Endometritis is one of the major diseases causing a high sow elimination rate and 6.6% of breeding sows death in large-scale farms . Many pathogens, including virus such as Japanese encephalitis, porcine reproductive and respiratory syndrome virus, and pseudorabies virus, bacteria such as E. coli, Streptococcus, Staphylococcus, clostridia, Pseudomonas aeruginosa, and parasites (e.g. trichomonas), can cause pig endometritis [1,2,3,4,5,6]. If disinfection is not strict, the breeding process, including artificial insemination, birth, midwifery, and abortion, can lead to infections and result in endometritis [1, 16, 17].
To improve the breeding and reproduction rate, artificial insemination is widely used in intensive pig farms. However, semen is easily contaminated with microbial pathogens carried by boar or contaminated during semen fluids collection and storage process [16, 17]. In the current study, the clinical endometritis cases were likely correlated with contamination of semen and artificial insemination because symptoms of endometritis emerged just 3 to 5 days after insemination. Therefore, we carried out an etiological examination to determine its causes.
In order to ensure the accuracy of detection results, we combined phenotypic and molecular biological approaches to determine the causative agent of sow endometritis, and set up vaginal secretion samples of clinical healthy sows and gilts in the same farm as the case controls. In addition to bacteriological testing, PCR was employed for detecting classical swine fever virus (CSFV), porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV2) and porcine circovirus type 3 (PCV3) in the sow vaginal secretions and did not find these viral nucleic acids (supplementary data). These results suggest that the sow endometritis was caused by a bacterial infection most likely due to contamination of semen and artificial insemination.
Indeed, we isolated and identified a Gram-positive Streptococcus from the vaginal secretions of the sick sows based on Gram-staining, microscopy, catalase and biochemical assays. The isolated Streptococcus with β-hemolysis in short chains arrangement under the microscope is negative to catalase, and positive to V-P test and to acid formation of sorbitol, mushroom sugar, sucrose and glucose, and hydrolysis of arginine and esculin. The most of these characteristics are consistent with those of Streptococcus porcinus in spite of exhibition of some variable reaction in the study [9, 10].
16S rRNA gene sequences contain highly and moderately conserved sequence regions, and highly variable sequence region, which is a powerful molecular target and widely used in classification and identification of bacteria from clinical isolates [18,19,20]. 16S rRNA gene sequencing is a very useful method for differentiating S. porcinus from S. pseudoporcinus isolates, and for determining the taxonomic status of isolates from animal, human and dairy sources [21, 22]. In order to further clarify the subspecies of the Streptococcus isolate in this study, we amplified the16S rRNA DNA sequence and conducted a comparative phylogenetic analysis. The results suggest that the Streptococcus isolate from vaginal secretions is likely to be Streptococcus porcinus because its 16S rRNA sequence is highly similar to those of the Streptococcus porcinus reference strains from both humans and pigs.
Bacterial resistance is an important global concern of public health especially in the last 20 years. The rapid increase in drug-resistant strains has caused a serious problem in clinical treatments of human and animal infectious diseases [23,24,25]. Resistant isolates of animal origin are very common and easily disseminated between animal and humans from contaminated food . Wang et al.  reported that multidrug-resistant Salmonella Typhimurium isolates dramatically increased among the salmonella isolated from human, animal and retail milk between 2011 to 2016. Resistances of S. porcinus for erythromycin, minocycline, sulfamethoxazole-trimethoprim, streptomycin and tetracycline were described in previous study . In this study, we observed that the S. porcinus strain had multi-drug resistance to antibiotics including aminoglycosides, quinolones, macrolides, and tetracyclines except being sensitive to some β- lactams such as penicillin G, cephalothin, cefazolin, cephradine and cefuroxime. These data suggest that sensitive antibiotics such as penicillin G or some cephalosporins could be used for treating this disease through local delivery into the uterus following washing and sterilizing vulva with disinfectant fluid. Meanwhile, it is necessary to implement more strict clinical veterinary medicine practice, including disinfection procedure during semen collection and artificial insemination.
Though an experimental proof would be of high interest for demonstrating the occurrence of endometritis during artificial insemination by detection of semen and instruments for insemination or animal experiment, we did not do these tests in this study because the semen and other relevant insemination materials had not been gotten then, and animal experiment was limited in fields. Nevertheless, we estimated that the occurrence of endometritis could be associated with artificial insemination since sows unfertilized artificially were not attacked by this disease in this farm. In addition, it happened just on days 3–5 after artificial insemination.
In summary, the identification of S. porcinus provides important information for determining the cause of sow endometritis. The evaluation of antibiotic susceptibility of the isolate could contribute to the treatment of sow endometritis from bacterial infection. It should be noted that bacterial multi-drug resistance is a great challenge to the current disease control.