Mouthrot in farmed Atlantic salmon

Mouthrot is a major health and welfare problem in farmed Atlantic salmon (Salmo salar) smolts in the Pacific Northwest (West Coast of North America), particularly in the first few months following saltwater transfer. This disease, associated with the bacterium Tenacibaculum maritimum, is the main reason the Atlantic salmon farming industry in this region continues to use antibiotics. Mouthrot results in large economic losses due to direct fish mortality, as well as the cost of treatments and poor performance of treated fish. Affected smolts die with very little external or internal clinical signs other than characteristic small yellow plaques in the mouth. This clinical presentation is visibly different to that of tenacibaculosis, the disease commonly associated with T. maritimum in other regions of the world or in other marine fish species. T. maritimum is the most extensively studied member of the Tenacibaculum genus; however, its role in causing mouthrot in British Columbia (BC) has not been the focus. The main objective of this study was to gain more knowledge about T. maritimum in BC and its connection to mouthrot in the Pacific Northwest, and to make steps towards developing management tools that would help decrease the use of antibiotic treatments and improve fish welfare. Genotyping of T. maritimum isolates collected from natural outbreaks of mouthrot on Atlantic salmon farms in BC showed the presence of two genetic strains of the bacterium based on 11 housekeeping genes. These strains are most closely related genetically to strains collected from lumpsuckers (Cyclopterus lumpus) with skin lesions and Atlantic salmon in Norway, as well as Atlantic salmon gills in Chile. The division of the BC isolates into two genetic groups is further supported by a serological analysis that showed that there are two serological groups that match the genetic strains. Representative isolates from the two identified BC genetic strains were used to develop a bath challenge model with Atlantic salmon smolts, which is necessary for testing management tools such as treatments and vaccines. Through these experiments, it was demonstrated that T. maritimum is the causative agent of mouthrot in BC without the need for other stressors or co-infections. The main pathology in Atlantic salmon smolts infected with BC T. maritimum strains are mouth lesions that damage the tissues surrounding the teeth causing a disease that is similar to periodontal disease in mammals. The pathological changes are focal, severe, and occur very rapidly with very little associated inflammation. A cohabitation experiment also showed that T. maritimum readily transfers from infected smolts to naïve ones. As mouthrot mainly affects smolts, further studies are needed to investigate the link between smolt status and susceptibility to developing mouthrot. With the knowledge gained from the genetic characterisation and serological analysis of the BC T. maritimum isolates, whole cell inactivated adjuvanted vaccines were created and tested using the developed challenge model. Despite giving an antibody response in immunised fish, the vaccines did not protect the smolts against mouthrot induced through a bath infection. Future research needs to focus on preventative tools, including other types of vaccines such as immersion or live-attenuated. This will require a better understanding of the pathogenesis of mouthrot.