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

Abstract

Co-infection of rainbow trout with infections haematopoietic necrosis virus (IHNV)and Flavobacterium psychrophilum is known to occur, and it has been speculated that a combined infection can result in dramatic losses. Both pathogens can persist in fish in an asymptomatic carrier state, but the impact of co-infection has not been well characterized or documented. In this study, it was hypothesized that fish co-infected with F. psychrophilum and IHNV would exhibit greater mortality than fish infected with either pathogen alone. To test this, juvenile rainbow trout were co-infected with low doses of either IHNV or F. psychrophilum, and at 2 days post-initial challenge, they were given a low dose of the reciprocal pathogen. This combined infection caused high mortality (76.2%-100%), while mortality from a single pathogen infection with the same respective dose was low (5%-20%). The onset of mortality was earlier in the co-infected group (3-4 days) when compared with fish infected with F. psychrophilum alone (6 days) or IHNV (5 days), confirming the synergistic interaction between both pathogens. Co-infection led to a significant increase in the number of F. psychrophilum colony-forming units and IHNV plaque-forming units within tissues. This finding confirms that when present together in co-infected fish, both pathogens are more efficiently recovered from tissues. Furthermore, pathogen genes were significantly increased in co-infected groups, which parallel the findings of increased systemic pathogen load. Extensive tissue necrosis and abundant pathogen present intracellularly and extracellularly in haematopoietic tissue. This was pronounced in co-infected fish and likely contributed to the exacerbated clinical signs and higher mortality. This study provides novel insight into host-pathogen interactions related to co-infection by aquatic bacterial and viral pathogens and supports our hypothesis. Such findings confirm that mortality in fish exposed to both pathogens is greatly elevated compared to a single pathogen infection. K E Y W O R D S co-infection, Flavobacterium psychrophilum, IHNV, rainbow trout How to cite this article: Ma J, Bruce TJ, Oliver L, Cain KD. Co-infection of rainbow trout (Oncorhynchus mykiss) with infectious haematopoietic necrosis virus and Flavobacterium psychrophilum. J Fish Dis. 2019;42:1065-1076. https ://doi.

Aquaculture Reuse Water, Genetic Line, and Vaccination Affect Rainbow Trout (Oncorhynchus mykiss) Disease Susceptibility and Infection Dynamics

“…This is in line with the finding of Ma et al. ( 11 ) that mortality was higher when IHNV proceeded F psychrophilum infection, compared to vice versa. Regardless of the mechanism, there was strong evidence in our study that co-infection diminished vaccine efficacy, both in the field and laboratory.…”

“…These pathogens can co-occur at farm sites, although the natural co-infection rate of individual fish is unclear. Under laboratory conditions, reciprocal low dose challenge with both pathogens resulted in high mortality, reduced time-to-death, and exacerbated clinical signs (11) consistent with farm site observations of high mortality when both pathogens are present.…”

“…Others have reported a co-infection effect between F. psychrophilum and IHNV where presence of one exacerbates disease caused by the other, as well as pathogen loads. The mechanism(s) driving this phenomenon are unknown but immune suppression has been postulated ( 11 ). The lower F. psychrophilum external loads in IHNV vaccinated fish observed here highlight the likely importance of IHNV as a primary driver of co-infection.…”

In Vivo Experiments Provide Evidence That Flavobacterium psychrophilum Strains Belonging to Multilocus Sequence Typing Clonal Complex ST191 Are Virulent to Rainbow Trout

“…This is particularly important since this bacterium is frequently isolated alongside other fish pathogens (Evensen and Lorenzen 1997; Ma et al. 2019a), including other flavobacterial species (Kämpfer et al. 2012; Loch et al.…”

“…Contrary to CC-ST10 strains, however, laboratory-controlled experiments confirming the virulence of most CC-ST191 strains have not been performed. This is particularly important since this bacterium is frequently isolated alongside other fish pathogens (Evensen and Lorenzen 1997;Ma et al 2019a), including other flavobacterial species (Kämpfer et al 2012;Loch et al 2013;Loch and Faisal 2015), making its role as the primary etiological agent inconclusive.…”