Treatment of fish parasites. 9. Effects of a medicated food containing malachite green on Ichthyophthirius multifiliis Fouquet, 1876 (Hymenostomatida, Ciliophora) in ornamental fish

Lethal and sublethal exposure of Hemichromis bimaculatus (Gill, 1862) to malachite green and possible implications for ornamental fish

Malachite green (MG) is a triphenylmethane dye that is widely used in aquaculture as a fungicide, bactericide, ectoparasiticide, and antiprotozoal. There is great debate regarding the potential for this compound to trigger adverse effects. Here, we review the previous findings and then evaluate the lethal and sublethal effects of MG in the species Hemichromis bimaculatus (jewelfish). The lethal concentration for 50% of the fish in 96 h was 1 mg/L. We observed a dose-dependent increase in the percentage of fish mortality as well as physical and behavioral changes. We further found that the highest tested sublethal dose significantly increased the DNA damage index identified using the comet assay (74.97 ± 13.8 at a significant level of P < 0.05 for the 0.75 mg/L concentration), but did not significantly alter the results of the micronucleus test. Although our results suggest that MG confers risks on exposed fish, the findings were significant only at the highest exposure concentration (0.75 mg/L). At lower concentrations (0.25 mg/L and 0.5 mg/L), no adverse effect was observed. The maximum MG concentration recommended for use in ornamental fish farming is 0.2 mg/L. Therefore, our results suggest that, specifically for the parameters analyzed in this work, MG does not have any adverse effect when users strictly adhere to the recommended concentration criteria for ornamental fish.

The in vitro and in vivo effect of tannic acid on Ichthyophthirius multifiliis in zebrafish (Danio rerio) to treat ichthyophthiriasis

The in vitro antiparasitic effect of polyphenol tannic acid (TA) on Ichthyophthirius multifiliis theronts and tomonts was evaluated. In vitro antiparasitic assays revealed that TA in a dose- and time-dependent pattern through the damage of parasite plasma membrane could be 100% effective against I. multifiliis theronts at concentrations of 8 and 11 ppm during all the exposure times (45-270 min). The tomonts proliferation was completely inhibited by penetrating TA (at least 15 ppm for 22-hr exposure) into encysted tomont across the cyst wall. However, 10 ppm TA could result in a ninefold decrease in the population of live tomonts compared to the control group (p < 0.05). Although at theront concentrations of over 6,000 per zebrafish (Danio rerio), a 100% prevalence of ichthyophthiriasis during a 5-day exposure was recorded, results of in vivo tests showed that the parasite that pretreated up to 10 ppm TA for 70 min had not any capability to infect the studied zebrafish population. The acute toxicity (96 hr-LC₅₀ ) of TA for zebrafish was 19.51 ppm. Thus, TA can be considered as a natural therapeutant to safely and efficiently improve the health of aquatic systems by controlling ichthyophthiriasis.

Identification and effect of two flavonoids from root bark of Morus alba against Ichthyophthirius multifiliis in grass carp

Ichthyophthirius multifiliis (Ich) is an important ciliate that parasitizes gills and skin of freshwater fish and causes massive fish mortality. In this study, two flavonoids (kuwanons G and O) with anti-Ich activity were isolated by bioassay-guided fractionation from the root bark of Morus alba, an important plant for sericulture. The chemical structures of kuwanons G and O were elucidated by spectroscopic analyses. Kuwanons G and O caused 100% mortality of I. multifiliis theronts at the concentration of 2 mg/L and possessed a median effective concentration (EC50) of 0.8 ± 0.04 mg/L against the theronts. In addition, kuwanons G and O significantly reduced the infectivity of I. multifiliis theronts at concentrations of 0.125, 0.25, 0.5, and 1 mg/L. The median lethal concentrations (LC50) of kuwanons G and O to grass carp were 38.0 ± 0.82 and 26.9 ± 0.51 mg/L, which were approximately 50 and 35 times the EC50 for killing theronts. The results indicate that kuwanons G and O have the potential to become safe and effective drugs to control ichthyophthiriasis.

Parasiticidal effects of Morus alba root bark extracts against Ichthyophthirius multifiliis infecting grass carp

Ichthyophthirius multifiliis (Ich), an important fish parasite, can cause significant losses in aquaculture. To find efficacious drugs to control Ich, the root bark of white mulberry Morus alba was evaluated for its antiprotozoal activity. Bark was powdered and extracted with 1 of 5 organic solvents: petroleum ether, chloroform, ethyl acetate, acetone, or methanol. The extracts were concentrated, dissolved in 0.1% (v/v) DMSO, and used for anti-Ich trials. Acetone and ethyl acetate extracts significantly reduced the survival of Ich tomonts and theronts. In vitro, acetone extract at 25 mg l-1 killed all non-encysted tomonts, at 50 mg l-1 eradicated all encysted tomonts, and at 8 mg l-1 caused mortality of all theronts. Ethyl acetate extract at 50 mg l-1 eliminated all non-encysted tomonts, at 100 mg l-1 killed all encysted tomonts and terminated tomont reproduction, and at 8 mg l-1 killed all theronts. Low concentrations (2 and 4 mg l-1) of acetone and ethyl acetate extracts could not kill all theronts after 4 h exposure, but a significant decrease in theront infectivity was observed following 30 min of pretreatment with the extracts. The 96 h LC(50) values of acetone and ethyl acetate extracts to grass carp were 79.46 and 361.05 mg l-1, i.e. much higher than effective doses for killing Ich theronts (8 mg l-1 for both extracts) and non-encysted tomonts (12.5 and 25 mg l-1, respectively). Thus M. alba extract may be a potential new, safe, and efficacious drug to control Ich.

Antiprotozoal activity of medicinal plants against Ichthyophthirius multifiliis in goldfish (Carassius auratus)

Ichthyophthiriasis is a widespread disease in aquaculture and causes mass mortalities of fish. The development of new antiprotozoal agents for the treatment of Ichthyophthirius multifiliis infections is of increasing interest. The aim of the present study was to investigate the efficacy of 30 medicinal plants against I. multifiliis. The results showed that the methanol extracts of Magnolia officinalis and Sophora alopecuroides displayed the highest antiprotozoal activity against theronts, with 4-h LC(50) values estimated to be 2.45 and 3.43 mg L(-1), respectively. Concentrations of 2.5, 5.0, 10.0, and 20.0 mg L(-1) of M. officinalis extracts resulted in tomont mortality of 9.7, 43.7, 91.3, and 100% at 20 h, respectively. From 40 to 320 mg L(-1) of S. alopecuroides extracts, tomont mortality increased from 29.7 to 100%. Antiprotozoal efficacy against settled tomonts (2 and 10 h) was also applied; the results indicated that encysted I. multifiliis tomonts were less susceptible to these plant extract treatments. In vivo experiments demonstrated that high concentrations of M. officinalis and S. alopecuroides extracts could kill tomonts, and M. officinalis significantly reduced its reproduction (P < 0.05). These results suggested that the methanol extracts of M. officinalis and S. alopecuroides have the potential to be used as an eco-friendly approach for the control of I. multifiliis.

Ichthyophthirius multifiliis Fouquet and Ichthyophthiriosis in Freshwater Teleosts

The ciliate Ichthyophthirius multifiliis is an important pathogen of freshwater teleosts occurring in both temperate and tropical regions throughout the world. The disease, ichthyophthiriosis, accounts for significant economic losses to the aquaculture industry, including the ornamental fish trade, and epizootics in wild fish populations can result in mass kills. This review attempts to provide a comprehensive overview of the biology of the parasite, covering the free-living and parasitic stages in the life cycle, host-parasite interactions, and the immune response of host and immune evasion strategies by the parasite. Emphasis on the immunological aspects of infection within the fish host, including molecular studies of i-antigens, reflects the current interest in this subject area and the quest to develop a recombinant vaccine against the disease. The current status of methods for the control of ichthyophthiriosis is discussed, together with new approaches in combating this important disease.

Parasites in cultured and feral fish

Parasites, causing little apparent damage in feral fish populations, may become causative agents of diseases of great importance in farmed fish, leading to pathological changes, decrease of fitness or reduction of the market value of fish. Despite considerable progress in fish parasitology in the last decades, major gaps still exist in the knowledge of taxonomy, biology, epizootiology and control of fish parasites, including such 'evergreens' as the ciliate Ichthyophthirius multifiliis, a causative agent of white spot disease, or proliferative kidney disease (PKD), one of the most economically damaging diseases in the rainbow trout industry which causative agent remain enigmatic. Besides long-recognized parasites, other potentially severe pathogens have appeared quite recently such as amphizoic amoebae, causative agents of amoebic gill disease (AGD), the monogenean Gyrodactylus salaris which has destroyed salmon populations in Norway, or sea lice, in particular Lepeophtheirus salmonis that endanger marine salmonids in some areas. Recent spreading of some parasites throughout the world (e.g. the cestode Bothriocephalus acheilognathi) has been facilitated through insufficient veterinary control during import of fish. Control of many important parasitic diseases is still far from being satisfactory and further research is needed. Use of chemotherapy has limitations and new effective, but environmentally safe drugs should be developed. A very promising area of future research seems to be studies on immunity in parasitic infections, use of molecular technology in diagnostics and development of new vaccines against the most pathogenic parasites.