Epidemiology
GEOGRAPHICAL DISTRIBUTION AND HOSTS
Chlamydophila psittaci primarily infects birds and has been demonstrated in about 465 species of 30 different orders. The disease has a worldwide distribution in wild bird species and may be more prevalent in gregarious and colonial nesting species.
It is considered that many species have their own strains of the bacterium and that most species are probably susceptible to natural infection. Human infection was recorded frequently in the 20th century, with psittacines probably the commonest identified source of this infection. Disease occurrences in humans associated with poultry (ducks, geese and turkeys) are infrequent but may be severe. Modern diagnostic methods, in particular polymerase chain reaction (PCR), are extending the known host species range in wild birds and revealing high prevalences of infection within species. Recovery from clinical disease in wild birds may be followed by persistence of infection, with highly virulent strains being carried and extensively shed in the absence of apparent clinical signs in the hosts themselves. Chlamydophilapsittaci comprises six avian serovars, and all should be considered transmissible to humans and potentially capable of causing disease.Serovar A is endemic among psittacine birds and has caused sporadic disease in mammals, including humans, and tortoises. Serovar B is usually associated with pigeons but has also been isolated from turkeys. Additionally, it has been identified as the cause of abortion in a dairy herd(1). Serovar C has primarily been isolated from waterfowl, and Serovar D mainly from turkeys. The host range of Serovar E includes pigeons, ratites, ducks, turkeys and occasionally humans; avian strains only occasionally jump to mammalian hosts.
S erovar F has been recovered from psittacines and turkeys. Two mammalian isolates (WC and M56) originated from epizootics in cattle (WC) and muskrats (Ondatra zibethicus; M56), respectively1-2).
European studies on C. psittaci in feral pigeons ( Columba livia var. domestica) conducted from 1983 to 2007 in Bosnia and Herzegovina, Bulgaria, Croatia, France, Germany, Italy, Spain, Slovenia and Switzerland revealed seroprevalences ranging from 19.4 to 95.6%(3). Studies performed in the UK using complement fixation test (CFT) serology in archived blood samples from family groups of wild and domesticated birds showed high titres only in pigeons, collared doves (Streptopelia decaocto) and psittacines. Of 1,549 pigeons tested, 47.3% were seropositive for C. psittaci, whereas wild collared doves (n = 37) showed 51% seropositivity(4). This study incidentally showed lower titres and prevalences in domesticated poultry, whereas in wild ducks the seroprevalence was 23% and in game birds 29%. In Scotland, Chlamydophila psittaci was demonstrated in gulls (Larus spp.), blue tits (Parus caeruleus), crows ( Corvus frugilegus), chaffinches (Fringilla coelebs) and European robins (Erithacus rubecula), with respiratory disease, airsacculitis and hepatitis(5). Surveys conducted on 20 species of free-living raptors in the Berlin and Brandenburg area, Eastern Germany revealed antibodies to C. psittaci in 267/422 (63%) of the sera tested. Chlamydophila psittaci was also detected in 74% of spleen and lung samples from buzzards (Buteo spp., Pernis apivorus), hawks (Accipiter spp.), falcons (Falco tinnunculus and F peregrinus), kites (Milvus spp.), white-tailed sea eagles (Haliaeetus albicillai and owls ( Tyto alba, Strix aluco) tested by PCR but C. psittaci infection could not be correlated with gross lesions found in the birds(6). In psit- tacines the prevalence rate ranges from 16 to 81% of samples collected in various studies, and a mortality rate of more than 50% has been reported. Psittacidae are major
reservoirs of chlamydiae, especially in captive populations. An incident in which multiple deaths occurred in a garden over several months in a range of passerine species led to the conclusion that more than one infectious agent was involved1-5).
In other incidents, the clinical significance of chlamydial infection detected in tissues sampled at necropsy, e.g. in blue tit nestlings and a gull (Laridae), was unclear(5).Although the role of some chlamydial species in birds, domestic animals and humans is well established, only sparse data are available on the situation in wild mammals. A German study on wild boar (Sus scrofa) from Thuringia provides a subtle hint for a possible C.psittaci reservoir. C. psittaci-DNA was detected in 57% of all animals examined. More than 80% of females were PCR-positive, compared to 43% of male boar. Chlamydophila psittaci DNA was amplified most frequently from lung, but also from pulmonary lymph node, intestine and uterus(7). In clinically healthy alpine ibex (Capra ibex ibex) from Switzerland, 31% were found to be antibody-positive for C. psittaci(8). A mass die- off among snowshoe hares (L epus americanus) and muskrats was attributed to C. psittaci infection in Canada(9). Investigations into the normal conjunctival bacterial flora in the North American opossum (Didelphis virginiana) and raccoon (Procyon lotor) revealed Chlamydia-positive PCR results in 18% of opossum and 30% of raccoon samples investigated(10).
HOST FACTORS
A trend from lower seroprevalences in young birds to higher rates in older individuals has been statistically con- firmed(11). A slightly higher seroprevalence in common buzzards (B uteo buteo) was attributed to different hunting and feeding behaviour (broad spectrum of prey, including carrion)(11).
EPIDEMIOLOGICAL ROLE OF WILD ANIMALS
Avian strains of C. psittaci relatively rarely jump to mammalian hosts, and mammal- to- mammal transmission of these strains is infrequent.
TRANSMISSION
Chlamydiae may be present in the conjunctiva, upper respiratory, urogenital and intestinal tracts of mammals and birds, indicating that transmission may be direct or indirect, via aerosol, contact with excretions and the venereal route. Faecal shedding occurs intermittently and can be activated through stress caused by nutritional deficiencies, prolonged transport, overcrowding, breeding, egg laying, treatment or handling. Periods of excretion during natural infection may vary depending on strain virulence, infection dose and host immune status. In ruminants, shedding of chlamydiae is highest at the time ofparturition/ abortion. Although transmission is possible by several routes, it primarily occurs by the respiratory route. Ingestion may not be so important, and vertical transmission and transmission by ectoparasites are probably relatively unusual occurrences.