Bird Flu Infects Second Human as Outbreak Continues


While US health officials continue to monitor the outbreak in dairy cattle across the country, a second human recently tested positive after a mutation of the bird flu virus.

A dairy farmer in Michigan tested positive for H5N1 (bird flu) this past week on May 22. Despite the CDC’s indications that a human outbreak is still unlikely, this marks the second case originating from dairy cattle in the US since April 1. Since the first case of bird flu in 2024, reported by a Texas dairy farmer, experts are concerned the virus has mutated and is more susceptible to infecting humans than previously.

bird flu graphic

The most recent human infection of bird flu marks the second case originating from dairy cattle in the US since April 1. | image credit: Berit Kessler /

What’s the Issue?

Since 2022, this marks just the third reported case of the bird flu in the US. While the first was reported from exposure to poultry in late April 2022, the 2 most recent cases stray from the ongoing outbreak among at least 67 dairy herds across 9 US states.1

From the first case in 2022 up until now, US health officials—specifically the US Department of Agriculture (USDA) and CDC—have reported the virus in US dairy products and allocated nearly $200 million to contain its spread.

“While the current public health risk is low, CDC is watching the situation carefully and working with states to monitor people with animal exposures,” wrote the CDC.1 This sentiment has been consistent from the CDC since the 2 most recent human cases purportedly originated from the same dairy cattle outbreak.

And while humans’ health risks are low, the virus is continuously adapting. The government has recently ramped up containment efforts and is currently monitoring about 350 people who’ve reached close contact with any of the infected herds across the country.2

Why It Matters

In just the past 2 months, a lot of information has been rapidly evolving surrounding the bird flu outbreak in the US. Despite multiple humans having reported an infection from the virus and its detection in US supermarket dairy products, the CDC has maintained its stance that the risk of an outbreak amongst humans is still very low. However, as recent containment efforts continue to be introduced, the bird flu outbreak has sounded alarms in the 9 states with infected herds.

  • Unrelated to the dairy cattle outbreak in 9 US states, poultry farms have also experienced an outbreak among chickens. Most recently, an outbreak of the virus was detected at a farm in Sioux County, Iowa, calling for the slaughter of 4.2 million chickens across the state of Iowa.3 Despite the 2-fold outbreak among dairy cattle and poultry farms, there has only been 1 case of a bird transmitting the disease to humans, which was the first case reported back in 2022. But between the recent dairy-related cases in the US and the worry of the virus living in dairy products and cow meat, the CDC has identified the cattle outbreak to be the greatest concern for human transmission.1
  • In both reported cases in the US this year, neither individual tested positive for bird flu after a nasal swab. Both patients who contracted the virus, however, reported eye irritation and an eye swab from each of them showed positive traces of the virus.4

Expert Commentary

  • “Viruses detected in both cows and the two human cases maintain primarily avian genetic characteristics and lack changes that would make them better adapted to infect or transmit between humans. The genome of the human virus from Michigan did not have the PB2 E627K change detected in the virus from the Texas case, but had one notable change (PB2 M631L) compared to the Texas case that is known to be associated with viral adaptation to mammalian hosts, and which has been detected in 99% of dairy cow sequences but only sporadically in birds,” wrote the CDC in a technical update of the bird flu’s recent genetic mutations.5
  • “This is exactly why I’ve been trying to call attention to deep sequencing of each virus from cows, cats, and people infected with H5N1, which is needed for a rapid and effective global response on vaccine and antiviral development,” Rick Bright, PhD, immunologist and influenza expert, told The Telegraph.2

In Depth Insights

  • As previously mentioned, US government agencies have dedicated almost $200 million to contain the current spread of bird flu. One of those agencies, the USDA, has particularly had its hands full with facilitating testing and other expenses for monitoring the outbreak among US dairy cattle. In another recent report, dairy cattle meat following its slaughter also tested positive for the virus, adding to the existing concern of the virus’s potential prevalence in US dairy products.2 “USDA have stressed that the US meat supply is safe, but have encouraged meat-eaters to cook beef to an internal temperature of at least 160 degrees Fahrenheit—which is classified as ‘well done,’” wrote Maeve Cullinan for The Telegraph.

Extra Reading

READ MORE: Infectious Disease Resource Center

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1. Avian influenza current situation summary. CDC. May 28, 2024. Accessed May 29, 2024.
2. Cullinan M. H5N1 virus in latest human case has mutated, officials say. The Telegraph. May 28, 2024. Accessed May 29, 2024.
3. Associated Press. 4.2M chickens must be killed after bird flu hits Iowa farm. TIME. May 28, 2024. Accessed May 29, 2024.
4. CDC reports second human case of H5 bird flu tied to dairy cow outbreak. CDC. May 22, 2024. Accessed May 29, 2024.
5. Technical update: summary analysis of the genetic sequence of a highly pathogenic avian influenza A(H5N1) virus identified in a human in Michigan. CDC. May 24, 2024. Accessed May 29, 2024.
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