In infectious pancreatic necrosis virus carrier Atlantic salmon, Salmo salar L., post-smolts, almost all kidney macrophages ex vivo contain a low level of non-replicating virus.

The level of infection by infectious pancreatic necrosis virus (IPNV) of kidney macrophages from 12 asymptomatic carrier Atlantic salmon post-smolts was studied. Kidney leucocytes were fractionated on 34/51% Percoll gradients, allowed to adhere to plastic wells overnight, washed to remove non-adherent cells and cultured for up to 7 days with or without renewal of medium on day 3. On day 1, supernatants were harvested, macrophages were counted, lysed and IPNV in the supernatants and lysates was titred in chinook salmon embryo (CHSE-214) cells. The multiplicity of infection ranged between 1:2.2 and 1:7.4 (virus:macrophages). On day 3, the titres of IPNV in macrophage lysates decreased and in wells where the medium was renewed on day 3, IPNV was no longer detectable on day 7. In the supernatants, one fish was positive for IPNV on day 1, four fish on day 3 but none were detectably positive on day 7. In parallel wells in which the medium was not renewed, on day 7 IPNV was detected in macrophage lysates of three fish and the supernatants were also IPNV positive in two of these fish. This suggests that virus might be shed from infected macrophages and then reinfect other macrophages. When macrophages were serially diluted in wells and cultured for 24 h, IPNV could be cultured from macrophage lysates of wells containing between two and 70 macrophages. These results indicate that a very high proportion of the adherent kidney macrophages must be infected with very few non-replicating virions.