Chlamydial infections in feral pigeons in Europe: Review of data and focus on public health implications

Feral pigeons (Columba livia domestica) which thrive in most European towns and cities are commonly infected with the zoonotic bacterium Chlamydophila psittaci, the agent of psittacosis (also known as ornithosis) in humans. A number of surveys carried out in the last thirty years across Europe allowed to detect high seropositivity values and high percentages of infection in feral pigeon populations. Overall, considering data from 11 European countries, seropositivity values to C. psittaci in the sampled populations ranged from 19.4 to 95.6 %. In most surveys, antibodies were detected with the complement fixation test in a percentage of samples from 19.4 to 66.3 %, with a median of 46.1 %. An indirect immunofluorescence test and an ELISA were employed less frequently, and allowed to detect higher percentages of seropositivity (23.7 – 67.7 % and 56 – 95.6 %, respectively). Attempts to grow C. psittaci in cell cultures or in embryonated chicken eggs were successful in 2 – 42.3 % and 0 – 57.1 % of samples respectively, antigen detection methods were positive in 2.3 – 40% of samples, while conventional PCR and real-time PCR with different genomic targets detected the organism in 3.4 – 50 % of samples. Twenty-five C. psittaci strains were typed as genotype B (n=14), E (n=10) and E/B (n=1). The huge increase of feral pigeon populations in Europe is a major cause of concern for the detrimental effect of pigeon droppings to environmental hygiene, in addition to the extensive damage due to the fouling of buildings and monuments. The most important pathogenic organism transmissible from feral pigeons to humans is C. psittaci, with 101 cases of disease reported in the literature. Exposure to C. psittaci-contaminated dust, direct contact with pigeons through handling and, to a lesser extent, through pigeon feeding have been identified as hazardous exposures in more than half cases of human disease, while loose or transient contacts with feral pigeons have been referred in about 40 % cases. Education initiatives as to the communication of a health risk deriving from contact with pigeons and pigeon excreta should primarily be targeted to workers that may be exposed to C. psittaci-contaminated dust, such as demolition/construction labourers. Recommendations to this category of workers include wearing protective clothes with hood, boots, gloves and air face filter masks when removing pigeon faeces from roofs, garrets and buildings, especially if working indoors. Monitoring for C. psittaci infections over time should also be considered in these workers. Children should be warned not to handle sick or dead pigeons and immunocompromised individuals should be educated to carefully limit their contact with feral pigeons. The culling of pigeons by shooting or poisoning is both unethical and useless as the place of the killed birds in the population is quickly filled by new juveniles or by birds immigrating from neighbouring areas. Pigeon-deterring systems such as nets and plastic or metal spikes applied to buildings and monuments will prevent their fouling, and the administration of contraceptive drugs may allow to resize the pigeon populations, but the measure that will ultimately lead to their permanent reduction and to establish healthy sustainable populations is the restriction of their indiscriminate feeding by pigeon lovers. The building of dovecotes and artificial breeding facilities should be considered for providing shelter and a balanced diet to the birds, as well as a chance of interaction for pigeon lovers in a hygienically-controlled environment.