Avian influenza (AI) has reemerged in the United States and presents a direct threat to the poultry industry. However, there are ways to protect flocks from exposure. Many of the recommended actions are founded on an understanding of how the disease is transmitted.

Dr. Algis Martinez | Courtesy Cobb-Vantress

Previous highly pathogenic outbreaks of AI (HPAI) were thought to be the result of flock exposure to a lowly pathogenic strain of AI (LPAI) and during the infection process the virus mutated to the more detrimental HPAI strain. The HPAI was then transferred to other farms through breaches in biosecurity appearing as clusters of farm infections in a region. This mode of infection and transmission is still valid. 

The current strain of HPAI infection appears to have adapted to wild birds. Surveillance of migratory bird mortalities has detected the HPAI Eurasia H5N1 in U.S. wild bird populations. Wild birds with HPAI symptoms have been found along the east coast from northern Canada all the way to Florida. Surveillance also detected H5N1 positive wild birds in areas around the Ohio, Mississippi, and Missouri Rivers. Likewise, sporadic and clustered infections of back yard, hobby, and commercial poultry flocks across the Midwest have been reported. 

Wild bird migration routes across North America are generally shaped by geography, forming 4 flyways (Atlantic, Mississippi, Central and Pacific). The increasing day length with the onset of spring stimulates the migration of birds in the southern states to the far northern regions of North America. Similar migration events occur simultaneously in Europe as birds migrate within the East Atlantic Flyway that extends from southwest Africa northward through Europe and includes Central Siberia, Greenland, and Canada. This creates the opportunity for disease to migrate from one continent to another and may explain the presence of Eurasia H5N1 in wild bird populations across the US east coast in 2022. 

There is concern that AI will further spread in wild bird populations. One area of significant wild bird concentration is the Artic Refuge located in northern Alaska. It is here that 200 species of wild migratory birds from all 4 US flyways along with the East Asian-Australasian and West Pacific Flyway come together to build nesting sites. Long days support the growth of forage and insect populations necessary to feed offspring. The onset of winter drives birds south to warmer environments and forage. The accumulation of wild birds in the Artic Refuge allows transmission of both LPAI and HPAI within and across species. It is expected that winter migration will cause an increase in AI during the fall.

Figure 1. Illustration of the four wild bird flyways across the United States and the location of high poultry producing states. (Courtesy Cobb-Vantress)

When considering an effective biosecurity program, it is important to understand the species and how it moves. 

(1) Be cautious of waterfowl and shorebirds that are susceptible to influenza viruses but may be asymptomatic carriers or birds experiencing minor symptoms. This has been observed in wild birds across the Atlantic and Mississippi flyways.   

(2) Be aware of bird species that are abundant and widespread. The risk of infection comes from the opportunity for contact.  

(3) Birds that are migratory or nomadic serve as viral vectors. Resident waterfowl (birds that do not migrate) can be infected by interacting with migrating waterfowl. Since resident fowl stay in a defined location, they will continue to shed virus that can then be transmitted through fecal contamination of shoes and clothing to commercial poultry and back yard flocks.

(4) Be aware of bird species that cluster together to form high density flocks. Clustering may be a daily or seasonal event that increases the opportunity for bird-to-bird transmission within and across species. Note that there are times of the year where waterfowl will cluster during periods not associated with migration. It is not unusual for waterfowl to group together during the post-breeding, flightless molt period. 

(5) The opportunity for direct contact with feces of infected waterfowl is greater for free ranging, mixed flocks. It is possible that an intermediate vector, such as songbirds, can become infected and transmit it to poultry by food and water sources. Since songbirds can carry H5N1, eliminate nesting sites in outdoor and commercial poultry facilities. Likewise, it is recommended that outdoor flocks be confined to indoors during periods of AI movement.     

(6) H5N1 is often spread through droplets and therefore easily transmitted in water. Assume that water visited by waterfowl is contaminated with AI or other disease-causing agents. Never use open water (stream, river, pond, or lake) as a water source for your birds. 

Mixed species of waterfowl may be present in the same water source which can facilitate inter-species transmission of avian influenza. (Courtesy Cobb-Vantress)

Fortunately, AI is vulnerable to cleaners and disinfectants as well as good biosecurity practices. For example, H5N1 is easily killed by acidic (pH < 5.5) or alkaline (pH > 8.0) conditions and disinfectants are very effective against H5N1.    

Environmental conditions associated with temperature and humidity are important for AI survival. The virus survives best under cool, dry conditions and longer in soil and feces than on hard surfaces. 

Considering the mode of infection in a biosecurity plan can improve protection. Following the basics will help prevent exposure to AI.

1. The most important step is to limit contact with outside birds. 

2. The chances of AI transmission from wild birds are minimized by physical barriers.

3. Everyone and everything that passes through the farm gate must be cleaned and disinfected. 

 4. Once on the clean side of the farm, it is important to remain clean. 

5. Each house should be treated separately. 

6. Consider materials that must be brought onto the farm.

There is a great deal of planning that must go into developing a comprehensive biosecurity plan. Unfortunately, even the best plans fail without commitment from all team members. Sometimes plans fail because they are too complicated and unreasonable. Sometimes the plan is too expensive to maintain. Often the plan fails because team members are in a hurry or impatient with the process. For these reasons, create a program that works for the farm and team members.

Algis Martinez, DVM, is an American College of Poultry Veterinarians Diplomate and a member of the Cobb-Vantress Global Veterinary Services team.