Viral Replication and Lesions in BALB/c Mice Experimentally Inoculated with Porcine Circovirus Isolated from a Pig with Postweaning Multisystemic Wasting Disease

Abstract

Abstract. Eight-week-old BALB/c mice were either sham inoculated (control mice) or were inoculated intraperitoneally (IP) and intranasally (IN) with a single (sPCV mice) or multiple (mPCV mice) doses of porcine circovirus 2 (PCV2). Four control mice and 4 sPCV mice were sacrificed 7, 14, 28, and 42 days postinoculation (PI). All 4 mPCV mice were sacrificed 42 days PI. In addition, 7-day and 14-day pregnant BALB/c mice were either sham inoculated (control mice) or were inoculated IP and IN with a single dose of PCV2. Newborn mice were euthanatized 1, 8, and 15 days after birth. Necropsies were performed on all euthanatized mice and tissues were collected for histopathology, electron microscopy, in situ hybridization, and polymerase chain reaction (PCR). PCV2 replicated in 8-week-old BALB/c mice that were inoculated with PCV2 and caused fetal infection when inoculated into pregnant BALB/c mice at 7 days and 14 days of gestation. PCV was detected by in situ hybridization and PCR in sPCV mice on days 7, 14, 28, and 42 PI; in mPCV mice on day 42 PI; and in newborn mice from mothers inoculated with PCV at 7 days and 14 days of gestation at 1, 8, and 15 days after birth, but not in control mice. No clinical signs or gross lesions were found in sPCV or mPCV mice during the study. Microscopic lesions in sPCV mice and mPCV mice were characterized by expansion of germinal centers in lymphoid organs with large numbers of histiocytic cells and lymphoblasts, apoptosis of histiocytic cells in germinal centers, and mild lymphoid depletion of the paracortex. PCV nucleic acid was detected in the nuclei and cytoplasm of histiocytes and apoptotic cells in germinal centers in lymphoid tissues as well as in the nuclei of hepatocytes in the liver, in the nuclei of renal tubular epithelial cells, and in the cytoplasm of single lymphocytes in the thymus. Congenitally infected mice only had PCV nucleic acid detected in putative Kupffer cells in livers.

Experimental inoculation of porcine circoviruses type 1 (PCV1) and type 2 (PCV2) in rabbits and mice

“…The results of this study are in contrast to those obtained by Kiupel et al [11], who found some evidence of PCV2 replication in tissues of BALB/c mice. The use of different mice strains, PCV2 isolates, experimental designs and doses of viral inoculation may explain the differences observed in both experiments.…”

“…Phylogenetic analysis of all PCV2 isolates show the relationship of isolates from Europe, while North American sequences display a separate cluster [13]. This variation may have important implications for PCV2 diagnosis and research [7] and, therefore, it cannot be discarded that the PCV2 isolate variation may account for some of the differences observed between the present results and those from the study of Kiupel et al [11]. In our study, the inoculum was of relatively low titre and a single inoculation was performed; however, no references were available on the amount of virus adequate to infect non-porcine species, and the titration system used in the study from Kiupel et al [11] was not comparable with the one used in the present study.…”

“…From all mice, only one PCV1 inoculated mouse showed a very low serological titre to PCV1 at 20 days PI. The absence of an obvious seroconversion could be related to the short duration of the experimental period and antibodies may have appeared later, as previously described [11]. However, it cannot be elucidated whether this positive titre to PCV1 is a seroconversion related to the viral infection or whether it is due to the immunisation of the mouse.…”

“…This variation may have important implications for PCV2 diagnosis and research [7] and, therefore, it cannot be discarded that the PCV2 isolate variation may account for some of the differences observed between the present results and those from the study of Kiupel et al [11]. In our study, the inoculum was of relatively low titre and a single inoculation was performed; however, no references were available on the amount of virus adequate to infect non-porcine species, and the titration system used in the study from Kiupel et al [11] was not comparable with the one used in the present study. Another relative difference between the two experimental studies on PCV2 inoculation is the route of inoculation; Kiupel et al [11] used a combination of intranasal and intraperitoneal routes while in this study we only used the intraperitoneal route of inoculation.…”

“…However, Nayar et al [15] suggested that PCV2 can also infect bovids; PCV2 was demonstrated by PCR but tested tissues were negative by immunohistochemistry. Furthermore, a recent publication has shown the presence of very mild lymphoid lesions (enlargement of germinal follicles and very mild lymphocyte depletion) associated to PCV2 infection in BALB/c mice [11]. Porcine circovirus type 1 (PCV1) serological studies in non-porcine species have also led to controversial results.…”