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Significance of “Backyard” Flocks in the Epidemiology of HPAI


Following the editorial on the possibility of vaccination against avian influenza, Dr. Donald Reynolds of the University of Nebraska insightfully raised the issue of “backyard” flocks and their epidemiologic significance.


USDA has recorded 247 outbreaks of H5N1 strain HPAI on commercial farms in 2022 compared to 305 cases in “backyard” flocks.  The number of commercial flocks affected is greater than during the 2015 epornitic but the number of “backyard” flocks has increased disproportionately. 


The first question is whether the profound increase in cases during 2022 compared to 2015 represents actual infections or increased surveillance given heightened concern with attendant publicity over HPAI coupled with the availability of diagnostic resources. The subjective conclusion from crude data collected to date suggests more widespread dissemination of H5N1 in 2022.


The second question concerns the definition of “backyard” flocks since location, diversity and purpose influences their epidemiologic significance.  “Backyard” flocks can be sub-classified as:-

  • Suburban flocks: Comprise up to 20 hens maintained as either companion birds or to supply eggs for a family - The significance of this category lies in the fact that they serve as sentinels for the presence of virus and are essentially end hosts.  They have little significance in the context of commercial outbreaks since most of these flocks are urban or suburban and remote from commercial farms.
  • Game fowl – This category could represent an important link between wild bird shedders of virus and commercial farms.  This is based on the fact that many game fowl flocks exceed 100 birds.  They are invariably allowed outside access with a complete absence of biosecurity.  The role of game fowl in perpetuation and dissemination of Exotic Newcastle disease (END) was demonstrated in the 2002 and 2019 outbreaks in California even though most birds were solidly vaccinated against END.  The danger relating to game fowl is that they are frequently moved for competition and traded over extensive distances.
  • Show birds: Represent a third category – Usually suburban but occasionally in rural       locations - Individual birds may be of high value and are held in relatively small flocks, frequently confined.  Owners of show birds are amenable to advice on biosecurity.  The expedient of cancelling shows limits movement of this category of fowl.
  • Multispecies flocks: Collections of galliform and anseriform birds of diverse species are frequently encountered in rural areas.  These birds are generally free ranging often with access to water and are susceptible to exposure to AI virus from migratory birds that cohabit with these flocks.


A number of observers have questioned why there have been more cases among “backyard” flocks in 2022 compared to 2015.  It is suggested that, based on the duration of outbreaks extending from February through the present, that H5N1 strain carrying Eurasian genes has been transferred from migratory waterfowl and sea birds to domestic wild birds.  Reports of mortality in raptors and corvids and possibly some passeriformes confirm species susceptibility but results of any structured survey on the shedding status of non-migratory birds have not been published.


EGG-NEWS has consistently urged APHIS to conduct structured surveys on HPAI infection over a range of wild birds.  Data made available during the spring of 2022 reflected hunter-killed migratory waterfowl.  From an epidemiologic perspective, it is critical to understand the dynamics of avian influenza in migratory and endemic species that may come into contact with commercial poultry.  Both the regional epidemiology of H5N1 infection and the molecular composition of viruses isolated from wild and commercial species requires evaluation to develop an appreciation of the extent and mechanisms of how the virus is disseminated.


If USDA-APHIS in cooperation with state and federal wildlife agencies cannot conduct these studies and to provide interim results, support should be provided to Land Grant universities to conduct the research.  The industry needs valid information in order to plan and execute modifications to biosecurity programs.  The interaction among migratory species, domestic birds and “backyard” flocks and extending to susceptible mammals is required, given the likely prospect of persistent or frequently reocurring infection.  Obviously, the approach to the control and prevention of H5N1 HPAI including any consideration of vaccination would be modified if it were recognized that the infection is no longer exotic but possibly is seasonally endemic or de facto endemic.