PAHO reports continued circulation of avian influenza A(H5N1) in the Americas

Washington.– The Pan American Health Organization (PAHO) reported that avian influenza A(H5N1) continues to circulate in the Americas region, with new outbreaks in birds, mammals, and sporadic human infections associated with exposure to the virus. Clade 2.3.4.4b, predominant since 2021 in the region, continues to drive the activity recorded in recent years. The most recent data indicates that 19 countries and territories have reported 5,136 outbreaks in animals since 2022, according to the epidemiological update published yesterday. Since mid-October, 73 additional outbreaks were recorded, mainly in domestic and wild birds. In 2025, nine countries have confirmed 508 outbreaks in birds, along with thousands of detections in wild birds, especially in the United States and Canada. Geographic expansion and the high frequency of outbreaks in poultry and wild birds continue to be a challenge for biosecurity and food production in the region. The update also indicates activity in mammals. Canada and the United States have reported 77 outbreaks in wild mammals and domestic animals so far this year. The United States has recorded cases in dairy cattle since March 2024, with confirmed infections in 18 states and more than a thousand herds affected. The detection of the virus in non-avian species – including dairy cattle – reinforces the need to consider surveillance activities in production animals, such as cattle or pigs, and wildlife according to the risk and epidemiological context. The PAHO indicates that human cases remain rare. Since 2022, countries in the continent have reported 75 H5N1 infections, including two deaths. In 2025, three human cases were recorded in the United States and one in Mexico. Additionally, one human case of zoonotic influenza A(H5N2) was confirmed in Mexico and one human case of zoonotic influenza A(H5N5) in the United States, the latter being the first reported globally. These recent events in humans highlight the importance of early detection through epidemiological, virological, and genomic surveillance (both human and animal) to promptly identify any changes in the virus, as well as the timely clinical management of cases and contact tracing. Most cases have been linked to direct contact with infected animals, with no evidence of person-to-person transmission. Globally, clade 2.3.4.4b of the H5N1 virus has caused extensive mortality in birds since 2020 in Africa, Asia, and Europe, and subsequently spread to North America, Central America, and South America. Outbreaks in mammals have been identified on three continents. The virus’s expansion to 22 countries across three continents, including the Americas, which have reported outbreaks in mammals to the World Organisation for Animal Health (WOAH) confirms that the threat is not limited exclusively to the avian field. Since 2003, the WHO has received 991 reports of human cases of H5N1, with a high lethality (48%). PAHO, in collaboration with WOAH and FAO, recommends that countries strengthen animal and human surveillance in affected areas, as well as strengthen biosecurity protocols, improve intersectoral coordination, and ensure the integrated analysis of epidemiological and virological information. Countries are also urged to actively monitor human populations at higher risk of exposure, including poultry and backyard workers, veterinarians, and operational staff on farms and involved in wildlife management, to detect early clinical signs compatible with infection, such as respiratory illness, conjunctivitis, or associated neurological conditions. The importance of applying strict biosecurity measures and the use of personal protective equipment on farms, in laboratories, and in health services is also emphasized. The update recalls that there is no evidence of transmission of the virus to humans through the consumption of properly cooked poultry products, and insists on the importance of having solid mechanisms for detection, notification, and response to new outbreaks or suspected cases.

The Avian Influenza Virus

Avian influenza is caused by some subtypes of the Influenza A virus, which mainly affect birds, although they can sometimes infect mammals, including humans. Influenza A viruses are classified into subtypes according to the proteins on their surface, hemagglutinin (H) and neuraminidase (N), such as H5N1 or H5N2. Wild aquatic birds constitute their main reservoir, from where it is sometimes transmitted to poultry, causing outbreaks with an impact on livestock production. Avian influenza viruses are also classified into genetic clades, which correspond to lineages defined by specific mutations in their genome. Currently, clade 2.3.4.4b of H5N1 predominates in the Americas and has been responsible for most of the recent outbreaks in birds and mammals. Genomic surveillance is key to identifying changes in the virus that may influence its spread, severity, or ability to infect new species.