What factors (risk or protective factors) regulate the time of the infection, the clinical presentation and the magnitude of an SRS outbreak? If these exist, how do they act or interact?
Retrospective studies have determined that water temperature, sea water time and a distance of less than 10 km from a farming center handling an SRS outbreak are possible risk factors for the appearance of an outbreak of this disease. Furthermore, the efficacy of antibiotic treatments have been evaluated against SRS outbreaks, demonstrating that a high pre-treatment mortality level is related to a lower probability of success for florfenicol or oxytetracycline treatments. Additionally, a weight increase in the specimens treated with florfenicol is directly related to an increase in pre-treatment mortality rates, as well as to a higher number of treatment failures.
What role do vector density and non-salmonid reservoirs play in the P. salmonis dynamic?
Samplings of native fish from zones with aquaculture activities showed various species susceptible to P. salmonis, primarily sea bass (Eleginops maclovinus) and silverside (Odontesthes regia), which presented a clear phylogenetic relationship with P. salmonis isolates linked to outbreaks in farming centers. Exposure of sea bass to P. salmonis extracts showed this species’ low susceptibility for developing SRS.
How do P. salmonis strains differ in terms of pathogenicity and virulence, and how does the host respond to these differences?
P. salmonis isolates of different origins were used to evaluate the effect of the external membrane vesicles in virulence. A difference in the protein expressions of the distinct P. salmonis strains was shown, possibly related to the different virulence that each isolate presented. Moreover, a study evaluating the pathogenicity of the SLGO-95 strain in rainbow trout showed that this species is highly susceptible to this strain. This allowed for an evaluation of the progression of the infection, starting from the attachment of the bacteria to the epithelium and the gills, to the spreading by means of blood vessels into deeper tissues, up until distribution in the whole organism.
What are the processes related to the emergence of resistance to drugs and what is the timeframe for the development of this resistance in P. salmonis?
The determinants that generate resistance to antimicrobials in P. salmonis have not been entirely explained; however, it has been shown that certain strains of P. salmonis present a mutation in the gyrA gene, associated with an alteration in quinolone functioning against these isolates, in addition to presenting various genes that express proteins, intensifying the resistance to florfenicol and tetracyclines by exporting these compounds. It was demonstrated that the use of florfenicol generates an overexpression of acrAB genes related with efflux systems in gram-negative bacteria.
What are the conditions that determine whether P. salmonis establishes an infection or whether it is removed by the immune system?
Various factors have been associated with the adaptation or persistence of P. salmonis in the host’s cells, mainly the overexpression of virulence factors. Recent studies have noted an increase in the expression of clpB and bipA genes, as well as of Toll-like receptors (TLR’s) and interleukin 1β (IL-1β) during P. salmonis infections in in vitro models of Atlantic salmon. Similarly, an increase in the expression of TLR’s, IL-1β, IL-10, GBP1 and in the proteins associated with the Icm/Dot secretion system has been noted during P. salmonis infections in experimental models derived from rainbow trout. When comparing the susceptibility to P. salmonis for Atlantic salmon specimens both resistant and susceptible to SRS, it has been shown that a pro-inflammatory immune response prevails in the susceptible specimens, whereas the mononuclear phagocyte system acts to a greater extent in resistant fish. Interestingly, hepcidin expression is higher following infection from inactivated P. salmonis isolates rather than with infective isolates. Another study demonstrated a difference in susceptibility between two Atlantic salmon populations having distinct gene expressions for iron homeostasis at a cellular level, showing that the quantity of intracellular iron diminishes in resistant strains, serving as a defense mechanism against P. salmonis infections; likewise, gene expression related to iron absorption, usage, storage and regulation was showed in P. salmonis, as well as various other genes related to the survival of bacteria inside the host. Using sequencing methods, it was possible to determine the differences in the presentation of genes linked to virulence factors between strains with different degrees of pathogenicity; however, it remains impossible to comprehensively determine what exactly causes the difference in virulence between different P. salmonis strains. The identification of molecules present in external membrane vesicles generated by P. salmonis helped determine the fact that these molecules contain virulence factors that help with the pathogenicity of the bacteria, although the mechanism is not yet completely understood. Concerning the P. salmonis entry and intracellular invasion, it was possible to determine that clathrin is the protein necessary for internalization, and that actin is a fundamental protein during a bacterial invasion in cytosol, as well as during an immune evasion.
What is the correlation between bacterial dynamics, damage and inflammation and the host? What is the correlation between the bacterial load (the abundance of P. salmonis) and the magnitude of the antibacterial immune response?
The evaluation of the P. salmonis load in different tissues coming from resistant specimens and from those susceptible to infection revealed that the bacterial load was significantly higher in susceptible specimens, and that regardless of the degree of resistance to P. salmonis, the muscle was a tissue that presented a high bacterial load in terms of kidney deterioration.
What is the mechanism involved in the immune response against multiple pathogens? What role do stress and the threshold for immunity play?
A study of the effect of a P. salmonis and C. rogercresseyi co-infection showed that there are significant differences in mortality for P. salmonis infections and for P. salmonis and C. rogercresseyi double infections, demonstrating the importance of the Caligidosis in the SRS dynamic. In addition, it was shown that the resistance mechanisms for P. salmonis infections and for co-infections with both pathogens were genetically different. Another study demonstrated that an increase in ovigerous females could be indirectly related to stress generated by diseases caused by P. salmonis or Neoparamoeba perurans.
What is the most effective antibiotic currently available? What is its sensitivity like?
The extended use of antimicrobials in aquaculture has caused the appearance of various strains resistant to different types of antibiotics. A study evaluated the susceptibility of different P. salmonis isolates to quinolone treatments, and enrofloxacin was the treatment that showed the best results in isolates both susceptible and resistant to flumequin and oxolinic acid deterioration. A comparison of strains with different years of origin showed that younger strains present higher levels of resistance to florfenicol, the most used antibiotic in the national industry, and that in the majority of the cases the resistance to florfenicol is higher than the resistance to oxytetracicline. However, another study researching 20 different P. salmonis strains showed that oxytetracicline and florfenicol are the most effective antibiotics, florfenicol being more effective than oxytetracicline.
How should alternative treatments (prebiotics, additives, immunostimulants, etc.) be developed and used? How can their optimal functioning be assured?
Different compounds have been evaluated as potential immune system stimulators. Prolactin has been evaluated for its immunostimulant function, demonstrating that it permits an overexpression of TLR’s and of other molecules associated with the immune response. Regarding plant or seaweed derived compounds, in vitro tests showed that molecules derived from Heliotropium spp. have an antimicrobial effect on in vitro P. salmonis. Another approach helped demonstrate that a fucoidan and diterpene-based phytopharmaceutical administered as a food additive against a P. salmonis pathogenic strain helped increase the inflammatory cytokine expression and diminish the cytotoxic effect in vitro, as well as diminish the mortality in in vivo tests. Similarly, the use of antibodies obtained from buds immunized against P. salmonis substantially improved the protection against P. salmonis in in vitro and in vivo tests, showing its potential as a possible therapeutic agent. However, when using arachidonic acid and vitamin E, no significant changes or improvements in the immune system were noted.
What are the areas of the genome (the genetic markers) that codify the resistance mechanisms against C. rogercresseyi and P. salmonis?
Recently, various studies regarding the Atlantic salmon genome have related certain genetic markers with resistance to P. salmonis, mainly genes involved in the immune response.
How can genetic selection for resistance to diseases interfere with ideal productive and sanitary characteristics?
When evaluating the correlation between the characteristics that determine resistance to P. salmonis and the harvest weight for Coho salmon, it was shown that the selection of characteristics that foment resistance for this pathogen are negatively related to the characteristics that promote a higher crop weight. Similarly, it was demonstrated that the selection of characteristics in Atlantic salmon that favor a higher weight for a given specimen are related slightly negatively to the characteristics that confer resistance to P. salmonis, and they are positively related with characteristics that determine resistance to C. rogercresseyi.
What are the areas of the genome (the genetic markers) for the pathogens that codify resistance mechanisms against drugs?
It has been shown that the expression of genes associated with aquaporins is different during an infection in Atlantic salmon and in Coho salmon. An evaluation of genes associated with resistance to C. rogercresseyi was carried out and showed that a gene associated with the alpha-1 collagen, linked to cytokine signaling, is expressed in resistant families.The overexpression of genes associated with iron homeostasis was shown in Atlantic salmon infected with C. rogercresseyi, however there is no clear evidence as to whether the parasite depends on the use of heme groups during its life cycle. It was estimated that the maximum amount of parasites tolerated by Atlantic salmon before generating a physiological imbalance is 6.
How can antigen absorption, processing and presentation for the fish’s immune system be improved at the mucosa level as a booster vaccine strategy?
An oral vaccine against P. salmonis was recently developed which allows the generation of adequate IgM levels in the specimens for a duration of approximately 2800 and 3200 ATU (Accumulated thermal units) , allowing for the maintenance of optimal antibody levels during the fattening stage, following the decline of IgM levels caused by the intraperitoneal vaccine. The scientific information published between 2014 and 2016 is for the most part based on experimental research. However, there are still many questions to be answered through applied research, preferably by means of observational studies that evaluate the most relevant aspects of previous experimental studies.