Porcine reproductive and respiratory syndrome virus: a persistent infection

Abstract

Persistent infection with porcine reproductive and respiratory syndrome virus (PRRSV) was shown in experimentally infected pigs by isolation of virus from oropharyngeal samples for up to 157 days after challenge. Four 4 week old, conventional, PRRSV antibody-negative pigs were intranasally inoculated with PRRSV (ATCC VR-2402). Serum samples were collected every 2 to 3 days until day 42 post inoculation (PI), then approximately every 14 days until day 213 PI. Fecal samples were collected at the time of serum collection through day 35 PI. Oropharyngeal samples were collected at the time of serum collection from 56 to 213 days PI by scraping the oropharyngeal area with a sterile spoon, especially targeting the palatine tonsil. Turbinate, tonsil, lung, parotid salivary gland, spleen, lymph nodes and serum were collected postmortem on day 220 PI. Virus isolation (VI) on porcine alveolar macrophage cultures was attempted on all serum, fecal and oropharyngeal samples, as well as tissues collected postmortem. Postmortem tonsil tissues and selected fecal samples were also assayed for the presence of PRRSV RNA by the polymerase chain reaction (PCR). Serum antibody titers were determined by IFA, ELISA and SVN. Virus was isolated from all serum samples collected on days 2 to 11 PI and intermittently for up to 23 days in two pigs. No PRRSV was isolated from fecal samples, but 3 of 24 samples were PCR positive, suggesting the presence of inactivated virus. Oropharyngeal samples from each pig were VI positive 1 or more times between 56 and 157 days PI. Oropharyngeal samples from 3 of 4 pigs were VI positive on days 56, 70 and 84 PI. Virus was isolated from one pig on day 157 PI, 134 days after the last isolation of virus from serum from this animal. Virus was isolated from oropharyngeal samples for several weeks after the maximum serum antibody response, as measured by IFA, ELISA and SVN tests. All tissues collected postmortem were VI negative and postmortem tonsil samples were also negative by PCR. An important element in the transmission of PRRSV is the duration of virus shedding. The results of this study provided direct evidence of persistent PRRSV infection and explain field observations of long-term herd infection and transmission via purchase of clinically normal, but PRRSV infected, animals. Effective prevention and control strategies will need to be developed in the context of these results.

Characterization of the carrier state in porcine reproductive and respiratory syndrome virus infection

“…Selection of samples to be assayed for virus was based on previous reports of persistent infection (Benfield et al, 1997;Wills et al, 1997 (Beaver et al, 2000). Prior to euthanasia, serum, peripheral blood leukocytes, and oropharyngeal scrapings (Wills et al, 1997) were collected.…”

“…Prior to euthanasia, serum, peripheral blood leukocytes, and oropharyngeal scrapings (Wills et al, 1997) were collected. Following euthanasia, lung, bronchoalveolar lavage, tonsil, and tracheobronchial lymph node samples were collected.…”

Introduction, persistence and fade-out of porcine reproductive and respiratory syndrome virus in a Dutch breeding herd: a mathematical analysis

“…Although it was not shown to be transmissible, virus was isolated from oropharyngeal samples of individual pigs for up to 157 days after experimental infection [13]. Nevertheless, virus transmission to susceptible contacts was already shown for up to 56 days after experimental infection [5].…”

“…Persistence can occur when R 1 and new susceptible animals are added to the population at a sufficiently high rate. Although several studies have demonstrated that PRRSV persists in individual pigs [10][11][12][13][14], little is known about PRRSV transmission among pigs [5,15,16] and about its persistence within herds. The objective of this longitudinal study was to investigate the population dynamics of PRRSV infection and to quantify transmission within a breeding herd.…”

Different immunological features of two genetically distinct type 2 porcine reproductive and respiratory syndrome (PRRS) viruses

“…Passively acquired SVN antibody alone was proven to prevent viremia and reproductive failure in pigs challenged with virulent PRRSV strains (Osorio et al, 2002), suggesting the critical role of SVN antibody in the control of virus infection. Nonetheless, cell-mediated immunity (CMI) appears to be essential for the clearance of PRRSV since PRRSV has been detected in lungs and lymph nodes despite the presence of SVN antibodies in serum or bronchoalveloar lavage fluid (Wills et al, 1997;Labarque et al, 2000;Bierk et al, 2001). The mechanisms for the weak induction of protective immunity by PRRSV have been explored in previous studies.…”