Beyond the barnyard: How H5N1 crosses species and borders

During the August 2025 Health Watch USA(sm) webinar “Solving epidemic problems“Several lectures focused on H5N1 or avian flu, topics that were outdated at the time. Many in the public, including on social media, attributed the disappearance of avian flu over the summer to the implementation of effective suppression strategies. The threat of the past is now long gone.

Unfortunately, as we have seen with seasonal flu for decades, all we have observed is the normal disappearance of influenza during the summer months.

On October 24, Reuters reported that avian flu outbreaks were at their highest in decades in several European countries. Culling millions of birds again. Each country is monitoring the situation together with the World Health Organization (WHO). France is taking action by launching its third annual avian flu vaccination campaign, starting with farm ducks. Belgium has separated its poultry flocks.

in USA, bird flu The deaths of about 7 million farm birds since September have made a comeback. That includes 1.3 million turkeys, threatening our supply just before Thanksgiving. And three states have new outbreaks in dairy cattle.

Dr. Richard WebbyDirector of the WHO Collaborating Center for Influenza Ecology Research in Animals and Birds; and Cinda Crawford, DVM, PhDFredrica Saltzman, professor of conservation medicine at the University of Florida, presented what we have learned and know about avian influenza. We hope that the following summary of this information will help you prepare for and face the challenges that will almost certainly be posed by H5N1 in the months and years ahead.

First, the name “avian flu” is a misnomer. All influenza strains, whether human seasonal flu or swine flu, originate from birds. At any given time, about 15% of wild birds may be infected with a strain of avian flu. Only a very small percentage of these people become ill, and only a very small percentage of the virus is transmitted to other species. Generally, when humans are infected, they are infected through an intermediate host, such as a pig with ‘swine flu.’ Therefore, these viruses are best referred to based on their hemagglutinin and neuraminidase structures. In ‘Bird Flu’ we will discuss the H5N1 virus.
Infections in birds: Before 2021, H5N1 was relatively free in North America. The virus then developed the ability to spread widely through migratory birds and was reintroduced to North America in 2021-2022. H5N1 virus by 2025 It has infected poultry in all 50 states, decimating the bird population by more than 175 million birds. The states of California, South Dakota, and Minnesota were particularly hard hit. Chickens are very susceptible to viruses Severe pneumonia develops with a rapid increase in mortality over a period of hours rather than days. Reducing the herd population is considered most humane.

Infections in mammals: The current H5N1 variant has an increased ability to infect mammals, primarily through predatory consumption of infected birds. To date, more than 200 different mammalian species have been infected, and it has spread to mammals. It’s so common in America. Mammals once thought to be immune to the virus, such as seals and dolphins, have also become infected. However, mammal-to-mammal transmission was limited.

The emergence of H5N1 clade 2.3.4.4b was associated with mammalian infections. This new clade has been reported to not only bind to the avian alpha-2,3 sialic acid receptor but also rarely bind to the alpha-2-6 receptor found in mammalian cells. The respiratory tract of many mammalian species contains a mixture of these two receptors.
Infections in dairy cattle: Dairy barns are open to the outside, exposing cows to migratory birds. Cattle were not considered susceptible to avian influenza, but as H5N1 viruses continued to mutate and form clades B3.13 and D1.1, dairy Cattle have become very vulnerable.. In dairy cows, the virus appears to be transmitted through milk rather than through respiration. This is because cow udders have virus receptors that are closer to those found in birds than the human respiratory system.

In cattle, the clinical manifestation of infection is mastitis rather than pneumonia. Milk production has decreased significantly and the milk has a high content of the cream-yellow virus. Infections are usually not fatal. The virus spreads through movement of infected cows, contaminated milking equipment, and milk spills. Additionally, when changing milking equipment, the virus can become airborne and cause worker infection.

Infections in domestic cats: Cats are highly susceptible to H5N1. Most cats become infected by consuming infected bird carcasses and, in dairy farms, by consuming infected milk. Consumption of raw milk is an example of mammal-to-mammal transmission. Infections in cats have increased in clade 2.3.4.4b, and unlike dairy cattle, infected cats develop a severe and highly fatal neurological disease. Clinical signs are similar to rabies. infected cats Mortality rate up to 67% The average time from symptom onset to death is only 2 days. There is no evidence of transmission from cats to humans.

Cats most at risk for bird flu are those that live outdoors and those that live on dairy or poultry farms. A new route of transmission that also affects indoor cats is purchasing raw cat food made from raw milk or chicken from commercial suppliers.

Human infection: H5N1 is not well adapted to infect humans. This virus binds to slightly different sugar receptors than those commonly found in humans. Human H5N1 infections tend to manifest primarily as conjunctivitis rather than respiratory disease, presumably from milk splashes or chicken dander getting into farm workers’ eyes. Unlike human lungs, human eyes are rich in receptors to which avian flu viruses like to bind. Therefore, most human cases are mild. To date, there has been no sustained human-to-human transmission.
Personal accidents may occurHowever, previous high human mortality rates may be related to the exposure of health care workers to very high viral doses and the underreporting of mild cases.

H5N1 had sporadic mutations that could have facilitated human transmission, but outbreaks of these viruses were completely eliminated. If H5N1 spreads in pig populations, it will promote adaptation to humans because pigs are efficient intermediate hosts for human infection.

Spread Control: Because the H5N1 virus is parasitic in migratory birds and cannot be controlled, strategies to prevent its spread are limited. Carcasses can still be removed and locally farmed species can still be protected. Improving biosecurity and culling farm birds can help protect both the agricultural industry and farm workers. Culling of chickens is necessary because of the extremely high mortality rate of the infection, to protect farm workers, and to prevent the virus from increasing its ability to infect humans.

There is evidence that vaccination can protect the health of both farm animals and humans. However, in the United States, H5N1 vaccination is not routinely administered to poultry due to trade agreements with other countries. Exporting vaccinated poultry is difficult. The H5N1 vaccine is not approved for use in farm animals in the United States.
However, at least 20 H5N1 vaccines have been authorized worldwide and at least 32,000 people have been vaccinated. Additionally, mRNA technology can rapidly bring strain-specific H5N1 vaccines to market. Unfortunately, the virus requires several mutations for human-to-human transmission to occur. We must prepare for these eventualities and make effective plans. Fight against this epidemic problem When it happens.