This study investigated and compared the antimicrobial resistance patterns and ribotypes of Staphylococcus aureus isolated from pig tonsils and cow’s milk in China. A total of 90 isolates of S. aureus was included: 42 strains were isolated from tonsils of pigs and 48 from half-udder milk. The broth microdilution method and the double-disc diffusion test (D test) were used for antimicrobial susceptibility testing. The mecA gene for methicillin-resistant S. aureus (MRSA) and the ermA, ermB, ermC, and msrA genes for erythromycin-resistant strains were detected by polymerase chain reaction (PCR). The isolates were ribotyped with the Riboprinter system. The highest frequency of resistance was observed with clindamycin (91.1%), followed by penicillin (90.0%), and erythromycin (85.6%). All strains were susceptible to vancomycin and trimethoprim-sulfamethoxazole. The D test showed that 54.5% (42/77) of erythromycin-resistant isolates had the constitutive resistance phenotype and 45.5% (35/77) had the inducible resistance phenotype to clindamycin. A higher proportion of resistance to cephalosporins, macrolides, fluoroquinolones, and pleuromutilins was observed in pig isolates than in milk isolates (P < 0.05). The mecA gene was detected in all MRSA isolates; 89.6% of erythromycin-resistant strains harbored the ermC gene and 16.9% harbored the ermB gene. A total of 35 different ribogroups was found among the isolates investigated; 83.3% of pig strains belonged to 1 cluster with a similarity coefficient of 0.84. In contrast, 3 main clusters were observed among 68.8% of milk strains, which indicates a high degree of host specificity.

Encephalomyocarditis virus (EMCV) can infect many host species and cause acute myocarditis and sudden death in preweaned piglets. In this study, an EMCV strain (NJ08) was isolated from newborn pigs with clinical signs on a pig farm in mideastern China. It was identified by indirect immunofluorescence assay and reverse-transcription polymerase chain reaction. Experiments showed that the isolate could cause severe clinical symptoms and pathological changes in mice but no obvious clinical and pathological changes in commercial piglets. Complete genomic sequencing showed that the NJ08 strain was 78.3% to 100% identical with other isolates in regions coding for various proteins. Phylogenetic analysis showed that the NJ08 isolate belonged to subgroup Ia. This study confirmed that an EMCV isolate from pigs could be fatal to mice and provided new epidemiologic data on EMCV in China.

From September 2008 to December 2010, 112 Haemophilus parasuis strains were isolated from 536 pigs with clinical signs of Glässer's disease in South China, for a frequency of 21%. The 112 strains were subjected to serovar analysis by gel diffusion (GD) and indirect hemagglutination (IHA) tests and to genotype analysis by means of pulsed-field gel electrophoresis (PFGE). With a combination of the GD and IHA results, serovars 5 and 4 were found to be the most prevalent, at 23% and 17%, respectively, followed by serovars 2 (8%), 15 (7%), 13 (6%), and 12 (5%); 20% of the strains were nontypeable. The 112 strains were genetically diverse, with 85 genotypes identified (discriminatory index 0.992). The 89 typeable isolates belonged to 15 H. parasuis serovars displaying 63 different PFGE profiles. The 23 nontypeable strains displayed 22 different PFGE profiles. These findings confirmed that 15 serovars and diverse genotypes of H. parasuis were widely distributed in southern China.