This week I want to follow up on two viruses I didn’t cover in my last article. The first is H5N1, also known as bird flu. H5N1 is a highly pathogenic avian influenza. It affects multiple internal organs, with mortality of 90 to 100 percent in chickens, often within 48 hours. Ducks and some wild bird species can be infected without any signs of illness. These healthy-appearing birds may still transmit illness.
Since 2022, H5N1 has led to the loss of more than 166 million poultry, including 19 million over the past 30 days.[1] Outbreaks in 2024 differed from prior outbreaks by affecting conventional caged, cage-free, and certified organic eggs.
The usual method of managing infections during recent years has been depopulation, or culling, of infected flocks, coupled with aggressive surveillance and quarantine of nearby flocks.[2]
All this might make us feel bad for the chickens. It also makes us wince at the cost of eggs. But as it turns out, H5N1 can cross species. Cattle and cats were the next to be impacted.
An H5N1 variant has been found in cattle. Data indicate that bovine-to-bovine transmission does occur, but the disease so far appears mild, causing anorexia and reduced production of abnormal milk. Symptoms appear in less than 10 percent of cows within a herd. Mortality is typically less than 2 percent. The virus can also spread from dairy cattle back to nearby poultry.[3] Cats, in contrast, have a very high mortality.[4]
H5N1 is zoonotic, and infects humans. As of last Friday, there have been 70 confirmed cases of human H5N1, and one death.[5] Usual human mortality is reportedly up to 50 percent. The deceased had exposure to sick and dead birds. No person-to-person spread of H5N1 has been detected – yet.
Folks who know the global history of rinderpest, an eradicated virus once impacting cattle, are probably screaming, “why not vaccinate?” That’s actually a great question. As it turns out, there is now a vaccine – but it’s for chickens, not people.
But vaccination is not easy as it seems. Many countries won’t buy vaccinated fowl and still require sellers to prove their birds are free of bird flu. Vaccines can mask the presence of the virus. Vaccines prevent severe disease, but don’t prevent infection. Lobbying groups such as the Congressional Chicken Caucus have also protested vaccination, noting that “vaccinating any poultry sector … will jeopardize the entire export market.”[6]
The next virus is hantavirus. These have been known since the Korean War, but they attracted little attention until the recent death of Betsy Arakawa, the wife of the late actor Gene Hackman. Transmission of hantavirus is through aerosols and droplets containing rodent excretions, as well as food, bites, and scratches. Human-to-human transmission does not occur. The usual hantavirus disease in the U.S. is hantavirus pulmonary syndrome, or HPS.
Prognosis for HPS depends on the stage at presentation but is generally poor. Fatality rates range from 30 to 60 percent. Death usually occurs 2–10 days after the onset of illness. Full recovery takes months and can be as long as two years.
The infectious disease threats in the U.S. are very real. We need to be highly vigilant, especially as chaos in the federal government may limit our ability to recognize and plan for emergent infectious threats.
[1] https://www.cidrap.umn.edu/avian-influenza-bird-flu/usda-rolls-out-5-step-plan-battle-avian-flu-poultry
[2] https://www.cdc.gov/bird-flu/virus-transmission/avian-in-birds.html#:~:text=HPAI%20A(H5)%20or%20A,without%20any%20signs%20of%20illness.
[3] and-welfare/animal-health/avian-influenza/avian-influenza-virus-type-h5n1-us-dairy-cattle#:~:text=While%20avian%20influenza%20virus%20type,2%25%
[4] https://www.avma.org/resources-tools/animal-health-and-welfare/animal-health/avian-influenza/avian-influenza-h5n1-cats
[5] https://www.cdc.gov/bird-flu/situation-summary/index.html
[6] https://www.nationalchickencouncil.org/wp-content/uploads/2025/02/Letter-to-USDA-Re-HPAI-Vaccine.pdf
