Avian Influenza Activity Expands Across Americas: PAHO

Avian influenza A(H5N1) continues to expand across the Americas, with growing impacts on animal health, food systems, and surveillance demands, according to a new epidemiological update from the Pan American Health Organization (PAHO). The agency reports that outbreaks in birds, mammals, and isolated human cases remain linked to clade 2.3.4.4b, the strain that has predominated in the region since 2021.

PAHO says that 19 countries and territories have reported more than 5,100 outbreaks in animals since 2022. The virus has continued to spread this year, with nine countries confirming 508 outbreaks in birds in 2025 and thousands of detections in wild birds, particularly in the United States and Canada. The organization noted that the pace and geographic reach of outbreaks pose ongoing challenges for biosecurity and food production across the region.

Activity in mammals has also persisted. Canada and the United States have confirmed 77 outbreaks in wild and domestic mammals in 2025. Since March 2024, US authorities have detected infections in dairy cattle across 18 states, affecting over 1,000 herds. PAHO says these developments underscore the need to expand surveillance strategies to livestock and wildlife as epidemiological patterns evolve.

Human infections remain infrequent, with 75 cases and two deaths recorded in the Americas since 2022. This year, three human infections were detected in the United States and one in Mexico. Mexico reported one human case of A(H5N2) and the first global human infection with A(H5N5) in the United States. Most human cases have been associated with direct contact with infected animals, and PAHO says there is no evidence of person-to-person transmission.

The case observed in Mexico, observed in April 2025, involved a three-year-old girl from Durango. The case was verified on April 1 by the National Institute for Diagnosis and Epidemiological Reference (InDRE), prompting a coordinated response across several government agencies to contain any potential spread.

Globally, clade 2.3.4.4b has caused widespread mortality in birds across multiple regions since 2020 and has expanded across North, Central, and South America. Outbreaks in mammals have now been reported in 22 countries across three continents. WHO data shows that since 2003, 991 human cases of H5N1 have been reported worldwide, with a case-fatality rate of 48%.

PAHO, together with the World Organization for Animal Health (WOAH) and the Food and Agriculture Organization (FAO), is calling on governments to strengthen surveillance systems in both animals and humans, reinforce biosecurity protocols, and improve coordination among sectors. The agency also advises countries to closely monitor populations at higher risk of exposure, including poultry farmers, veterinarians, farm workers, and wildlife specialists, and to ensure appropriate use of personal protective equipment.

The organization says that properly cooked poultry products remain safe for consumption. It emphasizes that early detection, timely reporting, and coordinated response mechanisms are essential for managing new outbreaks and assessing possible changes in the virus.

Avian influenza primarily affects birds but can infect mammals, including humans. Subtypes such as H5N1 and H5N2 are classified based on two surface proteins, hemagglutinin and neuraminidase. Genetic clades, including the dominant 2.3.4.4b lineage, reflect viral evolution and are key for monitoring mutations that could influence transmission, severity, or species adaptation.