SIR, – Avian influenza viruses are known to circulate mostly among migratory waterfowl (ducks, geese), gulls and shore-birds, which act as reservoirs of the virus (Webster and others 1992). Although the vast majority of strains have been recovered from ducks, influenza viruses have been isolated from many other wild birds including starlings, pigeons, sparrows, crows and swallows (Stallknecht and Shane 1988). We would like to report the isolation of an influenza virus in our laboratory in Pavia from the tissues of a saker falcon (Falco cherrug) owned by a falconer living near Milan, northern Italy. The raptor, a 10-year-old male, presented with a sudden onset of depression, weakness and anorexia the day after normal hunting activity and died two days later without further clinical signs. On postmortem examination, copious clear fluid distended the pericardial sac and the abdomen, and the small intestine was congested and filled with melena. No other gross lesions were observed. Coccidiosis was ruled out by microscopic observation of intestinal mucosa smears, and bacteriological cultures of the intestine, liver and pericardial fluid resulted in a few colonies of Escherichia coli from the intestine only. Virological investigations included inoculation of chicken embryonated eggs with pooled liver and brain of the falcon, resulting in death 48 hours later. A haemagglutinating agent was isolated and subsequently identified as an influenza virus by a haemagglutination inhibition (hi) test performed with anti-avian influenza virus sera. The virus – which has been designated pv 2476 – was also detected in the allantoic fluid of the embryonated eggs by an elisa targeting the viral nucleoprotein (Siebinga and de Boer 1988) and then identified as serotype h7 by a hi test performed with monospecific antisera. An intravenous pathogenicity index (ivpi) was calculated by inoculating six-week-old chickens, and the virus was classified as highly pathogenic (ivpi = 3·00). The virus isolation was reported to veterinary officials and restrictive measures were adopted according to the Directive 92/40/eec (cec 1992). Antibodies to influenza viruses were not detected in blood samples collected from three other falcons reared in the same facilities; cloacal swabs and faeces were also collected and tested negative. The susceptibility of falcons to an agent that most probably was an influenza virus has been reported in a paper published in 1903 by the Italian researchers Maggiora and Valenti; however, that paper dealt only with the experimental transmission of the infection to several species of birds, including falcons (E. F. Kaleta, personal communication). In addition, although circumstantial evidence suggests that the transmitted agent was an influenza A virus, its identity could not be verified at that time, nor has the agent been preserved in the laboratory for further studies. More recently, a highly pathogenic avian influenza virus of h7n3 subtype has been isolated from a peregrine falcon in the United Arab Emirates (D. J. Alexander, personal communication). Since raptors have occasionally been found to be infected with Newcastle disease virus as a consequence of their predation activity (Graham and Halliwell 1986), their natural infection with an influenza virus could also be related to the ingestion of a prey. As for the clinical signs and mortality, the old age of the bird in this case and the stress related to hunting may have contributed to the onset of disease. Since an epidemic of highly pathogenic avian influenza has recently spread and caused serious losses in turkey and chicken flocks reared in northern Italy (Capua and others 2000), there is a need to clarify the possible epidemiological relation between the outbreaks in domestic poultry and this case of fatal disease in the falcon. Further tests, including the molecular characterisation of the isolate, are currently in progress.
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Magnino et al. (2000) studied this question.