Effects of a Pseudomonas H6 surfactant on rainbow trout and Ichthyophthirius multifiliis: In vivo exposure

The Pseudomonas H6 lipopeptide is a surfactant, which is able to eliminate various parasitic pathogens including the ciliate Ichthyophthirius multifiliis in vitro. This suggests an application for aquaculture purposes. However, further information on efficacy of the compound and possible immune modulation of surfactant-exposed fish should be gathered before usage at farm level is considered. We performed an in vivo infection experiment using rainbow trout fry (mean weight 4.6 g, mean length 7.6 cm) as hosts and I. multifiliis theronts as the parasitic pathogen. We compared infection level, immune gene regulation and immune cell density in gills of 1) no exposed control fish, 2) parasite exposed but untreated fish, 3) surfactant treated fish without parasite exposure, and 4) fish exposed both to parasites and surfactant. The surfactant concentration was 10 mg/L, the infection dosage 1000 theronts/fish and the exposure period 6 h. The parasite infection was recorded and samples were taken from rainbow trout gills at day 0 and 10 post-exposure. We performed an immunohistochemical investigation (detecting cells positive for MHC II, SAA, CD8, IgM, IgT and IgD) and measured the expression of genes encoding cathelidin-1, CD8, hepcidin, IFN γ, IgDs, IL-1β, IL-6 and SAA. Theront exposed fish (without surfactant treatment) became heavily infected whereas concomitant surfactant treatment (10 mg/l), along with parasite exposure, could prevent infection. A significant inflammation (upregulation of il-1β, il6, ifn γ, cathelicidin, hepcidin) was elicited in non-treated and parasite exposed fish but it was prevented by the surfactant treatment. When investigated 10 days after treatment no immune gene regulation was seen in fish exposed to surfactant only. The therapeutic effect may be due to a direct parasitical action of the surfactant, but it cannot be excluded that a modulation of the host immune reaction may influence the infection success

Comparative In Vitro and In Vivo Effects of Feed Additives on Rainbow Trout Response to Ichthyophthirius multifiliis

Control of the skin parasitic ciliate Ichthyophthirius multifiliis is currently based on laborious chemical and mechanical approaches, thus advocating for alternative control measures. Here, we show that the early development of trophonts (feeding stage residing in the epidermis) in the Rainbow Trout Oncorhynchus mykiss epidermis was inhib-ited at 5 d postinfection after the administration of feed additives, including garlic Allium sativum, oregano Origanum onites, thyme Thymus vulgaris, coriander Coriandrum sativum, and astaxanthin (a carotenoid derived from the alga Haematococcus pluvialis). However, no inhibition was observed at 8 d postinfection. We investigated whether the early inhibition was caused by (1) the feed additives’ direct effect on the parasite and/or (2) stimulation of the Rainbow Trout immune responses. Garlic exhibited the highest in vitro killing capacity toward theronts (the infective stage of the parasite), followed by oregano, thyme, and astaxanthin, whereas coriander had no in vitro parasiticidal effect. Immune reactions were measured by plasma lysozyme activity in Rainbow Trout after feeding and by recording immune gene expression in trout leukocytes that were exposed to feed additives. Oregano-fed fish showed a signifi-cantly (P < 0.05) elevated plasma lysozyme activity. Oregano and thyme—and to a lesser extent, garlic, astaxanthin, and coriander—induced a significant regulation of innate and adaptive immune genes in vitro. The results suggest that the investigated additives inhibit early parasite development directly as well as indirectly (i.e., by elevating the host immune response)