Epidemiological surveillance and genetic evolution of Riemerella anatipestifer in Guangdong Province

Objective  To clarify serotype, drug resistance and genetic evolution of Rimerella anatipestifer in Guangdong.

Method  The isolates of R. anatipestifer were isolated and identified from samples of the large-scale duck farms. Serotype was identified by glass agglutination test. The minimum inhibitory concentration was tested by double dilution method in test tube, and the drug sensitivity was analyzed. Whole genome sequencing technology was used to analyze the sequence characteristices and construct the genetic evolution tree of the core genome.

Result  A total of 168 isolates of R. anatipestifer were isolated and identified. All serotype 1, 2, 3, 5, 6, 7, 8 and 10 were prevalent. Serotype 1 was dominant, reaching 54.17%(91/168), followed by serotype 2 (27.97%, 47/168). The 48 representative isolates were used for drug sensitivty test and sequence analysis. The isolates showed high drug resistance to gentamicin, kanamycin and ciprofloxacin hydrochloride, and the drug resistance rates were all more than 80%. The drug resistance rates to oxytetracycline, tetracycline hydrochloride, chlortetracycline hydrochloride, ofloxacin, norfloxacin, sulfadimidine and sulfamethoxydiazine were all more than 60%, and the drug resistance rates to amoxicillin, cefotaxime and spectinomycin were all less than 30%. The 48 representative isolates were resistant to 5−12 kinds of drugs, and there were 44 types of drug resistance spectrum. The whole genome sequences of 46 isolates were obtained successfully, and six drug-resistant genes were detected. The detection rates of drug-resistant geneserm(F) and tet(X) were 73.91% (34/46) and 82.60% (38/46), respectively. In addition, 95.65% (44/46) of the isolates carried more than two drug-resistant genes at the same time. Total 18 isolates (39.13%, 18/46) were typed successfully with 11 kinds of ST. Further genetic tree construction based on the core genome showed that all the sequenced isolates had a high similarity with the isolates from China in the multi-locus sequence typing (MLST) database, and mainly existed in the dominant clones of Clade 1 and Clade 3.

Conclusion  Serotype 1 is superior in the investigated R. anatipestifer isolates, and the drug resistance is serious. The drug-resistant genes and resistant phenotype have certain relevance. ST type has a high diversity. The genetic background of tested strains was similar with Chinese strains in MLST database. The results of this study can provide a basis for vaccine immunity prevention and drug therapy of R. anatipestife disease as well as for understanding the genetic evolution characteristics of R. anatipestife.