Evaluation of antimicrobial treatment strategies against Chlamydia psittaci using a bovine respiratory infection model

Background Chlamydiae are obligate intracellular pathogens that can cause a variety of diseases in many different mammalian and avian hosts. The successful antimicrobial treatment of chlamydial infections is still an unresolved issue in both, human and veterinary medicine. Aim and design of the project The project was designed to evaluate regimens commonly used for the treatment of chlamydial infections (i.e., tetracyclines, macrolides and quinolones), and to compare them to the most promising treatment regimens that were identified in vitro (i.e., combination of the antimicrobial substances with rifampicin). As animal model we chose a recently developed bovine model of an acute respiratory C. psittaci infection. In order to perform the treatment studies in the afore mentioned animal model, bronchoscopic sampling methods had to be adapted for the use in calves aged 6-10 weeks and for the use under experimental conditions in a biosafety level 2. Animals, material and methods For STUDY 1, four male Holstein-Friesian calves were repeatedly bronchoscoped under general anesthesia beginning at the age of 6 weeks. The following methods were evaluated: bronchial brushing, broncholaveolar lavage and transbronchial lung biopsy. For STUDY 2 and STUDY 3, a total of 80 conventionally raised, male Holstein-Friesian calves were intrabronchially inoculated with 108 inclusion forming units C. psittaci strain DC15. At 30 h post inoculation (pi), the animals were assigned either to the untreated control groups or to one of 11 different treatment groups. All animals were clinically examined on a daily basis. Results were summarized using a clinical scoring system. Venous blood was sampled throughout the study. Bronchial brushings, bronchoalveolar lavage fluid (BALF) and pharyngeal swabs were sampled under general anesthesia. At the end of the study on 14 dpi, all animals were euthanized and necropsied. Levels of antimicrobial substances were determined. The presence of the inoculated pathogen in treated and untreated animals was assessed by recultivation from bronchial brushings and by quantitative real-time PCR testing of blood, lung and mediastinal lymph node and swabs (i.e., nasal, conjunctival, pharyngeal, and rectal swabs). The hosts’ reaction was characterized by the clinical scores and signs of local and systemic inflammation. Results The intrabronchial inoculation with C. psittaci led to acute respiratory disease with fever in all animals that resolved until 10 dpi. The established protocol of obtaining bronchial brushings, BALF and transbronchial lung biopsies proved suitable for the use under experimental conditions. Sufficient antibiotic levels were detected in in blood and tissue samples of all treated animals. Recultivation results revealed that viable Chlamydiae could more often be isolated from untreated than from treated animals. Single drug therapy inhibited chlamydial growth in the same extent as combination therapy with rifampicin. Clinical score, white blood cell count, LBP concentration in the blood, and BALF cell count revealed acute respiratory and systemic disease in all animals, but again, no differences were visible between treated and untreated animals. All 80 infected animals included in the project regained clinical health by the end of the study, regardless if they were treated or not.