Chlamydiosis as a zoonotic threat: Are severe infections linked to wild birds in Europe

Members of the family Chlamydiaceae, such as Chlamydia psittaci and C. abortus, are obligate intracellular zoonotic pathogens that are difficult to diagnose and have an underestimated public health impact.

C. psittaci commonly infects birds, especially psittacines, ducks, and pigeons, often without showing symptoms, but it can cause severe, sometimes fatal, respiratory infections in humans. Recent single-nucleotide polymorphism (SNP)-based genotyping has identified eight C. psittaci lineages, including Mat116. This genotype was previously considered rare but is now being detected increasingly frequently in human cases in Sweden, the Netherlands, France, and, likely, Denmark. Preliminary findings suggest that garden birds, particularly great tits, may act as a reservoir.

C. abortus is classically associated with reproductive disorders in ruminants, but it also poses serious risks to pregnant women and immunocompromised individuals. Avian strains have recently been detected in wild birds and have been linked to human infections, including a cluster of cases in a family and ten additional historical cases in the Netherlands.

Chlamydial infections in humans are underdiagnosed, with species- or genotype-level identification rarely performed. Furthermore, patients often overlook or fail to report contact with wild birds, especially common garden species, which limits clinical suspicion and obscures transmission sources.

This collaborative project brings together four CoVetLab laboratories, clinicians, and ecologists to address these issues. By sharing detection and genotyping methods, conducting field investigations and comparing avian and human strains circulating in Europe, we aim to clarify the role of wild birds in zoonotic transmission and reinforce surveillance, diagnosis and risk assessment within a One Health framework.