Climate change, unplanned urbanization, sprawling cities, and the El Nino effect all converged to make 2024 a “historic” year for dengue transmission. With increased opportunities for Aedes aegypti mosquitoes–the insects that carry dengue– to breed, cases reached a record 12.7 million cases in the WHO’s Americas Region, nearly three times more than in 2023.

This translates to roughly 21,000 severe cases and over 7700 deaths across northern, central and southern regions of the continent.  And more than a third of the severe cases occurred in children, warned Dr Jarbos Barbosa, Pan-American Health Organization (PAHO) Director, in a press conference last week. 

“In countries like Guatemala, 70% of dengue-related deaths have occurred in children,” Barbosa noted. Barbosa’s home country of Brazil accounted for nearly 80% of cases in the Americas, followed by Argentina, Colombia, and Mexico. 88% of deaths from dengue occurred in these four countries.

At the close of this historic dengue season, Pan-American Health Organization (PAHO) leadership discussed reasons for the surge in cases, and the tools to combat next season’s caseload. Of concern is the geographic expansion of dengue-susceptible regions into countries like Argentina, Uruguay, and the United States. “This increase in cases is directly associated with climate events, including droughts, floods, and warmer climates that favour the proliferation of mosquito breeding sites,” said Barbosa. The director also cited population growth, unplanned urbanization, poor living conditions, and inadequate water supply and waste disposal as major drivers of dengue transmission. 

“Despite these challenges, we are not defenseless,” argued Barbosa. Vector surveillance, improved case management, community engagement, and the rollout of dengue vaccines in targeted populations have meant PAHO is optimistic about next year’s season. “We don’t have a crystal ball,” said Dr Sylvian Aldighieri, PAHO director of the Department of Prevention, Control, and Elimination of Transmissible Diseases, referring to 2025 projections. But because 2024 had such high transmission rates, much of the population has now acquired immunity to the most prevalent serotypes, he noted. 

Migrants at risk of dengue and other diseases – gaps in data and health care coverage are challenges

While dengue is typically characterized as an urban disease–where mosquitoes breed in discarded plastic bottles and sidewalk puddles, the large migrant population moving through central America to southern and central Mexico [Mesoamerica] towards the United States, is also especially vulnerable to infectious diseases.

“This is a big throughway for persons,” noted Dr Thais dos Santos, Regional Advisor on Surveillance and Control of Arboviral Diseases, in response to a Health Policy Watch query. “Dengue surveillance has a long tradition in the region of the America, but as it evolves, we realize that there are some data gaps.” Healthcare facilities do not typically collect information on a patient’s migrant status, meaning there is limited data to understand the burden of dengue in migrants. Dos Santos cautioned, thus, that PAHO has limited data to understand the scope of migrant health statuses, especially with regard to vector-borne diseases.

Dengue is not the only communicable disease of concern for migrants. “We are assisting countries in preventing any reintroduction of malaria that can be spread through the flow of migrants,” said Dr Andrea Vicari, PAHO Head of the Infectious Hazard Management Unit. “It’s a population at risk, living in very vulnerable conditions,” Vicari noted. 

Strengthening migrant health remains a PAHO priority, as record-breaking numbers of asylum-seeking migrants traverse the Darien Gap through Panama. The year 2023 saw more than half a million people transit through Mesoamerica en route to the United States. This population is burdened by a host of health threats, including sexual and gender-based violence, food insecurity, HIV, and malaria. PAHO’s November 2024 migrant health report called for increased access to emergency, maternal, pre and postnatal, and mental health services. 

Oropouche, avian flu emerge as new threats in 2024

While dengue contributed to the largest burden of mosquito-borne diseases, PAHO leadership also brought attention to Oropouche, a rare but increasingly circulating virus in the Americas. The year 2024 saw nearly 12,000 cases, mostly in Brazil, representing a dramatic increase since last year. The virus, an arbovirus like dengue, Zika, and chikungunya, is spread through bites by certain midge mosquitos. Oropouche symptoms are similar to these other arboviruses with fever, rashes, muscle aches, and headaches common. Symptoms typically last 5 to 7 days, but more recently, the virus has been linked to severe fetal outcomes, including congenital abnormalities and death. 

Notably, cases were seen in regions not typically associated with the disease, mirroring dengue’s march into new geographic areas.  Brazil also reported several instances of transmission from mother to child earlier this year, with 8 cases of infant microcephaly and fetal death, causing the US Centers for Disease Prevention and Control (CDC) to issue travel warnings for pregnant women to the affected regions in the country. 

PAHO leadership also provided an updated report on the ongoing H5N1, or avian flu, outbreak in North America. The United States has already reported 58 human cases, mostly poultry and dairy workers, while in Canada a teenager with no known contact with infected birds or cattle also fell ill recently – reviving longstanding concerns about new virus mutations that could facilitate more human-to-human transmission.

“Surveillance, including genomic characterization, is crucial to track the virus across species, understand its evolution and risks to humans, and guide our actions,” said Barbosa. The director argued that intersectoral collaboration with a One Health approach is critical – as this influenza strain has now jumped to dairy cows for the first time, meaning that measures to reduce animal exposures as well as to protect people must be addressed together.

Also among the longstanding WHO recommendations is the pasteurization of milk to contain the public health risk. “The pasteurization of the milk has been described and used for the last 150 years. This is one of the great achievements of the public health sector…with pasteurized milk, we should not have any major issues of transmission of this disease to people,” mentioned Dr Vicari.

“Countries must strengthen their surveillance and continue sharing information. We must work across borders to monitor new cases and support health systems to respond,” emphasized Barbosa.

Image Credits: Matias Delacroix/MSF, PAHO/WHO, PAHO/WHO/David Spitz.