Ichthyophthirius multifiliis (Ciliophora) Exit from Gill Epithelium1

The first change in the sequence of morphological events occurring as fully developed Ichthyophthirius multifiliis trophonts spontaneously left gill epithelium or as younger trophonts departed, following experimentally induced death of the fish, was the separation of parasites from overlying host cells. Discharge of contractile vacuoles may have played a role in this process. Spaces then appeared between host cells, and individual epithelial cells became vacuolated. Finally, the epithelium ruptured and the parasites swam free. In induced exit after three days residence in the host, departure of the trophont was evident only after autolysis of epithelium had occurred. Induced departure of trophonts after four days residence was more rapid, suggesting an active role for the parasite in exit. Changes in parasite and epithelium observed in induced exit were similar to those in spontaneous departure after five days residence.     

Ichthyophthiriasis: emphases on the epizootiology

Ichthyophthiriasis (white spot disease) is an economically important protozoosis caused by Ichthyophthirius multifiliis in freshwater fish. Medication prevention and curation are the main methods to control this disease with vaccines in laboratory, but the efficacy of drugs practically acts on the free‐living (nonparasitic) stage of I. multifiliis and can be easily impaired by a variety of environmental factors. Thus, study on the biological properties of I. multifiliis and the complicated ecological relationships between I. multifiliis and other biotic or abiotic factors that influence epidemicity of ichthyophthiriasis will contribute to integrated control of ichthyophthiriasis. In this article, some I. multifiliis biological properties, such as systematic position and life cycle of I. multifiliis, are briefly reviewed, and the seemingly abnormal phenomenon associated with in vitro cultures is specially discussed; then, the epizootiology of ichthyophthiriasis is emphasized, which involves various biotic or abiotic factors that impact the life and action of I. multifiliis. The susceptibility and stress reaction of fish to I. multifiliis infection are stated. Also, the pathogenicity and diagnosis of ichthyophthiriasis were covered, and an overall assessment is finally made on ichthyophthiriasis control.     

Immune response of fishes to ciliates

Ciliates are highly evolved protists comprising a phylum of diverse species, many of which are opportunistic or obligate parasites. Ciliates parasitic to fish consist of salt and freshwater forms with endo- or ectoparasitic modes of infection. Some of the more commonly encountered genera include Chilodonella, Brooklynella, Ophryoglenina, Ichthyophthirius, Cryptocaryon, Uronema, Tetrahymena, Epistylus, and Trichodina. Species range from obligate parasites and commensals to opportunistic, facultative forms. Some parasitic ciliates are highly pathogenic and fishes in closed environments such as aquaria and farm ponds are particularly susceptible to high mortalities. Nevertheless, fish have evolved an immune system capable of mounting an effective protective response against parasite challenge. Much of the experimental research on immunity against ciliates has been carried out with Ichthyophthirius multifiliis, on obligate parasite that invades surface epithelia of virtually all freshwater fish species. Interest in the immune response against I. multifiliis stems from the fact that convalescent fish become resistant to subsequent challenge (suggesting the possibility of immunoprophylaxis), and the need to curtail severe losses caused by this parasite in intensively farmed fishes. Furthermore, I. multifiliis has proven to be a useful experimental model because it is amenable to study under laboratory conditions. In this review cellular and humoral factors involved in both innate and acquired immunity against ciliates are covered and include natural killer cells, phagocytic cells, and antibody responses. Current ideas on the mechanisms of antibody-mediated cutaneous immunity against I. multifiliis are discussed and approaches toward the development of vaccines against this and other ciliate parasites are presented.     

Diversity of parasites in Cichlasoma amazonarum Kullander, 1983 during rainy and dry seasons in eastern Amazon (Brazil)

This study investigated the influence of the rainy and dry seasons on the parasite communities of Cichlasoma amazonarum (Kullander, 1983) in a tributary of the Amazon River system, northern Brazil. Of 112 fish examined, 95.5% were parasitized by Ichthyophthirius multifiliis, Piscinoodinium pillulare, Gussevia disparoides, Posthodiplostomum sp., Procamallanus (Spirocamallanus) inopinatus, and Echinorhynchus paranensis, as well as by the Glossiphoniidae leech. Ichthyophthirius multifiliis and P. pillulare were the dominant parasite species, with I. multifiliis the most prevalent and abundant; the leech (Glossiphoniidae) was the least prevalent and least abundant. Parasites presented an aggregate dispersion pattern and seasonal variations in infestation levels, influenced by the environmental conditions in the rainy season. Variation in the parasite dynamics created overall changes in the parasite communities, characterized by greater diversity, species richness and evenness during the rainy season. However, there was a high similarity (99.8%) in the parasite community structure between the rainy and dry seasons. During the dry season the hosts had predominantly 1–3 parasite species compared to 3–4 parasites in the rainy season. Some parasites in the eastern Amazon undergo population changes relating to seasonality. These results thus indicate the correct season to apply adequate prophylactic measures to reduce negative impacts of parasites in this wild ornamental fish when captured.     

Stage-Dependent Protein Composition in the Life Cycle of Synchronous Ichthyophthirius multifiliis,a Ciliate Fish Parasite1

The sequential development of Ichthyophthirius multifiliis Fouquet, 1876 at 22°C was monitored from detachment from the fish through to infective tomites using one-dimensional polyacrylamide gel electrophoresis. Methods are described for synchronizing I. multifiliis cells to permit the collection of large numbers of cells at stages during mitotic division. The electrophoretic protein patterns of the encysted stage differ from those of the tomite stage as well as from those of the other measured intervals.     

Experimental treatments with diflubenzuron and deltamethrin of sea bass, Dicentrarchus labrax L., infected with the isopod, Ceratothoa oestroides

Sea bass with approximate average weights of 5 and 20 g were treated against Ceratothoa oestroides infection with: (i) medicated pellets of diflubenzuron PC90 at a dosage of 3 mg kg^?1 body weight (BW) per day for 14 days. Lice were counted at the beginning of treatment and 19 days after treatment. The drug cleared all lice in the treated group; in the control group, infection remained high 30 days after beginning the experiment. It was concluded that medicated pellets containing 3 mg kg^?1 BW diflubenzuron effectively cleared pre‐adult and adult stages of the isopod parasite over a 14‐day period. No adverse effects were recorded in treated sea bass during the trials and no reinfection occurred 15 days after end of the treatment. (ii) Deltamethrin by means of bath treatments in infected sea bass kept in experimental tanks at 20°C. Before treatment, toxicity on healthy fish was preliminarily assessed by testing five fish from each size group at concentrations of 30, 10, 5, 3, 1, 0.1, 0.05 and 0.01 mg L^?1 for 30 min. The therapeutic concentrations tested were: 10, 5, 3, 0.15, 0.1, and 0.05 μg L^?1 and assessed at 1, 24 and 48 h. Best results were achieved with the 10 μg L^?1 (0.01 mg L^?1) dose, where prevalence was reduced from 100 to 0% over 24 h in both large and small fish. No parasite recovery was observed at 48 h. The dose of 5 μg L^?1 reduced prevalence from 100 to 11.7% and to 0% for small and large fish, respectively. Finally, with the 3 μg L^?1 dose, prevalence was reduced from 100 to 37.5% (small fish) and to 13.3% (large fish). Lower doses were ineffective on the parasites at either 24 or 48 h.     

小瓜虫在澳洲龙纹斑鳃器官上的分布及其影响

【目的】小瓜虫病是澳洲龙纹斑苗种阶段危害巨大的寄生虫病。探究小瓜虫在澳洲龙纹斑鳃器官上的分布及其影响可以丰富小瓜虫的致病性及病理学方面的研究内容,也可以为渔业生产中小瓜虫病的防治提供参考。【方法】采用光镜及扫描电镜技术确定病原,并观察小瓜虫在澳洲龙纹斑鳃丝、鳃小片及鳃盖上的分布情况及这些器官的变化情况。【结果】小瓜虫侵染澳洲龙纹斑的鳃器官后,分布在鳃丝、鳃小片及鳃盖的表面,上皮细胞之下及鳃小片之间,或是包裹在黏液细胞里,但侵染后期在鳃丝及鳃小片上的数量明显少于鳃盖。侵染后期,鳃丝、鳃小片及鳃盖出现一定程度的膨胀变形,黏液细胞分泌增多,鳃小片末端膨大变形甚至黏连融合,顶端充血呈球状或棒状。【结论】对于鳃部而言,侵染后期小瓜虫主要分布在澳洲龙纹斑的鳃盖上。小瓜虫主要通过侵染引发澳洲龙纹斑鳃部器官的变形膨大,使其丧失正常的功能,造成血液循环受阻、渗透压调节失衡,最终导致鱼体缺氧死亡。     

Differential effects of exposure to parasites and bacteria on stress response in turbot Scophthalmus maximus simultaneously stressed by low water depth

The stress response of turbot Scophthalmus maximus was evaluated in fish maintained 8 days under different water depths, normal (NWD, 30 cm depth, total water volume 40 l) or low (LWD, 5 cm depth, total water volume 10 l), in the additional presence of infection–infestation of two pathogens of this species. This was caused by intraperitoneal injection of sublethal doses of the bacterium Aeromonas salmonicida subsp. salmonicida or the parasite Philasterides dicentrarchi (Ciliophora:Scuticociliatida). The LWD conditions were stressful for fish, causing increased levels of cortisol in plasma, decreased levels of glycogen in liver and nicotinamide adenine dinucleotide phosphate (NADP) and increased activities of G6Pase and GSase. The presence of bacteria or parasites in fish under NWD resulted in increased cortisol levels in plasma whereas in liver, changes were of minor importance including decreased levels of lactate and GSase activity. The simultaneous presence of bacteria and parasites in fish under NWD resulted a sharp increase in the levels of cortisol in plasma and decreased levels of glucose. Decreased levels of glycogen and lactate and activities of GSase and glutathione reductase (GR), as well as increased activities of glucose‐6‐phosphate dehydrogenase (G6PDH), 6‐phosphogluconate dehydrogenase (6PGDH) and levels of nicotinamide adenine dinucleotide phosphate (NADPH) occurred in the same fish in liver. Finally, the presence of pathogens in S. maximus under stressful conditions elicited by LWD resulted in synergistic actions of both type of stressors in cortisol levels. In liver, the presence of bacteria or parasites induced a synergistic action on several variables such as decreased activities of G6Pase and GSase as well as increased levels of NADP and NADPH and increased activities of GPase, G6PDH and 6PGDH.     

Some remarks on parasitic infections of the invasive Neogobius spp. (Pisces) in the Hungarian reaches of the Danube River, with a description of Goussia szekelyi sp. n. (Apicomplexa: Eimeriidae)

During a survey of the parasite fauna Neogobius spp., which only recently invaded Hungarian reaches of the Danube River upstream from Budapest, it was found that Neogobius melanostomus and N. fluviatilis had become common species. Neogobius kessleri was less frequent and the previously widespread Proterorhinus marmoratus was drastically reduced. The parasite fauna of the newcomer fish species was characterized by ubiquitous parasites of a wide host range, such as the trematode Nicolla skrjabini, the metacercariae of Metagonimus yokogawai and Apatemon cobitidis, the larval stages of the acanthocephalan Pomphorhynchus laevis and the nematode Rhaphidascaris acus, the glochidia of an Anodonta sp. (Mollusca) and the ciliophoran Ichthyophthirius multifiliis. The specific parasites of Neogobius spp. were represented by three intestinal coccidia: Eimeria daviesae, Goussia kessleri, and a new species described here as Goussia szekelyi sp. n. This survey showed that the rapid invasion of Ponto‐Caspian gobies in the northern Danube caused important changes in the fish fauna composition of the Danube River and enriched the parasite fauna of Hungarian fishes.     

Critical Periods in Development of Ichthyophthirius multifiliis (Ciliophora) Populations1

ABSTRACT. Three periods in development that strongly influence population dynamics of Ichthyophthirius multifiliis were identified in experimental infections of channel catfish. The first occurred upon establishment within the host, 0 to 10 min postexposure (PE), when the parasite population that gained entrance declined 50%. Survival from 10 to 45 min PE, however, was constant. The second period identified came after I. multifiliis left the host and the free-living tomont encysted. The third occurred during reproduction. Although survival of encysting tomonts approached 100% among individuals departing after three to five days residence in the host, theront production varied significantly with parasite size, culture temperature during development, and length of residence by the trophont in the host. Theront production per tomont increased daily and on days three, four, and five PE was significantly higher for parasites developing at 24°C than for those at 21°C. At five days PE, mean production was 562 theronts/tomont and 240 theronts/tomont, respectively, and production by tomonts of equal size was greater for parasites maintained at 24°C.     

Density of Parasites in Various Organs and the Relation to Numbers of Trypomastigotes in the Blood during Acute Infections of Trypanosoma cruzi in Mice*

Quantitative methods were used to (a) determine the density of Trypanosoma cruzi in organs of CF₁ mice following intraperitoneal inoculation of 50,000 trypomastigotes of a Brazil strain of T. cruzi and (b) study the relation of the numbers of these intracellular stages to the numbers of trypomastigotes in the blood. Tissue stages (predominantly amastigotes) in heart, skeletal muscle (triceps), diaphragm, cerebrum, cerebellum, and musculature of stomach, duodenum, esophagus, jejunum, cecum, and rectum increased in numbers during the 1st 3 weeks of infection, reached maximum density 21–28 days after inoculation and subsequently declined in numbers until mice were histologically negative for intracellular parasites by 30–40 days. The density of tissue stages in the urinary bladder, uterine body, and ileum was similar with the exception that maximum numbers of parasites were observed slightly earlier at 15 days. The greatest density of intracellular stages was seen in heart, urinary bladder, diaphragm, and triceps muscle where mean counts of 44.6–60.0 × 10⁶ parasites/cc of muscle were recorded while maximum density of parasites in the uterine body, cerebrum, stomach, cerebellum, duodenum, esophagus, jejunum, ileum, cecum, and rectum was 13.0 × 10⁶/cc of muscle or less. Amastigotes were not observed in sections of lymph node, thymus, salivary glands, liver, spleen, or kidney and only a single pseudocyst containing 5 amastigotes was seen in lung. With the exception of the brain and lung, intracellular parasites were located exclusively in the musculature. Trypomastigotes in the blood increased during the 1st 3 weeks of infection, reached maximum numbers 21–28 days after initiation of infection, and subsequently decreased until by 30–40 days parasites were observed only rarely in the blood of a few animals. Thus generally close correlation was noted between the numbers of intracellular stages of T. cruzi in the organs and trypomastigotes in the blood throughout acute Chagas’ disease in mice as evidenced by the concomitant increase in numbers of both stages, the coincidence of days of maximum parasite levels, and the simultaneous decline in numbers of both stages. The mean number of parasites/pseudocyst section varied in the organs studied. Of the 15 positive organs studied, the pseudocyst sections in skeletal muscle contained the highest mean number of parasites (64.3 parasites/pseudocyst section) and those pseudocyst sections seen in the musculature of the small intestine contained the lowest mean number (5.5–6.8 parasites/pseudocyst section respectively in ileum and jejunum). Serial sections of skeletal muscle, heart, urinary bladder, and stomach revealed the largest pseudocysts in skeletal muscle while those in the musculature of the urinary bladder were the smallest.     

Seasonality affects the parasitism levels in two fish species in the eastern Amazon region

Fish parasite communities have strong interactions with the environment, the host fish, and the aquatic invertebrate communities. Thus, factors directly involving their different life cycles, such as different host populations and environments, as well as seasonal fluctuations in water levels can cause different responses. The main factors structuring parasite communities may be the hydrodynamic variations, together with seasonal variations in the availability of infectious stages of parasites in the environment. The purpose of this study was therefore to investigate the influence of the rainy and dry seasons on parasite infracommunities in fish species in the Amazon River System of Brazil. Between October 2009 and April 2011, specimens of Colossoma macropomum and Colossoma macropomum × Piaractus brachypomus hybrids (tambatinga) were examined for the presence of parasites. The parasite communities were similar in these two species that both showed aggregate dispersion, although the hybrid specimens were less parasitized. For both hosts, the Brillouin diversity, species richness, evenness and Berger‐Parker dominance indices were similar in the rainy season and dry season, except that the Berger‐Parker dominance index for the C. macropomum × P. brachypomus hybrid was higher during the dry season. Ichthyophthirius multifiliis was the dominant parasite, followed by Piscinoodinium pillulare, in both hosts. However, C. macropomum had a higher prevalence and abundance in the dry season, while in the C. macropomum × P. brachypomus hybrid, only the I. multifiliis prevalence increased. The abundance of monogenean species (Anacanthorus spathulatus, Linguadactyloides brinkmanni, Mymarothecium and Notozothecium janauachensis) was higher during the dry season in the gills of C. macropomum. Despite overdispersion of the lernaeid Perulernaea gamitanae, a seasonal pattern was seen only in the C. macropomum × P. brachypomus hybrid, with higher prevalence in the rainy season. The low prevalence and abundance of the ectoparasites Tetrahymena sp., Trichodina sp. and Braga patagonica, as well as of the endoparasites Procamallalus (Spirocamallanus) inopinatus and Neoechinorhynchus buttnerae, were not affected by seasonality. This information is applicable in farming practice for these economically important fish, as it indicates the best time for prophylactic management and treatment against parasites, in order to prevent economic losses in fish farms.     

Effects of the sea louse Lepeophtheirus salmonis on temporal changes in cortisol,sex steroids,growth and reproductive investment in Arctic charr Salvelinus alpinus

Groups of mature (5+ year old) Arctic charr Salvelinus alpinus held in sea water were exposed for 34 days to either a high (mean ±s.e . 0·15 ± 0·01 sea lice Lepeophtheirus salmonis g^?1 fish mass) (HI), medium (0·07 ± 0·00 sea lice g^?1 fish mass) (MI) or no [control (C)] sea‐lice infection during early stages of gonad development (June to July). Infection with sea lice resulted in increased plasma cortisol concentrations and this was related to intensity of infection; females tended to have higher cortisol concentrations than males at high infection intensities (HI group: female c. 130 ng ml^?1; male c. 80 ng ml^?1). Plasma osmolality (C c. 330, MI c. 350 and HI c. 415 mOsm) and chloride concentrations (C c. 135, MI c. 155 and HI c. 190 mM) increased significantly with infection intensity, indicating osmoregulatory problems in infected fish. A strong positive relationship between plasma osmolality and cortisol concentration was recorded. Plasma sex‐steroid concentrations were influenced negatively by sea‐lice infection, particularly in the HI group, and were inversely related to plasma cortisol concentrations. The most heavily infected fish postponed the initiation of reproductive development until exposed to fresh water and timing of ovulation tended to be delayed in these fish. Growth rate and condition were negatively influenced by sea‐lice infection and growth rate was inversely related to plasma cortisol concentrations. Sea‐lice infection resulted in mortality among females in the HI group, and the proportion of maturing females was lower in the MI group (46%) than in the controls (85%). Egg production in the MI and HI groups was c. 50 and 30% of the C group. Egg size, embryonic survival and fry mass did not differ across groups. Sea lice influence reproductive development and egg production in S. alpinus, and consequently these parasites may influence populations via sublethal effects on broodfish, affecting growth and condition, and their reproductive output.     

Grass carp (Ctenopharyngodon idella) TRAF6 and TAK1: Molecular cloning and expression analysis after Ichthyophthirius multifiliis infection

Ichthyophthirius multifiliis, a pathogenic ciliate parasite, infects almost all freshwater fish species and causes significant economic losses. Tumor necrosis factor receptor-associated factor 6 (TRAF6) and transforming growth factor-β-activated kinase 1 (TAK1) are two important signaling molecules involved in toll-like receptor (TLR) signal transduction. To date, the roles of TRAF6 and TAK1 in host defense against fish parasites are still poorly understood. In the present study, TRAF6 (CiTRAF6) and TAK1 (CiTAK1) were identified from grass carp (Ctenopharyngodon idella). The full-length cDNA sequence of CiTRAF6 (2250 bp) includes an open reading frame (ORF) of 1629 bp, which shows a high similarity to that of Cyprinus carpio TRAF6 and encodes a putative protein of 542 amino acids containing one RING domain, two zinc fingers, one coiled-coil region, and one MATH domain. The full-length CiTAK1 cDNA sequence is 2768 bp and includes an ORF of 1626 bp that encodes a putative protein of 541 amino acids containing a conserved serine/threonine protein kinase catalytic domain and a coiled-coil region. Phylogenetic analysis showed that CiTRAF6 and CiTAK1 were clustered with TRAF6 and TAK1 of other teleosts, respectively. CiTRAF6 and CiTAK1 were both constitutively expressed in all examined tissues but with varied expression levels. The highest expressions of CiTRAF6 and CiTAK1 were in the head kidney and spleen, respectively. The expression profiles of CiTRAF6 and CiTAK1 were detected in grass carp after I. multifiliis infection. Expressions of both genes were significantly up-regulated in the skin, gill, head kidney, and spleen at most time points after infection, indicating that CiTRAF6 and CiTAK1 may play essential roles in grass carp defense against I. multifiliis.     

Ecosystem consequences of fish parasites*

In most aquatic ecosystems, fishes are hosts to parasites and, sometimes, these parasites can affect fish biology. Some of the most dramatic cases occur when fishes are intermediate hosts for larval parasites. For example, fishes in southern California estuaries are host to many parasites. The most common of these parasites, Euhaplorchis californiensis, infects the brain of the killifish Fundulus parvipinnis and alters its behaviour, making the fish 10–30 times more susceptible to predation by the birds that serve as its definitive host. Parasites like E. californiensis are embedded in food webs because they require trophic transmission. In the Carpinteria Salt Marsh estuarine food web, parasites dominate the links and comprise substantial amount of biomass. Adding parasites to food webs alters important network statistics such as connectance and nestedness. Furthermore, some free‐living stages of parasites are food items for free‐living species. For instance, fishes feed on trematode cercariae. Being embedded in food webs makes parasites sensitive to changes in the environment. In particular, fishing and environmental disturbance, by reducing fish populations, may reduce parasite populations. Indirect evidence suggests a decrease in parasites in commercially fished species over the past three decades. In addition, environmental degradation can affect fish parasites. For these reasons, parasites in fishes may serve as indicators of environmental impacts.     

The intranuclear mitosis of the ciliates Paracineta limbata and Ichthyophtirius multifiliis

Electron microscope studies on the premetaphase stages of micronuclear divisions of Paracineta limbata and Ichthyophtirius multifiliis showed that spindle material also exists during interphase. In the case of I. multifiliis scattered microtubule fragments persist in the nuclear space; in P. limbata the micronuclei contain a small paracrystalloid which is suggested to be microtubular protein. Wide microtubules, varying in diameter from 300 to 400 Å develop during intranuclear prophase near the nuclear envelope in both cases. There are good reasons to assume that they function as a kind of stem body during the enlargement of the surface area of the nuclear envelope. Later micronuclear prophase stages of both species show a some-what different development. In I. multifiliis, there are scattered groups of short microtubular segments, partly in parallel array, whereas in P. limbata the wide tubules are transformed into normal microtubules of 180–200 Å diameter. The nuclei of both species are similar at late prophase and prometaphase stages. Bundles of interpolar microtubules run between the chromosomes, and single microtubules, presumably induced by the chromosomes, cross them at different angles. The chromosome-induced microtubules appear a short time after the interpolars. At prometaphase stage all microtubules show a highly parallel arrangement and therefore it is suggested that chromosomal tubules reach their final polar orientation by the action of cross-bridges.     

Ichthyophthirius (Ciliophora): Population Studies Suggest Reproduction in Host Epithelium1

Evidence that Ichthyophthirius multifiliis trophonts may reproduce within the epithelium of the host was obtained from experimental infections of channel catfish. Mean number of parasites spontaneously leaving the fish increased from 0 on day 3 postexposure (PE) to 66.5 per fish on day 7 PE. Mean population density in fin, however, increased five-fold from day 3 to day 5 PE in the absence of opportunity for reinfection. At day 3 PE, 100% of parasite loci in fin and gill arches contained solitary trophonts. At day 4 PE, 10% of loci in fin contained clusters of two trophonts; at day 7 PE, 69% contained clusters of two or more trophonts. The first clusters of four trophonts in fin were observed day 5 PE and of eight trophonts, day 8 PE. Trophonts in clusters were flattened against one another.     

Histopathological analyses of native silver catfish Rhamdia quelen (Quoy and Gaimard, 1824) immunized against and challenged with live theronts of Ichthyophthirius multifiliis

As an alternative to treating with chemicals, immunization using Ichthyophthirius multifiliis as a vaccine has been studied in fishes that were often affected with white spot diseases also to understand the possible changes to the tissue caused by the vaccine. The focus of this study was the analysis of the influence of immunization via intraperitoneal injection (i.p.) and via immersion bath (im.b.) on the histopathology of R. quelen after being challenged with live theronts of I. multifiliis distributed in: control (non‐immunized and non‐challenged); non‐immunized and challenged with 12,000 theronts/fish; non‐immunized and challenged with 22,000 theronts/fish; immunized and challenged with 12,000 theronts/fish; immunized and challenged with 22,000 theronts/fish. Water quality was measured in each assay, with 300 fingerlings distributed among 15 tanks with 20 fish in each of three replicates. Six days after challenge, samples for histopathological and parasitological analyses were collected. In both i.p. and im.b. fish the prevalence of I. multifillis in the gills was higher in the non‐immunized fish (33.33% and 27.77%, respectively). Melanomacrophages were present in 53% of the samples of i.p. non‐immunized fish. Fish im.b. immunized and challenged showed more atrophied areas in the hepatocytes. Higher numbers of melanomacrophages in the i.p. non‐immunized fish kidneys were observed compared to control. The results showed no difference in the gill lesions of either immunized or non‐immunized fish compared to control. Histological alterations in the organs of silver catfish were considered light, except in the liver that presented significant atrophy and hypertrophy of hepatocytes after immunization via i.p.     

Infection status of masu salmon Oncorhynchus masou by the parasite Salmonema cf. ephemeridarum (Nematoda,Cystidicolidae)

Nematodes that parasitize salmonids are found in both seawater and freshwater. Unlike seawater species such as those in family Anisakidae, freshwater species have not been well studied. In particular, the influences of these nematodes on the body condition of salmonids remain unclear. We studied the effects of Salmonema cf. ephemeridarum on the body condition of masu salmon Oncorhynchus masou. We found a positive relationship between the number of parasites and fish fork length. In contrast, we found a negative relationship between the body condition (condition factor) of fish and the number of parasites. These results suggest that nematode infection could affect host energy reserves for future growth.