Dengue Serotype 3's Return Raises Concerns Over Outbreak Severity in Brazil

The reappearance of dengue virus serotype 3 (DENV-3) in Brazil, after a 17-year absence, is raising alarms about the potential for heightened outbreaks of the disease. This resurgence occurs at a time when the population remains unvaccinated against this particular serotype, and serotypes 1 and 2 (DENV-1 and DENV-2) continue to circulate in the region.

Researchers from the São José do Rio Preto Medical School (FAMERP) have shared their findings in the Journal of Clinical Virology, highlighting the risks associated with this situation. The last major epidemic of DENV-3 in Brazil took place over 15 years ago, particularly in São José do Rio Preto in 2007. The ongoing circulation of DENV-1 and DENV-2, alongside the reemergence of DENV-3, could lead to severe dengue outbreaks if the virus establishes itself further.

According to Maurício Lacerda Nogueira, a professor at FAMERP, the city has been the focus of a two-decade-long Thematic Project dedicated to genomic and epidemiological surveillance of dengue and other insect-borne viruses. The warm and humid climate of São José do Rio Preto--with an average annual temperature of 25°C and about 2,000 millimeters of rainfall--provides ideal breeding conditions for mosquitoes, making it a key area for monitoring arboviruses, including dengue.

Active surveillance in the city has revealed a rise in DENV-3 cases since late 2023, with 31 blood samples collected between November 2023 and November 2024 testing positive for this serotype. Common symptoms reported include muscle pain, headaches, and fever. During the 2023-2024 epidemic, São José do Rio Preto initially saw DENV-1 as the primary cause; however, by mid-2024, DENV-1 cases had diminished, giving way to DENV-2 and subsequently, a marked increase in DENV-3 infections.

Previous studies indicate that the last dengue outbreak in Brazil in 2021 was predominantly linked to DENV-1. Notably, sequential infections involving DENV-3 have been associated with increased severity during outbreaks. However, the current study did not observe a corresponding rise in severity among the patients involved.

Additionally, the researchers sequenced the genomes of viral isolates derived from patients displaying acute fever. Their analysis indicated that the DENV-3 strain identified in Brazil is genetically similar to strains found in Florida and the Caribbean, suggesting that recent outbreaks in those regions may have facilitated the reintroduction of the virus into Brazil.

These developments underline the critical importance of molecular and genomic surveillance of circulating dengue strains as part of public health preparedness and response strategies in the face of increasing case numbers. Dengue transmission has expanded in tropical and subtropical areas globally, largely driven by climate change and the increasing distribution of the Aedes aegypti mosquito, which spreads the virus.

Brazil has consistently reported high rates of dengue, being the most affected nation in the Americas. Previously, DENV-1 and DENV-2 were the dominant serotypes circulating within the country. However, the resurgence of DENV-3--which had seen fewer than 100 reported cases between 2010 and 2022--has escalated, with cases rising from 106 in 2023 to 1,008 in 2024. Observations from ongoing studies suggest a pattern akin to the emergence of SARS-CoV-2 during the COVID-19 pandemic, wherein new serotypes can evade existing immunity, leading to subsequent epidemics.