Positive identification rates of Salmonella enterica in hatcheries and upstream breeder farms were 16.4% (36/220) and 3.0% (6/200), respectively. Among the Salmonella serovars identified in the hatcheries, S. enterica ser. Albany (17/36, 47.2%) was the most prevalent, followed by the serovars S. enterica ser. Montevideo (11/36, 30.6%) and S. enterica ser. Senftenberg (5/36, 13.9%), which were also predominant. Thirty-six isolates showed resistance to at least one antimicrobial tested, of which 52.8% (n = 19) were multidrug resistant (MDR). Thirty-three isolates (enrofloxacin, MIC ≥ 0.25) showed point mutations in the gyrA and parC genes. One isolate, S. enterica ser. Virchow, carrying the bla_CTX-M-15 gene from the breeder farm was ceftiofur resistant. Pulsed-field gel electrophoresis (PFGE) showed that 52.0% S. enterica ser. Montevideo and 29.6% S. enterica ser. Albany isolates sourced from the downstream of hatcheries along the broiler chicken supply chain carried the same PFGE types as those of the hatcheries. Thus, the hatcheries showed a high prevalence of Salmonella isolates with high antimicrobial resistance and no susceptible isolate. The AMR isolates from hatcheries originating from breeder farms could disseminate to the final retail market along the broiler chicken supply chain. The emergence of AMR Salmonella in hatcheries may be due to the horizontal spread of resistant isolates. Therefore, Salmonella control in hatcheries, particularly its horizontal transmission, is important.

This study investigated the effect of enrofloxacin (ENR) administration on the prevalence and antimicrobial resistance of E. coli, Salmonella, and Campylobacter isolated from broiler chickens under field conditions. The isolation rate of Salmonella was significantly lower (p < 0.05) on farms that administered ENR (6.4%) than on farms that did not (11.6%). The Campylobacter isolation rate was significantly higher (p < 0.05) in farms that administered ENR (6.7%) than in farms that did not (3.3%). The ratio of resistance to ENR was significantly higher (p < 0.05) in E. coli isolates from farms that used ENR (88.1%) than farms that did not (78.0%). The respective ratio of resistance to ampicillin (40.5% vs. 17.9%), chloramphenicol (38.0% vs. 12.5%), tetracycline (63.3% vs. 23.2%), and trimethoprim/sulfamethoxazole (48.1% vs. 28.6%) and the ratio of intermediate resistance to ENR (67.1% vs. 48.2%) were significantly higher (p < 0.05) in Salmonella isolates from the farms that used ENR than farms that did not. In conclusion, the use of ENR at broiler farms was an important factor in decreasing the prevalence of Salmonella but not Campylobacter and caused ENR resistance among E. coli and Salmonella but not Campylobacter. Exposure to ENR could have a co-selective effect on antimicrobial resistance in enteric bacteria in the field.

Infectious bursal disease (IBD), caused by IBD virus (IBDV), is an extremely contagious immunosuppressive disease that causes major losses for the poultry industry worldwide. Recently, the novel variant IBDV (G2d) has been highly prevalent in Korea, but the current vaccines against this very virulent IBDV have limited efficacy against this novel variant. To develop a vaccine against this variant IBDV, a recombinant virus designated rHVT-VP2 was constructed by inserting the IBDV (G2d) VP2 gene into herpesvirus of turkeys (HVT) using CRISPR/Cas9 gene-editing technology. The PCR and sequencing results obtained showed that the recombinant virus rHVT-VP2 was successfully constructed. Vaccination with rHVT-VP2 generated IBDV-specific antibodies in specific pathogen-free chickens starting from 2 weeks post-immunization. Seven days after the challenge, the autopsy results showed that the bursa atrophy rates of the rHVT-VP2, HVT, vaccine A, and positive control groups were 0%, 100%, 60%, and 100%, respectively, and the BBIX values were 1.07 ± 0.22, 0.27 ± 0.05, 0.64 ± 0.33, and 0.32 ± 0.06, respectively. These results indicate that rHVT-VP2 can provide 100% protection against a challenge with the IBDV (G2d), whereas vaccine A only provides partial protection. In conclusion, vaccination with the recombinant virus rHVT-VP2 can provide chickens with effective protection against variant IBDV (G2d).

Background/Objective: Duck virus hepatitis (DVH), caused by duck hepatitis A virus (DHAV), poses significant challenges to duck farming due to high mortality rates in young ducklings. Despite the widespread use of live attenuated vaccines, the genetic diversity within DHAV strains has diminished their cross-protection efficacy. This study aimed to evaluate the cross-protective efficacy of current DHAV-1 and DHAV-3 vaccines against genetically divergent wild strains. Methods: Phylogenetic analyses of the VP1 genes from DHAV-1 and DHAV-3 were conducted. Both DHAV-1 and DHAV-3 vaccines were tested in ducklings, with and without maternal-derived antibodies (MDA), through challenge trials with homologous and heterologous strains. Results: In the phylogenetic analysis, compared to vaccine strains, DHAV-1 and DHAV-3 field variant strains were classified into different genotypes. In ducklings without MDA, the DHAV-1 vaccine provided 60% survival against homologous strains by 2 days post-vaccination (DPV) and complete protection by 4 DPV, while survival rates against heterologous strains ranged from 40 to 60%. In ducklings with MDA, the DHAV-1 vaccine provided full protection with an additional vaccination for day-old ducklings against heterologous strains. The DHAV-3 vaccine conferred complete protection against both homologous and heterologous strains by 2 DPV, regardless of MDA presence. Conclusions: The DHAV-3 vaccine demonstrated robust cross-protection across genotypes, while the DHAV-1 vaccine showed limitations against genetically divergent strains. These findings highlight the necessity for genotype-matched vaccines and optimized immunization strategies to enhance protection against evolving DHAV field strains.

Antimicrobial resistance and pulsed-field gel electrophoresis (PFGE) genotypes of collected S. enterica ser. Gallinarum isolates were investigated to examine the epidemiological relationship between field outbreak isolates of S. enterica ser. Gallinarum. Thirty S. enterica ser. Gallinarum isolates collected from poultry farms with FT outbreaks from 2013 to 2018 in South Korea were analyzed. All isolates were resistant to at least 3 of the 18 antimicrobials tested and exhibited an MDR phenotype. All isolates showed resistance to streptomycin, sulfisoxazole, and colistin. One isolate was resistant to 9 antimicrobials. The antimicrobial resistance profile, streptomycin-sulfisoxazole-colistin-nalidixic acid-ciprofloxacin-gentamicin (18/30, 60.0%), was the most prevalent. PFGE types were classified into 10 groups with a 100% correlation cutoff in dendrograms for 30 field isolates. The dominant PFGE types were 1 (8/30, 26.7%), 4 (7/30, 23.3%), and 9 (5/30, 16.7%). Interestingly some isolates collected from the same and different companies had the same PFGE type. We reported a high MDR rate in S. enterica ser. Gallinarum isolates. The present study highlights the occurrence of horizontal spread and cyclic contamination of MDR S. enterica ser. Gallinarum within the same company. Furthermore, we showed cross-contamination between different companies. The characterization of these isolates would be helpful in the development of prevention and control strategies for MDR S. enterica ser. Gallinarum infection in South Korea.

Duck circovirus (DuCV) infections cause immunosuppression in ducks, potentially leading to significant economic losses for the duck farming industry. This study investigates the prevalence, genetic characteristics, and evolutionary trends of DuCV in Korea between 2013 and 2022. Samples from 184 farms across seven provinces were analyzed using polymerase chain reaction (PCR). The overall DuCV infection rate was 29.4% (54/184), with Jeollanam-do showing the highest prevalence (37.5%, 15/40). Ducks aged 3–6 weeks were most susceptible to infection, while ducklings younger than one week were rarely infected. Whole-genome sequencing was performed on 24 positive samples with phylogenetic analysis revealing that DuCV-1b is the predominant subtype in Korea (23/24 strains). Notably, a Korean DuCV-1a subtype strain was identified for the first time, showing close genetic relatedness to Chinese DuCV 1a strains. Novel subtype-specific amino acid variations in ORF1 and ORF2 were statistically analyzed and classified. Recombination analysis suggested some Korean DuCV-1b strains may have resulted from recombination events involving strains from different countries. This comprehensive study provides crucial insights into the current prevalence, genetic diversity, and evolutionary dynamics of DuCV in Korea, offering valuable data for developing effective control strategies and understanding the global epidemiology of this economically important avian pathogen.