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Highly pathogenic avian influenza H5N1 subtype in mute swans in Italy

SIR, – The mute swan (Cygnus olor) is a semi-sedentary, migratory species that is particularly common in Eurasia and Europe north of the 46° parallel. In Italy, the nesting population does not exceed 500 couples, but as an overwintering species it is relatively common. The overwintering birds migrating from eastern Europe generally prefer the wetlands in northern/central Italy. However, due to persistent low temperatures during the past winter, some individuals reached the coastal regions of southern Italy. The first cases of dead birds or of birds with suspect clinical signs were reported simultaneously from three southern regions, Calabria, Sicily and Puglia, to the Italian Reference Laboratory in the first week of February. The dead or sick birds were part of small flocks of 10 to 20 birds that included several young individuals. The general condition of the majority of the birds was poor, as is normally seen at the end of the winter season. Ill birds exhibited nervous signs such as somnolence, incoordination and ataxia. Other clinical signs observed were greenish diarrhoea and conjunctivitis. Gross pathology in dead or euthanased birds included hyperaemia and haemorrhage of the mucosal surface of the proventriculus and duodenum, airsacculitis, catarrhal rhinosinusitis and serohaemorrhagic tracheobronchitis. In total, the presence of the highly pathogenic H5N1 virus was confirmed in 16 swans all originating from the south of Italy. Although the birds did not have any identification tags, migration routes indicate that they most probably originated from the Black Sea. Phylogenetic analysis of the isolates from the Italian swans revealed a very high homology to other highly pathogenic avian influenza H5N1 isolates from wild birds in Asia and Europe (Table 1). Information is lacking on the incubation period, the duration and extent of shedding and other aspects of infection, which makes any forecast of the role of these birds in avian influenza epidemiology impossible. The greatest concern is that infection could have spread to resident wild bird populations, generating an endemic cycle. Increased surveillance in wild bird populations across Europe is crucial to the early detection of infection, which is a prerequisite to the prevention of infection spreading to rural and intensively reared poultry.