| Summary: | 碩士 === 國立屏東科技大學 === 獸醫學系 === 88 === Abstract
A high prevalence of porcine reproductive and respiratory syndrome virus (PRRSV) infection has been noted in recent years. This disease causes significant damage to pig producers in Taiwan. Results from experimental and field studies have demonstrated that PRRSV could cause persistent infections in pigs. The purpose of this study was to establish a method for quantifying PRRSV and applying this data to research regarding the persistent infections by this disease. To understand the distribution of PRRSV in the pig population, sequential sera were obtained from pigs at various ages in PRRSV infected pig herds. Serum neutralization (SN) antibody tests, isolation of virus and viral quantitation using reverse transcription - quantitative competitive polymerase chain reaction (RT-qcPCR) was performed for collected sera. Viral isolation and quantitation were also performed for organs or secretions collected from naturally infected pigs to investigate the distribution of PRRSV in individual pigs. Using locally isolated PRRSV (strain THF-1), plasmid target sequence (pTS) and internal control (pIC) were constructed and a standard curve (y = 4.94x + 0.18, r 2 = 0.98) was established for RT-qcPCR. This assay could measure PRRSV cDNA levels ranging from 102 to 108 copies/ml. Percentages of pigs with positive SN titers dropped from 81.3% at the age of 1 week old to the lowest level of 6.3% at the age of 9 weeks old. The percentages of pigs with viremia reached a maximum of 100﹪at the age of 9 weeks. The serum concentrations measured ranged from 104 to 107.3 copies/ml . For some pigs, viremia started at the age of 3 weeks old and persisted for over 12 weeks. In naturally infected pigs, the highest mean concentration of PRRSV was found in the lungs with a range between 105.7 to 108.9 copies/ml. Over 105 copies/ml of PRRSV also existed in samples such as sera, tonsils, inguinal lymph nodes, pulmonary lymph nodes, mesentery lymph nodes and spleens. PRRSV concentrations of 105.8 and 104.8 copies/ml were observed in the nasal and oral secretions, respectively. The concentrations of PRRSV in alimentary tracts and urine were within the levels of 102.5 and 103.5 copies/ml. There was no significant difference (P>0.1) among the mean concentrations of PRRSV in pigs with various stages of infection. These results indicated that pigs reached the highest percentage of viremia when the passive immunity dropped to the lowest level. Lungs and lymphoid tissues were the major target organs for PRRSV. PRRSV present in the nasal and oral secretions, alimentary tracts and urine was the main source for infection and resulted in the persistence of PRRS.
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