector-borne viral diseases which spread primarily through the female Aedes aegypti mosquitoes. There are four distinct and closely related serotypes of the virus. 

Recovery from infection by one of these four (DEN-1, DEN-2, DEN-3, and DEN-4) provides the infected person lifelong immunity against that particular serotype and cross-immunity for 6-12 months to the other serotypes. 

If the person is infected by other serotypes subsequently, then the risk of severe dengue increases. There is an FDA approved vaccine -- Dengvaxia -- which is approved for children within nine-15 years of age who had previously been infected with dengue. However, there are reservations among physicians regarding this vaccine. 

Aedes is not only the carrier of dengue virus but is also the vector for Chikunguniya, Zika, West-Nile, and Yellow Fever viruses. Back in 2017,chikunguniya was rampant in Dhaka. A study suggested that it was possible that the same mosquito carried both dengue and chikunguniya viruses. 

The disease travelled to our country and is now an endemic, and we will continue to get seasonal outbreaks till a permanent solution, like a vaccine covering all serotypes, is developed. Singapore, one of the most developed countries in the world, continues to battle with the yearly outbreaks and has reported over 4,000 cases already this year. 

The World Health Organization recommends vector control as the key strategy to contain or prevent the transmission of dengue virus. Any protocol will need coordinated effort between disease control agencies and communities. 

People are familiar with the personal protective measures of using mosquito repellents, mosquito nets, and the biting habits of the vector but survey shows that there is a gap in knowledge about the breeding habitats of the vector, the timing of breeding, and how far the disease can travel.  

Why is dengue so difficult to eliminate? 

We may not be able to defeat this little monster anytime soon, but knowing more about it can definitely help us prepare better. Only infected female Aedes mosquitoes can infect humans, and the mosquito gets infected after it feeds on a dengue infected person. The virus is then transmitted to other people and vertically to the mosquito ova, which means that the eggs hatched are also infected with the dengue virus. 

Adult female mosquitoes lay eggs on the inner walls of containers with water, above the waterline, but can lay eggs in dry containers as well. The eggs are hardy and can survive in dry conditions for several months to a year.

Mosquito larvae require very little water to hatch and can remain in this stage for months if there is adequate water. It takes approximately seven days for a larva to grow into an adult mosquito. 

Within its lifespan of 14 days to a month, the mosquito lays down five batches of eggs at different sites, after continuously feeding on human blood. Each time, around 100 to 200 eggs are produced.

They can only cover a small distance and remain close to the place of its birth. The vertical transmission of the virus in the ova, the hardiness of the eggs, and the sheer number of eggs at different places make this disease difficult to eliminate.

When is the dengue season?

It is highly climate sensitive and does not survive below 10C or beyond 40C and ceases to bite below 15C. This explains why there are almost no dengue cases in December and January. 

The optimum temperature range for mosquito development is 25-29.5C at 80% humidity. With the rise in temperature, for instance at 32C to 35C, the mosquitoes develop faster, the virus within the mosquitos replicate rapidly, and the biting rate of the vector is also higher, which results in swifter disease transmission. 

Now, it probably makes sense why Bangladesh is the perfect melting pot for this disease from May through November with the peak season being July to October.  

What are the breeding sites?

Aedes mosquitoes are a hazard of unplanned urbanization, and nowadays also thrive outdoors if the temperature and humidity are conducive. The mosquito mates, feeds, rests, and lays eggs in and around urban human habitation, within the living spaces of humans. 

They generally spend their entire lives in and around the houses, and breed mostly in clear water stagnating in man-made containers like buckets, tires, barrels, plastic drums, water storage pots, plant saucers, tank, discarded water bottle, cans, water coolers,birdbaths, water storage jars, vase, flowerpots,rooftop gutters, septic tanks, etc.

It is a day-time feeder; its peak biting periods are early in the morning and in the evening before dusk.

What can be the short-term interventions to prevent an outbreak?

Identifying and localizing the area of disease outbreak is extremely important to take targeted vector control intervention measures. Rainfall during monsoon raising the river water level is a marker of a high number of cases anywhere, and a prolonged dry season preceding the rainy season can potentially cause an epidemic. 

Pre-monsoon, monsoon, and post-monsoon entomological surveys are conducted every year by Centers for Disease Control and Prevention (CDC) to determine the spatial density and distribution of the vectors. The eventual disease hot spots correlate to the vector distribution and both are highest during monsoon and post-monsoon periods.

City corporations play the most crucial role -- they must always ensure proper waste management to remove small water reservoirs and use the surveillance report for identifying areas where larvicides and pesticides are needed to control the vector population across the seasons.

We are currently in the quiet phase of the disease onset, which will switch the moment the rainy season commences. It is indeed an ideal time for taking pre-monsoon larvae control measures. 

Infographics and community awareness drives are mandatory for the city-dwellers to keep the surroundings free of the breeding habitats, use mosquito repellents, cover the body with light coloured garments, cover the inhouse water reservoirs, etc. 

The Singapore government has legislative actions it can carry out to control a dengue outbreak. We need not be as strict, but citizens need to follow guidelines made mandatory by the government. 

What can be the long-term management?

A comprehensive approach to predict, prepare, and allocate resources to implement disease control activities needs to be set as the standard operating protocol for the country. 

As a country, by now we have a clear understanding about the different variables that link to a dengue outbreak. Machine learning can be applied using variables like temperature, humidity, rainfall, wind, people movement, and past record of cases to develop an Early Warning System (EWS). 

Countries like Singapore, Thailand, the Philippines, Malaysia, Vietnam, and Brazil use such prediction analytical tools for country preparedness.

A data gathering platform is already being launched nationwide to record all the dengue cases being treated in both public and private facilities. Information from this platform will be uploaded in a server from where reports will get generated.

An app and web-based data analytical tool will pull this data in real time for a dashboard and a heat map to visualize the dengue hot spots and the total number of cases in the country at any given time.The qualitative and quantitative value of this report will influence data-based decision making for all the stakeholders.

Relevant authorities like city corporation officials, health care providers, and disease control personnel will access the dashboard, receive alerts of any localized outbreaks, and take measures to control it. Geographic information system (GIS) mapping installed in the app can guide the citizensto their nearest hospital and diagnostics. 

It also prepares hospitals and diagnostics to organize their resources like beds and testing kits. The dashboard will clearly identify the public and private health facilities providing better care and encourage healthy competition between them for improved disease management.

There were many cases of dengue hemorrhagic fever and DSS in 2019. Not all Bangladeshi health care providers are skilled at managing DHF and DSS. More training with adequate management guidelines and medical equipment is needed that will help the providers manage the severe cases better. 

Schools, social advocacy institutions, and urban planning authorities play a crucial role in disease control and sensitization. Rules and regulation must be set in place for city infrastructure and buildings under construction, where no temporary or permanent structures are allowed that can store stagnant water. 

Finally, sero-surveillance will need to be studied in the future to determine herd immunity against different serotypes. 

Covid-19 has brought the country to a standstill and forced the policy-makers to focus on the existing shortfalls of the health care system. The crisis has, however, also changed the personal hygiene behaviour of the entire nation overnight. 

Massive scale hand-washing programs to create awareness about its benefits will probably not be needed for some time to come. 

If this heightened hygienic awareness can be extended, with coordinated effort by all concerned authorities, to mobilize citizens to keep their surroundings clean in a relevant manner, then we can possibly expect some success in controlling a dengue outbreak for the year. 

However, if I may add, it is high time that we re-evaluate our whole health care delivery system. 

Given our resource shortages, which cannot be improved overnight, utilization of innovative digital tools is ever more important to identify gaps at all levels and build an effective and accountable health care system. 

Dr Maliha Mannan Ahmed is the Founder and Executive Director of Organikare. She has an MBBS, MBA and a Masters in Healthcare Leadership.

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