Why vaccines fail against Piscirickettsiosis in farmed salmon and trout and how to avoid it: A review

Abstract

Piscirickettsiosis is the most severe, persistent, and damaging disease that has affected the Chilean salmon industry since its origins in the 1980s. As a preventive strategy for this disease, different vaccines have been developed and used over the last 30 years. However, vaccinated salmon and trout frequently die in the sea cages and the use of antibiotics is still high demonstrating the low efficiency of the available vaccines. The reasons why the vaccines fail so often are still debated, but it could involve different extrinsic and intrinsic factors. Among the extrinsic factors, mainly associated with chronic stress, we can distinguish: 1) biotic including coinfection with sea lice, sealions attacks or harmful algal blooms; 2) abiotic including low oxygen or high temperature; and 3) farm-management factors including overcrowding or chemical delousing treatments. Among the intrinsic factors, we can distinguish: 1) fish-related factors including host’s genetic variability (species, population and individual), sex or age; 2) pathogen-related factors including their variability and ability to evade host immune responses; and 3) vaccine-related factors including low immunogenicity and poor matches with the circulating pathogen strain. Based on the available evidence, in order to improve the development and the efficacy of vaccines against P. salmonis we recommend: a) Do not perform efficacy evaluations by intraperitoneal injection of pathogens because they generate an artificial protective immune response, instead cohabitation or immersion challenges must be used; b) Evaluate the diversity of pathogen strains in the field and ensure a good antigenic match with the vaccines; c) Investigate whether host genetic diversity could be improved, e.g. through selection, in favor of better and longer responses to vaccination; d) To reduce the stressful effects at the cage level, controlling the co-infection of pathogens and avoiding fish overcrowding. To date, we do not know the immunological mechanisms by which the vaccines against P. salmonis may or may not generate protection. More studies are required to identify what type of response, cellular or molecular, is required to develop effective vaccines.

The Effect of an Attenuated Live Vaccine against Salmonid Rickettsial Septicemia in Atlantic Salmon (Salmo salar) Is Highly Dependent on Water Temperature during Immunization

“…Active propagation of the vaccine strain is likely to be a prerequisite for exploiting the full potential of live attenuated vaccines. These results therefore identify an important pitfall in the use of AJ LiVac SRS and could provide an explanation for some of the recent controversy around the efficacy of this vaccine [8,9]. Surveillance programs and scientific projects that aim at optimizing the use of this and other live vaccines for poikilothermic animals must take this factor into account and preferably utilize tools that can verify that the animals have indeed been exposed to a propagating vaccine organism.…”

“…Around 95% of Atlantic salmon in Chilean regions X and XI (stocked with 141 million individuals) were vaccinated with AJ LiVac SRS in 2022. A decline in Chilean antibiotic use was observed in the years after the introduction of the vaccine, but in recent years, the use of antibiotics has started to rise again [8]. This has led to some controversy regarding the efficacy of the vaccine, and Chilean researchers recently reported that they found no effect of vaccination with AJ LiVac SRS in a laboratory challenge of vaccinated fish [9].…”

IgYs: on her majesty’s secret service

“…The hyperimmune IgYs were able to increase the survival rate of shrimps from 40% (normal diet) to 75% (0.5% IgY in diet), 7 days after infection with V. Parahaemolyticus ( 418 ). A similar approach with the antigen-heteroantibody complex used as an immunogen was applied in the case of the invention related to the production of egg yolk antibodies specific to intracellular bacteria Piscirickettsia salmoni ( 419 ), the causative agent of piscirickettsiosis, a highly severe and prevalent disease in salmon ( 420 ). The immunogen composed of inactivated cells from P. salmonis and the complex of these cells and the mouse IgG against them.…”

The effect of PACAP administration on LPS-induced cytokine expression in the Atlantic salmon SHK-1 cell line.

“…These clearly represent a significant economic loss in a critical food industry. While fish are vaccinated, these vaccines are not as effective as they could be due to a variety of reasons [18] , [19] , [20] . Thus, it is common practice to treat potential disease with antibiotic enhanced feed.…”

“…Thus, it is common practice to treat potential disease with antibiotic enhanced feed. Indeed in 2021, the Chilean aquaculture industry alone used 450 tons of antibiotics [20] . Given that fish do not eat all the feed and excrete excess antibiotics, this can lead to antibiotic resistance in both fish and human pathogens.…”