The host range of IPNV is very broad, and the virus has been isolated from a wide range of aquatic organisms. The virus also replicates in a variety of piscine cell lines. Other birnaviruses show a more restricted host range both in vivo and in vitro. IBDV, for instance, is only known to infect poultry and replicates in vitro in a few avian primary cell cultures , although the virus replicates in several cell lines of mammalian origin [16, 17]. The entomobirnavirus DXV most likely also has a narrow host range, as it is thus far only isolated from fruit fly . This indicates that there are important differences in the molecular mechanisms governing the infection processes of these birnaviruses. Investigating the IPNV host range could reveal some of these mechanisms.
Six cell lines of human, leporid, monkey or bovine origin were inoculated with IPNV and investigated by IFAT. Virus specific staining was detected in all cell lines after incubation at 15°C, 25°C and 37°C. IFAT does not distinguish between internalized or surface associated virus. Confocal microscopy analysis was therefore performed on IPNV infected HE and Vero cells. This clearly demonstrated that IPNV was present in the cytoplasm in punctate structures not associated with the plasma membrane. These structures may represent cytoplasmic structures such as endosomes or caveolae . In addition, VOPBA showed that IPNV bound in a specific manner to a membrane protein of approximately 85 kDa from RK-13 cells. Taken together, these findings indicate that IPNV is able to enter into these cells by specific endocytic mechanisms. The difference in molecular size between the IPNV binding proteins from piscine cells and the rabbit cell line could indicate that IPNV attaches to different receptors, and may even represent different entry mechanisms. This, in turn, could explain the broad host range of IPNV.
An early step in viral replication is the production of viral mRNA. IPNV mRNA lacks a poly-A tail, making discrimination between viral genomic RNA and mRNA difficult by PCR. A significant increase in viral RNA over time would, however, indicate that replication has taken place. Thus, the cell cultures were inoculated with low titres of IPNV, and viral RNA levels were detected by RRT-PCR over time. The amount of target nucleic acids was identical in each reaction, and a good correlation was found between the two parallels that were run in the RRT-PCR. Therefore, the Ct values could be used as an indicator of the relative amount of IPNV specific RNA. No significant change in the IPNV RNA level was observed at 5 dpi in HE and Vero cells. As the cells did not grow well at 25°C, incubation at this temperature was terminated at 5 dpi. When the cells were kept at 37°C, on the other hand, they grew to confluence within 7 dpi. No cpe was observed in any of the cell cultures at 7 dpi, even after inoculation with high levels of IPNV. We therefore did not expect any virus in the cell supernatants when inoculating cells with low levels of IPNV. Instead of passaging the virus medium onto fresh cultures, the infected cells cultures were passaged with co-cultivation. For each cell passage, we observed a decrease in the viral RNA levels, most likely due to a dilution of the RNA with each passage. This indicates that no significant level of replication occurs in these cell lines. The gradual decrease in IPNV RNA observed after 2 weeks and longer incubation suggests that the virus multiplication process is arrested before replication of viral nucleic acid starts. Even though the RdRp has been shown to be active at 37°C , other factors, such as structural differences between piscine and mammalian proteins or antiviral activity in the mammalian cells, could explain the absence of IPNV replication. The virus may also simply be trapped in the punctate structures described above. When studying RGNNV infection in human cell cultures, Adachi et al. (2008)  showed that circumventing some obstacles early in the infection process may lead to the production of progeny RGNNV.