Decreased water temperature enhance Piscine orthoreovirus genotype 3 replication and severe heart pathology in experimentally infected rainbow trout

Piscine orthoreovirus genotype 3 (PRV-3) was first discovered in Denmark in 2017 in relation to disease outbreaks in rainbow trout (Oncorhynchus mykiss). While the virus appears to be widespread in farmed rainbow trout, disease outbreaks associated with detection of PRV-3 have only occurred in recirculating aquaculture systems, and has predominantly been observed during the winter months. To explore the possible effects of water temperature on PRV-3 infection in rainbow trout, an in vivo cohabitation trial was conducted at 5, 12, and 18 degrees C. For each water temperature, a control tank containing mock-injected shedder fish and a tank with PRV-3 exposed fish were included. Samples were collected from all experimental groups every 2nd week post challenge (WPC) up until trial termination at 12 WPC. PRV-3 RNA load measured in heart tissue of cohabitants peaked at 6 WPC for animals maintained at 12 and 18 degrees C, while it reached its peak at 12 WPC in fish maintained at 5 degrees C. In addition to the time shift, significantly more virus was detected at the peak in fish maintained at 5 degrees C compared to 12 and 18 degrees C. In shedders, fish at 12 and 18 degrees C cleared the infection considerably faster than the fish at 5 degrees C: while shedders at 18 and 12 degrees C had cleared most of the virus at 4 and 6 WPC, respectively, high virus load persisted in the shedders at 5 degrees C until 12 WPC. Furthermore, a significant reduction in the hematocrit levels was observed in the cohabitants at 12 degrees C in correlation with the peak in viremia at 6 WPC; no changes in hematocrit was observed at 18 degrees C, while a non-significant reduction (due to large individual variation) trend was observed at cohabitants held at 5 degrees C. Importantly, isg15 expression was positively correlated with PRV-3 virus load in all PRV-3 exposed groups. Immune gene expression analysis showed a distinct gene profile in PRV-3 exposed fish maintained at 5 degrees C compared to 12 and 18 degrees C. The immune markers mostly differentially expressed in the group at 5 degrees C were important antiviral genes including rigi, ifit5 and rsad2 (viperin). In conclusion, these data show that low water temperature allow for significantly higher PRV-3 replication in rainbow trout, and a tendency for more severe heart pathology development in PRV-3 injected fish. Increased viral replication was mirrored by increased expression of important antiviral genes. Despite no mortality being observed in the experimental trial, the data comply with field observations of clinical disease outbreaks during winter and cold months.

Automated summary

Piscine orthoreovirus genotype 3 (PRV-3) was found in rainbow trout and it was discovered that low water temperature allowed for significantly higher PRV-3 replication in rainbow trout. In addition, more severe heart pathology development was observed in PRV-3 injected fish at low water temperature. These findings align with field observations of disease outbreaks during winter and cold months.