Freshwater Bathing Research Articles

Anthelmintic effects of 75 ppm hydrogen peroxide treatment on the monogeneans Benedenia seriolae, Neobenedenia girellae, and Zeuxapta japonica infecting the skin and gills of greater amberjack Seriola dumerili

Infection by the capsalid monogeneans Benedenia seriolae and Neobenedenia girellae, and/or the microcotylid monogenean Zeuxapta japonica can be a problem when the greater amberjack Seriola dumerili (Carangidae) is commercially cultured. Effective control measures against these monogeneans that are safe for S. dumerili, especially during the summer when the ocean temperature increases, are still needed. Therefore, we investigated: 1) the anthelmintic effects of 75 and 300ppm hydrogen peroxide on these monogeneans in vitro trials, 2) the safety of hydrogen peroxide for S. dumerili at various concentrations for 6h, 3) the anthelmintic effects of 75 and 300ppm hydrogen peroxide on N. girellae infecting S. dumerili in vivo at water temperatures of 25 and 30°C, and 4) the efficacy of 75ppm hydrogen peroxide against the monogeneans infecting S. dumerili in a summer field trial. We found that for the in vitro trials, B. seriolae and N. girellae that were exposed to 75ppm hydrogen peroxide for 30 or 60min were more strongly affected than were those exposed to 300ppm hydrogen peroxide for 3min. In addition, the bodies of all Z. japonica contracted, the extent of contraction in those exposed to 75ppm hydrogen peroxide for 30min was greater than those exposed to 300ppm hydrogen peroxide for 3min. Uninfected fish suffered severe effects when exposed to 150 or 300ppm hydrogen peroxide, but when exposed to 75ppm hydrogen peroxide for 6h, their survival, appetite, and swimming behavior were unaffected. We found that N. girellae infection levels in fish exposed to 75ppm hydrogen peroxide for 30min in vivo were significantly lower than those exposed to 300ppm hydrogen peroxide for 3min at 25 and 30°C (P<0.01). In addition, complete elimination of B. seriolae, N. girellae, and Z. japonica was achieved under the condition of 75ppm hydrogen peroxide, 30min, and a water temperature of 28.5°C in the field trial. This treatment did not influence the behavior of the fish in the in vivo trials. Thus, treatment of cultured S. dumerili in a bath of 75ppm hydrogen peroxide for 30min should control these monogeneans. Statement of relevanceS. dumerili is a major commercially cultured fishes in Japan. These fish are cultured in floating net pens or cages, but the capsalid monogeneans B. seriolae and N. girellae can infect their skin and are, therefore, destructive parasites. Additionally, the microcotylid monogenean Z. japonica, which infects the gills of S.dumerili, is also a destructive parasite. To eliminate these parasites, fish may be fed praziquantel-supplemented feed, submerged in a freshwater bath for 2- to 5-min, or placed in a hydrogen peroxide bath. However, none of these measures simultaneously dislodge B. seriolae, N. girellae, and Z. japonica. Orally administered praziquantel is markedly poor against N. girellae infecting S. dumerili and Seriola quinqueradiata, and the freshwater treatment is not very effective when the fish are infected with Z. japonica. Additionally, the hydrogen peroxide treatment negatively affects fish health and viability, especially in the summer when water temperatures are higher and the parasite doubling and maturation rates increase. Thus, any parasite control measure must be performed frequently during the summer.For the study reported in our enclosed manuscript, we investigated: 1) the anthelmintic effect of hydrogen peroxide treatment at concentrations of 75 and 300ppm on those monogeneans in vitro, 2) the effects of hydrogen peroxide at various concentrations for 6h on S. dumerili, and 3) the anthelmintic effect of hydrogen peroxide treatment at concentrations of 75 and 300ppm against N. girellae in vivo and the effect of 75ppm hydrogen peroxide against these monogeneans in a field trial.This study shows that 75 ppm hydrogen peroxide for 30min could be for a way to eliminate B. seriolae, N. girellae, and Z. japonica when they infect commercially cultured S. dumerili. In addition, the treatment with 75ppm hydrogen peroxide for 6h did not negatively impact fish survival, swimming ability, and appetite. Thus, treatment of cultured S. dumerili in a bath of 75ppm hydrogen peroxide for 30min should control these monogeneans.

Influence of fresh water bathing on stress response of yellowtail Seriola quinqueradiata

Influence of fresh water bathing on stress response of yellowtail Seriola quinqueradiata

Shading of net cage is an effective control measure against skin fluke Neobenedenia girellae infection in chub mackerel Scomber japonicus

The skin fluke Neobenedenia girellae has become a serious problem in Japan since the 1990s. Present control methods focus on the removal of the attached parasite and these post-infection treatments are often labor intensive, time consuming, and/or stressful to fish. Chub mackerel Scomber japonicus are highly susceptible to N. girellae. However, because of their sensitive nature, bath treatments may cause mortality. In this study, we investigated the efficacy of cage shading to reduce skin fluke infection and the frequency of conventional post-infection treatments. Juvenile mackerel were reared in cages with or without shade for 3 months and their skin fluke infections were monitored. We performed either freshwater baths or oral administration of praziquantel if fluke intensity exceeded the given criteria. In unshaded cages, 3 total bath treatments or 6 total drug treatments were conducted. In contrast, no treatment was required for the shaded cage. The overall fluke intensity in the shaded cage was less than half that of the unshaded cages, despite the lack of treatments. This study demonstrated for the first time the practical use of shading in fish farms to reduce skin fluke infection.

Diseases of cultured marine fishes caused by Platyhelminthes (Monogenea, Digenea, Cestoda).

Mariculture is a rapidly developing industrial sector. Generally, fish are maintained in net cages with high density. Cage culture systems allow uncontrolled flow of sea water containing potentially infectious stages of fish parasites. In such culture conditions, prevention of such parasitic infections is difficult for parasites with life cycles that complete within culture sites, among which monogeneans and blood flukes are the most important platyhelminthes. Intense monogenean infections induce respiratory and osmo-regulatory dysfunctions. A variety of control measures have been developed, including freshwater bath treatment and chemotherapy. The potential to control monogenean infections through selective breeding, modified culture techniques to avoid infection, and general fish health management are discussed. It should be noted that mariculture conditions have provided some host-specific monogeneans with a chance to expand their host ranges. Blood flukes sometimes induce mass mortality among farmed fish. In-feed administration of praziquantel is the best solution to treat infected fish. Some cases are described that show how international trade in marine fish has resulted in the spread of hitherto unknown parasites into indigenous farmed and wild fish.

First record of Neobenedenia “melleni” – like species (Monogenea: Capsalidae) in Goliath grouper Epinephelus itajara in Brazil

Neobenedenia melleni (MacCallum, 1927) (Monogenea) is a pathogen widely spread in marine teleost cultures around the world. The present paper recorded this parasite species in the Goliath grouper Epinephelus itajara for the first time Brazil. The use of freshwater bath for parasite control was successful for the Goliath grouper.

Outbreak of mortality among cage-reared cobia (Rachycentron canadum) associated with parasitism

This study reports a disease outbreak among juvenile cobia (Rachycentron canadum) farmed in cages in the state of Rio de Janeiro, Brazil, caused by the dinoflagellate Amyloodinium ocellatum and the monogenean Neobenedenia melleni. Two thousand five hundred fish were stocked at 0.4 kg/m3 in a set of 12 m3 tanks, in autumn (mean weight 15.0 ± 7.3 g) and in winter (mean weight 43.0 ± 5.6 g). Freshwater baths were administered as a routine treatment, as the symptoms were detected followed by two collection samples. Firstly in May 2011 (n = 5) and secondly in September 2011 (n = 10). In the first sample, the prevalence of N. melleni on the body surface was 100% and the mean intensity was 42.0 ± 1.7, while in the second sample the prevalence was 60% with a mean intensity 3.0 ± 0.2 and mean abundance 1.8 ± 0.4. Amyloodinium ocellatum was only found in the second sample, at a prevalence 100% and mean intensity 46.8 ± 3.4. The cause of fish mortality was possibly associated with a decrease in fish resistance after the first contact with monogenean parasites, allied with respiratory difficulty caused by the presence of A. ocellatum in the gills.

A year-long study of Cryptosporidium species and subtypes in recreational, drinking and wastewater from the central area of Spain

A year-long longitudinal study was undertaken to evaluate the presence of Cryptosporidium spp. in drinking water treatment plants (DWTPs), wastewater treatment plants (WWTPs) and freshwater bathing beaches (FBBs) from the central area of Spain. Water samples were collected according to USEPA Method 1623, and concentrated by the IDEXX Filta-Max® system. Cryptosporidium species were detected based on PCR-restriction fragment length polymorphism and sequence analyses of the ssuRNA gene. C. hominis and/or C. parvum isolates were subtyped by DNA sequencing of the Gp60 gene. Among 150 samples, 23 (15.3%) were positive by IFAT and 40 (26.7%) by PCR. Cryptosporidium spp. was more frequent in WWTPs (26.2 and 50.8%) and FBBs (12.5 and 17.5%) by IFAT and PCR respectively. Effluent waters from DWTPs were negative for this parasite suggesting that they are suitable for public use. Tertiary treatment in the WWTPs demonstrated a high removal efficiency of Cryptosporidium in the samples evaluated. Cryptosporidium species identified included C. hominis, C. parvum, C. ubiquitum, C. andersoni and C. muris. Subtyping analysis revealed C. hominis IbA10G2 and IeA11G3T3 alleles, which is the first report of the latter in water samples. Cryptosporidium highest frequency was observed in winter and spring. Our data provide information about the occurrence and diversity of Cryptosporidium in water of human use from the central area of Spain.

The use of siliceous baths for the treatment of the patients presenting with arterial hypertension

This article is focused on the benefits of drug-free therapeutic technologies, such as the use of artificial general siliceous baths for the treatment of patients with arterial hypertension. This approach results in the anti-hypertensive effect simultaneous with the correction of lipid metabolism and lipid peroxidation. The positive changes due to the specific action of silicic acid were much more pronounced than those induced by the hydrostatic water pressure or water temperature known to be the main factors producing the therapeutic effect in fresh-water baths. General siliceous baths with the silicic acid concentration of 50-100 mg/I are recommended for the treatment of patients presenting with grade I-II arterial hypertension (stage II-III) and high risk of development of cardiovascular complications associated with coronary heart disease. One of the advantages of the proposed modality is it does not only produce the hypotensive effect but also exerts specific influence on lipid metabolism.

Efficacy of Bath Treatments of Formalin and Copper Sulfate on Cultured White Bass, Morone chrysops, Concurrently Infected by Onchocleidus mimus and Ichthyophthirius multifiliis

Concurrent ectoparasite infections on fishes are commonly encountered in the wild and in aquaculture (Rohde 1984; Hoffman 1999), for example, infections of both Ichthyophthirius multifiliis Fouquet, 1876 (Hymenostomatida: Ichthyopththiriidae; Colorni 2008) and gill‐infecting monogenoids (Platyhelminthes: Monogenoidea; Whittington and Chisholm 2008). The decision to aggressively treat infections of either of these parasites can be prudent in aquaculture systems. Both taxa have direct life cycles and short generation times, which may result in high infection intensities that can debilitate or kill the fish host (Noga 2010). Various chemical therapies have been evaluated to prevent, reduce, or eliminate such infections. Typical treatments for killing monogenoids include: freshwater or saltwater baths, formalin, copper sulfate (CuSO4), hydrogen peroxide, mebendazole, trichlorphon, and praziquantel (Whittington and Chisholm 2008). Treatments for infections of I. multifiliis (Ich) include: temporary exposure to high temperature, formalin, malachite green, CuSO4, potassium permanganate, and sodium chloride (Colorni 2008). Despite the common occurrence of these parasites on fish, little species‐specific information exists for the vast majority of fish–parasite combinations across the diversity of culture systems and captive settings. Formalin is approved by the US Food and Drug Administration (US FDA) for use as a treatment for external monogenetic trematodes in fish at up to 250 mg/L for up to 1 h (US FWS/AFS 2008). The ambiguity of this recommendation suggests a wide therapeutic range with no interpretation of repeated applications. Likewise, little information exists on the safety or efficacy of repeated formalin treatments on fish, and CuSO4 has an indeterminate approval status but is widely accepted as having high efficacy. In October 2009, white bass, Morone chrysops (Rafinesque, 1820; Perciformes: Moronidae), in a flow‐through culture system were diagnosed with concurrent infections of the gill‐infecting monogenoid Onchocleidus mimus (Mueller, 1936; Monogenoidea: Ancyrocephalidae) and Ich. O. mimus has a direct life cycle with no intermediate host required and as the intensity of infection increases can become problematic to a population. This study reports an opportunistic evaluation of the efficacy of repeated applications of formalin and CuSO4 to reduce, control, or eradicate these infections in cultured white bass.

Preliminary success using hydrogen peroxide to treat Atlantic salmon, Salmo salar L., affected with experimentally induced amoebic gill disease (AGD)

Currently, the only effective and commercially used treatment for amoebic gill disease (AGD) in farmed Tasmanian Atlantic salmon is freshwater bathing. Hydrogen peroxide (H₂O₂), commonly used throughout the aquaculture industry for a range of topical skin and gill infections, was trialled in vitro and in vivo to ascertain its potential as an alternative treatment against AGD. Under in vitro conditions, trophozoites of Neoparamoeba perurans were exposed to three concentrations of H₂O₂ in sea water (500, 1000 and 1500 mg L⁻¹) over four durations (10, 20, 30 and 60 min) each at two temperatures (12 and 18 °C). Trophozoite viability was assessed immediately post-exposure and after 24 h. A concentration/duration combination of 1000 mg L⁻¹ for >10 min demonstrated potent amoebicidal activity. Subsequently, Atlantic salmon mildly affected with experimentally induced AGD were treated with H₂O₂ at 12 and 18 °C for 15 min at 1250 mg L⁻¹ and their re-infection rate was compared to freshwater-treated fish over 21 days. Significant differences in the percentage of filaments affected with hyperplastic lesions (in association with amoebae) and plasma osmolality were noted between treatment groups immediately post-bath. However, the results were largely equivocal in terms of disease resolution over a 3-week period following treatment. These data suggest that H₂O₂ treatment in sea water successfully ameliorated a clinically light case of AGD under laboratory conditions.

Classroom measurements of sound speed in fresh/saline water

A tabletop apparatus permitting demonstrations and hands-on student exercises in the measurement of sound speed in water as a function of both temperature and salinity, at atmospheric pressure, is described. By measuring sound speed using a differential technique with a precisely measured path length difference Δx, errors due to uncertainty in original path length are minimized. The apparatus can be used to measure sound speed in freshwater baths of varying temperature and room temperature baths of varying salinities to provide the student a clear picture of sound speed variation along two axes.

Efficacy of oral praziquantel treatment against the skin fluke infection of cultured chub mackerel, Scomber japonicus

Outbreaks of skin flukes and associated mortalities have been observed in cultured chub mackerel, Scomber japonicus. Although freshwater bathing effectively removes the parasites, large post-treatment mortalities are occasionally observed especially under high water temperature conditions due to the stress from handling and confinement. Since outbreaks of the skin fluke infections often occur in high temperature seasons, development of an effective and safe control method is essential for the advancement of mackerel aquaculture. The present study aims to: 1) identify the skin fluke of cultured chub mackerel and 2) develop an alternative control measure using oral drug administration. The skin fluke was identified as Neobenedenia girellae by morphology and molecular data targeting ITS region of rDNA. Two trials of oral administration of praziquantel (PZQ), a common anthelmintic, were conducted and parasite intensity was compared before and after the treatment and between treated and untreated control fish. Fish rejected PZQ-coated commercial pellets, but oral administration was successfully achieved using frozen krill as a basal diet. The three-day administration with a dose of 150 mg·kg −1 BW·day −1 resulted in over 80% reduction in worm intensity. However, some proportion of the skin flukes survived the drug treatment. The resistance to PZQ does not relate to worm's developmental stage. Freshwater bathing was more effective and eradicated the parasite, but some post-treatment mortality of host fish was observed. Moreover, the parasite intensity drastically increased after the freshwater bathing, possibly due to stress and loss of mucus during the bathing. The study indicates the PZQ oral treatment is effective to control N. girellae in chub mackerel aquaculture when the drug is properly administrated with an appropriate feeding technique.

Gill disease in marine farmed Atlantic salmon at four farms in Ireland

A study of four marine salmon farms was undertaken in Ireland in 2008, with a focus on gill health and disease. All four farms suffered severe gill disease resulting in...

Understanding amoebic gill disease.

Abstract Amoebic gill disease (AGD) is a severely debilitating disease, which mainly affects the salmonid industry. It causes high production losses worldwide, including Tasmania, where it is the main health problem in farmed Atlantic salmon. Without the use of control procedures such as fresh water bathing and/or by maintaining fish population densities at a level commensurate with proper hygiene, this disease can often lead to the death of over 50% of infected salmon. AGD is caused by the parasitic amoeba, Neoparamoeba perurans , which binds to and inhabits the gill epithelium of growing fish. Pathology is associated with severe epithelial hyperplasia, fusion of gill lamellae, lowering of oxygen tension and pH of the blood and eventual death of the fish. In order to understand this disease process, research has been carried out, to study the immune response of fish to primary and secondary infections, the genetic basis of resistance to infection as well as how the parasite binds to the host gill tissue and induces a severe host response. Results have shown that N. perurans contains within its surface glycocalyx, high-molecular-weight mucin-like glycoproteins. These surface mucins are immunodominant, according to studies showing that they are the major antigens recognized by antibody in serum from fish exposed to the parasite, as well as when immunized with either whole parasites or subcellular fractions. Thus far, attempts to vaccinate salmon using the purified, high-molecular-weight surface antigen (HMWA) have failed to induce significant protective immunity. However, these studies have greatly aided our understanding of the molecular basis of the amoeba binding and infection process.

Geographical distribution of cercarial dermatitis in Norway

Bird schistosomes can cause a disease called cercarial dermatitis, or swimmer's itch, in humans. The disease occurs when people have direct contact with fresh water or sea water containing the free-swimming cercariae of the flukes. The symptoms are well known, and include intense itching, maculae, papulae, urticariae and, in some cases, local oedema with enlarged lymph nodes and fever. In this study, we present the geographical distribution of freshwater cercarial dermatitis in Norway. The study is based on random reports obtained from both individuals and physicians treating patients with itching skin rash after freshwater bathing. The first case of cercarial dermatitis in Norway was reported in 1980 and was traced to a lake near Trondheim in the central part of Norway. In the following years, an increasing number of cases were reported, especially in southern Norway. However, case reports are distributed almost all over the country, even from lakes in northern Norway. As far as we know, these are the northernmost case reports in Europe. So far, only one fluke species (Trichobilharzia franki) from a single infected snail (Radix auricularia) has been identified in Norway. However, unidentified schistosomatid ocellate cercariae have been found on several occasions in snails collected from six lakes where swimmer's itch is frequently reported. Future studies should be performed to identify the fluke species, as well as the most important snail and bird hosts, in Norwegian lakes.

Assessment of acquired protection levels against the parasite Neobenedenia girellae (Monogenea) between body surface sites including fins of amberjack Seriola dumerili (Carangidae) and the skin in response to the parasite infection

This study investigated acquired protection against the parasite Neobenedenia girellae (Capsalidae) at multiple body surface sites and fins of the amberjack Seriola dumerili (Carangidae). We also assessed histopathology of skin between naïve and primed fish. Fish, which were previously infected with N. girellae and treated by freshwater bath, was referred to as “primed fish”. The number of N. girella was significantly lower on primed S. dumerili than on naïve fish at 4 days and 8 days after exposure to N. girellae oncomiracidia when they were exposed in the same aquarium. Parasite numbers on pectoral and pelvic fins and on the dorsal (region 2) and belly side of fore body (region 3) were also significantly lower on primed fish than on naïve fish. In addition, the length of parasites was significantly shorter from primed S. dumerili fins than from naïve fish. However, in the head region (region 1), parasite number and length were similar. These results show that acquired protection was high in pectoral and pelvic fins and in regions 2 and 3 but low in region 1. Epidermis from regions 2 and 3 of oncomiracidia-exposed naïve fish at 4 days and 8 days after exposure was significantly thinner than the skin of an uninfected naïve fish, whereas epidermis of oncomiracidia-exposed primed fish at 8 days was significantly thicker than skin of uninfected primed fish. Furthermore, mucus cell numbers in regions 2 and 3 of primed fish was markedly increased at 4 days post-exposure as compared to naïve fish. Taken together, these results suggest that increased numbers of mucus cells and increased mucus secretion in fish skin may have contributed to acquired protection against N. girellae infection.

Two thousand parasites on a single ray: An infection with two species of skin monogeneans on a blotched fantail ray kept in an aquarium

AbstractA blotched fantail ray, Taeniurops meyeni (Müller et Henle, 1841), was captured in New Caledonia, South Pacific, and kept in a tank for quarantine before exhibition at the Nouméa public aquarium. After 24 days, the ray exhibited a heavy infection with two species of skin monogeneans. A freshwater bath allowed the collection of 1,914 monogeneans, including 1,453 capsalids, Neoentobdella taiwanensis Whittington et Kearn, 2009, on the ventral surface, and 461 monocotylids, Dendromonocotyle pipinna Chisholm et Whittington, 2004, on the dorsal surface. More than 300 monogeneans were prepared on slides to allow precise measurements. Capsalids and monocotylids occupied about 6% and 0.2% of the total ventral and dorsal ray surfaces, respectively.

A cross comparison of QPCR to agar-based or defined substrate test methods for the determination of Escherichia coli and enterococci in municipal water quality monitoring programs

Molecular methods such as quantitative, real-time polymerase chain reaction (QPCR) are intended to shorten the period between sampling and publicly available results. Cross comparison studies in Racine, WI, USA evaluated QPCR against agar-based (US EPA Method 1600) and defined substrate (IDEXX Colilert-18 ®) methods for the detection and quantification of Escherichia coli and enterococci in a variety of aqueous environments (wastewater, stormwater, and surface water). Regulatory outcomes were also compared based on choice of indicator and method. Positive correlation was seen between QPCR cell equivalents and viable cells through the wastewater treatment process and in all surface water samples (river or freshwater bathing beach) but not in direct stormwater discharge. For surface water samples, correlation improved with the application of a site-specific corrective factor, with regulatory action correctly predicted 98% of the time at bathing beaches. This study suggests the potential utility of QPCR for certain water quality monitoring applications.

Genetic variation of gross gill pathology and survival of Atlantic salmon ( Salmo salar L.) during natural amoebic gill disease challenge

Survival in an experimental disease challenge test or to natural disease challenge is utilised by aquaculture breeding programs as the selection trait for disease resistance. However, these trials are expensive and do not offer the ability to retest animals. The aim of this study was therefore to estimate genetic parameters for resistance to amoebic gill disease (AGD) measured by a categorical scale of gross gill signs (“gill score”) and survival in a field challenge in order to establish whether gill score provides adequate measurement of genetic variation for AGD resistance compared to an AGD challenge survival. A total of 1504 Atlantic salmon smolt, representing 140 full-sib families, was transferred to a marine site in SE Tasmania. The gills were assessed by gill score prior to freshwater bathing on the first two rounds of infection, and then the disease was allowed to develop until mortalities began. Gill score was reassessed after 50 days and mortality was allowed to continue until it had reached a plateau at 100 days. The overall survival rate was 32.3% but varied from 0% to 69% between families. Estimated narrow sense heritability for AGD resistance assessed by gill score varied between 0.23 and 0.48 over the three rounds of infection. Heritability of AGD survival challenge was 0.40 to 0.49 on the observed scale using binary and longitudinal measures. Gill score and survival showed a weak (− 0.19) to strong (− 0.96) negative genetic correlation which improved when assessed closer to the survival challenge. Estimated genetic gains by selection of the top one hundred estimated breeding values for gill score indicated that up to 82% of the expected gain in survival can be achieved when compared to estimated gain by selection upon survival (days to death), thus minimising selection costs and improving fish welfare whilst allowing repeat measures to be made. The results show that genetic variation of gill score at the early onset of losses closely compares with survival results if the disease is allowed to progress without subsequent freshwater bathing. Gill score may therefore be utilised as a nondestructive and repeatable selection trait for breeding Atlantic salmon with greater resistance to AGD.

The effect of oral antibiotic treatment and freshwater bath treatment on susceptibility to Neobenedenia girellae (Monogenea) infection of amberjack ( Seriola dumerili) and yellowtail ( S. quinqueradiata) hosts

Amberjack Seriola dumerili (Carangidae) and yellowtail S. quinqueradiata, important commercial fish in Japan, are susceptible to parasitic infection with Neobenedenia girellae (Capsalidae) when cultured in floating net cages. To prevent N. girellae infection, a 2- to 5-min freshwater dip is often practiced. In addition, many bacterial diseases, such as vibriosis, pasteurellosis, streptococcosis, mycobacteriosis and nocardiosis, also afflict cultivated Seriola species and oral administration of antibiotics is often necessary. The present study investigated whether oral antibiotics and freshwater bath treatments will influence the susceptibility to N. girellae infection of these two species. The infection intensity and growth of N. girellae on S. dumerili and S. quinqueradiata did not differ significantly between the control group and oxytetracycline, florfenicol, ampicillin, erythromycin or sulfamonomethoxine-treated groups, suggesting that oral antibiotics do not influence the susceptibility to N. girellae infection of S. dumerili or S. quinqueradiata hosts when administered at recommended doses. In contrast, freshwater bath treatment for 2 min significantly increased N. girellae infection on both S. dumerili and S. quinqueradiata compared to control groups exposed to oncomiracidia in the same aquarium. Thus, the susceptibility to N. girellae infection of these species was shown to be increased by freshwater bath treatment. These results may be well considered for controlling N. girellae infections when cultivating Seriola species.