A recent study found a highly pathogenic avian influenza (HPAI) H5N1 virus identified in an infected dairy worker proved to be lethal in lab mice and ferrets, as human HPAI cases continue to spread.
USDA and Oregon state officials are investigating an H5N1 outbreak in an Oregon backyard farm with mixed poultry and livestock, while the USDA’s Animal and Plant Health Inspection Service (APHIS) plans to expand testing and monitoring efforts.
Study
A recent study published in Nature explored a HPAI H5N1 virus, known as huTX37-H5N1, that infected a farm worker exposed to dairy cows. This virus, isolated from the worker’s eye after they experienced mild conjunctivitis, was lethal in lab mice and ferrets. Notably, it could also spread through the air between separated ferrets and replicate in human respiratory tract cells, raising concerns about its ability to potentially spread between humans.
In lab experiments, the huTX37-H5N1 virus was able to reproduce in human eye and lung cells. Researchers found that a very small amount of this virus—less than one plaque-forming unit (PFU)—was lethal in mice, whereas it took 31.6 PFU of a similar virus from cow milk to be fatal. Additionally, huTX37-H5N1 spread to all 15 types of mouse tissues tested, with the highest concentration found in the lungs and other respiratory tissues.
The huTX37-H5N1 virus has a mutation, PB2-E627K, often seen in avian influenza viruses that helps them adapt to mammals. This mutation allows the virus to reproduce more efficiently, making it critical for scientists to monitor similar viruses.
Researchers recently studied how the huTX37-H5N1 virus affects ferrets, which are similar to humans in how they react to the flu. After infecting ferrets with a high dose of the virus, all of them died within five days, with high virus levels found in their respiratory systems. This virus also spread through the air to nearby ferrets, though only 17-33% of these exposed ferrets were infected, showing that airborne spread is possible, but limited.
Researchers tested antiviral drugs on huTX37-H5N1, finding that it was susceptible to the drugs favipiravir and baloxavir marboxil (Xofluza), and zanamivir (Relenza), but less sensitive to oseltamivir (Tamiflu).
Researchers think past exposure to human flu viruses might give people slight protection against H5N1, though more research is needed.
Cases
As of Nov. 4, the Centers for Disease Control (CDC), in collaboration with state health departments, confirmed 44 human cases of H5N1 avian influenza in the U.S. Nineteen cases were linked to infected poultry, and 24 were tied to exposure to sick or infected dairy cows. Among these 44 cases, 20 are dairy farm workers in California, and nine are poultry farm workers in Washington.
Four probable cases were also reported but did not meet CDC’s confirmatory testing. People infected have shown only mild symptoms like eye irritation or mild respiratory issues, and none required hospitalization. According to the CDC, the risk to the public remains low, though exposure to infected animals increases infection risk.
Since March 2024, USDA reports show 440 dairy herds across 15 states have been infected with H5N1 avian influenza, with most new cases in California. Infected poultry have also been confirmed in 45 commercial and 30 backyard flocks, impacting over 22 million birds. The USDA continues monitoring the spread closely as cases rise among dairy herds and poultry flocks.
USDA monitoring
USDA’s APHIS announced plans to enhance testing and monitoring of H5N1 avian influenza in dairy herds. Partnering with state veterinarians, USDA will implement a tiered milk sampling strategy to track where H5N1 is present. This will help improve biosecurity measures and guide states in protecting farm workers exposed to infected animals.
USDA said this proactive testing approach follows the agency’s steps to control the virus’ spread following its detection in dairy cattle in March. USDA’s strategy includes bulk milk testing, modeled on programs like the one used to eradicate brucellosis, and will expand this testing in states where the virus persists. USDA has also instituted a federal order requiring cattle testing before interstate movement, which has helped reduce the number of affected states from 14 to two.
APHIS said field trials are transpiring to approve a H5N1 vaccine. The two vaccine trials involve non-viable, non-replicating vaccines that don’t lead to virus shedding or transmission to humans, animals or the environment. USDA expects more submissions and approvals for field trials as vaccine development progresses.
Pig infections
The Oregon Department of Agriculture reported on Oct. 25 the first detection of H5N1 in poultry in Crook County, OR. On Oct. 29, USDA confirmed that a pig tested positive on an Oregon backyard farm with mixed poultry and livestock, marking the first U.S. H5N1 case in swine.
APHIS and Oregon state veterinary officials began investigating on Oct. 30. Although the pig showed no symptoms, it was euthanized for further analysis, with test results negative for two other pigs and pending for the remaining two.
Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, noted that it remains unclear whether the pig was genuinely infected or if its nasal passages were contaminated by environmental exposure. Osterholm explained that a necropsy would determine whether the pig was infected by checking for the virus deep in its lungs. He also noted that previous research indicates the virus does not typically infect pigs easily.
Agriculture Secretary Tom Vilsack said at a recent press conference that the avian influenza found in the pig was a different strain than the H5N1 detected in cattle and is distinct from what’s typically seen in poultry. Vilsack emphasized that the food supply remains safe and no new precautions are necessary for large-scale hog operations. — Charles Wallace, WLJ contributing editor
