Long-term Monitoring for Avian Influenza Viruses in Wild Bird Species in Italy

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  • Veterinary Research Communications, 27 Suppl. 1 (2003) 107114 2003 Kluwer Academic Publishers. Printed in the Netherlands

    Long-term Monitoring for Avian Inuenza Viruses in Wild Bird

    Species in Italy

    M.A. De Marco1*, E. Foni2, L. Campitelli3, E. Rani4, M. Delogu5 and I. Donatelli31Istituto Nazionale per la Fauna Selvatica, Ozzano Emilia (BO); 2IstitutoZooprolattico Sperimentale della L ombardia e dellEmilia Romagna, Parma;3Department of V irology, Istituto Superiore di Sanita`, Rome; 4Istituto ZooprolatticoSperimentale della L ombardia e dellEmilia Romagna, L ugo (RA); 5Department ofPublic Health and Animal Pathology, Faculty of Veterinary Medicine, University ofBologna, Italy*Correspondence: Istituto Nazionale per la Fauna Selvatica A. Ghigi , 9 V ia CaFornacetta, 40064 Ozzano Emilia (BO), ItalyE-mail: infs.adem@iperbole.bologna.it

    Keywords: avian inuenza, Italy, serological surveys, virological surveys, wild birds

    Abbreviations: HA, hemagglutinin; HI, haemagglutination inhibition; HPAI, highly pathogenic avianinuenza; IVPI, intravenous pathogenicity index; LPAI, low-pathogenicity avian inuenza; N, neura-mindase; NP-ELISA, nucleoprotein ELISA


    The rst reported isolation of an inuenza virus from wild birds was obtained in 1961from common terns (Sterna hirundo) in South Africa, where a highly pathogenic avianinuenza (HPAI) virus (H5N3 subtype) caused the death of about 1300 birds(Stallknecht and Shane, 1988). Although evidence for the infection of wild birds withinuenza viruses existed prior to the 1970s, it was not until then that systematicsurveillance studies revealed the widespread distribution of inuenza viruses in wildavian populations (Webster et al., 1992). To date a vast number of low pathogenicityavian inuenza (LPAI) viruses have been isolated from a wide variety of birds. TheHPAI viruses have rarely been isolated from feral birds, and when they have it hasusually been from wild birds found dead in the vicinity of farms infected by HPAIviruses (Capua et al., 2000). The above-mentioned mass mortality among commonterns represents the only case reported to date of inuenza infection in free-livingbirds that is associated with obvious signs of disease and is unrelated to outbreaks ofHPAI viruses in domestic birds (Alexander, 2000).Aquatic birds are considered to be the main reservoir of inuenza A viruses in

    nature, harbouring in their populations viruses of all 15 (H1H15) haemagglutinin(HA) and all 9 (N1N9) neuraminidase (NA) subtypes, with almost every possiblecombination of antigenic subtypes. In particular, among wild waterfowl, ducks are ofparamount importance for the ecology of this infection, providing, by various mecha-nisms, most of the gene pool that is the source of all inuenza viruses both in human


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    and in animal populations (Webster et al., 1992). Wild waterfowl are often consideredto be the primary source of poultry outbreaks H5 or H7 LPAI viruses circulating inwild populations could infect poultry reared in semi-wild conditions. The kind ofbreeding, such as that where poultry species are kept in bird-proof connement,provides strategies for the prevention of the spread of infection by wild birds(Alexander, 2000).


    Numerous studies suggest the important role of waterfowl in the ecology of avianinuenza viruses; therefore, in 1992 we initiated a long-term monitoring programmeto evaluate the presence and diusion of inuenza viruses in waterfowl caught forringing purposes in two neighbouring wildlife refuges (Laguna di Orbetello andLago di Burano WWFOases) located on the west coast of Italy, about 140 km northof Rome. Both represent important wintering sites for migratory and resident water-fowl; the main breeding sites of these migrant populations are wetlands in central andnorth-eastern Europe. Serum samples and cloacal swabs were collected from ducksand coots during consecutive wintering periods, when numerous ocks of these birdswere concentrated in a few clustered sites. These studies were performed to evaluatethe circulation of avian inuenza viruses in wild reservoirs in the Mediterranean area,and to investigate possible epidemiological relationships with inuenza in poultry.The main serological and virological results are described below.

    Serological results

    In order to detect antibodies against type A inuenza virus, serum samples weretested using a double antibody sandwich ELISA, nucleoprotein-ELISA (NP-ELISA).The haemagglutination-inhibition (HI) assay was performed to assess the presence ofantibodies against dierent H subtypes.The NP-ELISA (De Marco et al., 1996) was used to test 1040 serum samples

    obtained from 591 ducks and 449 coots caught during six wintering periods, from1992 through 1998. The bird age was identied whenever possible. According to themain biological features of the avian species, birds were divided into four groups:(1) mallards (Anas platyrhynchos), 407 specimens (this species is employed in heavyrestocking activities; at least 71 400 birds are reared and released in hunting groundseach year in Italy); (2) other dabbling ducks (including Tadorna tadorna, Anas penel-ope, Anas crecca, Anas acuta), 70 specimens; (3) diving ducks (including Aythya ferina,Aythya nyroca, Aythya fuligula), 114 specimens; (4) coots (Fulica atra), 449 specimens.The overall seroprevalences detected for the four bird groups were 63.9% (mallards);31.4% (other dabbling ducks); 22.8% (diving ducks); 7.1% (coots). Figure 1 shows the

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    Figure 1. Seroprevalences of inuenza A viruses detected in free-living waterfowl (1040 speci-mens) caught in Italy between 1992 and 1998

    seroprevalences observed during the study period. Statistical analysis (De Marcoet al., 2000) showed that mallards were more positive than the two other duck groupsand that ducks in general were more positive than coots. No age group dierenceswere noted among ducks; however, the seroprevalence was higher for adult cootsthan for the juvenile ones. The size of the coot wintering population is similar to thatof the duck population; nevertheless, the seroprevalence is lower and more adultsthan juveniles are positive. These results indicate a very slow spread of inuenzainfection in the sampled coots.Using the NP-ELISA as a screening assay, the type A-positive serum samples,

    when available, were tested by HI (Anonymous, 1982; Alexander, 1989). The HI testwas performed to evaluate the inuenza virus subtypes circulating in duck and cootpopulations and, in particular, the occurrence of H5 and H7 subtypes that couldrepresent risk factors for poultry. Fourteen reference strains of avian inuenza viruseswere used in the HI test (from H1 to H14 subtypes). Serum samples were also testedusing the LPAI virus A/Turkey/Italy/6423-1/99 (H7N1). Figure 2 shows the percen-tage of HI-seropositive birds. These percentage values were calculated on NP-ELISA-seropositive ducks (278 specimens) and coots (24 specimens). From 1992 through1998, the ducks sampled showed HI antibodies against most of the H subtypes, exceptfor the H3, H4, H7 and H12 antigenic subtypes. In contrast, the coots were seroposi-tive against the H10 subtype only.

    V irological results

    In order to evaluate the circulation of avian inuenza viruses in wild reservoirs,detailed virological studies were carried out in North America (Hinshaw et al., 1985)and northern Europe (Suss et al., 1994).

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    Figure 2. Percentage of HI-seropositive birds. The percentage values were calculated onNP-ELISA-seropositive ducks (278 specimens) and coots (24 specimens) caught in Italybetween 1992 and 1998. Fourteen reference strains (from H1 to H14) were used in the HI test.Serum samples were also tested using the H7N1 LPAI virus A/Turkey/Italy/64231/99 (H7*)

    To study the epidemiology of avian inuenza in a Mediterranean area, between1993 and 1998, a total of 802 cloacal swabs were collected, concomitantly with bloodsamples, from 469 ducks and 333 coots. Virus isolation and characterization wereperformed as described (Anonymous, 1982; De Marco et al., 2000). A total of 22viruses were isolated, as shown in Table I. The results obtained showed the importantrole in these Italian wetlands of ducks in the inuenza ecology. The low overallisolation prevalence observed in ducks (18/469) showed only a few individuals har-bouring the virus during the winter period, whereas the high seroprevalences observedby the NP-ELISA in the absence of age group dierences, indicated that the presentstudies were performed after a very rapid spread of the infection, occurring towards

    TABLE IAvian inuenza viruses isolated from waterfowl wintering in Italybetween 1993 and 1998

    Bird species Inuenza virus subtype No. isolates

    Fulica atra H3N8 1H10N8 2H1N1 1

    Anas platyrhynchos H5N2 1H1N1 16

    Tadorna tadorna H1N1 1

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    the end of the breeding season, when the susceptible population is mainly representedby newly hatched ducks.


    In addition to the main research activities performed on waterfowl, we carried outepidemiological surveys on other wild bird species.


    The limited number of cases of inuenza in free-living Galliformes suggests thedynamics of a self-limiting infection within these populations (Stallknecht and Shane,1988). Nevertheless, the fact that the pheasant is resistant to some inuenza virusstrains that are pathogenic for poultry (Wood et al., 1985), makes this species a riskfactor for the spread of infection. During February 1992 and February 1994, 194 and185 serum samples, respectively, were collected from free-living pheasants (Phasianuscolchicus) caught in an area of the Po Valley. Furthermore, since epidemiological dataon free-living quails (Coturnix coturnix


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