A Novel "Small Round Virus" Inducing Poult Enteritis and Mortality Syndrome and Associated Immune Alterations

Abstract

The role of a novel "small round virus" (SRV) isolated from poult enteritis and mortality syndrome (PEMS) cases in inducing PEMS and associated immune alterations was examined in this study. Specific-pathogen-free and conventional poults were orally challenged with SRV and/or turkey coronavirus and monitored for clinical signs. Intestines, thymus, bursa, and spleens were examined for SRV antigen at various days postinoculation (DPI). Peripheral blood lymphocytes (PBLs), thymocytes, and splenic lymphocytes from inoculated poults or lymphocytes isolated from healthy poults after incubation with SRV in vitro were examined for lymphoproliferative potential against concanavalin A (Con A). The incidence of lymphocyte subpopulations in the peripheral blood and thymic lymphocytes of SRV-challenged poults was examined by flow cytometry. The results of these studies showed that the SRV challenge induced diarrhea, growth suppression, and atrophy of thymus and bursa resembling those of PEMS in field and/or experimental infections. The SRV antigen was detected in intestinal tissues soon after infection (i.e., at 2 and 4 DPI), whereas lymphoid tissues such as thymus, bursa, and spleen were positive for SRV antigen starting at 4 DPI until 8 DPI, suggesting virus translocation to lymphoid organs. The responsiveness of PBLs to Con A at 2 DPI was significantly reduced in all virus challenge groups (e.g., 28% and 22% in the SRV-alone group in studies 1 and 2, respectively) below the uninfected group. However, this suppressed response was no longer evident in the SRV group by 7 DPI. The SRV incubation with normal thymocytes and splenocytes in vitro resulted in significantly reduced lymphoproliferative response against Con A (41.2% and 10.49% reductions at 1:50 SRV dilution vs. controls in thymocytes and splenocytes, respectively). Flow cytometry analysis revealed a sudden decline at 2 DPI in the numbers of CD4- CD8+ lymphocyte subset in PBLs of SRV-infected poults. However, by 8 DPI, SRV-challenged poults had relatively higher CD4- CD8+ lymphocytes in PBLs. On the contrary, thymocytes had higher percentages of CD4- CD8+ lymphocytes at 2 and 4 DPI and reached comparable levels at 8 DPI in controls and SRV-infected poults. No differences were observed in CD4+ CD8- lymphocyte numbers in controls vs. SRV-infected poults. The findings of these studies imply that SRV may be a promising primary etiologic agent of PEMS. Furthermore, the SRV infection may compromise the lymphocyte-mediated immune defenses by reducing lymphoproliferation and the CD4- CD8+ (presumably T-cytotoxic cells) lymphocytes during the acute stage of SRV infection.