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Priya Venkatesan
13 human cases of highly pathogenic avian influenza (HPAI) A(H5) have been confirmed in the USA so far in 2024, as of July 26. All were mild illnesses occurring in farm workers exposed to infected dairy cattle or poultry. Although the current risk to the general public is low, increased vigilance and pandemic preparedness efforts have been encouraged by public health authorities.
Outbreaks of HPAI viruses with zoonotic potential have been increasing globally in bird populations since 2020; in July 2024 alone, more than 100 outbreaks in multiple species of birds were noted in Asia, the Americas, and Europe. Concerningly, spillover events of A(H5) infections have also occurred in unprecedented numbers in several mammalian species, including raccoons, dolphins, and foxes. In 2024 (as of July 26), 171 dairy cow herds in 13 US states reported confirmed cases of A(H5N1) virus infections, and 18·37 million birds in commercial and backyard flocks were affected. Additionally, 13 human cases of A(H5) were reported in three US states—Colorado, Michigan, and Texas—since April 2024, including ten human cases of A(H5N1). Four of the human cases were reported in farm workers exposed to infected dairy cattle and nine were reported in workers with exposure to A(H5N1)-infected poultry. All these individuals reported mild symptoms (predominantly conjunctivitis) and are recovering after being provided antiviral medication. There have been no cases of human-to-human transmission of A(H5N1) HPAI so far during the 2024 US outbreak.
Genomic data for the A(H5N1) from US farm workers show the clade 2.3.4.4b of genotype B3.13. Sequencing of the virus from human cases from Colorado, the state with the greatest number of human cases so far, shows that the sequence maintains predominantly avian genetic characteristics without mutations that would enable viral transmission among humans, although sequencing of a virus from an individual from Texas showed the mutation PB2 E627K that is known to be associated with adaptation to mammalian hosts and enhanced viral replication in mammalian cells.
Although government technical groups from other countries, such as the UK Health Security Agency, view the US animal A(H5N1) outbreak as sustained transmission with increasing human zoonotic cases, the general consensus is that the risk of zoonotic influenza transmission to the general public is low, although the risk to occupationally exposed individuals such as farm workers is low to moderate. Noah Kojima (Keck School of Medicine, University of Southern California, Los Angeles, CA, USA) commented “The main point is that we are now finding H5N1 viruses among humans now that we have looked in humans who have had exposure to infected poultry and dairy cows. The overall risk to the general public is still low; however, it is clear that people with occupational exposures to poultry and dairy cows will more likely have exposure of H5N1 infection given its spread among the animals”.
Jeanne Marrazzo (National Institute of Allergy and Infectious Diseases [NIAID], Bethesda, MD, USA) added “Currently, the human risk of acquiring highly pathogenic H5N1 avian influenza is low, and thankfully we have seen no indication that the virus currently affecting US dairy cattle and poultry has acquired any sort of genetic changes to allow for easier transmission among people”. Marrazzo emphasised that people should continue to avoid close or unprotected exposures to sick or dead animals and reiterated the US Centers for Disease Control and Prevention (CDC) recommendation that farm workers in close contact with animals use personal protective equipment, such as gloves and masks.
Part of the problem in containing the current US outbreak is that the mode of transmission of the A(H5) virus between and within species is unclear; therefore, it is difficult to determine the correct mitigation strategies needed. Enhanced surveillance in the USA is ongoing, with more than 4000 people monitored based on their exposure to infected or potentially infected animals and at least 200 people who developed influenza-like symptoms tested as part of a targeted approach: “none of these individuals were confirmed to have H5N1 HPAI”, emphasised Marrazzo. The CDC is also coordinating with state-led farm investigations to detect infections and the US Department of Agriculture is assessing the prevalence of the virus among dairy herds.
Good vaccine production pipelines are thought to be in place, according to Marrazzo: “The current H5N1 influenza vaccines in the US national stockpile are well-matched to the current strain circulating in affected US dairy cows and poultry.” Additionally, WHO launched an initiative on July 29, 2024, to advance mRNA A(H5N1) vaccine development in low-income and middle-income countries. Marrazzo commented “[NIAID] are working with our colleagues in the US government to end the current outbreak while actively monitoring for any changes, and intensifying our pandemic preparedness efforts with this situation in mind”.
13 human cases of highly pathogenic avian influenza (HPAI) A(H5) have been confirmed in the USA so far in 2024, as of July 26. All were mild illnesses occurring in farm workers exposed to infected dairy cattle or poultry. Although the current risk to the general public is low, increased vigilance and pandemic preparedness efforts have been encouraged by public health authorities.
Outbreaks of HPAI viruses with zoonotic potential have been increasing globally in bird populations since 2020; in July 2024 alone, more than 100 outbreaks in multiple species of birds were noted in Asia, the Americas, and Europe. Concerningly, spillover events of A(H5) infections have also occurred in unprecedented numbers in several mammalian species, including raccoons, dolphins, and foxes. In 2024 (as of July 26), 171 dairy cow herds in 13 US states reported confirmed cases of A(H5N1) virus infections, and 18·37 million birds in commercial and backyard flocks were affected. Additionally, 13 human cases of A(H5) were reported in three US states—Colorado, Michigan, and Texas—since April 2024, including ten human cases of A(H5N1). Four of the human cases were reported in farm workers exposed to infected dairy cattle and nine were reported in workers with exposure to A(H5N1)-infected poultry. All these individuals reported mild symptoms (predominantly conjunctivitis) and are recovering after being provided antiviral medication. There have been no cases of human-to-human transmission of A(H5N1) HPAI so far during the 2024 US outbreak.
Genomic data for the A(H5N1) from US farm workers show the clade 2.3.4.4b of genotype B3.13. Sequencing of the virus from human cases from Colorado, the state with the greatest number of human cases so far, shows that the sequence maintains predominantly avian genetic characteristics without mutations that would enable viral transmission among humans, although sequencing of a virus from an individual from Texas showed the mutation PB2 E627K that is known to be associated with adaptation to mammalian hosts and enhanced viral replication in mammalian cells.
Although government technical groups from other countries, such as the UK Health Security Agency, view the US animal A(H5N1) outbreak as sustained transmission with increasing human zoonotic cases, the general consensus is that the risk of zoonotic influenza transmission to the general public is low, although the risk to occupationally exposed individuals such as farm workers is low to moderate. Noah Kojima (Keck School of Medicine, University of Southern California, Los Angeles, CA, USA) commented “The main point is that we are now finding H5N1 viruses among humans now that we have looked in humans who have had exposure to infected poultry and dairy cows. The overall risk to the general public is still low; however, it is clear that people with occupational exposures to poultry and dairy cows will more likely have exposure of H5N1 infection given its spread among the animals”.
Jeanne Marrazzo (National Institute of Allergy and Infectious Diseases [NIAID], Bethesda, MD, USA) added “Currently, the human risk of acquiring highly pathogenic H5N1 avian influenza is low, and thankfully we have seen no indication that the virus currently affecting US dairy cattle and poultry has acquired any sort of genetic changes to allow for easier transmission among people”. Marrazzo emphasised that people should continue to avoid close or unprotected exposures to sick or dead animals and reiterated the US Centers for Disease Control and Prevention (CDC) recommendation that farm workers in close contact with animals use personal protective equipment, such as gloves and masks.
Part of the problem in containing the current US outbreak is that the mode of transmission of the A(H5) virus between and within species is unclear; therefore, it is difficult to determine the correct mitigation strategies needed. Enhanced surveillance in the USA is ongoing, with more than 4000 people monitored based on their exposure to infected or potentially infected animals and at least 200 people who developed influenza-like symptoms tested as part of a targeted approach: “none of these individuals were confirmed to have H5N1 HPAI”, emphasised Marrazzo. The CDC is also coordinating with state-led farm investigations to detect infections and the US Department of Agriculture is assessing the prevalence of the virus among dairy herds.
Good vaccine production pipelines are thought to be in place, according to Marrazzo: “The current H5N1 influenza vaccines in the US national stockpile are well-matched to the current strain circulating in affected US dairy cows and poultry.” Additionally, WHO launched an initiative on July 29, 2024, to advance mRNA A(H5N1) vaccine development in low-income and middle-income countries. Marrazzo commented “[NIAID] are working with our colleagues in the US government to end the current outbreak while actively monitoring for any changes, and intensifying our pandemic preparedness efforts with this situation in mind”.