Chlamydia caviae is a member of the family of gram-negative intracellular bacteria Chlamydiaceae. Bacteria of this family cause a broad spectrum of diseases in animals and humans and are currently subdivided into a single genus and 11 species.¹ Among these species, C. psittaci and C. abortus are known for their zoonotic potential, causing psittacosis and abortion, respectively, in humans.^2,3 In contrast, only one case of mild conjunctivitis has previously been documented in association with C. caviae.⁴

Background: Feral pigeons (Columba livia domestica) live and breed in many city centres and contact with their droppings can be a hazard for human health if the birds carry Chlamydia psittaci. Objective: The aim of this study was to establish whether pigeon droppings in two Dutch cities (Utrecht and Haarlem) contain C. psittaci and/or C. avium, which could be a potential hazard for transmission to humans. Methods: In May 2017 seven feral pigeon ‘hot spots’ with between 5 and 40+ pigeons present were identified in two cities by visual observations over two days. During the following ten days fresh droppings were collected at these hot spots and the samples were pooled per three droppings to achieve 40–41 samples per city. Samples were analysed for Chlamydia DNA with a broad range 23S Chlamydiaceae Real-Time PCR and positive samples were tested with a specific C. psittaci and C. avium Real-Time PCR. Positive C. psittaci samples were genotyped. Results: C. psittaci and C. avium were detected in both cities. For C. psittaci the prevalences in Utrecht and Haarlem were 2.4% and 7.5%, respectively; for C. avium 36.6% and 20.0%, respectively. One sample contained both species. All C. psittaci samples belonged to genotype B. Conclusion: C. psittaci and C. avium are present in feral pigeon droppings in Utrecht and Haarlem. Human contact with droppings from infected pigeons or inhalation of dust from dried droppings represent a potential hazard to public health.

In poultry several Chlamydia species have been detected, but Chlamydia psittaci and Chlamydia gallinacea appear to be most prevalent and important. Chlamydia psittaci is a well-known zoonosis and is considered to be a pathogen of poultry. Chlamydia gallinacea has been described more recently. Its avian pathogenicity and zoonotic potential have to be further elucidated. Within the Netherlands no data were available on the presence of Chlamydia on poultry farms. As part of a surveillance programme for zoonotic pathogens in farm animals, we investigated pooled faecal samples from 151 randomly selected layer farms. On a voluntary base, 69 farmers, family members or farm workers from these 151 farms submitted a throat swab. All samples were tested with a generic 23S Chlamydiaceae PCR followed by a species specific PCR for C. avium, C. gallinacea and C. psittaci. C. avium and psittaci DNA was not detected at any of the farms. At 71 farms the positive result could be confirmed as C. gallinacea. Variables significantly associated with the presence of C. gallinacea in a final multivariable model were ‘age of hens,’ ‘use of bedding material’ and ‘the presence of horses.’ The presence of C. gallinacea was associated with neither clinical signs, varying from respiratory symptoms, nasal and ocular discharges to diarrhoea, nor with a higher mortality rate the day before the visit. All throat swabs from farmers, family members or farm workers tested negative for Chlamydia DNA, giving no further indication for possible bird-to-human (or human-to-bird) transmission.

Chlamydia gallinacea is an obligate intracellular bacterium that has recently been added to the family of Chlamydiaceae. C. gallinacea is genetically diverse, widespread in poultry and a suspected cause of pneumonia in slaughterhouse workers. In poultry, C. gallinacea infections appear asymptomatic, but studies about the pathogenic potential are limited. In this study two novel sequence types of C. gallinacea were isolated from apparently healthy chickens. Both isolates (NL_G47 and NL_F725) were closely related to each other and have at least 99.5% DNA sequence identity to C. gallinacea Type strain 08-1274/3. To gain further insight into the pathogenic potential, infection experiments in embryonated chicken eggs and comparative genomics with Chlamydia psittaci were performed. C. psittaci is a ubiquitous zoonotic pathogen of birds and mammals, and infection in poultry can result in severe systemic illness. In experiments with embryonated chicken eggs, C. gallinacea induced mortality was observed, potentially strain dependent, but lower compared to C. psittaci induced mortality. Comparative analyses confirmed all currently available C. gallinacea genomes possess the hallmark genes coding for known and potential virulence factors as found in C. psittaci albeit to a reduced number of orthologues or paralogs. The presence of potential virulence factors and the observed mortality in embryonated eggs indicates C. gallinacea should rather be considered as an opportunistic pathogen than an innocuous commensal.

Chlamydia (C.) caviae is a known pathogen in guinea pigs, causing conjunctivitis, respiratory infections and abortions. Recently, a C. caviae-induced zoonotic link was identified as the etiology of severe community-acquired pneumonia in humans. Here, 784 conjunctival and rectal swabs originating from 260 guinea pigs and 110 rabbits from 64 husbandries in Switzerland, as well as 200 composite conjunctival swabs originating from 878 guinea pigs from 37 husbandries in The Netherlands were examined by real-time PCR followed by conventional PCR and sequencing. Chlamydiaceae were detected in 2.3% (18/784) and 12.5% (25/200) of all Swiss and Dutch samples, respectively. An overall C. caviae occurrence was detected in 2.7% (7/260) and 8.9% (78/878) of all Swiss and Dutch guinea pigs, respectively. OmpA genotyping of 64 C. caviae-positive samples resulted in 33 sequences sharing 100% nucleotide identity with the strains isolated from the zoonotic transmission cases in The Netherlands. However, all ompA sequences of this study were distinct from the C. caviae GPIC reference strain. C. caviae was not detected in rabbits but C. psittaci genotype A was identified in guinea pigs and rabbits, raising concerns about the importance of these animal species as novel zoonotic sources for C. psittaci.

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.