Coinfection takes its toll: Sea lice override the protective effects of vaccination against a bacterial pathogen in Atlantic salmon

Abstract

Vaccination is considered crucial for disease prevention and fish health in the global salmon farming industry. Nevertheless, some aspects, such as the efficacy of vaccines, can be largely circumvented during natural coinfections. Sea lice are ectoparasitic copepods that can occur with a high prevalence in the field, are frequently found in co-infection with other pathogens, and are highly detrimental to fish health. The aim of this case-control study was to evaluate the interaction between the detrimental effects of coinfection and the protective effects of vaccination in fish. We used the interaction between the sea louse Caligus rogercresseyi, the bacterial pathogen Piscirickettsia salmonis, and their host, the Atlantic salmon Salmo salar, as a study model. Our results showed that coinfection decreased the accumulated survival (AS) and specific growth rate (SGR) of vaccinated fish (AS = 5.2 ± 0.6%; SGR = −0.05 ± 0.39%) compared to a single infection of P. salmonis (AS = 42.7 ± 1.3%; SGR = 0.21 ± 0.22%). Concomitantly, the bacterial load and clinical signs of disease were significantly increased in coinfected fish. Coinfection may explain the reduced efficacy of vaccines in sea cages and highlights the need to test fish vaccines in more diverse conditions rather than with a single infection.

The nature and consequences of co‐infections in tilapia: A review

“…These authors demonstrated that the vaccinated Nile tilapia experimentally parasitized with T. heterodentata, G. cichlidarum and I. multifiliis showed lower relative protection survival (RPS) than vaccinated fish without further parasitic infection. This finding is similar to what has been reported in Atlantic salmon, Salmo salar , where sea louse, Caligus rogercresseyi , may explain the reduced efficacy of vaccines against the bacterial pathogen, Piscirickettsia salmonis , in salmon sea cages (Figueroa et al, 2017).…”

“…Moreover, immunosuppression can cause vaccines to lose their protectiveness (Figueroa et al, 2017; Martins et al, 2011). For example, it has been demonstrated tilapias vaccinated against S. iniae and parasitized with several ectoparasites such as Trichodina , Gyrodactylus and I. multifiliis showed lower relative percentage survival (RPS) in comparison with vaccinated and parasite‐free fish (Martins et al, 2011).…”

Co‐infection of rainbow trout (Oncorhynchus mykiss) with infectious hematopoietic necrosis virus and Flavobacterium psychrophilum

“…It would be recommended that diagnostic approaches for fish pathologists routinely include screening for both bacterial and viral pathogens. Vaccination may be option to potentially mitigate the severity of co‐infection, although it has been recently reported that parasitic co‐infections may compromise vaccine efficacy for some bacterial pathogens (Figueroa et al, ). Further experimentation and documentation of the incidence and impact of co‐infections in fish would be important aid in the development of new strategies for disease control or prevention.…”

Methods for the diagnosis of bacterial fish diseases

“…Of course, there will be implications for diagnosis and immunoprophylaxis (Kotob et al 2016). In particular, it has been considered to be highly likely that co-infection may well reduce the beneficial effect of vaccination (Figueroa et al 2017). Examples of co-infection have reached the scientific literature, and incidences appear to be increasing possibly as scientists become aware of, and actively look for, evidence.…”