Pestology Blog

Understanding the ways that we prevent disease is important in our line of work. But, much like understanding the biology and behavior of the pest can contribute to the success of your management strategy, understanding the ways disease can spread can be just as important when discussing prevention.  

H5N1, more commonly known Highly Pathogenic Avian Influenza recently came to the forefront in North America in 2024 as it quickly spread through not only poultry farms and dairy farms, but also featured at least 71 human cases in the United States as well. The researchers of this paper sought to understand how this strain of avian influenza spread across North America. They did so by taking data from positive samples collected by USDA APHIS across November 2021-September 2023 and examined the genomes of the positive viral samples from domestic birds, wild birds, and mammals. Specifically, the researchers compared the DNA make-up of the protein hemagglutinin in these samples.  By comparing the DNA that codes from the protein hemagglutinin, the researchers could trace the path of how and when this virus likely spread by tracking those changes to the genetic make-up of the protein over time. Hemagglutinin is a great candidate for examining how a virus spreads as it is the first protein that binds with a host’s red blood cells and represents the first step in the infection process. By using this genetic backtracking known as viral phylodynamics, researchers can understand and pinpoint how and why these outbreaks occur- and more importantly, where these viruses may go next. 

Photo Credit: NIAID

First, most of the North American sequences they examined descended from a single introduction that was likely from migratory gulls coming in from Europe in late 2021. However, they were also able to track additional introductions from both Asia and Europe for an estimated total of at least nine introductions across 2021-2023. Repeated introductions mean a larger opportunity for this virus to spread across populations. 

Similarly, the researchers were also able to document similar genetic strains of the virus across the four migratory pathways across North America. This suggests that not only can we track the potential movement of the virus, but we can also show that these wild migratory bird pathways are extremely important in the dissemination of the virus. This is also supported by the fact that case detections of avian influenza peaked in the fall and the spring, which coincides with migration timing. In terms of the main suspects and drivers of this viral spread a-la-migration, unsurprisingly the order Anseriformes were the main suspects. This order consists of the waterfowl, such as ducks, geese, swans, and others, and are known for their migratory patterns. However, in terms of spreading the virus to other species, shorebirds as well as the order Galliformes, which includes species like chickens, turkeys, pheasants, and quail were the top suspects. 

As a result, documented outbreaks in domestic birds appeared to be driven by repeated introductions from wild birds, with the estimates suggesting that there were at least 106 introductions from wild birds into domestic birds, and 4 introductions from domestic birds to wild birds.  

Lastly, the researchers examined the likelihood of “backyard birds” to become infected compared to commercial birds. With the increase in popularity in home-raised domesticated poultry, this becomes an important avenue to consider for infection of avian influenza from wild birds to domesticated birds. From examining the genetics of the positive samples collected, researchers were able to document not only that backyard birds were infected earlier on average compared to commercial birds, but also featured a higher variety of introductions, suggesting that they are encountering wild birds more than commercial birds.  

Wild birds, particularly waterfowl, played an important role in the recent Highly Pathogenic Avian Influenza outbreak. This group, along with commercial birds, are both groups that pest management professionals may encounter in their regular work. Therefore, understanding the ways in which a disease may spread through these groups can be the first line of defense in our role of protecting public health as well as the safety of our industry.