Dengue virus cases on the rise. Global warming among main causes

By Edgar Maciel

Dengue virus cases on the rise. Global warming among main causes

Approximately 50% of the world’s population faces the potential of being infected by the mosquito-transmitted dengue virus, with an estimated 100–400 million infections occurring annually. The World Health Organization (WHO) highlighted that infection rates of the virus increased eightfold in 2022 compared to 2000, and instances are being recorded for the first time in certain areas. Climate change adds to the factors exacerbating the severity and frequency of outbreaks.

In January, the WHO warned that dengue is the most rapidly spreading tropical disease in the world and poses a “pandemic threat”. Cases reported to the WHO reached a historic high in 2019 – 5.2 million in 129 countries. This year, the world is on track for “over 4 million” cases, depending primarily on the monsoon season in Asia, the organization warns.

What is dengue?

Dengue, also referred to as bone-break fever, is a viral infection resulting from the dengue virus which is transmitted from mosquitoes to humans. While the majority of infections show no symptoms or result in mild illness, the disease can progress to a severe stage, potentially leading to a fatal outcome.

Symptoms vary from a mild to incapacitating high fever accompanied by severe headache, eye pain, muscle and joint discomfort, and the development of a rash. In its severe form, dengue can manifest as shock, respiratory difficulties, bleeding, and potential organ dysfunction.

What causes dengue?

Long-term factors contributing to the escalating transmission of dengue include urbanization and the surging global mobility, both of which facilitate the proliferation of mosquito breeding grounds and the broadening of their territorial range. Climate change is also widely attributed to be exacerbating the severity and frequency of outbreaks.

The elevated global temperatures act as a catalyst, fueling the intensified replication of the dengue virus within mosquitoes. This phenomenon results in an augmented reproductive capacity for the mosquitoes leading to increased biting frequencies. It also extends the duration of transmission seasons as substantiated by multiple studies.

While scientists hesitate to attribute this year’s surge directly to El Niño, it is of note that the warmer temperatures associated with this weather phenomenon in Asia can have comparable short-term impacts on transmission.

Dr Raman Velayudha, Unit Head of the Veterinary Public Health, Vector Control and Environment and Neglected Tropical Diseases at the WHO, explains that urbanization and climate change have had a huge impact on the spread of dengue across the world. Additionally, the disruptions caused by COVID-19 have also had an impact on mosquito control efforts.

Velayudha explains that temperatures over 45 degrees Celsius (113 degrees Fahrenheit) “should kill the mosquito more than breeding it, but the mosquito is a very clever insect and it can breed in water storage containers where the temperature doesn’t rise that high.”

Most affected countries

Before 1970, the mosquito vector responsible for transmitting the disease was limited to just a handful of countries. However, the global incidence of dengue has seen a significant rise in recent decades. Dengue is now prevalent in over 100 countries spanning WHO regions in Africa, the Americas, the Eastern Mediterranean, South-East Asia, and the Western Pacific. Recently, the disease was found in the capital of Sudan, Khartoum, for the first time, according to a report in March.

During 2023, the world has experienced a significant upsurge in dengue cases with a staggering total of over 3.7 million new infections being documented by August according to the European Centre for Disease Prevention and Control. The figure is higher than in 2019 when 3.1 million cases were reported, including 28,203 severe cases and 1,823 fatalities.

Brazil, the worst hit

The bulk of these cases, over 3.3 million, have been concentrated in the southern cone with Brazil alone contributing to 80% of the total. However, notably high transmission rates have also been observed in other regions of the continent including the Andean region, where over 400,000 cases have been reported, accompanied by an elevated case fatality rate.

The largest country in South America saw a 22% increase in the number of dengue cases in the first half of this year compared to the same period last year when 900,008 cases were reported, and a 73% rise compared to the average of the last five years. Moreover, over 800 people have died from dengue in Brazil this year.

The Brazilian Ministry of Health acknowledges the concern and explains that the COVID-19 pandemic disrupted national efforts to combat the Aedes aegypti mosquito.

“The pandemic had a significant impact. Our endemic disease control agents had their actions greatly hindered during this period. We experienced a discontinuity in the purchase of some products, such as insecticides used in vector control,” explained Ethel Maciel, Secretary of Health Surveillance and Environment (SVSA) at the Ministry of Health.

Dengue typically follows a cyclical pattern, causing larger outbreaks every three years. However, the impact of the pandemic and the continuation of the upward trend this year following the high records in 2022 is raising concern.

Innovative prevention method

In addition to vaccines and prevention campaigns, Brazil has also been implementing the Wolbachia method.

Developed by Fiocruz, a Rio de Janeiro-based institution for research and development in biological sciences, this innovation involves the construction of a biofactory capable of producing up to 100 million mosquitoes per week, totaling 5 billion annually. The investment for this initiative amounts to 180 million Brazilian Reais (approximately US$36 million).

Wolbachia is a bacterium naturally found in about half of all insects but not naturally present in Aedes aegypti mosquitoes. However, when introduced into these mosquitoes, it prevents the dengue, Zika, and chikungunya viruses from developing within them. Consequently, it reduces the likelihood of these pathogens being spread among the population, thus decreasing the incidence of these diseases in the areas where these mosquitoes circulate.

Over time, the modified mosquitoes reproduce, gradually leading to a population dominated by those carrying the Wolbachia bacterium in that particular location.

“We hope that within four years, we can have at least 70% of the municipalities currently facing the highest disease burden covered by this new technology,” envisioned Ethel Maciel of SVSA.

This technology has also been introduced in 11 other countries across Asia, Oceania, and the Americas, benefiting nearly 11 million people, including 3 million Brazilians.

The results of implementing this technology have been promising. A recent study published in the New England Journal of Medicine assessed the dengue incidence two years after the release of these mosquitoes in Indonesia and found a 77% reduction in the disease cases and an 86% decrease in hospitalizations.