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Significance of Backyard Flocks in HPAI Outbreak in France 2016-2017


The role of backyard flocks in the incidence and dissemination of highly pathogenic avian influenza (HPAI) was investigated subsequent to the extensive outbreak of H5N8 clade in Southwestern France. Approximately 480 farms were affected in a limited geographic area from late November 2016 to late March 2017. The initial case was presumed to have been infected by migratory birds. The outbreak was characterized by a low incidence rate in backyard flocks compared to commercial farms, reminiscent of the 2015 HPAI epornitic in the U.S.

The study involved 70 backyard flocks in the Gers Department. This administrative region reported 20 percent of the cases recorded with 55 percent of 96 cases diagnosed between December 11th 2016 and January 4th 2017. A retrospective serologic survey was carried out on

70 backyard flocks comprising 608 birds. Of all backyard flocks evaluated, 26 percent were seropositive for AI with 11 percent demonstrating antibodies against subtype H5. Backyard ducks showed a 41 percent seroprevalence against AI and 27 percent specifically against subtype H5. This may be compared to backyard flocks holding only chickens yielding a 19 percent seropositive rate against AI and with 4 percent showing antibodies against H5.

The authors noted that backyard flocks with a connection to commercial farms were likely to be infected suggesting the role of humans in transmission. The odds ratio associated with a link with commercial poultry was 5.8 for avian influenza and 20.5 for subtype H5, denoting deficiencies in biosecurity on commercial farms. The findings in France correspond with observations made in the Netherlands following an H7N7 epornitic. Following this outbreak it was demonstrated that infection in backyard flocks was essentially a spillover from commercial units.

The results in France should be related to the current situations involving velogenic viscerotropic Newcastle disease in Southern California. In the case of backyard flocks in France and the Netherlands, a limited number of birds in each location had minimal indirect contact with commercial poultry and represented a situation of minimal risk. In contrast in Southern California, the so-called "backyard" and "exhibition" flocks comprise game-fowl (fighting-cocks). These birds are by their purpose frequently moved and have direct contact with game-fowl from other locations. In addition there is frequent movement of owners and caretakers among and between flocks. This has contributed to the high incidence rate in San Bernardino and Riverside counties including four commercial units and extension from the three-county area to one case in each of Alameda County in Northern California and to Utah County, Utah.

Souvestre, M. et al Role of backyard flocks in transmission dynamics of highly pathogenic avian influenza A(H5N8) clade, France, 2016-2017 Emerging Infectious Diseases 25: 551-554 (2019)