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In southwestern France, during the winter of 2016–2017, the rapid spread of highly pathogenic avian influenza H5N8 outbreaks despite the implementation of routine control measures, raised the question about the potential role of airborne transmission in viral spread. As a first step to investigate the plausibility of that transmission, air samples were collected inside, outside and downwind from infected duck and chicken facilities. H5 avian influenza virus RNA was detected in all samples collected inside poultry houses, at external exhaust fans and at 5 m distance from poultry houses. For three of the five flocks studied, in the sample collected at 50–110 m distance, viral genomic RNA was detected. The measured viral air concentrations ranged between 4.3 and 6.4 log10 RNA copies per m3, and their geometric mean decreased from external exhaust fans to the downwind measurement point. These findings are in accordance with the possibility of airborne transmission and question the procedures for outbreak depopulation.

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I nfluenza A viruses are enveloped viruses of the Alphainfluenzavirus genus in the Orthomyxoviridae family. Their negative-stranded RNA genome consists of 8 segments encoding a total of 10-14 proteins. Avian influenza viruses (AIVs) are classified on the basis of antigenic differences in their surface glycoproteins, hemagglutinin (H1-H16) and neuraminidase (N1-N9) (1). H5 and H7 subtypes can become highly pathogenic avian influenza (HPAI) viruses after the evolution of multiple basic amino acids in the cleavage site of hemagglutinin protein (2,3). This mutation enables the virus to replicate efficiently in all organs, causing a severe and often fatal systemic disease. In contrast, the cleavage site of hemagglutinin in low pathogenicity AIVs lacks these multiple amino acids, restricting viral replication to the respiratory and digestive tracts. Low pathogenicity AIVs cause subclinical or mild disease that can be aggravated by secondary infections (4,5). Because H5 and H7 AIVs can evolve to be highly pathogenic, the diseases caused by these subtypes are notifiable to national and international bodies (6). Since 1996, highly pathogenic H5 viruses of the A/goose/Guangdong/1/96 (Gs/GD/96) lineage have caused recurrent outbreaks with high death rates in birds. These HPAIs are categorized into 10 distinct clades (0-9) on the basis of hemagglutinin sequences (7). These clades are found in Asia; a few have spread to Africa, Europe, and North America (8-10). Europe experienced major introductions of H5N1 of clade 2.2 during 2005-2007 and H5N8 of clade 2.3.4.4 during 2014-2020 (11-14). Many reassortments were observed on Gs/Gd/1/96-like viruses, especially within clade 2.3.4.4. The reassortments generated several subtypes including H5N1, H5N2, H5N5, H5N6, and H5N8 (11,15-17). During winter 2016-17, twenty-nine countries in Europe reported 1,576 cases of Gs/Gd/1/96like H5N8 infections in wild birds and 1,134 in poultry, especially domestic ducks (18). During this outbreak, researchers identified 6 HPAI A(H5N8) genotypes in Europe; 2 of these genotypes were identified using 6 sequences from infected birds in France (19). France had 539 cases of HPAI A(H5N8) infections, 51 in wild birds and 488 in poultry flocks, most of which occurred at duck farms producing foie gras (18). A previous study used spatiotemporal analysis of clinical cases comprising 2 distinct epizootic periods in southwestern France (20). The first period spanned November 28, 2016-February 2, 2017 and comprised 4 spatiotemporal clusters (20). The second period spanned February 3-March 23, 2017 and comprised a single spatiotemporal cluster (20).

In autumn/winter 2016–2017, HPAI-H5N8 viruses belonging to the A/goose/Guandong/1/1996 (Gs/Gd) lineage, clade 2.3.4.4b, were responsible for outbreaks in domestic poultry in Europe, and veterinarians were requested to reinforce surveillance of pigs bred in HPAI-H5Nx confirmed mixed herds. In this context, ten pig herds were visited in southwestern France from December 2016 to May 2017 and serological analyses for influenza A virus (IAV) infections were carried out by ELISA and hemagglutination inhibition assays. In one herd, one backyard pig was shown to have produced antibodies directed against a virus bearing a H5 from clade 2.3.4.4b, suggesting it would have been infected naturally after close contact with HPAI-H5N8 contaminated domestic ducks. Whereas pigs and other mammals, including humans, may have limited sensitivity to HPAI-H5 clade 2.3.4.4b, this information recalls the importance of implementing appropriate biosecurity measures in pig and poultry farms to avoid IAV interspecies transmission, a prerequisite for co-infections and subsequent emergence of new viral genotypes whose impact on both animal and human health cannot be predicted.