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subtypes resulted in histologically demonstrated mild pneumonia, although clinical signs were not evident Cooley et al., 1989. On the other hand, experimental infection of com-
mon starlings and house sparrows with an H7N7 influenza virus isolate from Australian chickens caused high mortality 100 in starlings and 30 in house sparrows and trans-
mission to contact birds, although it caused no mortality in ducks Nestorowicz et al., 1987. In Italy, highly pathogenic avian influenza A HPAI strain H7N1 was isolated
from two synanthropic birds a house sparrow and a collared dove, Streptopelia decaocto found dead, close to a chicken farm with an HPAI epizootic Capua et al., 2000. In addi-
tion to HPAI viruses H5 and H7, LPAI strains circulate at a much higher frequency in such wild waterfowl as mallards Anas platyrhynchos, and could convert to HPAI after
being transmitted to poultry Munster et al., 2005.
South-east Asia is regarded as a region where influenza viruses co-circulate in urban and rural environments among people, domestic pigs and ducks, which provides the opportu-
nity for interspecies transmission and genetic exchange among viruses Webster et al., 1992. An extensive HPAI epornitic an outbreak of disease in a bird population, caused
by an H5N1 strain, started in domestic poultry in South-East Asia in 1997 and has raised special concern, because of its very rapid spread across Eurasia in 2005 and 2006 Perdue
Swayne, 2005; WHO Global Influenza Program Surveillance Network, 2005. Some wild urban and suburban birds have also been involved – for example, feral pigeons and
tree sparrows Passer montanus died from HPAI in China, Hong Kong SAR, Thailand and the Russian Federation Ellis et al., 2004; Brown et al., 2005; FAO Avian Influenza
Task Force, 2005; Morris Jackson, 2005; Nguyen et al., 2005.
HPAI is mainly a veterinary and economic problem, and bird-to-human transmissions have been only sporadically reported: 271 human cases 165 of them fatal have been
confirmed in some countries in Asia and Africa but, as of 3 February 2007, no cases have been confirmed in Europe or North America WH O, 2007. Obviously, people can
become infected with HPAI only under very special circumstances: when they inhale or ingest a massive dose of virus, resulting from very intimate contact with infected domes-
tic birds. Moreover, no human HPAI infection acquired from a wild bird has been des- cribed before April 2006. In brief, free-living urban birds do not present a risk of infec-
ting people with HPAI.
8.2.2. Bacteria
8.2.2.1. Chlamydiaceae
Chlamydophila psittaci causes ornithosis chlamydiosis, also known as psittacosis in gulls,
pigeons, passerines and other birds, especially young ones. Adult birds are more resistant to the disease, but they can carry and intermittently shed the infectious agent for months
Roberts Grimes, 1978. Clinical symptoms in birds are extremely variable, ranging from infection without symptoms to death caused by septicaemia. Some avian species
may remain serologically negative despite active chlamydiosis. The importance of chla- mydiosis to the population dynamics of wild birds is probably underrated. Epizootic epi-
sodes of disease – that is, the rapid spread among animals – with high mortality have been described; such episodes have occurred in gulls in North America Brand, 1989.
rion crows in Egypt Work, Hurlbut Taylor, 1953; Hurlbut et al., 1956, black-headed gulls Larus ridibundus in Slovakia Greˇsíková, Sekeyová Prazniaková, 1975; Ernek et
al., 1977, rooks Corvus frugilegus in Ukraine, carrion crows in the southern part of the European part of Russia, American crows Corvus brachyrhynchos in North America,
and the common blackbird Turdus merula in Azerbaijan Gaidamovich Sokhey, 1973; Lvov Ilyichev, 1979; Vinograd et al., 1982; Eidson et al., 2001. WNV sometimes cau-
ses a clinically manifest disease in feral pigeons and other free-living birds, with an occa- sional death H urlbut et al., 1956. Nevertheless, mass mortality of corvids mostly
American crows and other birds has been observed during recent outbreaks of West Nile fever in North America, due to a high virulence of the virus strain for birds; the
dying crows have therefore been used to monitor the epidemiological situation in urban areas of the United States Eidson et al., 2001; Komar et al., 2003; Caffrey, Smith
Weston, 2005, and they probably contributed to the rapid east–west spread of WNV across North America since 2001. However, no WNV infections of people have been
reported as attributable to, or directly associated with, urban birds.
8.2.1.2.3. Flaviviruses of the tick-borne encephalitis complex The principal hosts of tick-borne encephalitis TBE complex viruses are forest rodents,
while birds are occasional hosts and disseminators of preimaginal larval and nymphal stages of ixodid ticks infected with T BE viruses. One of these viruses, the central
European encephalitis virus CEEV, was isolated occasionally from a few synanthropic birds – for example, the common blackbird and the song thrush Turdus philomelos in
Finland Brummer-Korvenkontio et al., 1973 and from common blackbirds and the great tit Parus major in the eastern Baltic region Lvov Ilyichev, 1979. Two CEEV
strains were recovered from nymphal Ixodesricinus castor-bean ticks collected on com- mon blackbirds in Slovakia Ernek et al., 1968. Most bird species are resistant to CEEV,
including synanthropic house sparrows and great tits Greˇsíková Ernek, 1965; certain species, however, have been observed to develop viraemia that lasted several days after
experimental infection with CEEV: common starling, song thrush, common blackbird Saikku, 1973; Lvov Ilyichev, 1979. Morbidity, mortality, and shedding of TBE viru-
ses have been observed occasionally in experiments with the house sparrow Lvov Ilyichev, 1979. No human cases of TBE, however, have been reported as attributable to,
or directly associated with, urban birds.
8.2.1.3. Orthomyxoviridae: genus Orthomyxovirus
Influenza A virus has often been isolated from wild birds worldwide, mainly from ducks, gulls, terns, shearwaters and shorebirds, and less often from passerines Lvov Ilyichev,
1979; Wobeser, 1997; Lipkind, Shihmanter Shoham, 1982; H inshaw et al., 1985; Stallknecht Shane, 1988; Webster et al., 1992; Munster et al., 2005, most strains being
so-called low pathogenic avian influenza LPAI viruses. Wild aquatic birds are the pri- mordial source and reservoir of all influenza viruses for other vertebrates Hinshaw et al.,
1981, and these birds perpetuate all known antigenic subtypes of influenza A viruses H1–H16 and N1–N9. In wild ducks, influenza viruses replicate in the cells of the intes-
tinal tract and are excreted in high concentrations in the faeces; the virus shedding can continue for 2–4 weeks Slemons Easterday, 1977; Hinshaw et al., 1980. Experimental
infection of adult mallard ducks with the H5N2, H5N3 and H5N9 influenza A virus
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the song thrush in North America and Europe Anderson et al., 1986; Magnarelli, Stafford Bladen, 1992; H umair et al., 1993; H ubálek, Juˇricová H alouzka, 1995;
Hubálek et al., 1996; Hanincová et al., 2003, Stern et al., 2006. Tick larvae and nymphs have parasitized some species of birds as frequently as they have parasitized the white-
footed mouse Peromyscus leucopus, the principal reservoir host of B. burgdorferi s.s. in North America. Bird-feeding larval and nymphal deer ticks were removed from their
hosts, left to molt, and shown to transmit B. burgdorferi trans-stadially that is, from one stage of the life-cycle to the next into the adult stage Anderson, Magnarelli Stafford,
1990. In Switzerland, 16 of larval and 22 of nymphal castor-bean ticks collected from passerine birds were infected with borreliae; the highest infection rate was observed in
ticks removed from thrushes Turdidae Humair et al., 1993. In the Czech Republic, borreliae were detected in 7 of larval and 12 of nymphal castor-bean ticks collected
from wild birds; positive ticks were collected from synanthropic species: the common blackbird, the European robin Erithacus rubecula and the great tit. Three massively
infected ticks with hundreds of borreliae were collected from robins and blackbirds, and Borrelia garinii
was isolated from a nymphal tick, collected from a young blackbird Hubálek et al., 1996. A high rate of infection with borreliae mainly B. garinii and
Borrelia valaisiana was also found in preimaginal ticks, collected from Turdidae in
Slovakia Hanincová et al., 2003. Moreover, some synanthropic bird species are compe- tent amplifying hosts for B. burgdorferi s.l. – for instance, the common blackbird
Humair et al., 1998 and the American robin Richter et al., 2000; Ginsberg et al., 2005. However, no B. burgdorferi s.l. infections of people have been reported as attributable to,
or directly associated with, urban birds.
8.2.2.5. Campylobacteraceae
