Pathogenicity of Chlamydia gallinacea in chickens after oral inoculation

Chlamydia gallinacea is a recently discovered and widespread obligate intracellular bacterium in chickens. In chickens, infections appear to be asymptomatic, but can result in reduced weight gain in broilers. Molecular typing revealed C. gallinacea is genetically diverse which might lead to differences in pathogenic potential between strains. However, studies about the pathogenesis of different C. gallinacea strains are still limited. In this study, the pathogenesis of C. gallinacea strain NL_G47 was investigated in three consecutive animal experiments. The first experiment served as a pilot in which a maximum culturable dose was administered orally to 13 chickens. Excretion of chlamydial DNA in cloacal swabs was measured during 11 days post infection, but no clinical signs were observed. The second and third experiment were a repetition of the first experiment, but now chickens were sacrificed at consecutive time points to investigate tissue dissemination of C. gallinacea. Again excretion of chlamydial DNA in cloacal swabs was detected and no clinical signs were observed in line with the results of the first experiment. PCR and immunohistochemistry of tissue samples revealed C. gallinacea infected the epithelium of the jejunum, ileum and caecum. Furthermore, C. gallinacea could be detected in macrophages in the lamina propria and in follicular dendritic cells (FDCs) of the B cell follicles in the caecal tonsil. Results of serology showed a systemic antibody response from day seven or eight and onward in all three experiments. The experiments with strain NL_G47 confirmed observations from field studies that C. gallinacea infection does not result in acute clinical disease and mainly resides in the epithelium of the gut. Whether the presence of C. gallinacea results in chronic persistent infections with long term and less obvious health effects in line with observations on other infections caused by Chlamydiae, needs further investigation.