On the biology and epidemiology of the feral pigeon (Columba livia)

Feral pigeons (Columba livia, Gmelin 1789) are among the most abundant vertebrates in the urban environment of almost every city in the world. Close contacts to humans bear the risk of transmission of zoonotic pathogens and parasites. The most important of these zoopathogens is the bacterium Chlamydia psittaci, the agent of avian chlamydiosis and human psittacosis/ornithosis. Many human infections reported in the medical literature were attributed to brief and transient contacts to feral pigeons in the urban environment. The aim of the present thesis is to investigate the prevalence of C. psittaci in the feral pigeon population of Basel, to identify possible routes of transmission in the city and to propose measures for the prevention of zoonotic C. psittaci transmissions.
In a preliminary study, 47 faecal samples were collected from nest boxes in a feral pigeon loft in Basel, Switzerland. In addition 34 samples were collected from the feather dust film on the water surface of public fountains, where feral pigeons regularly bathe. All 81 samples were tested for the presence of chlamydial antigen by use of an antigen-ELISA assay. Initially the ELISA assay yielded a high proportion of positive results: 8 out of 47 (23.5 %) faecal samples and 26 out of 34 (76.5 %) of the water film samples were tested positive. However, a subsequent blocking test revealed only one true positive faecal sample, which could be confirmed by microarray. This ELISA assay was not used for further studies.
In the main study, the prevalence of C. psittaci shedding in free ranging feral pigeons living in a pigeon loft in Basel was investigated. A total of 202 individual birds were tested on four different time points between 2007 and 2009 by analysis of pharyngeal and cloacal swabs. Some of the birds could be tested repeatedly. All samples were analyzed by use of a species-specific nested PCR assay targeting the ompA gene of C. psittaci. The tested feral pigeons were shedding C. psittaci by respiratory secretions, since 9 out of 447 (2.0 %) of the pharyngeal swabs were tested positive. Furthermore, 11 out of 348 (3.2 %) of the cloacal swabs were tested positive, indicating that these birds are shedding C. psittaci in their faecal droppings. In total, C. psittaci was documented in 17 of 202 individual birds (8.4 %). The majority of the positive birds were shedding the pathogen intermittently. At present, this is the first study to test individual free-living feral pigeons repeatedly and therefore the first to prove intermittent shedding of C. psittaci in these birds. Genotyping of the positive samples by real-time PCR revealed C. psittaci genotype B in 7 of the birds, as well as a mixed infection with the genotypes A, B and E/B in one bird. A mixed infection with three different chlamydial genotypes could be documented for the first time. Seven of the birds that tested positive immigrated into the pigeon loft as adults, including the bird with the mixed infection. Thus, it could be proven how the interconnectedness of feral pigeon subpopulations favours the spread of pathogens. Additionally, 620 faecal samples from public sites in the city were analyzed. However, C. psittaci could not be detected in these faecal samples from the urban environment.
Feral pigeons on building facades, on balconies and window ledges, or in open attics pose a health risk. Avoiding attractive building structures already during building design is the best and most cost-effective way to prevent problems with feral pigeons in the future. We identified the structural parameters required to proof a building against feral pigeons. It could be demonstrated that feral pigeons are not able to pass through an outlet width of 4 cm, the respective outlet height is 5 cm, and a pigeon-safe square opening size is not exceeding 6 × 6 cm. A feral pigeon is not able to sit on a ledge if it’s width is 4 cm or smaller. The pigeon-safe angle of inclination for smooth construction materials (tinplate, glass, plastics) is 25°, for medium rough materials (wood, plane concrete) 35°, and for rough materials (sandstone, rough concrete) at least 50°. With these essential data, required to proof a building against feral pigeons, we contribute to the prevention of disease transmissions and parasite infestations due to these birds.
The low proportion of our feral pigeons in Basel, which are shedding C. psittaci, could be documented. However, C. psittaci could not be detected in faecal samples from public sites in the urban environment. The regular cleaning of streets and squares in Basel additionally contributes to public health, since potentially infectious feral pigeon faeces are removed quickly. Considering the numerous opportunities of close contacts to feral pigeons in the urban environment, a transmission of the pathogen from feral pigeons to humans can never be ruled out. Therefore, feral pigeons present a significant potential health risk.