Background/Objectives: Chlamydia (C.) psittaci is an avian respiratory pathogen that regularly infects budgerigars (Melopsittacus undulatus) and is a known zoonosis. This study aimed to evaluate the efficacy of a nucleoside-modified mRNA vaccine formulated in lipid nanoparticles (LNPs), either with (mRNA Galsomes) or without (mRNA LNPs) the glycolipid antigen α-Galactosylceramide, in protecting budgerigars against C. psittaci genotype A infection. Methods: Three groups of eight budgerigars received two intramuscular vaccinations with PBS, mRNA LNPs or mRNA Galsomes, and were subsequently challenged via aerosol with the C. psittaci genotype A strain 90/1051. Vaccine efficacy was assessed over 14 days post challenge by monitoring clinical signs, macroscopic and microscopic lesions, pathogen excretion and chlamydial burden in organs. Antibody levels were evaluated at baseline, after vaccination and post challenge. Results: Both mRNA LNPs and mRNA Galsomes induced significant serum antibody responses post booster. Vaccination significantly reduced clinical signs, chlamydial burden in the lungs and macroscopic lesions in conjunctiva, conchae, lungs and thoracic airsacs, compared to controls. Additionally, mRNA Galsomes-treated birds showed a significantly reduced lung inflammation and fewer macroscopic lesions in abdominal airsacs and liver, compared to non-vaccinated animals. These animals also experienced a significantly lower chlamydial burden in the spleen, fewer clinical signs at day 11 and fewer fecal shedding at day 14 post challenge, compared to mRNA LNP-treated animals. Conclusions: This study demonstrated that mRNA vaccination confers partial protection against C. psittaci in budgerigars, with mRNA Galsomes appearing to provide enhanced efficacy. However, the absence of species-specific reagents for assessing cellular immunity in Psittaciformes limits a comprehensive understanding of vaccine-induced protection. The development of psittacine-specific T cell markers and cytokine assays is necessary to further elucidate immune mechanisms and optimize vaccine formulations.

Chlamydia psittaci is an established zoonotic agent causing respiratory disease in humans. An infection often remains asymptomatic but can also result in flu-like illness, pneumonia or even multi-organ failure. This paper describes three patients, hospitalised at AZ Sint-Lucas Hospital, with atypical pneumonia who were diagnosed with C. psittaci after an in-depth anamnesis and laboratory investigation in the midst of the COVID pandemic. All three infections were confirmed with PCR and serology, whereas viable bacteria were only present for one patient. Genotyping revealed the presence of genotype B for patient 1 and 2 whereas ompA genotyping was unsuccessful for patient 3. This case report demonstrates the importance of a thorough patient history as close contact with birds is one of the main risk factors to contract the pathogen. Once exposure to birds has been confirmed, a diagnosis by a combination of PCR and serology is essential in order to initiate a treatment with the proper antibiotics. As psittacosis is still an underestimated and underdiagnosed disease, communication between laboratory, clinicians and bird fanciers is encouraged.

The presence and zoonotic transfer of four different avian Chlamydia spp. was assessed in an epidemiological study in a psittacine bird population and its owners. Fecal swabs from 84 pet birds and pharyngeal swabs from 22 bird owners were collected from 21 locations in Flanders. Samples were examined using established and novel PCR platforms combined with culture on PCR-positive samples. Chlamydiaceae DNA was detected in 33 of 84 (39.3%) birds. The predominant part of the avian infections could be attributed to C. psittaci (22 of 84; 26.2%), followed by C. avium (11 of 84; 13.1%). C. gallinacea and C. abortus were not detected in birds or humans. C. psittaci was the only species detected in pet bird owners (4 of 22; 18.2%), stressing its zoonotic importance. This study showed that C. psittaci and the more recently discovered novel avian species C. avium are undoubtedly present in the Flemish psittacine bird population. Our results justify additional research in a larger psittacine bird population and its owners, focusing on C. psittaci and C. avium. In the meantime, increased awareness among pet bird owners and the implementation of preventive measures in the pet bird industry is advised to limit the circulation of established and novel emerging avian chlamydial species.

Chlamydia psittaci, Chlamydia gallinacea, and Chlamydia abortus are the most common Chlamydia spp. in chickens and have a confirmed or suggested zoonotic potential. No recent data are available on their prevalence and impact in the Belgian chicken industry or in the recreational chicken branch. Therefore, a cross-sectional epidemiological study was executed where samples were collected from both factory-farmed and backyard chickens. More specifically, pharyngeal chicken swabs were obtained from 20 chicken farms, 5 chicken abattoirs, and 38 different backyard locations and were analyzed using species-specific Polymerase Chain Reactions (PCRs) for the presence of the three avian Chlamydia spp. To investigate their zoonotic potential, samples were simultaneously collected from 54 backyard chicken caretakes and 37 professional chicken caretakers or abattoir employees and analyzed using species-specific PCRs as well. This study confirmed the presence of DNA of all three Chlamydia species in both the chicken industry and backyard settings. Chlamydia psittaci was the most prevalent in the industry chickens (11.0%), whereas Chlamydia gallinacea was the dominant species in the backyard chickens (14.5%). Chlamydia abortus infections were more common in the commercial chickens (9.0%) compared to the backyard chickens (2.6%). The DNA of all three species was also detected in humans (3.9% Chlamydia psittaci, 2.9% Chlamydia gallinacea, and 1.0% Chlamydia abortus).

Chlamydia suis, a close relative of the human pathogen C. trachomatis, can be detected in the porcine gut, yet its prevalence and viability across intestinal segments remain poorly defined. This study aimed to assess the segment-specific prevalence, isolation success, and tetracycline susceptibility of C. suis in grower-finisher pigs. Jejunal, ileal, and colonic samples (n = 200 per intestinal segment) were collected from 600 pigs at slaughter and analyzed using C. suis-specific real-time PCR and culture. PCR revealed significantly higher detection rates in the colon (40%) than in the jejunum or ileum (both 4.5%), accompanied by significantly higher calculated bacterial loads in colonic samples. In contrast, viable C. suis was most frequently isolated from ileal material, indicating that the ileum may provide a more favorable condition for active bacterial replication. Among 24 culture-confirmed isolates, 75% were susceptible to tetracycline (MIC/MBC < 2 µg/mL), 12.5% exhibited an intermediate phenotype (2 µg/mL < MIC/MBC < 4 µg/mL) and another 12.5% were resistant (MIC/MBC > 4 µg/mL). Intermediate phenotypes were recovered from the jejunum and ileum, whereas resistant isolates were found in the ileum and colon. These findings suggest that the porcine colon may serve as an intestinal reservoir for C. suis, while the ileum supports more robust bacterial replication. Overall, these data contribute to our understanding of the intestinal ecology of C. suis under field conditions and its tetracycline susceptible patterns.

Serology routinely serves as a diagnostic tool to confirm Chlamydia infections in humans. Particularly in delayed settings, such as post-outbreak scenarios where the acute phase of infection has subsided, serology is invaluable. Multiple studies, nonetheless, indicate deficiencies in specificity and sensitivity of current chlamydial antibody detection assays. Incorporation of multiple antigens per target is known to improve the accuracy of chlamydial serological assays. We, therefore, used the recomLine test (Mikrogen diagnostics) on serological samples of two cohorts, as it is the only commercially available test allowing detection of antibodies against three human pathogenic Chlamydia species (C. trachomatis, C. pneumoniae and C. psittaci) using multiple antigens per target. The first cohort (n = 156; samples collected between 2008 and 2022 during a C. trachomatis screening initiative) comprised women from the Netherlands (NL) with past exposure to C. trachomatis, while the second cohort (n = 44; samples collected in 2018 in a health examination survey) consisted of Belgian citizens (BE) with occupational or recreational exposure to chickens, representing a risk population for C. psittaci. The test indicated a statistically equivalent C. pneumoniae seroprevalence in both cohorts (39.10% in NL and 34.09% in BE; p = 0.337). As expected C. trachomatis seroprevalence was significantly higher (p < 0.001) in the Dutch cohort (48.72%), as compared to the Belgian cohort (4.55%). Lastly, C. psittaci seroprevalence did not significantly differ between the two groups (2.27% in BE and 1.92% in NL; p = 0.633), even though a higher prevalence was expected for the Belgian cohort. This prompts us to question whether the Belgian cohort truly constituted a C. psittaci risk population or whether the recomLine test is susceptible to cross-reaction of species-specific antibodies, thereby increasing C. psittaci prevalence in the Dutch cohort. We advocate for the development of affordable, highly sensitive antibody detection assays that can effectively distinguish between chlamydial species, addressing the increasing demand for enhanced serological testing methodologies.