Infectious pancreatic necrosis virus establishes an asymptomatic carrier state in kidney leucocytes of juvenile Atlantic cod, Gadus morhua L.

Juvenile Atlantic cod (10 g) were infected with infectious pancreatic necrosis virus (IPNV) by intraperitoneal injection and cohabitation. Fish showed no signs of disease but IPNV could be re-isolated from kidney tissue for up to 12 weeks. On weeks 2, 5, 8, 10, 11 and 12 following infection, kidney leucocytes were fractionated on Percoll gradients, and cells separated into plastic adherent and non-adherent cell populations after overnight incubation. IPNV was detectable in lysates of both cell populations and in supernatants by culture in CHSE-214 cells. Wells containing 105−106 macrophages had an IPNV TCID50 of about 103/well and in serially diluted macrophages the minimum number of cells required to detect virus ranged from 101 to 104. These data indicate that about one in 104 macrophages were infected and the mean number of virus/infected cell was about 10. Replication of IPNV in the macrophages was low as the titre of the virus in macrophage lysates did not increase between days 1 and 3 of culturing the macrophages, but virus was released into the supernatant over this time. [ABSTRACT FROM AUTHOR]