Avian Chlamydia abortus causing human respiratory infections in the Netherlands

A national research collaboration detected an avian Chlamydia abortus variant as the most likely causative agent of human pneumonia. The study was recently published in Lancet Infectious Diseases and supports that avian Chlamydia abortus strains, which were previously not detected in humans, can cause human infections.

In 2021, a novel group of Chlamydia strains in wild birds was described and based on phylogenetic analysis and animal host, were classified as avian Chlamydia abortus strains. Until now there was no evidence for the zoonotic potential of these avian Chlamydia abortus strains to infect humans.

An outbreak investigation of a family cluster of community acquired pneumonia prompted a research collaboration between experts from the Public Health Service region Utrecht, Radboudumc Zuyderland MC, St Antonius Hospital Nieuwegein, NVWA, RIVM and Wageningen Bioveterinary Research.

In the outbreak investigation they reported that an avian Chlamydia abortus strain caused a cluster of respiratory illness in four family members. Three patients were hospitalised with community-acquired pneumonia, one of whom was admitted to the intensive care unit. The strain could be isolated from sputum and bronchoalveolar lavage samples and with whole-genome sequencing three complete genomes were obtained. Phylogenetic analysis revealed that the genomes belonged to the avian subgroup of Chlamydia abortus.
The faeces of wild birds were considered a probable source for the index infection of the family cluster. Human-to-human transmission appeared to be a plausible explanation for at least two infections in this outbreak. Finally, circulation of this avian Chlamydia abortus strain dates back to at least 2010, based on the first documented human infection from ten retrospectively detected patients in the Netherlands.

Additional research should address questions regarding the avian reservoir, mode of transmission, and pathogenicity of avian Chlamydia abortus.

The article is linked to research program vector-borne diseases and zoonoses.

About the publication

Lancet Infect Dis. 2025 Feb;25(2):198-207. doi: 10.1016/S1473-3099(24)00529-2. Epub 2024 Oct 16.
Circulation of avian Chlamydia abortus in the Netherlands and community-acquired pneumonia: an outbreak investigation and retrospective cohort study 
Stijn Raven, Marloes Heijne, Jeroen Koomen, Gert Doornenbal, Miriam Maas, Pieter Jacobs, Ingrid Keur, Frederika Dijkstra, Daphne Reukers, Mark Platenburg,