Influenza A Viruses May Infect GI Tract and Cause Digestive Symptoms

Background

September 8, 2025 - A new CDC report summarizes data on how influenza A viruses might affect the gastrointestinal (GI) tract of mammals, including people, in the context of the A(H5N1) bird flu outbreak in the United States. A key question has been the potential health threat to people from eating or drinking food or beverages contaminated with avian influenza A(H5N1) viruses, such as contaminated raw (unpasteurized) milk. While no human infections with A(H5N1) virus have been attributed to consumption of raw cow's milk or products made from raw cow milk, the research summarized in this report shows that some seasonal and novel influenza A viruses may have potential to cause infection of the GI tract. Additionally, while the risk appears to be low to date, avian influenza A(H5N1) viruses may cause infection in mammals and have potential to cause infection in humans if the digestive tract were exposed to A(H5N1) viruses or virus-contaminated products. This study also notes the need to better understand how influenza A viruses may cause infection through other (non-respiratory) routes, including their potential impact on the GI tract.

Summary of report findings

Seasonal influenza A viruses are contagious respiratory pathogens in people and typically cause infection via the respiratory tract. In this report, CDC researchers analyzed data from scientific reports that studied (1) how often people infected with seasonal and novel influenza A viruses reported GI symptoms and (2) laboratory studies in which live mammals became infected with avian influenza A(H5N1) virus after eating or drinking contaminated products, including meat or milk. The collective results indicate influenza A viruses may cause infection in mammals through non-respiratory routes. Overall, the studies summarized here support that the risk of human infection following consumption of avian influenza A(H5N1) virus-infected materials is low, but theoretically possible.

Below are key findings from this review of studies:

• Researchers presented combined data from previous studies that reported the presence of influenza A viruses in the GI tissue of ferrets after they were infected through the respiratory/nasal tract, showing that seasonal and novel influenza A viruses may spread to the GI tract of mammals after infection through a respiratory route.
• Researchers summarized data from reports that showed dogs and cats that ingested A(H5N1) virus-containing material (typically eating virus-contaminated meat or drinking virus-contaminated milk) could develop a potentially severe and fatal infection. Numerous animal studies also have shown that eating A(H5N1) virus-contaminated meat or drinking A(H5N1) virus-contaminated milk can lead to infection and death in some mammals. However, there are limitations in translating these findings to people.
• CDC researchers reviewed studies of human cases of seasonal or novel influenza A virus infection in which patients had GI symptoms to better understand the impact on the human GI system. Clinical reports of GI symptoms and detections of viral genetic material in feces show the potential for both seasonal and novel influenza A viruses to cause GI symptoms in people.
• Additional studies found different influenza A virus subtypes are able to multiply within human GI tissues in laboratory settings, suggesting influenza A virus infection in the human GI tract may be possible. However, it is also important to note that laboratory settings do not have the same acidic conditions as the human GI tract, which may inactivate these viruses.

Conclusion and risk assessment

This review of available studies suggests exposure of the digestive system to HPAI A(H5N1) viruses represents a potential non-respiratory route for infection in people and other mammals. There are also unpublished anecdotal reports of raw duck blood consumption as a potential source of A(H5N1) virus infection in a small number of human cases in the past. While the risk of human infection of the GI tract from consuming HPAI A(H5N1) virus-contaminated products is likely low, further investigation is needed to understand how different seasonal and novel influenza A viruses may use the GI tract to possibly infect humans and mammals.

Based on the limited research and information available, we do not know at this time if avian influenza A viruses can be transmitted to people through consumption of raw milk and products (such as cheese) made from raw milk from infected cows. A(H5N1) virus has been found in commercially sold raw milk. Therefore, drinking raw milk or eating products made with raw milk, should be avoided. As a reminder, eating uncooked or undercooked poultry or beef, drinking raw (unpasteurized) milk, or consuming other uncooked products made from these animals, can make you sick. Additionally, products made from raw milk, including soft cheese, ice cream, and yogurt, can be contaminated with germs that can cause serious illness, hospitalization, or death. Cooking poultry, eggs, and beef to the appropriate internal temperature kills bacteria and viruses, including avian influenza A viruses. Choosing pasteurized milk and products made with pasteurized milk is an important way to keep you and your family safe. Pasteurization kills bacteria and viruses, like avian influenza A viruses, in milk. Make the best decision for your health and the health of your family by always choosing pasteurized milk and products made with it.

For more information on preventing H5N1 bird flu, including additional guidance on the safe consumption of milk and other dairy products, keep reading: Preventing Bird Flu Infections | Bird Flu | CDC