Dengue cases have been recorded across the Americas at over three million this year. It indicates that since 1980, when the Pan American Health Organization began collecting data on the number of cases, 2023 already has the second highest annual incidence of the disease.
The recent article published in Nature science journal revealed some of the main reasons behind the increase in dengue cases in America. Speaking to the publication, Cláudia Codeço, an epidemiologist at the Oswaldo Cruz Foundation, a life sciences and public health institution in Rio de Janeiro, Brazil, says: “We are observing an increase in cases beyond what was expected for this period.” How the disease spreads in Central and North America will determine whether the record 3.2 million cases reported in 2019 will be broken in 2023, as the majority of researchers believe the height of the dengue season in South America has passed. (Also read: Dengue in children on the rise; here’s what parents need to keep in mind )
The fact that dengue is transmitted by four serotypes of closely similar viruses makes it difficult to pinpoint the specific source of the upsurge. “There is an interaction between these serotypes, where immunity to one interferes with the others. When we put this together, it can lead to unpredictable dynamics,” says Codeço.
According to academics, however, the increase can also be explained by rising temperatures and changed precipitation patterns. The primary carrier of dengue, the Aedes aegypti mosquito, thrives in humid environments with temperatures around 30 °C, which have become increasingly prevalent in recent years due to exceptional heat records and other weather-related events. The disease, which can result in fever, headache and lethargy, has no specific treatment. Severe cases can be fatal; this year, dengue has claimed the lives of more than 1,300 people across the Americas.
Primary causes of the increase in dengue cases in the Americas
Areas where A. aegypti previously had no presence are now experiencing dengue outbreaks. The disease is spreading to the southern regions of Brazil, where this year there have been over 2.4 million registered cases, which were previously too cold for mosquitoes. According to research by Codeço and her colleagues1, 481 Brazilian cities have shown sustained local dengue transmission for the first time over the previous five years1. In addition, Mexico City, which is at an altitude of 2,240 meters, experienced its first A. aegypti invasion in 2015. “If you read books on the biology of Aedes aegypti, they say that the mosquito does not reproduce at altitudes above 1,200 meters ,” says José Ramos-Castañeda, a virologist at the Mexican National Institute of Public Health in Cuernavaca. “In that aspect, global warming affects the distribution of the vector and therefore the possible distribution of cases.”
The possibility of an epidemic of dengue – the likelihood that it would spread among individuals – in the late 2040s has been studied by researchers at the University of Michigan in Ann Arbor. According to computational epidemiologist Andrew Brouwer, one of the study’s authors, “what we found was that the epidemic potential was higher than today, regardless of the specific climate change scenario.” He claims that “in most places we’ve seen a 10-20% increase in epidemic potential.”
The problem is not only in South America. According to Brouwer, the areas where vectors and pathogens can survive will expand in both the southern and northern hemispheres. Dengue local transmission has already been documented in Florida, Texas, and Arizona in the continental United States.
Dengue cases often increase in the summer or rainy season and decrease in the winter or dry season. But as global temperatures rise, dengue seasons may lengthen. In the forecast made by Brouwer and his colleagues, “we found that the transmission seasons generally increased by about a month at each end”, claims Brouwer.
The impact of the El Niño weather event
Dengue may suffer in the near future as a result of the ongoing El Nio weather phenomenon, which is expected to bring record temperatures, droughts and floods. Tedros Adhanom Ghebreyesus, the head of the World Health Organization, issued a warning about the phenomena in late June, saying that it “may increase the transmission of dengue and other so-called arboviruses such as Zika and chikungunya.”
The areas of Central America and parts of North America that are now experiencing the rainy season are likely to be most affected by El Nio in terms of dengue incidence. The spread of dengue has been controlled using a number of tactics. They include eliminating open containers of standing water where the insects can breed, and using traps or pesticides to destroy the mosquito host. In addition, attempts are being made to create mutant mosquitoes that are immune to disease transmission.
Ramos-Castaeda argues that while all of these strategies can be helpful, “what can really affect transmission is immunity in the population.” Since 2015, two dengue vaccines have received official approval in specific regions; however, due to efficiency challenges, safety concerns, and high cost, they have not been widely used.