Current characterisation of Pasteurella spp. isolates from cultured lumpfish and salmon in Norway, shows that these isolates genetically separate into two distinct groups. Challenge experiments with the bacteria indicate that lumpfish may be infected by isolates from both groups. However, only one of the variants has been isolated from diseased salmon. Hence, when lumpfish are infected with Pasteurella spp., it is vital to identify the variant of the isolate, both to identify the potential risk of infection to salmon and to get an overview of the distribution of the different variants of Pasteurella spp. in Norwegian aquaculture. Therefore, Pharmaq Analytiq has developed a real-time RT-PCR assay to distinguish between these two known variants of Pasteurella spp., which seem to be a challenge, particularly on the western coast of Norway.
Pasteurellosis has sporadically occurred in Norwegian aquaculture since the first isolate was described in the late '80s. A few years later, the disease was described in Scottish salmon and was subsequently determined to be caused by Pasteurella skyensis. Along the Norwegian coast, the number of pasteurellosis outbreaks has been relatively stable up until the spring of 2018, when the number of salmon outbreaks increased markedly during 2018-2019. A high number of outbreaks has also been registered in 2020, and the prevalence of pasteurellosis seems to have become a persistent problem for producers. During the same time, the use of lumpfish for sea-lice control in salmon farming has increased, and there is speculation that the lumpfish may be a possible route of infection and the reason why the number of outbreaks in salmon is increasing. Accordingly, we wanted to take a closer look at whether Pasteurella spp. isolated from lumpfish and salmon are the same species, and characterise the variations. We then isolated Pasteurella spp. from lumpfish and salmon with clinical signs of pasteurellosis. The field isolates were further whole genome sequenced, and analyses shows that the two isolates are different, but closely related to Pasteurella skyensis, the closest relative.
Picture to the right: Lumpfish infected with Pasteurella spp. in challenge trials, with the characteristic white spots typical for Pasteurella infection.
Picture to the left: Nina Sandlund, PhD and Anita Rønneseth, PhD.
Photo: Nina Sandlund/Institute of Marine Research.
During spring 2020, a challenge experiment with lumpfish and salmon was completed in collaboration with the Institute of Marine Research (IMR), University of Bergen (UoB) and Pharmaq Analytiq. The main aim was to investigate possible infection of the two fish species with Pasteurella spp., and to determine whether clinical signs observed in the field could be reproduced in a controlled challenge experiment. Naïve salmon and lumpfish were challenged with Pasteurella isolates from both species. In addition, possible co-habitant infection was investigated, by placing naïve salmon together with Pasteurella-infected lumpfish.
In the challenge trial, the salmon did not develop any clinical signs of disease, regardless of which isolate was used. No clinical signs of disease were seen amongst salmon placed together with lumpfish with developing pasteurellosis either. Put together, this indicates that Pasteurella spp. infecting lumpfish, may not infect the salmon. This again may indicate that other factors, such as other microbes, extensive handling or lice treatment, may give Pasteurella spp. an opportunity to infect the already weakened salmon, which consequently develop pasteurellosis.
By contrast to salmon, the lumpfish developed clinical signs of pasteurellosis after being challenged with salmon or lumpfish isolates. From the current results, it seems like the salmon variant is more potent than the lumpfish variant, with manifestation of clinical signs and mortality appearing earlier for lumpfish in this challenge group.
As the salmon variant results in a more rapid and severe disease, it would be helpful to know whether you have the salmon variant or the lumpfish variant at your site. Pharmaq Analytiq offers an assay that can distinguish between the two known variants of Pasteurella spp. The assay is specific for Pasteurella spp. isolated from salmon and lumpfish, respectively. As the current knowledge of other possible variants of Pasteurella is scarce, we recommend screening for Pasteurella with our general assay and the type assay to find out which of the two variants is present. Masters student Lukas Lorentzen (Pharmaq Analytiq (PA) and UoB), is characterising Pasteurella isolates from both lumpfish and salmon. For this work, he needs more isolates to be sent from different production areas in Norway with outbreaks of pasteurellosis and growth medium for sampling of Pasteurella is available from Pharmaq Analytiq.