Abstract
AbstractControl of the skin parasitic ciliateIchthyophthirius multifiliisis 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 TroutOncorhynchus mykissepidermis was inhibited at 5 d postinfection after the administration of feed additives, including garlicAllium sativum, oreganoOriganum onites, thymeThymus vulgaris, corianderCoriandrum sativum, and astaxanthin (a carotenoid derived from the algaHaematococcus 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 significantly (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).
Highlights
The parasitic ciliate Ichthyophthirius multifiliis is the causative agent of white spot disease in freshwater fish worldwide (Buchmann et al 2001; Matthews 2005; Dickerson 2006; Jørgensen 2016)
Several of the compounds mentioned can partly control I. multifiliis infections but are less successful than malachite green treatment (Lahnsteiner and Weismann 2007). Treatments using chemicals such as formalin may damage the fish epidermis, leaving the fish susceptible to other pathogens (Buchmann et al 2004). Alternatives to this laborious practice may be found in functional feeding, as various feed additives have been shown to decrease the susceptibility of Rainbow Trout Oncorhynchus mykiss to I. multifiliis infection (Xueqin et al 2012)
The highest number of parasites was recorded in the control fish, and the lowest number was found in the groups receiving astaxanthin at 100 mg/kg feed
Summary
The parasitic ciliate Ichthyophthirius multifiliis is the causative agent of white spot disease ( known as ichthyophthiriosis) in freshwater fish worldwide (Buchmann et al 2001; Matthews 2005; Dickerson 2006; Jørgensen 2016). The infection is characterized by macroscopically visible white spots in the epidermis and may be associated with heavy mortality and morbidity (Hines and Spira 1973; Dickerson 2006; Buchmann 2019). Treatments using chemicals such as formalin may damage the fish epidermis, leaving the fish susceptible to other pathogens (Buchmann et al 2004). Alternatives to this laborious practice may be found in functional feeding, as various feed additives have been shown to decrease the susceptibility of Rainbow Trout Oncorhynchus mykiss to I. multifiliis infection (Xueqin et al 2012). Astaxanthin feeding was associated with reduced mortality due to Edwardsiella tarda infection (Kim et al 2012)
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