Infectious bursal disease virus tissue tropism and pathogenesis of the infection in chickens by application of in situ PCR, immunoperoxase and HE staining.

Infectious bursal disease is one of an OIE list of notifiable diseases. Chicken is the only host that manifests clinical signs and its pathogenicity is correlated with the distribution of antigens in organs. This study was conducted to determine disease pathogenesis and virus tissue tropism by in situ PCR, immunoperoxidase staining (IPS), and HE staining. Twenty four chickens were infected with very virulent Infectious Bursal Disease Virus (vvIBDV). Fifteen chickens were kept as a control group. Infected chickens were sacrificed at hrs 2, 4, 6, 12, days 1, 2, 4, and 6 post-inoculation (pi). While, control chickens were euthanized on days 0, 1, 2, 4, and 6 pi. Different tissues were collected, fixed in 10% buffered formalin, and processed. At hr 2 pi, virus was detected in intestinal, junction of the proventriculus and gizzard, cecal tonsil, liver, kidney, and bursa of Fabricius. At hr 4 pi, virus reached spleen, and at hr 6 pi, it entered thymus. At hr 12 pi, virus concentration increased in positive tissues. The latest invaded tissue was muscle on day 1 pi. Secondary viraemia occurred during 12-24 h pi. In situ PCR was the most sensitive technique to highlight obscure points of infection in this study.

Antimicrobial susceptibility and molecular characteristics of Mycoplasma bovis isolated from cases of bovine respiratory disease in Australian feedlot cattle.

To date, antimicrobial susceptibility has not been reported for Australian Mycoplasma bovis isolates. This study determined minimal inhibitory concentrations (MICs) for 12 different antimicrobials against Australian M. bovis isolates and used whole genome sequencing to screen those showing high macrolide MICs for point mutations in target genes. Most lung tissue/swab samples from bovine respiratory disease cases (61/76, 80.3%) tested positive for M. bovis. A set of 50 representative isolates (50/61, 82.0%) that showed adequate growth, was used for MIC testing. Uniformly, low MIC values were confirmed for enrofloxacin (≤ 4 µg/mL), florfenicol (≤ 8 µg/mL), gamithromycin (≤ 2 µg/mL), spectinomycin (≤ 4 µg/mL), tetracycline (≤ 8 µg/mL), tiamulin (≤ 4 µg/mL), and tulathromycin (≤ 0.5 µg/mL). A small proportion (10%) of isolates exhibited high MICs (≥ 32 µg/mL) for tildipirosin, tilmicosin, tylosin, and lincomycin, which were above the epidemiological cut-off values for each antimicrobial (≥ 4 µg/mL). These isolates, originating from three Australian states, underwent whole genome sequencing/multilocus sequencing typing and were compared with the reference strain PG45 to investigate mutations that might be linked with the high macrolide/lincosamide MICs. All five belonged to ST52 and two macrolide associated mutations were identified within the 23 S rRNA gene (A2058G in two sequenced isolates and G748A in all sequenced isolates). Four additional 23 S rRNA gene mutations did not appear to be linked to macrolide resistance. Whilst the majority of Australian M. bovis isolates appear susceptible to the tested antimicrobials, emerging macrolide resistance was detected in three Australian states and requires continued monitoring.