Note: The spread of this virus around the world is an evolving process; what is true today about avian influenza virus today may be out of date tomorrow. But the following is an overview of what we know so far.
The Evolution and Global Spread of H5N1
H5N1 is the influenza variant that is currently spreading across the globe. This is not a new virus. The current form of the virus evolved from a virus first documented in 1996. Since that date, the virus has undergone many evolutionary changes, which have changed how deadly the virus is, and what species can be infected with the virus. Migrating birds have spread the virus worldwide, and the virus is now, for the first time, infecting many different mammals. The virus is no longer confined just to birds. In 1995, this virus first attacked the US, with outbreaks limited to poultry farms. That outbreak was stopped. But this more recent outbreak is now so widespread that no amount of human effort will likely eliminate it.
H5N1 in Dairy Cows
In March 2024, H5N1 was first detected in US dairy cows. Infection with the virus causes the cows to produce less milk, which directly reduces the dairy farmer’s income. Currently, this virus has been detected on over 1000 farms. The virus can easily spread from one farm to the next, hitchhiking on farm equipment, food trucks, and even veterinarian or farmer’s shoes and clothing.
Devastation in Poultry Farms
Wild birds and cows have spread this virus to poultry farms. The virus rapidly kills poultry, and, like dairy farms, the virus can quickly spread from one chicken farm to the next. Due to the highly contagious nature of the virus, farmers kill all the chickens on a farm once the virus is detected (depopulation). Many large farms have a great many chickens living in close quarters; this makes the spread of a virus easy. It is shocking that to date 169 million birds have been killed, in an attempt to stop this disease outbreak. Such a significant loss of life is hard to comprehend. So far, 1700 farms in the US have been infected with this disease. These events cause major economic damage to farmers, and spikes in egg prices are caused by the loss of so many egg laying birds.
Cross-Species Transmission:
Cats, Wildlife, and Humans
Cats also seem to be susceptible to the H5N1 virus. First cases in cats were seen on dairy farms; some barn cats tested positive for the virus, and some died. But other cats with no exposure to dairy barns have also come up positive. How did these cats become infected? Some cases have been linked to owners feeding their cats a raw diet. Other cats may become infected by killing wild infected birds and then eating them. The risk for outdoor cats is not accurately known, because it is unknown how widespread the virus is in wild birds. Keeping cats indoors to limit exposure to wild birds would be a prudent first step.
The spread of the H5N1 virus to wildlife has been widespread. Many species of birds and mammals have now contracted the virus. Some of the documented species are predatory birds (bald eagle, snowy owls, peregrine falcons), seals that frequent our coasts, nesting bird colonies, captive zoo animals, and urban wildlife (skunks and raccoons).
Even humans are not spared! Luckily the cases in people have usually been mild or even asymptomatic. Some farm workers and large animal veterinarians showed evidence of exposure, but they were unaware of their exposure to the virus. Only 70 cases in people have been confirmed. There has been one death. This situation is concerning, because the virus could change into a form that is more dangerous or more contagious, setting off a pandemic.
Containment Efforts and Precautions
Many efforts have been undertaken to try to control the spread of this virus. Limiting exposure of domestic birds to wild birds is very important. Also increasing biosecurity on farms can significantly limit the spread of the disease from one farm to another. Unfortunately, there is no strategy to eliminate the virus from wild birds and mammals.
As already noted, this is an evolving story. Be safe: drink pasteurized milk, limit the spread of the virus from dairy and poultry farms, don’t feed your pets a raw diet, keep cats away from wild birds, and stay tuned for the rest of the story.
