Can We Decrease Dengue Transmission by Understanding Climate Factors?

This is a female Aedes aegypti mosquito, which is the primary vector for the spread of Dengue fever. The virus that causes Dengue is maintained in the mosquito’s life cycle, and involves humans, to whom the virus is transmitted when bitten. The female mosquito pictured here, was shown as she was obtaining a blood meal, by inserting the feeding stylet through the skin, and into a blood vessel. Blood can be seen being drawn up through the red colored stylet, and into the mosquito’s mouth.

Photo Credit: James Gathany

This is the first article in a new Contagion series, Climate Change and Infectious Disease. Please check out the website for ongoing topics about this emerging subject.

Often, when people who live outside of tropical regions hear of mosquito-borne diseases such as malaria and dengue, they do not understand the scope of the diseases in how many people are affected by it or equate its serious and sometimes fatal consequences. It may be believed such diseases do not happen here in the United States, and that they are an “over there” problem. However, dengue, for example, is common in Puerto Rico, the American Virgin Islands, and American Samoa, and has occurred in the continental US. ¹

According to the World Health Organization (WHO), “the number of dengue cases reported to WHO increased over 8 fold over the last two decades, from 505,430 cases in 2000, to over 2.4 million in 2010, and 5.2 million in 2019. Reported deaths between the year 2000 and 2015 increased from 960 to 4032, affecting mostly younger age groups.”²

And this is just the cases being reported. There might be more milder cases that go undiagnosed or case information might not be provided to WHO.

Dengue can develop into dengue hemorrhagic fever, and can cause bleeding, shock, and death. In the US, there is the Dengvaxia vaccine, which is indicated for children 9 through 16 years old with laboratory-confirmed evidence of a previous dengue virus infection and living in areas where dengue is common.

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

Dengue, transmitted by Aedes aegypti mosquitoes, are commonplace insects in the United States. They originated in Africa, but are now found in various tropical, subtropical, and temperate regions. However, their mosquito cousins, Aedes albopictus, are believed to have been brought to the US back in the 1980s by the used-tire industry with the mosquitos being transported in the rubber mostly from the Northern Asia region.³

And with the shifting climate, more vector-borne diseases may happen in new regions of the world.

Some species of Aedes aegypti mosquitoes are carriers of not only Dengue, but 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 nation of over 22 million people has a tropical climate and is roughly the size of the state of Georgia. There are areas in the country that see up to 98 inches of rain 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 it is where it was first serologically confirmed in 1962.⁵ As of June 7 of this year, authorities from Sri Lanka reported an increase in dengue with 19,339 cases reported nationwide from Jan. 1-June 3. These officials reported 5176 cases over a similar period in 2021.⁶

Sri Lanka reported 25,067 dengue cases 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 investigators studied Aedes mosquito activity for 9 years in 10 subdistricts in Kalutara, Sri Lanka, which is considered one of the most hyperendemic dengue areas within the country. In terms of treatment, Sri Lanka does not have vaccines or antiviral drugs, so the country relies on vector control as the only intervention to reduce transmission of Dengue.

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

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

The investigators report that knowing the amount of rainfall could be an indicator of vector prevalence in the same month. When looking at temperature and Oceanic Niño Index (ONI) they can serve as predictors of vector activity with lead times of 1–6 months, according to the investigators. And the latter variable, ONI, could predict seasonal prevalence of Aedes vectors with a lead time of 6 months.

“This information is useful for developing early warnings and spatial risk categorization to prioritize areas for more intense vector control interventions,” investigators wrote.

Although humans may not be able to control the day-to-day weather patterns, if they have an understanding of its patterns they may be able to establish prevention strategies to try to reduce vector-borne diseases and viruses.

“The evidence of the association between Aedes vector indices and climate would enable policy makers to set up medium-term and long-term targets to control dengue and other Aedes-borne diseases,” the investigators wrote. “In addition to temperature and rainfall variability, longer-term climate change can influence water management, land use, irrigation practices, and population movements.”

References

1. Dengue in the United States. Centers 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 Organization. Reviewed January 10, 2022. Accessed 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 in Used Tires from Northern Asia". Science. 236 (4805): 1114–1116. doi:10.1126/science.3576225
4. Transmission of Zika virus. Centers for Disease Control and Prevention. Archived from the original on 2017-09-20. Accessed 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. Am J Trop Med Hyg. 2002; 66: 765-773
6. Crisis 24. Sri Lanka: Health officials report elevated dengue fever activity nationwide 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