What Is H5N1 Bird Flu, How it Spreads to Humans, and Safety Concerns

Jun. 20, 2024

The H5N1 virus, also known as avian or bird flu, is once again in the news and on the radar of scientists and public health officials as a growing number of infections have been found in dairy cattle and farm workers.

David Topham, PhD, a professor of Microbiology and Immunology and director of the University of Rochester Translational Immunology and Infectious Disease Institute, has been studying the H5N1 and other influenza viruses for 27 years.

What is bird flu, and when did bird flu in cattle start?

David Topham

Topham: The H5N1 virus has been in North America for quite a long time in both wild birds and domestic poultry. A few years ago, a large outbreak in chickens and turkeys prompted farms to cull their entire flocks. Bird flu has been circulating in dairy cattle for at least five to six months and probably longer, but we don't know exactly when or where it jumped over.

We assume it’s a respiratory infection in cattle, and it also looks like the udders are infected because we’re finding a lot of virus in milk. There are also reports of infections in cats, which is concerning because they’re biologically closer to humans than cattle are.

The recent cases in humans have been dairy workers in a handful of states (not New York) who have close contact with the cattle. This is similar to what we’ve experienced since 1997, when H5N1 first emerged, and the people infected were either working with infected poultry or taking care of severely ill family members. So, transmission of H5N1 in humans so far resulted from very intense contact, as opposed to casually spreading.

The virus has been found in milk—should people worry about the food supply?

Topham: Viral particles have been detected in the milk supply, but people should rest assured that pasteurization kills microorganisms, including the H5N1 virus.

A lot of people feel that drinking raw milk is healthier than pasteurized milk and I respect that. But when there's potentially a pathogen floating around in that milk, that changes the equation, and I would recommend avoiding raw milk at present.

Are there any precautions people can take to limit risk?

Topham: Get vaccinated. Flu vaccines aren't perfect, but even a badly matched vaccine is better than no vaccine and will offer a degree of protection. It won't necessarily keep you from getting infected, but people that have more immunity tend to experience less-severe symptoms.

What are the warning signs that H5N1 is becoming a danger to human health?

Topham: Anytime these viruses start to pop up in environments where lots of people are present and there are signs of infection, that's a concern. Because the more often that happens, the more likely that two viruses, such as the human influenza virus and the avian H5N1 virus, will mix and a new virus can emerge.

This is just like reshuffling a deck of cards, and if the virus gets a better hand maybe it acquires the ability to spread from person to person via the respiratory route, and that would be really bad. This mixing event is the biggest concern and a major red line, because that’s historically how pandemic influenza has emerged.

How prepared is the medical community if H5N1 becomes more infectious to humans?

Topham: The good news is that we already have an H5N1 vaccine stockpile. We've developed vaccines against this strain that may not be a perfect match but will be close enough. In fact, the government has ordered manufacturers to begin increasing the stockpiles. So if the virus were to emerge, we could scale up very quickly and we would rapidly begin vaccinating people.

The COVID pandemic has accelerated the process of vaccine development, testing, and manufacturing and we are able to quickly ramp up the production even against newly emerged variants. We can do it in months instead of years. Tamiflu and the other antivirals against influenza look like they're effective against this virus.