Investigation On Major Bovine Mastitis-causing Bacteria With Emphasis On Coagulase-negative Staphylococci

Coagulase-negative staphylococci (CNS) have become the most common cause of bovine mastitis. Mastitis caused by CNS in most cases is subclinical, or has mild clinical signs.CNS mastitis increases milk somatic cell count (SCC) in the infected udder. The increase in milk SCC is usually moderate compared with mastitis caused by other pathogens such as Staphylococcus (S.) aureus and streptococci.However, high prevalence of CNS mastitis can affect bulk milk SCC in a herd. CNS comprises almost 50 different species. They are differentiated from S. aureus using a test to gauge the ability of bacteria to coagulate plasma. Many CNS species have been isolated from milk, hair coat, udder skin and teat canals in cows. CNS is normally not identified at species level.In this study, bovine mastitis caused by coagulase-negative staphylococci were studied, in order to determine the main CNS species caused in clinical mastitis and subclinical mastitis, and the effect of CNS infection to milk somatic cell, as well as the CNS to antimicrobial resistance. Such knowledge is essential for effective control and prevention of CNS mastitis.1.The main pathogens of bovine clinical mastitis.During November 2009～March 2010, after extensive investigation of clinical bovine mastitis conducted in Hangzhou, Jinhua and Taizhou of Zhejiang province, the incidence of clinical mastitis in Zhejiang province is about 7.00% totally,6.75%, 6.90%,7.69% in Hangzhou, Jinhua and Taizhou respectively.Acquisition milk samples of clinical mastitis for bacteriological isolation and identification. The results indicated that the main isolated pathogens were staphylococcus and streptococcus, and the infection rate CNS is about 40.8% totally, 43.3%,36.0%,42.9%in Hangzhou, Jinhua and Taizhou respectively.31 strains of CNS isolated from clinical mastitis were identified with TH-16S staphylococci biochemical systems into 9 sub-species, which were mainly as Staphylococci simulans, Staphylococci xylosus, Staphylococci epidermidis, Staphylococci capitis and Staphylococci lentus, accounting for 19.3%,16.1%,12.9%, 12.9% and 12.9%.2.The main pathogens of bovine subclinical mastitis, and the relationship with SCC.Milk samples (314) were randomly collected from a dairy farm in Anhui Province.The main mastitis-causing pathogens were isolated and identified, and the milk somatic cell counts (SCC) were detected. The sub-species and drug resistance of coagulase-negative staphylococci (CNS) with high proportion were studied. The results showed that only 8.33% of 120 milk samples with low SCC(SCC< 25×104cells/mL) was bacteria (CNS) isolated. While the rate of 194 milk samples with high SCC (SCC≥25×104cells/mL) was as high as 72.2%, including Staphykococcus aureus, CNS, E. coli, Streptococcus dysgalactiae and so on. CNS was the most frequently isolated (28.8%), mainly as Staphylococci sciuri, Staphylococci epidermidis, Staphylococci hominis. The milk samples'SCC of pathogen infection udder was significantly higher (P<0.05), Staphylococci sciuri and Staphylococci epidermidis infection can also cause the SCC significantly higher (P<0.05).3.The susceptibility test of CNS isolated from bovine mastitis against commonly used antibacterial agents.The susceptibility test of 31 strains of CNS isolated from clinical mastitis and 52 strains of CNS from sub-clinical mastitis was carried out by K-B method against 11 commonly used antibacterial agents, respectively. The results showed that the CNS isolated from clinical mastitis exhibited high resistance to Penicillin, Gentamicin, Erythromycin, with resistance frequency of 90.3%,80.6%,67.7%, respectively, and resistance to Oxacillin, Tetracycline, Ampicillin-sulbactam, Ciprofloxacin, Clindamycin and Cefoxitin, with resistance frequency of 64.5%,58.1%,54.8%, 38.7%,29.0% and 22.6%, respectively, but sensitive to Rifampicin and Vancomycin, with resistance frequency of 16.1%,12.9%, repectively; The CNS isolated from sub-clinical mastitis also exhibited high resistance to Penicillin, Gentamicin, Erythromycin, with resistance frequency of 71.2%,53.8%,82.7%, respectively, and resistance to Oxacillin, Tetracycline, Ampicillin-sulbactam, Clindamycin, Ciprofloxacin and Rifampicin, with resistance frequency of 44.2%,32.7%,26.9%, 23.1%,19.2% and 11.5%, respectively, but sensitive to Cefoxitin and Vancomycin, with resistance frequency all of 3.85%.In summary, CNS are a frequent cause of bovine clinical and sub-clinical mastitis in modern dairy herds, they have become the most common mastitis bacteria isolated from milk samples.CNS can cause persistent infection within the breast, resulting in milk somatic cells was significantly increased, causing economic losses can not be underestimated. CNS isolated from clinical mastitis was mainly as Staphylococci simulans, Staphylococci xylosus, Staphylococci epidermidis, Staphylococci capitis and Staphylococci lentus, and which was mainly as Staphylococci sciuri, Staphylococci epidermidis, Staphylococci hominis in sub-clinical mastitis.CNS effect well of antibiotic treatment, but easily develop drug resistance.CNS exhibited high resistance against commonly used antibacterial agents, such as Penicillin, Gentamicin, Erythromycin, but sensitive to Vancomycin.