Detection of Porcine Circovirus Type 2a and Pasteurella multocida Capsular Serotype D in Growing Pigs Suffering from Respiratory Disease

Simple Summary Bacterial and viral respiratory diseases are a major health problem in pig farms. Most often multiple infectious agents are involved in the development of clinical cases of respiratory diseases. It is necessary to conduct laboratory etiological tests on suspected cases. In January 2021, a 70-day-old pig herd in a pig farm in Hebei Province, China, had been suffering from respiratory problems. On the basis of this preliminary clinical diagnosis, this study carried out comprehensive laboratory detection including nucleic acid detection and sequencing, the isolation and identification of the pathogen, genetic evolution analysis, and genotype or serotype identification, as well as an analysis of the bacterial morphology characteristics and pathogenicity. The results showed that the co-infection of PCV2a and P. multocida capsular serotypes D was likely responsible for the disease. This study could provide important etiological information for effective control and treatment of the disease in pig farms. In order to diagnose a respiratory disease in a pig farm, the lungs, spleen, and lymph nodes of three dead pigs were collected for pathogen detection by PCR and isolation on the basis of preliminary clinical diagnosis. The virus isolate was identified by gene sequence analysis and Immunoperoxidase monolayer assay (IPMA). The bacterial isolate was identified by biochemical tests, 16S rDNA sequence analysis, and species- and serotype-specific PCR, and the pathogenicity was analyzed. Porcine circovirus type 2a (PCV2a) genotype from the lungs, spleen, and lymph nodes and Pasteurella (P.) multocida capsular serotypes D from the lungs were found. The PCV2a isolates could specifically bound the anti-PCV2-Cap polyclonal antibody. The 16S rDNA sequence of P. multocida isolates had 99.9% identity with that of the strain from cattle, and the isolate was highly pathogenic to mice. The results showed that the co-infection of PCV2a and P. Multocida capsular serotypes D should be responsible for the disease. The uncommon PCV2a is still prevalent in some pig farms besides the dominant PCV2d genotype. This study could provide important etiological information for effective control and treatment of the disease in pig farms.