Part 2: Answering reader questions on H5N1

Programming note: Data on influenza and Covid-19 were not updated last week due to the national day of mourning for President Carter. I will aim to publish an Outbreak Outlook published later this week.

In today’s post: This is the second installment in a short series for paid subscribers answering reader questions on H5N1. If you missed last week’s edition, you can find it here. Today I'll answer questions on transmission, individual prevention, and food safety. Future edition(s) will cover the current threat level and policy measures.

Transmission

What are the main transmission routes for H5N1, and which pose the greatest risk?

Influenza enters the body through your nose, mouth, or eyes. This can happen by breathing in viral particles, eating or drinking contaminated products, or touching your eyes with contaminated hands. Right now, people mostly get H5N1 through close contact with infected animals or their waste, which is why farmers are at highest risk. Of the 66 human cases reported in the United States so far, 64 were exposed through contact with animals.

A caveat: While there have not been any confirmed cases of infection from raw or unpasteurized milk, it is one possible route of infection. High concentrations of H5N1 have been found in unpasteurized milk, and laboratory mice and farm cats have become infected by drinking raw milk. Eating or preparing undercooked poultry has been linked to human cases of H5N1 in other countries (e.g., Cambodia), but not in the U.S.

Future concerns: Right now, H5N1 is not spreading from person to person. It may never gain the ability to do so, but the fear is that it might someday gain that ability, in which case it would spread via droplets and aerosols. This is how seasonal influenza and SARS-CoV-2 spread.

How long can the virus survive on different surfaces, and what implications does this have for infection control?

Keep in mind that these numbers are rough estimates and depend on a lot of variables. How long a virus lasts on a surface depends on factors like temperature, air circulation, humidity, and light exposure. Also, finding traces of a virus on a surface doesn’t necessarily mean it’s still infectious. And note that most of these studies pre-date the current H5N1 situation, so I am assuming the findings remain relevant. With those cautions in mind, here’s what some studies have found:

• In agricultural settings, preliminary research found that H5N1 can persist on dairy cow milking equipment for at least several hours. This could potentially enable virus transmission between cows (Le Sage et al. 2024).
• The virus can remain infectious in poultry feces for about 5 days at 77 degrees F and for about 7 weeks at 39 degrees F.
• This research also suggests that other, less-pathogenic types of influenza can be used as reasonable proxies for H5N1 (Le Sage et al. 2024)

Some research has found that H5N1 can last about a day on plastic surfaces, and several hours on human skin. Studies on influenza more generally (not specific to H5N1) also put it in the range of hours to days, depending on the surface and the environmental conditions.