Climate and dengue – Dr Shaukat Ali


Dengue outbreak is one of the major issues this summer, grabbing headlines across whole South Asia. Thousands of dengue cases have been reported in South Asian countries likes Pakistan, India, Nepal and Bangladesh which brings forth the fact that the South Asian region is grappling with the worst outbreak this year. Once confined to a tropical climate only, dengue fever is now being found also in several European countries where dengue was once marginal, while Latin American countries including Brazil, Colombia, Honduras and Nicaragua are tackling a surge in cases.

In recent weeks, outbreaks have been reported in the Philippines, causing 146,000 cases and 622 deaths. Bangladesh has experienced the worst outbreak since the year 2000, with 17,000 cases according to official figures. Sri Lanka has seen 234,000 cases and 47 deaths so far this year, according to the Sri Lankan government. Singapore and Vietnam are also seeing outbreaks. On average, 390 million dengue infections are reported every year around the world. However, the number of cases of the mosquito-borne illness is expected to continue to grow in the future according to WHO, as climate change and other environmental factors provide more suitable habitats for disease-carrying mosquitoes to breed.

The situation in Pakistan has also become worse this summer, with a major outbreak seen in Punjab and Khyber Pakhtunkhwa. Different sources report thousands of dengue-affected patients being admitted in hospitals in Rawalpindi, Lahore and Peshawar. The months of September and October have been declared as ‘sensitive’ by the health department in this regard. But the most alarming element about Pakistan is that every year the dengue outbreak comes with increased intensity.

According to the WHO, since 2010, Pakistan has been experiencing an epidemic of dengue fever that has caused 16, 580 confirmed cases and 257 deaths in Lahore and nearly 5,000 cases and 60 deaths reported from the rest of the country. While it is a bit early to conclude, in 2019 the worst outbreak with highest dengue cases has been reported so far in all the major cities of Pakistan.

While the endemic outbreak every year around the world can be attributed to urbanization, poor sanitation and more, the major and prevalent indirect reason for this widespread outbreak at such an accelerated rate is climate change and global warming. Extreme weather events like elevated temperatures and changing rainfall patterns are recognized as favourable conditions for dengue vector proliferation. The warmest July recorded this year has been linked with a surge in dengue cases.

Any variability in climate largely determines the distribution and population dynamics of the vectors. Hence, rising temperatures, humidity levels and rainfall patterns (which provide breeding habitats) variously affect the replication, maturation, transmission and viability of the pathogen, the vector organism (where applicable), and the range and abundance of any reservoir or intermediate animal species. For example, disease-transmitting insects are ‘coldblooded’ and thus very sensitive to temperature. They are also easily desiccated if conditions become too dry. At higher temperatures they showed a higher biting rate, low extrinsic incubation period, low mortality rate and higher reproduction rate.

Studies have also revealed that when the temperature is warmer mosquitoes breed more quickly; increases in the speed of virus replication inside the mosquito also predicted that an increase of three to four degrees Celsius in average temperatures may double the reproduction rate of the viruses. Consequently, several cases are being reported at higher altitude mountainous regions in the world due to increased warming conditions. This issue has raised concern among the scientific community.

Against this backdrop, a study is underway by this author bu using CMIP5 Global Climate Models downscaled by Quantile Delta Mapping (QDM) methods to find the correlation of future extreme temperature and precipitation events with dengue occurrence in Pakistan. The results show that changing climate and consequent high temperature extremes provide favourable conditions for mosquitoes to spread vector-borne diseases like dengue which lead to many causalities in Pakistan every year.

Also, due to seasonal shifts and changing temperature conditions, Islamabad and Karachi are likely to encounter a more prolonged season for dengue in the future. Moreover, as the temperature is expected to increase over the whole country including the northern areas, dengue is likely to occur in high altitude areas of KP and Punjab since increased climate warming in the future will provide suitable conditions for its proliferation. This will make those areas vulnerable to dengue where it does not exist at the moment.

Apart from measures to control the dengue outbreak, government should also initiate joint research projects on a regional level to explore the linkage of changing climate with increased dengue outbreak. Moreover, other regional research organisations should also support research in climate change and health.The Asia Pacific Network on Global Change Research, which supports regional research projects on changing climate and its implication for sustainable development, should also promote research in this field as the issue is ravaging the whole region. In this regard, the APN should take the initiative to support regional research in order to unveil the future vulnerability of the region to dengue due to the changing climate. Also, communication with particularly vulnerable individuals and populations, as well as with healthcare professionals and public health officials tasked with protecting communities, too require further research to tackle this issue effectively.

Since there exist insufficient research on this topic, researchers should grab opportunities to explore the link between the changing climate and the increased occurrence of dengue. Timely preventive measures coupled with dedicated research and effective communication on a large scale are needed to produce some promising mitigation of the dengue outbreak.

The writer is a researcher at the Global Change Impact Studies Centre (GCISC), Ministry of Climate Change.