Can we reduce dengue transmission by understanding climate factors?


This is the first article in a new series of Contagion, Climate Change and Infectious Disease. Check the website for ongoing topics on this emerging topic.

When people living outside tropical regions hear about mosquito-borne diseases such as malaria and dengue, they often fail to understand the magnitude of the diseases in the number of people affected or equate their serious and sometimes fatal consequences. One can assume that such diseases are not present here in the United States and that they are a ‘there’ problem. For example, dengue is common in Puerto Rico, the US Virgin Islands, and American Samoa, and has also occurred in the continental US. 1

According to the World Health Organization (WHO), “the number of dengue cases reported to WHO has increased eight-fold over the past two decades, from 505,430 cases in 2000 to more than 2.4 million in 2010 and 5.2 million in 2019. Reported deaths between the year 2000 and 2015 increased from 960 to 4032, which mainly affects a younger age group.”2

And these are just the cases that are reported. There may be more milder cases that go undiagnosed or information about the case may not be provided to WHO.

Dengue can progress to dengue hemorrhagic fever and can cause bleeding, shock and death. In the US, there is the Dengvaxia vaccine, which is indicated for children ages 9 to 16 with lab-confirmed evidence of a previous dengue virus infection and who live in areas where dengue is common.

One of the important lessons from the COVID-19 pandemic is that if something happens internationally, it certainly could happen here, especially with the ability for humans and disease vectors to travel around the world and spread disease quite easily.

Dengue, transmitted by Temples of the Egyptians mosquitoes, are common insects in the United States. They originated in Africa but are now found in various tropical, subtropical and temperate regions. But their cousins, mosquitoes, Aedes albopictus, are believed to have been brought to the US in the 1980s by the used tire industry, transporting the mosquitoes in the rubber mainly from the region of North Asia.3

And with the changing climate, there may be more vector-borne diseases in new regions of the world.

some species Temples of the Egyptians mosquitoes are not only carriers of dengue, but also of yellow fever, chikungunya and the Zika virus.4

Studying climate and mosquito growth in Sri Lanka

Sri Lanka is an island country south of India and located in the Indian Ocean. The country of more than 22 million inhabitants has a tropical climate and is about the size of the state of Georgia. There are areas in the country where up to 98 inches of rain falls and Sri Lanka experiences tropical cyclones.

Dengue is a major public health problem in Sri Lanka. The country is ground zero for the disease and is where it was first serologically confirmed in 1962.5 On June 7 this year, authorities from Sri Lanka reported an increase in dengue with 19,339 cases reported nationwide from January 1 to June 3. These officials reported 5,176 cases over a comparable period in 2021.6

Sri Lanka reported 25,067 cases of dengue in 2021; 31,162 cases in 2020; and 105,049 cases in 2019. Dengue is a year-round risk in the country, but transmission rates are typically highest in May-July and October-January.6

A new study published in The Lancet Planetary Health looked at specific climate factors and the local mosquito growth in Sri Lanka. The researchers studied Aedes mosquito activity for 9 years in 10 sub-districts in Kalutara, Sri Lanka, which is considered one of the most hyperendemic dengue areas in the country. In terms of treatment, Sri Lanka has no vaccines or antiviral drugs, so the country relies on vector control as an intervention to reduce Dengue transmission.

The researchers of the study found that 3 climate factors – rainfall, temperature and El Niño events – can predict mosquito population growth for up to 6 months, thus influencing the proliferation of Dengue’s primary vector.

“This information, along with knowledge of breeding site distribution, is useful for spatial risk prediction and implementation of effective Aedes control interventions,” the researchers wrote.

The researchers report that knowing the amount of rain can be an indicator of vector prevalence in the same month. If we look at temperature and Oceanic Niño Index [ONI]) they can serve as predictors of vector activity with turnaround times of 1-6 months, according to the researchers. And the last variable, ONI, could predict the seasonal prevalence of Aedes vectors with a lead time of 6 months.

“This information is useful for developing early warning and spatial risk categorization to prioritize areas for more intensive vector control interventions,” researchers wrote.

While humans may not be able to control the weather, if they understand its patterns, they can devise prevention strategies to reduce vector-borne diseases and viruses.

“Evidence of the association between Aedes vector indices and climate would allow policymakers to set medium and long-term goals to combat dengue and other Aedestransmitted diseases,” the researchers wrote. “In addition to temperature and rainfall variability, longer-term climate change may affect water management, land use, irrigation practices and population movements.”

References

  1. Dengue in the United States. Center for Disease Control and Prevention. Reviewed October 7, 2020. Accessed July 7, 2022. https://www.cdc.gov/dengue/areaswithrisk/in-the-us.html
  2. Dengue and severe dengue. World Health Organisation. Reviewed January 10, 2022. Retrieved July 7, 2022.https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue
  3. Hawley, William A.; Reiter, Paul; Copeland, Robert S.; Pumpuni, Charles B.; Craig, George B. (1987-05-29). “Aedes albopictus in North America: probable introduction into used tires from northern Asia”. Science† 236 (4805): 1114-1116. doi: 10.1126/science.3576225
  4. Transmission of the Zika virus. Center for Disease Control and Prevention. Archived from the original on 9-20-2017. Accessed on July 6, 2022.
  5. Messer WB, Vitarana UT, Sivananthan K, et al. Epidemiology of dengue in Sri Lanka before and after the emergence of epidemic dengue hemorrhagic fever. Ben J Trop Med Hyg. 2002; 66: 765-773
  6. Crisis 24. Sri Lanka: Health officials report increased activity of dengue fever across the country through June. June 7, 2022. Accessed July 7, 2022. https://crisis24.garda.com/alerts/2022/06/sri-lanka-health-officials-report-elevated-dengue-fever-activity-nationwide-through-june
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