#18,865
One of the topics we've touched on repeatedly over the past twenty years has been the - as yet, unquantified - risks of preparing, and consuming animal products (meat, eggs, milk, etc.) which may be infected or contaminated with HPAI viruses.
We've known for more than 20 years what happens when cats are fed raw (fresh or frozen) H5N1 contaminated, poultry (see HPAI H5: Catch As Cats Can), with the most recent report issued less than a week ago (see FDA Issues New Warning On H5N1 Detected In Cat Food).
Since then, we've seen evidence that other mammalian species are susceptible to HPAI virus via the GI tract as well (see Research: Ferret H5N1 Infection Via Consumption Of Infected Chicken).
In 2008's Clinical Case Review Of 26 Chinese H5N1 Patients, we saw a review that mentioned a number of `atypical' symptoms in HPAI cases, citing:While reports of human infection via the consumption of contaminated food products are harder to pin down, we've seen a number of `suspicious' incidents where ingestion and absorption of HPAI via the GI tract could not be ruled out (see 2007's Don't Order The Beggar's Chicken).
Diarrhea was present in only two H5N1 cases at admission, but developed in a quarter of cases during hospitalization. Diarrhea was a common presenting symptom among H5N1 cases in Vietnam and Thailand, but was reported infrequently among cases in Hong Kong SAR, China, and Indonesia .
H5N1 virus and viral RNA have been detected in feces and intestines of human H5N1 cases . Whether the gastrointestinal tract is a primary site for H5N1 virus infection is currently unknown.
Two years later, in H5N1 Can Replicate In Human Gut, we looked at the following study published in the J. Infectious Diseases.
Avian influenza A(H5N1) viruses can directly infect and replicate in human gut tissues
Yuelong Shu 1, Chris Ka-fai Li, Zi Li, Rongbao Gao, Qian Liang, Ye Zhang, Libo Dong, Jiangfang Zhou, Jie Dong, Dayan Wang, Leying Wen, Ming Wang, Tian Bai, Dexin Li, Xiaoping Dong, Hongjie Yu, Weizhong Yang, Yu Wang, Zijian Feng, Andrew J McMichael, Xiao-Ning Xu
Affiliations ExpandPMID: 20210629
DOI: 10.1086/651457
Abstract
The human respiratory tract is a major site of avian influenza A(H5N1) infection. However, many humans infected with H5N1 present with gastrointestinal tract symptoms, suggesting that this may also be a target for the virus. In this study, we demonstrated that the human gut expresses abundant avian H5N1 receptors, is readily infected ex vivo by the H5N1 virus, and produces infectious viral particles in organ culture. An autopsy colonic sample from an H5N1-infected patient showed evidence of viral antigen expression in the gut epithelium. Our results provide the first evidence, to our knowledge, that H5N1 can directly target human gut tissues.
While anecdotal reports suggest that a significant number of H5N1 cases over the past 20 years may have consumed chicken, they almost always had other environmental exposures (raising birds, visiting `live' markets, slaughtering or preparing poultry, etc.) that make it impossible to single out consumption as the route of infection.
Recently in Cambodia we've seen a spate of H5N1 cases (and deaths) linked to preparing, cooking, or consuming `sick or dead' poultry. But once again, the exact route of infection remains unknown (see Cambodia: Food Insecurity, Food Safety & H5N1).
All of which brings us to a recent CDC study, published last month in the the Journal Virology, which looks at the available evidence for influenza A infection via the GI tract, and a CDC Summary/Risk Assessment published yesterday.
I've posted links and excerpts from both reports, after which I'll have a bit more.
The (digestive) path less traveled: influenza A virus and the gastrointestinal tract
Trent A. Bullock, Claudia Pappas, Timothy M. Uyeki, Nicole Brock, Troy J. Kieran, Sonja J. Olsen, Todd C. Davis, Terrence M. Tumpey, Taronna R. Maines , Jessica A. Belser jbelser@cdc.govAuthors Info & Affiliations
https://doi.org/10.1128/mbio.01017-25
PDF/EPUB
ABSTRACT
Influenza A virus (IAV) infection of the respiratory tract can cause both respiratory and non-respiratory symptoms. Gastrointestinal (GI) symptoms such as diarrhea, vomiting, and abdominal pain can occur in persons with seasonal influenza A or novel IAV infections, but the extent to which IAVs can infect and replicate in GI tissues is understudied.
The ongoing outbreak of A(H5N1) IAV in US dairy cattle associated with sporadic human infections has highlighted the potential public health threat posed by the introduction of infectious virus into materials that may be consumed by humans, such as milk. Here, we review epidemiologic reports documenting the frequency of GI complications in humans infected with seasonal and novel IAVs and present laboratory studies supporting the capacity of IAV to replicate in mammalian GI tissues, with an emphasis on A(H5N1) viruses. Studies assessing the ability of IAV to cause mammalian infection following consumption of virus-containing material are also presented.
Collectively, these studies suggest that gastric exposure represents a potential non-respiratory route for A(H5N1) IAVs in mammals that can lead to infection and support that IAV may be detected in mammalian intestinal tissues following multiple exposure routes.
From the CDC's Website.
Influenza A Viruses May Infect GI Tract and Cause Digestive Symptoms
For Everyone
Sept. 8, 2025
At a glance
A new CDC report highlights the potential for influenza A viruses to cause gastrointestinal (digestive) symptoms in humans and infect mammals through other (non-respiratory) routes.
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.
(SNIP)
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
- In 2017 tons of H5N8 contaminated poultry products were shipped from a single infected turkey farm in the Rostov region of Russia sparking a massive months-long operation(see Rosselkhoznador: HPAI Contaminated Poultry Shipped To At Least 9 Regions Of Russia).
- The following year (2018) an outbreak of avian H5N8 led Bulgaria To Recall 1 Million Eggs Due To HPAI H5N8.
- We've also seen contaminated chicken confiscated from shops in Hong Kong (see Hong Kong FEHD Finds Another Shop Selling H5 Contaminated Poultry).
Transmission
The most common way for the virus to enter a territory is through migratory wild birds. The main risk factor for transmission from birds to humans is direct or indirect contact with infected animals or with environments and surfaces contaminated by feces. Plucking, handling infected poultry carcasses, and preparing poultry for consumption, especially in domestic settings, may also be risk factors.
And last year the WHO published Interim Guidance to Reduce the Risk of Infection in People Exposed to Avian Influenza Viruses, which lists a number of `risk factors', including:
- keep live poultry in their backyards or homes, or who purchase live birds at markets;
- slaughter, de-feather and/or butcher poultry or other animals at home;
- handle and prepare raw poultry for further cooking and consumption;
