| Subclinical mastitis(SCM), as known as subclinical mammitis, is a kind of inflammation reaction, induced by inflammatory factor, occurs in mammary gland. This kind of inflammation presents no visible clinical symptoms, only with the change of physicochemical property and the SCC in milk. Therefore SCM is usually difficult to observe. Pathogenic microorganism infection is the most important reason for SCM, over 150 pathogenic microorganisms are related with SCM, and Staphylococcus, E. coli and Streptococcus are the 3 main pathogenic microorganisms. SMC leads to a reduction of lactation yield and low quality of milk products, causing huge economic losses in dairy enterprise. In the mean time, the SCM milk contains pathogenic microorganisms, causing threaten to food safety. Therefore, the actively screening and treatment of SCM has signality meaning for improve economic returns, guarantee food safety and human health protection.Antibiotic therapy is the preferred treatment for SCM. But with the long-term use of drugs, more and more bacteria presented resistance to antibiotics, brings difficulties to treatment of disease. Intrinsic resistance and acquired resistance are the 2 main routes for antibiotic resistance. And due to the broad spectrum resistance, acquired resistance is the most important mechanism for bacteria resistant. Integron and gene cassettes resistance system is composed by integron and gene cassettes 2 parts. Integron could not move by itself, the movement of integron achieved by phage and plasmid. Gene cassettes, unlike integron, is a kind of moveable gene element, exists in the environment harboring resistance genes. Once the gene cassettes are captured by integron, the resistance gene would be integrated and the bacteria will present antibiotics resistance. There are 9 kinds of integrons widely distributed in bulk of bacteria, acquire and transmit antibiotic resistance by capturing more than 40 kinds of gene cassettes which harboring different kinds of resistance genes.In this study, 9 dairy cattle breeding farms(with different breeding scale) in the main dairy cattle breeding area of Liaoning province were chosen, and SCC were tested to evaluate the prevalence rate of SCM. Furthermore, E. coli isolates were separated and identified. The results showed that the prevalence rate of SCM in this area was 34.7%. And the prevalence rate of Shenyang, Fuxing and Jinzhou area was 34.5%, 34.2% and 35.8% respectively. 88 E. coli isolates were obtained from SCM cases, the separation rate was 63.3%.The minimal inhibitory concentration(MIC) of 21 antibiotics(commonly used in clinical treatment) against 88 E. coli were tested following the method suggested by CLSI. The results showed that isolates were highly resisted to thiamphenicol, florfenicol, sulfamonomethoxine and sulfamethoxazole, the resistance rate was 97.7%, 88.6%, 87.5% and 73.9% respectively. Isolates were sensitive to mequindox, ampicillin, ceftiofur, ofloxacin and ciprofloxacin, the sensitive rate was 95.5%, 94.3%, 94.3%, 93.2% and 90.9%. All isolates were sensitive to gentamicin and amikacin, no resistance isolate was observed.Using specific primers and PCR method to detect the containing situation of integron-gene cassettes system in 88 E. coli isolates obtained from dairy cows with SCM. The results showed that 21 isolates were positive for class 1 integron, the positive rate was 23.9%, no isolate was positive for class 2 integron. 5 kinds of gene cassettes were detected from the class 1 integron positive isolates. The antibiotics resistance genes were dfr A12-aad A2, aar-3, dfr A15-aad A2, aad A1-dfr A15 and smr, the separation rate was 81.0%, 9.5%, 61.9%, 19.0% and 47.6% respectively.According to the pathogenic microorganism separation, identification and antibiotic sensitivity test results, confirmed the treatment plans for SCM. All dairy cows with SCM were divided into 4 groups, they were untreated group, penicillin-streptomycin group, ceftiofur sodium group and gentamicin group. The results showed that, in untreated group the self-cure rate was 8.1%, and the cure rate in other 3 groups was 76.9%, 96.9% and 80.6% respectively. Ceftiofur sodium presented the best therapeutic effect. While the cure rate of gentamicin group, which treatment plan was established by the results of bacteria isolation and MIC tests, was higher than the traditional penicillin-streptomycin group.In conclusion, the treatment plan established by bacteria identification, antibiotic sensibility tests could ensure the treatment effect of dairy cow with SCM, and reduced the use of broad spectrum and extended-spectrum antibiotics, providing solid theoretical foundation for the treatment plan of dairy cow SCM. |
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