Abstract: 【Objective】 This study was conducted to understand the main pathogenic bacteria causing abortion in Yili horses,and provide scientific basis for their clinical guidance of medication. 【Method】 In this experiment,35 tissue samples of heart,liver,spleen,lung,kidney,umbilical cord and amniotic membrane from 5 spontaneously aborted foals were collected for bacterial enrichment and culture,purification,PCR identification and sequencing,and the isolated strains were tested for drug resistance and pathogenicity. 【Result】 Through bacterial isolation and culture,morphological observation,6 strains were isolated from the tissue of the aborted foal.One strain was a large grayish-white mucus colony on the agar plate,easy to stretch when picked,Gram-negative,short,thick,rod-shaped,individually or in pairs arranged bacilli,suspected to be Klebsiella sp.One strain was glossy surface,with an untidy edge of semi-transparent or opaque,Gram-negative,short,bluntly rounded at the ends,suspected to be Bacillus sp. One stain was stered into clusters of cocci,suspected to be Staphylococcus spp. One stain was round,white or creamy-white colony with smooth surfaces,Gram-negative,and long rods with bluntly rounded ends,suspected to be Escherichia coli.One strain was greyish-white round colony with a small bulge and β-hemolysis ring on a sheep blood agar plate,Gram-positive cocci,in pairs or in long chains,suspected to be Streptococcus spp.One strain was a round colony with a smooth surface and purple color,with neat edges,in the Salmonella chromogenic medium,and was Gram-negative,and short bacilli with bluntly rounded ends,suspected to be Salmonella.The results of PCR amplification of 16S rRNA gene of six isolates showed that the strains all showed a single band at 1 540 bp.BLAST alignment of 16S rRNA gene sequencing results showed that one isolate had more than 95% similarity to Klebsiella sp. published in GenBank, and five isolates had more than 97% similarity to Citrobacter freundii, Staphylococcus lentus, Escherichia coli, Streptococcus equi subsp. zooepidemicus and Salmonella published in GenBank, respectively.The virulence gene detection results of each strain were consistent with the expected results.The six strains of isolated pathogenic bacteria were finally identified as Klebsiella sp., Citrobacter freundii,Staphylococcus lentus,Escherichia coli, Streptococcus equi subsp.zooepidemicus and Salmonella,and named 23-KLB-16S,23-NM-16S,23-HM-16S,23-DC-16S,23-ML-16S and 23-SM-16S，respectively.The similarity and genetic evolution results showed that the isolates 23-KLB-16S and Klebsiella sp.,23-NM-16S and Citrobacter freundii,23-HM-16S and Staphylococcus lentus,23-DC-16S and Escherichia coli,23-ML-16S and Streptococcus equi subsp.zooepidemicus,23-SM-16S and Salmonella reference strains were similar and closely related.The results of drug sensitivity showed that the isolates were multi-drug resistant,generally more sensitive to tetracyclines and fluoroquinolones,and resistant to lincosamide.The results of the mouse attack test showed that the LD₅₀ of the above six pathogenic bacteria were 1.54×10²,2.59×10⁷,3.62×10⁷,7.02×10⁷,6.08×10⁶ and 8.15×10⁵ CFU/mouse,respectively,with Klebsiella sp.,Streptococcus equi subsp.zooepidemicus,and Salmonella being more pathogenic.The autopsy results of mice showed varying degrees of bruising and enlargement of liver and spleen,and hemorrhage in lung. 【Conclusion】 Six main pathogenic strains,including Klebsiella sp.and Citrobacter freundii,were isolated from tissue samples of aborted foals,and mixed infections were found.All six strains were multidrug-resistant and generally sensitive to tetracyclines and fluoroquinolones antibiotics,but resistant to lincosamides.Among them,Klebsiella sp.,Streptococcus equi subsp. zooepidemicus and Salmonella had strong pathogenicity.

Key words: abortion; foal; histopathological changes; isolation and identification; pathogenicity