Avian Influenza A(H5N1) U.S. Situation Update and CDC Activities

CDC Update

April 19, 2024 – CDC continues to respond to the public health challenge posed by the outbreak of avian influenza A(H5N1) virus, or "H5N1 bird flu" in dairy cows and other animals in the U.S. CDC is collaborating with partners including the U.S. Department of Agriculture (USDA), the Food and Drug Administration (FDA), and state public health and animal health officials to address this emerging infectious disease using a One Health approach. USDA is now reporting that eight U.S. states have outbreaks in dairy cattle and that the virus has spread through cattle movement between herds and also from dairy cattle premises into nearby poultry premises and has infected a number of barn cats. However, only one associated human case to date has been linked with this outbreak in dairy cows and was reported by Texas on April 1, 20241. CDC's response to this unprecedented outbreak of influenza A(H5N1) in dairy cattle and other animals most recently includes:

• Supporting states monitoring people with exposure to cows, birds, or other domestic or wild animals infected or potentially infected with influenza A(H5N1) viruses. Testing for these people is being done by state or local officials using a CDC test, and CDC is conducting confirmatory testing when needed.
• Continuing to work in the laboratory to better characterize the virus from the human case in Texas. This week CDC completed susceptibility testing for influenza antiviral medications that are used for seasonal influenza (e.g., the neuraminidase inhibitors oseltamivir, zanamivir and peramivir). Testing confirmed that the A(H5N1) virus was susceptible to all commercially available FDA-approved and recommended neuraminidase inhibitor antivirals. Testing to confirm susceptibility to baloxavir marboxil, a different antiviral medication, takes longer and is ongoing.
• Studying human sera (blood) from people vaccinated against A(H5) to confirm that existing A(H5N1) candidate vaccine viruses (CVVs) will provide protection against the A(H5N1) virus isolated from the human case in Texas. Manufacturers could use these CVVs to make a vaccine if needed. Preliminary genetic analysis had suggested two existing CDC CVVs would offer protection against the virus isolated from the human case in Texas.
• Designing an epidemiological field study and preparing a multilingual and multidisciplinary team to travel on site to better understand the current outbreak, particularly the public health and One Health implications of the emergence of this virus in cattle.
• Engaging One Health partner organizations from public health, agriculture, milk regulatory officials, and others to share information and ensure preparedness to prevent and respond to this emerging infectious disease threat and for any potential human infections.
• Monitoring emergency department data and flu testing data in areas where A(H5N1) bird flu viruses have been detected in dairy cattle or other animals for any unusual trends in flu-like illness, flu, or conjunctivitis. So far, these data remain in expected ranges, and to date, surveillance systems do not show any unusual trends or activity.

This is a rapidly changing, emerging situation and CDC is committed to providing frequent and timely updates.

What Might Happen

The wide geographic spread of A(H5N1) bird flu viruses in wild birds, poultry, and some other mammals, including in cows, is creating additional opportunities for people to be exposed to these viruses. Therefore, there could be an increase in sporadic human infections resulting from bird, cattle, and other animal exposures, even if the risk of these viruses spreading to people has not increased. Sporadic human infections in the current context would not significantly change CDC's risk assessment.

What Would Increase Public Health Risk

Identification of multiple simultaneous instances of A(H5N1) bird flu viruses spreading from birds, cattle, or other animals to people or certain genetic changes in circulating viruses could change CDC's risk assessment because they could indicate the virus is adapting to spread more easily from animals to people. Additionally, if limited, non-sustained, person-to-person spread with this virus were to occur, that would also raise the public health threat because it could mean the virus is adapting to spread between people. Sustained person-to-person spread is needed for a pandemic to occur.

Because of the potential for influenza viruses to constantly change, continued surveillance and preparedness efforts are critical, and CDC is taking measures to be ready in case the current risk assessment for the general public changes. The immediate goal is to prevent further spread of this virus between animals and people. CDC will continue to monitor these viruses and update and adjust guidance as needed. As a reminder, while CDC believes the current risk of A(H5N1) infection to the general public remains low, people with close, prolonged, or unprotected exposures to infected birds, cattle, or other animals, or to environments contaminated by infected birds, cattle, or other animals, are at a greater risk of infection. CDC has interim recommendations for prevention, monitoring, and public health investigations of A(H5N1) viruses. CDC also has recommendations for worker protection and use of personal protective equipment (PPE) to reduce the risk of exposure. Compliance with these recommendations is central to containing the public health risk. Additionally, unpasteurized ("raw") milk from sick cattle has tested positive for A(H5N1) viruses. Consumption of raw milk can be dangerous and is not recommended. The FDA has Questions and Answers Regarding Milk Safety During Highly Pathogenic Avian Influenza (HPAI) Outbreaks.