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Avian chlamydiosis (‘psittacosis’) is caused by the intracellular bacterium Chlamydia (previously Chlamydophila) psittaci (Andersen and Franson 2007). Birds are natural hosts of C psittaci, and a wide range of avian species are susceptible to infection (Kaleta and Taday 2003). C psittaci causes potentially severe zoonotic disease, and captive psittacines have most often been implicated as the source of infection in humans (Vanrompay and others 1995).
While C psittaci infection is prevalent in wild columbiforms in Britain (Bracewell and Bevan 1986, Sharples and Baines 2009), its prevalence in wild passerines is unknown. Some studies in continental Europe have demonstrated a high prevalence of subclinical C psittaci infection in Paridae (tit species). For example, Holzinger-Umlauf and others (1997) detected Chlamydia sp. in 54 per cent of 399 free-living, apparently healthy Paridae in Germany, but Zweifel and others (2009) failed to detect C psittaci in any of the 527 free-living passerines (including 51 tits, 12 robins and 2 dunnocks) in Switzerland. Chlamydiosis is infrequently seen in wild birds (Simpson and Bevan 1989); the collared dove (Streptopelia decaocto) appears to be the most commonly affected British species (de Gruchy 1983, Gough and Bevan 1983). There have been only two previous reports of chlamydiosis outbreaks in songbirds (‘oscines’; suborder Passeri) in Britain. Simpson and Bevan (1989) diagnosed an outbreak in Cornwall (using ELISA and C psittaci isolation) in May 1988, in which ‘four robins (Erithacus rubecula), eight dunnocks (Prunella modularis), one great tit (Parus major) and a coal tit (Parus (Periparus) ater)’ were found dead. Also, chlamydiosis was diagnosed (using PCR and immunohistochemistry) in a robin and a chaffinch (Fringilla …