Wild bird migration and the movement of Avian Influenza virus
Emily Martin, Jane Parmley
Animal Health Laboratory, University of Guelph, Guelph, ON (Martin); Department of Population Medicine, University of Guelph, Guelph, ON (Parmley).
AHL Newsletter 2022;26(2):14.
Avian Influenza (AI) is caused by type A influenza virus (family Orthomyxoviridae, genus Influenzavirus A) and is classified into subtypes by 2 surface proteins: hemagglutinin (H1-H16) and neuraminidase (N1-N9). As an RNA virus, errors are made during replication that cause the virus to change and evolve. Sometimes these errors are minor, but other times larger pieces of genetic material can be traded with another AIV virus, increasing the risk of the virus becoming highly pathogenic (causing severe disease in poultry) or able to infect mammals (i.e., swine, humans). Avian Influenza Viruses (AIV) can be grouped as low-pathogenic (LPAIV) or highly-pathogenic (HPAIV, H5 and H7 subtypes). In countries such as Canada where LPAIVs are not endemic and are considered a Foreign Animal Disease (FAD), flocks are destroyed when HPAIV is identified to prevent further spread of the virus. The low pathogenic subtypes of H5 and H7 are handled in the same manner as HPAIV, as these subtypes can transform into HPAI as the disease spreads through the flock(s). In order to manage the risk of poultry flocks, swine or humans contracting this virus, potential risk factors must be considered. One of these risk factors is the wild bird population.
AIV naturally infects wild birds, and most wild bird species usually do not show signs of infection. All known subtypes of AIV have been isolated from wild birds, and LPAIVs are the most common. The virus is carried and replicates in the intestinal and respiratory tracts, spreading via feces, saliva, and nasal secretions. Domestic poultry are highly susceptible to AIVs. If flocks are infected with LPAIV, the birds may have mild signs, but the virus is more likely to transforms into HPAIV. If a flock is infected with HPAIV, the birds tend to have severe clinical signs and high mortality. This has significant impacts including economic loss, international trade sanctions, and potential spread to humans and other livestock.
Since wild birds can carry AIV without clinical signs, they can potentially pass AIV to other wild birds, including during migration. There are 8 migratory pathways worldwide, and many of these overlap (Fig. 1). This creates the potential for AIV to emerge and evolve, as well as to move long distances by transmitting the virus to other migratory or non-migratory birds, including poultry. Previous research tracking H5N1 HPAIV between migratory bird flyways identified a correlation between wild bird migration and H5N1 HPAIV infections in people. This suggests that transmission is strongly associated with avian migration. Wild birds are tested annually in Canada for AIVs. Most wild bird samples are collected during the fall and spring migrations, with identification of AIVs being more likely during the fall migration and at higher latitudes. It is thought this pattern of identification occurs due to the larger number of naïve, juvenile birds in the population during the fall migration. As the juveniles fly south, they become infected, recover, and are less likely to be carrying the virus once they reach lower latitudes.
There are certain migratory birds, predominantly ducks, followed by geese and gulls which are mainly responsible for annual movement and evolution of AIV. The virus is stable and can survive in the environment for weeks to months. One study showed that AIV in duck feces remained infective for at least 30 days at 4 °C and for 7 days at 20 °C. The aquatic environment is also a potential source of infection for other birds or mammals. While research has provided a better understanding of how AIVs move over long distances (i.e., migration pathways), it is still not well understood how AIVs move from wild birds into commercial poultry populations housed in closed barns.
To track the movement of AIVs, the strains are compared by looking at their genetic structure in comparison with other isolates. There is a world influenza database (GISAID – Global Initiative on Sharing Avian Influenza Data) in which the genetic sequences of AIVs can be uploaded and compared to other AIVs. This database can be used to compare AIVs from the same and different geographic areas, to look for mutations, and to determine if there is movement into bird or mammalian populations (i.e., swine or humans). Part of this comparison consists of constructing a phylogenetic (family) tree of the viruses to see how they cluster into groups; viruses that are more closely related will cluster closer together. As more information is gathered, we can have a clearer picture of how AIVs move within and between species of birds and mammals, and how the viruses change over time.
It should be noted that when humans become infected with Influenza A (the same group of viruses as AIV), virus mutations or recombinations can occur, and birds or swine can then be infected with the virus from humans (and vice versa). That is why poultry workers are encouraged to have a ‘flu shot’ every year to protect themselves and their flocks.
It should also be remembered that AI is not the only FAD that can be transmitted by wild birds. Cormorants play an important role in the maintenance and circulation of Newcastle disease virus (Avian Paramyxovirus-1) in North America, which is also designated a Foreign Animal Disease in Canada. NDV was detected in cormorants in Ontario in 2021.
Please be aware that there is currently a H5N1 HPAI outbreak in North America that has been detected in multiple wild bird species, poultry, and mammals (wild foxes). The strain has been identified as the H5 Gs/GD (Goose Guangdong) Eurasian lineage of HPAI that started circulating in 1996, has genetically evolved over time, and has gained wide geographic dispersion due to asymptomatic infections in migratory aquatic birds. The current strain circulating in North America is causing disease in a wider variety of wild birds, including waterfowl (ducks, geese, swans), shorebirds (gulls), raptors (hawks, owls), and scavengers (turkey vultures). Multiple Canadian provinces have had multiple commercial poultry and small flock poultry identified with H5N1. Current recommendations are to keep domestic poultry, or any captive avian species (i.e., pigeons, pet birds), away from wild birds to prevent transmission of H5N1.
For more information, please refer to the following links:
Canadian Wildlife Health Cooperative (CWHC):
http://www.cwhc-rcsf.ca/avian_influenza.php
CWHC diagnosis of H5N1 in wild foxes:
Poultry Industry Council (Biosecurity and Disease resource page):
https://www.poultryindustrycouncil.ca/resources/biosecurity-and-disease
Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA):
http://www.omafra.gov.on.ca/english/livestock/vet/facts/avian_influenza.htm
Canadian Food Inspection Agency (CFIA):
AHL
Figure 1. The 8 migration pathways. (https://pacificbirds.org/birds-migration/the-flyways/)
References
1. Blagodatski A, et al. Avian Influenza in wild birds and poultry: Dissemination pathways, monitoring methods, and virus ecology. Pathogens 2021;10:630.
2. Diskin ER, et al. Subtype diversity of Influenza A virus in North American waterfowl: A multidecade study. J Virol 2020;94: https://doi.org/10.1128/JVI.02022-19
3. Karamendin K, Kydyrmanov A. Cormorants as a potentially important reservoir and carrier of Newcastle disease virus on the Asian continent. Front Vet Sci 2021;8:648091.
4. https://www.ioes.ucla.edu/project/avian-influenza-virus-north-american-migratory-birds/